     [file defs.inc not included]

This file documents the GPGME library.

This is Edition EDITION, last updated UPDATED, of The `GnuPG Made Easy' Reference Manual, for Version VERSION.

Copyright © 2002--2008, 2010, 2012--2017 g10 Code GmbH.

     Permission is granted to copy, distribute and/or modify this document under the terms of the GNU General Public License as published by the Free Software Foundation; either version 3 of the License, or (at your option) any later version. The text of the license can be found in the section entitled ``Copying''.

This document is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License for more details.

Main Menu
*********

This is Edition EDITION, last updated UPDATED, of The `GnuPG Made Easy' Reference Manual, for Version VERSION of the GPGME library.

Introduction
************

`GnuPG Made Easy' (GPGME) is a C language library that allows to add support for cryptography to a program.  It is designed to make access to public key crypto engines like GnuPG or GpgSM easier for applications.  GPGME provides a high-level crypto API for encryption, decryption, signing, signature verification and key management.

GPGME uses GnuPG and GpgSM as its backends to support OpenPGP and the Cryptographic Message Syntax (CMS).

Getting Started
===============

This manual documents the GPGME library programming interface.  All functions and data types provided by the library are explained.

The reader is assumed to possess basic knowledge about cryptography in general, and public key cryptography in particular.  The underlying cryptographic engines that are used by the library are not explained, but where necessary, special features or requirements by an engine are mentioned as far as they are relevant to GPGME or its users.

This manual can be used in several ways.  If read from the beginning to the end, it gives a good introduction into the library and how it can be used in an application.  Forward references are included where necessary.  Later on, the manual can be used as a reference manual to get just the information needed about any particular interface of the library.  Experienced programmers might want to start looking at the examples at the end of the manual, and then only read up those parts of the interface which are unclear.

The documentation for the language bindings is currently not included in this manual.  Those languages bindings follow the general programming model of GPGME but may provide some extra high level abstraction on top of the GPGME style API.  For now please see the README files in the lang/ directory of the source distribution.

Features
========

GPGME has a couple of advantages over other libraries doing a similar job, and over implementing support for GnuPG or other crypto engines into your application directly.

it's free software
     Anybody can use, modify, and redistribute it under the terms of the GNU Lesser General Public License (see Library Copying).

it's flexible
     GPGME provides transparent support for several cryptographic protocols by different engines.  Currently, GPGME supports the OpenPGP protocol using GnuPG as the backend, and the Cryptographic Message Syntax using GpgSM as the backend.

it's easy
     GPGME hides the differences between the protocols and engines from the programmer behind an easy-to-use interface.  This way the programmer can focus on the other parts of the program, and still integrate strong cryptography in his application.  Once support for GPGME has been added to a program, it is easy to add support for other crypto protocols once GPGME backends provide them.

it's language friendly
     GPGME comes with languages bindings for several common programming languages: Common Lisp, C++, Python 2, and Python 3.

Overview
========

GPGME provides a data abstraction that is used to pass data to the crypto engine, and receive returned data from it.  Data can be read from memory or from files, but it can also be provided by a callback function.

The actual cryptographic operations are always set within a context. A context provides configuration parameters that define the behaviour of all operations performed within it.  Only one operation per context is allowed at any time, but when one operation is finished, you can run the next operation in the same context.  There can be more than one context, and all can run different operations at the same time.

Furthermore, GPGME has rich key management facilities including listing keys, querying their attributes, generating, importing, exporting and deleting keys, and acquiring information about the trust path.

With some precautions, GPGME can be used in a multi-threaded environment, although it is not completely thread safe and thus needs the support of the application.

Preparation
***********

To use GPGME, you have to perform some changes to your sources and the build system.  The necessary changes are small and explained in the following sections.  At the end of this chapter, it is described how the library is initialized, and how the requirements of the library are verified.

Header
======

All interfaces (data types and functions) of the library are defined in the header file `gpgme.h'.  You must include this in all programs using the library, either directly or through some other header file, like this:

     #include <gpgme.h>

The name space of GPGME is gpgme_* for function names and data types and GPGME_* for other symbols.  Symbols internal to GPGME take the form _gpgme_* and _GPGME_*.

Because GPGME makes use of the GPG Error library, using GPGME will also use the GPG_ERR_* name space directly, and the gpg_err*, gpg_str*, and gpgrt_* name space indirectly.

Building the Source
===================

If you want to compile a source file including the `gpgme.h' header file, you must make sure that the compiler can find it in the directory hierarchy.  This is accomplished by adding the path to the directory in which the header file is located to the compilers include file search path (via the -I option).

However, the path to the include file is determined at the time the source is configured.  To solve this problem, gpgme ships with a small helper program gpgme-config that knows about the path to the include file and other configuration options.  The options that need to be added to the compiler invocation at compile time are output by the --cflags option to gpgme-config.  The following example shows how it can be used at the command line:

     gcc -c foo.c `gpgme-config --cflags`

Adding the output of gpgme-config --cflags to the compiler command line will ensure that the compiler can find the GPGME header file.

A similar problem occurs when linking the program with the library. Again, the compiler has to find the library files.  For this to work, the path to the library files has to be added to the library search path (via the -L option).  For this, the option --libs to gpgme-config can be used.  For convenience, this option also outputs all other options that are required to link the program with GPGME (in particular, the -lgpgme option).  The example shows how to link foo.o with the GPGME library to a program foo.

     gcc -o foo foo.o `gpgme-config --libs`

Of course you can also combine both examples to a single command by specifying both options to gpgme-config:

     gcc -o foo foo.c `gpgme-config --cflags --libs`

If you need to detect the installed language bindings you can use list them using:

     gpgme-config --print-lang

or test for the availability using

     gpgme-config --have-lang=python && echo 'Bindings for Pythons available'

Largefile Support (LFS)
=======================

GPGME is compiled with largefile support by default, if it is available on the system.  This means that GPGME supports files larger than two gigabyte in size, if the underlying operating system can.  On some systems, largefile support is already the default.  On such systems, nothing special is required.  However, some systems provide only support for files up to two gigabyte in size by default. Support for larger file sizes has to be specifically enabled.

To make a difficult situation even more complex, such systems provide two different types of largefile support.  You can either get all relevant functions replaced with alternatives that are largefile capable, or you can get new functions and data types for largefile support added.  Those new functions have the same name as their smallfile counterparts, but with a suffix of 64.

An example: The data type off_t is 32 bit wide on GNU/Linux PC systems.  To address offsets in large files, you can either enable largefile support add-on.  Then a new data type off64_t is provided, which is 64 bit wide.  Or you can replace the existing off_t data type with its 64 bit wide counterpart.  All occurrences of off_t are then automagically replaced.

As if matters were not complex enough, there are also two different types of file descriptors in such systems.  This is important because if file descriptors are exchanged between programs that use a different maximum file size, certain errors must be produced on some file descriptors to prevent subtle overflow bugs from occurring.

As you can see, supporting two different maximum file sizes at the same time is not at all an easy task.  However, the maximum file size does matter for GPGME, because some data types it uses in its interfaces are affected by that.  For example, the off_t data type is used in the gpgme_data_seek function, to match its POSIX counterpart.  This affects the call-frame of the function, and thus the ABI of the library.  Furthermore, file descriptors can be exchanged between GPGME and the application.

For you as the user of the library, this means that your program must be compiled in the same file size mode as the library.  Luckily, there is absolutely no valid reason for new programs to not enable largefile support by default and just use that.  The compatibility modes (small file sizes or dual mode) can be considered an historic artefact, only useful to allow for a transitional period.

On POSIX platforms GPGME is compiled using largefile support by default.  This means that your application must do the same, at least as far as it is relevant for using the gpgme.h header file.  All types in this header files refer to their largefile counterparts, if they are different from any default types on the system.

On 32 and 64 bit Windows platforms off_t is declared as 32 bit signed integer.  There is no specific support for LFS in the C library.  The recommendation from Microsoft is to use the native interface (CreateFile et al.) for large files.  Released binary versions of GPGME (libgpgme-11.dll) have always been build with a 32 bit off_t.  To avoid an ABI break we stick to this convention for 32 bit Windows by using long there. GPGME versions for 64 bit Windows have never been released and thus we are able to use int64_t instead of off_t there.  For easier migration the typedef gpgme_off_t has been defined.  The reason we cannot use off_t directly is that some toolchains (e.g. mingw64) introduce a POSIX compatible hack for off_t.  Some widely used toolkits make use of this hack and in turn GPGME would need to use it also.  However, this would introduce an ABI break and existing software making use of libgpgme might suffer from a severe break.  Thus with version 1.4.2 we redefined all functions using off_t to use gpgme_off_t which is defined as explained above.  This way we keep the ABI well defined and independent of any toolchain hacks.  The bottom line is that LFS support in GPGME is only available on 64 bit versions of Windows.

On POSIX platforms you can enable largefile support, if it is different from the default on the system the application is compiled on, by using the Autoconf macro AC_SYS_LARGEFILE.  If you do this, then you don't need to worry about anything else: It will just work.  In this case you might also want to use AC_FUNC_FSEEKO to take advantage of some new interfaces, and AC_TYPE_OFF_T (just in case).

If you do not use Autoconf, you can define the preprocessor symbol _FILE_OFFSET_BITS to 64 before including any header files, for example by specifying the option -D_FILE_OFFSET_BITS=64 on the compiler command line.  You will also want to define the preprocessor symbol LARGEFILE_SOURCE to 1 in this case, to take advantage of some new interfaces.

If you do not want to do either of the above, you probably know enough about the issue to invent your own solution.  Just keep in mind that the GPGME header file expects that largefile support is enabled, if it is available.  In particular, we do not support dual mode (_LARGEFILE64_SOURCE).

Using Automake
==============

It is much easier if you use GNU Automake instead of writing your own Makefiles.  If you do that you do not have to worry about finding and invoking the gpgme-config script at all.  GPGME provides an extension to Automake that does all the work for you.

 -- Macro: AM_PATH_GPGME ([minimum-version], [action-if-found], [action-if-not-found])
 -- Macro: AM_PATH_GPGME_PTH ([minimum-version], [action-if-found], [action-if-not-found])
 -- Macro: AM_PATH_GPGME_PTHREAD ([minimum-version], [action-if-found], [action-if-not-found])
     Check whether GPGME (at least version MINIMUM-VERSION, if given) exists on the host system.  If it is found, execute ACTION-IF-FOUND, otherwise do ACTION-IF-NOT-FOUND, if given.

     Additionally, the function defines GPGME_CFLAGS to the flags needed for compilation of the program to find the gpgme.h header file, and GPGME_LIBS to the linker flags needed to link the program to the GPGME library.  If the used helper script does not match the target type you are building for a warning is printed and the string libgcrypt is appended to the variable gpg_config_script_warn.

     AM_PATH_GPGME_PTH checks for the version of GPGME that can be used with GNU Pth, and defines GPGME_PTH_CFLAGS and GPGME_PTH_LIBS.

     AM_PATH_GPGME_PTHREAD checks for the version of GPGME that can be used with the native pthread implementation, and defines GPGME_PTHREAD_CFLAGS and GPGME_PTHREAD_LIBS. Since version 1.8.0 this is no longer required to GPGME_PTHREAD as GPGME itself is thread safe.

     This macro searches for gpgme-config along the PATH.  If you are cross-compiling, it is useful to set the environment variable SYSROOT to the top directory of your target.  The macro will then first look for the helper program in the bin directory below that top directory.  An absolute directory name must be used for SYSROOT.  Finally, if the configure command line option --with-gpgme-prefix is used, only its value is used for the top directory below which the helper script is expected.

You can use the defined Autoconf variables like this in your Makefile.am:

     AM_CPPFLAGS = $(GPGME_CFLAGS)
     LDADD = $(GPGME_LIBS)

Using Libtool
=============

The easiest way is to just use GNU Libtool.  If you use libtool, and link to libgpgme.la, libgpgme-pth.la or libgpgme-pthread.la respectively, everything will be done automatically by Libtool.

Library Version Check
=====================

 -- Function: const char * gpgme_check_version (const char *REQUIRED_VERSION)
     The function gpgme_check_version has four purposes.  It can be used to retrieve the version number of the library.  In addition it can verify that the version number is higher than a certain required version number.  In either case, the function initializes some sub-systems, and for this reason alone it must be invoked early in your program, before you make use of the other functions in GPGME.  The last purpose is to run selftests.

     As a side effect for W32 based systems, the socket layer will get initialized.

     If REQUIRED_VERSION is NULL, the function returns a pointer to a statically allocated string containing the version number of the library.

     If REQUIRED_VERSION is not NULL, it should point to a string containing a version number, and the function checks that the version of the library is at least as high as the version number provided.  In this case, the function returns a pointer to a statically allocated string containing the version number of the library.  If REQUIRED_VERSION is not a valid version number, or if the version requirement is not met, the function returns NULL.

     If you use a version of a library that is backwards compatible with older releases, but contains additional interfaces which your program uses, this function provides a run-time check if the necessary features are provided by the installed version of the library.

     If a selftest fails, the function may still succeed.  Selftest errors are returned later when invoking gpgme_new or gpgme-data_new, so that a detailed error code can be returned (historically, gpgme_check_version does not return a detailed error code).

 -- Function: int gpgme_set_global_flag
     (const char *NAME,
     const char *VALUE)

       SINCE: 1.4.0

     On some systems it is not easy to set environment variables and thus hard to use GPGME's internal trace facility for debugging. This function has been introduced as an alternative way to enable debugging and for a couple of other rarely used tweaks.  It is important to assure that only one thread accesses GPGME functions between a call to this function and after the return from the call to gpgme_check_version.

     All currently supported features require that this function is called as early as possible --- even before gpgme_check_version.  The features are identified by the following values for NAME:

     debug
          To enable debugging use the string ``debug'' for NAME and VALUE identical to the value used with the environment variable GPGME_DEBUG.

     disable-gpgconf
          Using this feature with any VALUE disables the detection of the gpgconf program and thus forces GPGME to fallback into the simple OpenPGP only mode.  It may be used to force the use of GnuPG-1 on systems which have both GPG versions installed.  Note that in general the use of gpgme_set_engine_info is a better way to select a specific engine version.

     gpgconf-name
     gpg-name
          Set the name of the gpgconf respective gpg binary.  The defaults are GNU/GnuPG/gpgconf and GNU/GnuPG/gpg.  Under Unix the leading directory part is ignored.  Under Windows the leading directory part is used as the default installation directory; the .exe suffix is added by GPGME.  Use forward slashed even under Windows.

     require-gnupg
          Set the mimimum version of the required GnuPG engine.  If that version is not met, GPGME fails early instead of trying to use the existent version.  The given version must be a string with major, minor, and micro number.  Example: "2.1.0".

     w32-inst-dir
          On Windows GPGME needs to know its installation directory to find its spawn helper.  This is in general no problem because a DLL has this information.  Some applications however link statically to GPGME and thus GPGME can only figure out the installation directory of this application which may be wrong in certain cases.  By supplying an installation directory as value to this flag, GPGME will assume that that directory is the installation directory.  This flag has no effect on non-Windows platforms.

     This function returns 0 on success.  In contrast to other functions the non-zero return value on failure does not convey any error code.  For setting ``debug'' the only possible error cause is an out of memory condition; which would exhibit itself later anyway. Thus the return value may be ignored.

After initializing GPGME, you should set the locale information to the locale required for your output terminal.  This locale information is needed for example for the curses and Gtk pinentry.  Here is an example of a complete initialization:

     #include <locale.h>
     #include <gpgme.h>

     void
     init_gpgme (void)
     {
       /* Initialize the locale environment.  */
       setlocale (LC_ALL, "");
       gpgme_check_version (NULL);
       gpgme_set_locale (NULL, LC_CTYPE, setlocale (LC_CTYPE, NULL));
     #ifdef LC_MESSAGES
       gpgme_set_locale (NULL, LC_MESSAGES, setlocale (LC_MESSAGES, NULL));
     #endif
     }

Note that you are highly recommended to initialize the locale settings like this.  GPGME can not do this for you because it would not be thread safe.  The conditional on LC_MESSAGES is only necessary for portability to W32 systems.

Signal Handling
===============

The GPGME library communicates with child processes (the crypto engines).  If a child process dies unexpectedly, for example due to a bug, or system problem, a SIGPIPE signal will be delivered to the application.  The default action is to abort the program.  To protect against this, gpgme_check_version sets the SIGPIPE signal action to SIG_IGN, which means that the signal will be ignored.

GPGME will only do that if the signal action for SIGPIPE is SIG_DEF at the time gpgme_check_version is called.  If it is something different, GPGME will take no action.

This means that if your application does not install any signal handler for SIGPIPE, you don't need to take any precautions. If you do install a signal handler for SIGPIPE, you must be prepared to handle any SIGPIPE events that occur due to GPGME writing to a defunct pipe.  Furthermore, if your application is multi-threaded, and you install a signal action for SIGPIPE, you must make sure you do this either before gpgme_check_version is called or afterwards.

Multi-Threading
===============

The GPGME library is mostly thread-safe, and can be used in a multi-threaded environment but there are some requirements for multi-threaded use:

  * The function gpgme_check_version must be called before any other function in the library, because it initializes the thread support subsystem in GPGME.  To achieve this in multi-threaded programs, you must synchronize the memory with respect to other threads that also want to use GPGME.  For this, it is sufficient to call gpgme_check_version before creating the other threads using GPGME (At least this is true for POSIX threads, as pthread_create is a function that synchronizes memory with respects to other threads.  There are many functions which have this property, a complete list can be found in POSIX, IEEE Std 1003.1-2003, Base Definitions, Issue 6, in the definition of the term ``Memory Synchronization''.  For other thread packages other, more relaxed or more strict rules may apply.).

  * Any gpgme_data_t and gpgme_ctx_t object must only be accessed by one thread at a time.  If multiple threads want to deal with the same object, the caller has to make sure that operations on that object are fully synchronized.

  * Only one thread at any time is allowed to call gpgme_wait.  If multiple threads call this function, the caller must make sure that all invocations are fully synchronized.  It is safe to start asynchronous operations while a thread is running in gpgme_wait.

  * The function gpgme_strerror is not thread safe.  You have to use gpgme_strerror_r instead.

Protocols and Engines
*********************

GPGME supports several cryptographic protocols, however, it does not implement them.  Rather it uses backends (also called engines) which implement the protocol.  GPGME uses inter-process communication to pass data back and forth between the application and the backend, but the details of the communication protocol and invocation of the backend is completely hidden by the interface.  All complexity is handled by GPGME.  Where an exchange of information between the application and the backend is necessary, GPGME provides the necessary callback function hooks and further interfaces.

 -- Data type: enum gpgme_protocol_t
     The gpgme_protocol_t type specifies the set of possible protocol values that are supported by GPGME.  The following protocols are supported:

     GPGME_PROTOCOL_OpenPGP
     GPGME_PROTOCOL_OPENPGP
          This specifies the OpenPGP protocol.

     GPGME_PROTOCOL_CMS
          This specifies the Cryptographic Message Syntax.

     GPGME_PROTOCOL_GPGCONF
          Under development.  Please ask on gnupg-devel@gnupg.org for help.

     GPGME_PROTOCOL_ASSUAN
            SINCE: 1.2.0

          This specifies the raw Assuan protocol.

     GPGME_PROTOCOL_G13
            SINCE: 1.3.0

          Under development.  Please ask on gnupg-devel@gnupg.org for help.

     GPGME_PROTOCOL_UISERVER
          Under development.  Please ask on gnupg-devel@gnupg.org for help.

     GPGME_PROTOCOL_SPAWN
            SINCE: 1.5.0

          Special protocol for use with gpgme_op_spawn.

     GPGME_PROTOCOL_UNKNOWN
          Reserved for future extension.  You may use this to indicate that the used protocol is not known to the application.  Currently, GPGME does not accept this value in any operation, though, except for gpgme_get_protocol_name.

 -- Function: const char * gpgme_get_protocol_name (gpgme_protocol_t PROTOCOL)
     The function gpgme_get_protocol_name returns a statically allocated string describing the protocol PROTOCOL, or NULL if the protocol number is not valid.

Engine Version Check
====================

 -- Function: const char * gpgme_get_dirinfo (cons char *WHAT)
       SINCE: 1.5.0

     The function gpgme_get_dirinfo returns a statically allocated string with the value associated to WHAT.  The returned values are the defaults and won't change even after gpgme_set_engine_info has been used to configure a different engine.  NULL is returned if no value is available.  Commonly supported values for WHAT are:

     homedir
          Return the default home directory.

     sysconfdir
          Return the name of the system configuration directory

     bindir
          Return the name of the directory with GnuPG program files.

     libdir
          Return the name of the directory with GnuPG related library files.

     libexecdir
          Return the name of the directory with GnuPG helper program files.

     datadir
          Return the name of the directory with GnuPG shared data.

     localedir
          Return the name of the directory with GnuPG locale data.

     agent-socket
          Return the name of the socket to connect to the gpg-agent.

     agent-ssh-socket
          Return the name of the socket to connect to the ssh-agent component of gpg-agent.

     dirmngr-socket
          Return the name of the socket to connect to the dirmngr.

     uiserver-socket
          Return the name of the socket to connect to the user interface server.

     gpgconf-name
          Return the file name of the engine configuration tool.

     gpg-name
          Return the file name of the OpenPGP engine.

     gpgsm-name
          Return the file name of the CMS engine.

     g13-name
          Return the name of the file container encryption engine.

     gpg-wks-client-name
          Return the name of the Web Key Service tool.

 -- Function: gpgme_error_t gpgme_engine_check_version (gpgme_protocol_t PROTOCOL)
     The function gpgme_engine_check_version verifies that the engine implementing the protocol PROTOCOL is installed in the expected path and meets the version requirement of GPGME.

     This function returns the error code GPG_ERR_NO_ERROR if the engine is available and GPG_ERR_INV_ENGINE if it is not.

Engine Information
==================

 -- Data type: gpgme_engine_info_t
     The gpgme_engine_info_t type specifies a pointer to a structure describing a crypto engine.  The structure contains the following elements:

     gpgme_engine_info_t next
          This is a pointer to the next engine info structure in the linked list, or NULL if this is the last element.

     gpgme_protocol_t protocol
          This is the protocol for which the crypto engine is used.  You can convert this to a string with gpgme_get_protocol_name for printing.

     const char *file_name
          This is a string holding the file name of the executable of the crypto engine.  Currently, it is never NULL, but using NULL is reserved for future use, so always check before you use it.

     const char *home_dir
          This is a string holding the directory name of the crypto engine's configuration directory.  If it is NULL, then the default directory is used.  See gpgme_get_dirinfo on how to get the default directory.

     const char *version
          This is a string containing the version number of the crypto engine. It might be NULL if the version number can not be determined, for example because the executable doesn't exist or is invalid.

     const char *req_version
          This is a string containing the minimum required version number of the crypto engine for GPGME to work correctly.  This is the version number that gpgme_engine_check_version verifies against.  Currently, it is never NULL, but using NULL is reserved for future use, so always check before you use it.

 -- Function: gpgme_error_t gpgme_get_engine_info (gpgme_engine_info_t *INFO)
     The function gpgme_get_engine_info returns a linked list of engine info structures in INFO.  Each info structure describes the defaults of one configured backend.

     The memory for the info structures is allocated the first time this function is invoked, and must not be freed by the caller.

     This function returns the error code GPG_ERR_NO_ERROR if successful, and a system error if the memory could not be allocated.

Here is an example how you can provide more diagnostics if you receive an error message which indicates that the crypto engine is invalid.

     gpgme_ctx_t ctx;
     gpgme_error_t err;

     [...]

     if (gpgme_err_code (err) == GPG_ERR_INV_ENGINE)
       {
         gpgme_engine_info_t info;
         err = gpgme_get_engine_info (&info);
         if (!err)
           {
             while (info && info->protocol != gpgme_get_protocol (ctx))
               info = info->next;
             if (!info)
               fprintf (stderr, "GPGME compiled without support for protocol %s",
                        gpgme_get_protocol_name (info->protocol));
             else if (info->file_name && !info->version)
               fprintf (stderr, "Engine %s not installed properly",
                        info->file_name);
             else if (info->file_name && info->version && info->req_version)
               fprintf (stderr, "Engine %s version %s installed, "
                        "but at least version %s required", info->file_name,
                        info->version, info->req_version);
             else
               fprintf (stderr, "Unknown problem with engine for protocol %s",
                        gpgme_get_protocol_name (info->protocol));
           }
       }

Engine Configuration
====================

You can change the configuration of a backend engine, and thus change the executable program and configuration directory to be used.  You can make these changes the default or set them for some contexts individually.

 -- Function: gpgme_error_t gpgme_set_engine_info (gpgme_protocol_t PROTO, const char *FILE_NAME, const char *HOME_DIR)
       SINCE: 1.1.0

     The function gpgme_set_engine_info changes the default configuration of the crypto engine implementing the protocol PROTO.

     FILE_NAME is the file name of the executable program implementing this protocol, and HOME_DIR is the directory name of the configuration directory for this crypto engine.  If HOME_DIR is NULL, the engine's default will be used.

     The new defaults are not applied to already created GPGME contexts.

     This function returns the error code GPG_ERR_NO_ERROR if successful, or an error code on failure.

The functions gpgme_ctx_get_engine_info and gpgme_ctx_set_engine_info can be used to change the engine configuration per context.  See Crypto Engine.

OpenPGP
=======

OpenPGP is implemented by GnuPG, the GNU Privacy Guard. This is the first protocol that was supported by GPGME.

The OpenPGP protocol is specified by GPGME_PROTOCOL_OpenPGP.

Cryptographic Message Syntax
============================

CMS is implemented by GpgSM, the S/MIME implementation for GnuPG.

The CMS protocol is specified by GPGME_PROTOCOL_CMS.

Assuan
======

Assuan is the RPC library used by the various GnuPG components.  The Assuan protocol allows one to talk to arbitrary Assuan servers using GPGME.  See Using the Assuan protocol.

The ASSUAN protocol is specified by GPGME_PROTOCOL_ASSUAN.

Algorithms
**********

The crypto backends support a variety of algorithms used in public key cryptography. (Some engines also provide symmetric only encryption; see the description of the encryption function on how to use this.)  The following sections list the identifiers used to denote such an algorithm.

Public Key Algorithms
=====================

Public key algorithms are used for encryption, decryption, signing and verification of signatures.

 -- Data type: enum gpgme_pubkey_algo_t
     The gpgme_pubkey_algo_t type specifies the set of all public key algorithms that are supported by GPGME.  Possible values are:

     GPGME_PK_RSA
          This value indicates the RSA (Rivest, Shamir, Adleman) algorithm.

     GPGME_PK_RSA_E
          Deprecated.  This value indicates the RSA (Rivest, Shamir, Adleman) algorithm for encryption and decryption only.

     GPGME_PK_RSA_S
          Deprecated.  This value indicates the RSA (Rivest, Shamir, Adleman) algorithm for signing and verification only.

     GPGME_PK_DSA
          This value indicates DSA, the Digital Signature Algorithm.

     GPGME_PK_ELG
          This value indicates ElGamal.

     GPGME_PK_ELG_E
          This value also indicates ElGamal and is used specifically in GnuPG.

     GPGME_PK_ECC
            SINCE: 1.5.0

          This value is a generic indicator for ellipic curve algorithms.

     GPGME_PK_ECDSA
            SINCE: 1.3.0

          This value indicates ECDSA, the Elliptic Curve Digital Signature Algorithm as defined by FIPS 186-2 and RFC-6637.

     GPGME_PK_ECDH
            SINCE: 1.3.0

          This value indicates ECDH, the Eliptic Curve Diffie-Hellmann encryption algorithm as defined by RFC-6637.

     GPGME_PK_EDDSA
            SINCE: 1.7.0

          This value indicates the EdDSA algorithm.

 -- Function: const char * gpgme_pubkey_algo_name (gpgme_pubkey_algo_t ALGO)
     The function gpgme_pubkey_algo_name returns a pointer to a statically allocated string containing a description of the public key algorithm ALGO.  This string can be used to output the name of the public key algorithm to the user.

     If ALGO is not a valid public key algorithm, NULL is returned.

 -- Function: char * gpgme_pubkey_algo_string (gpgme_subkey_t KEY)
       SINCE: 1.7.0

     The function gpgme_pubkey_algo_string is a convenience function to build and return an algorithm string in the same way GnuPG does (e.g. ``rsa2048'' or ``ed25519'').  The caller must free the result using gpgme_free.  On error (e.g. invalid argument or memory exhausted), the function returns NULL and sets ERRNO.

Hash Algorithms
===============

Hash (message digest) algorithms are used to compress a long message to make it suitable for public key cryptography.

 -- Data type: enum gpgme_hash_algo_t
     The gpgme_hash_algo_t type specifies the set of all hash algorithms that are supported by GPGME.  Possible values are:

     GPGME_MD_MD5

     GPGME_MD_SHA1

     GPGME_MD_RMD160

     GPGME_MD_MD2

     GPGME_MD_TIGER

     GPGME_MD_HAVAL

     GPGME_MD_SHA256

     GPGME_MD_SHA384

     GPGME_MD_SHA512

     GPGME_MD_SHA224
            SINCE: 1.5.0

     GPGME_MD_MD4

     GPGME_MD_CRC32

     GPGME_MD_CRC32_RFC1510

     GPGME_MD_CRC24_RFC2440

 -- Function: const char * gpgme_hash_algo_name (gpgme_hash_algo_t ALGO)
     The function gpgme_hash_algo_name returns a pointer to a statically allocated string containing a description of the hash algorithm ALGO.  This string can be used to output the name of the hash algorithm to the user.

     If ALGO is not a valid hash algorithm, NULL is returned.

Error Handling
**************

Many functions in GPGME can return an error if they fail. For this reason, the application should always catch the error condition and take appropriate measures, for example by releasing the resources and passing the error up to the caller, or by displaying a descriptive message to the user and cancelling the operation.

Some error values do not indicate a system error or an error in the operation, but the result of an operation that failed properly.  For example, if you try to decrypt a tempered message, the decryption will fail.  Another error value actually means that the end of a data buffer or list has been reached.  The following descriptions explain for many error codes what they mean usually.  Some error values have specific meanings if returned by a certain functions.  Such cases are described in the documentation of those functions.

GPGME uses the libgpg-error library.  This allows to share the error codes with other components of the GnuPG system, and thus pass error values transparently from the crypto engine, or some helper application of the crypto engine, to the user.  This way no information is lost.  As a consequence, GPGME does not use its own identifiers for error codes, but uses those provided by libgpg-error.  They usually start with GPG_ERR_.

However, GPGME does provide aliases for the functions defined in libgpg-error, which might be preferred for name space consistency.

Error Values
============

 -- Data type: gpgme_err_code_t
     The gpgme_err_code_t type is an alias for the libgpg-error type gpg_err_code_t.  The error code indicates the type of an error, or the reason why an operation failed.

     A list of important error codes can be found in the next section.

 -- Data type: gpgme_err_source_t
     The gpgme_err_source_t type is an alias for the libgpg-error type gpg_err_source_t.  The error source has not a precisely defined meaning.  Sometimes it is the place where the error happened, sometimes it is the place where an error was encoded into an error value.  Usually the error source will give an indication to where to look for the problem.  This is not always true, but it is attempted to achieve this goal.

     A list of important error sources can be found in the next section.

 -- Data type: gpgme_error_t
     The gpgme_error_t type is an alias for the libgpg-error type gpg_error_t.  An error value like this has always two components, an error code and an error source.  Both together form the error value.

     Thus, the error value can not be directly compared against an error code, but the accessor functions described below must be used. However, it is guaranteed that only 0 is used to indicate success (GPG_ERR_NO_ERROR), and that in this case all other parts of the error value are set to 0, too.

     Note that in GPGME, the error source is used purely for diagnostical purposes.  Only the error code should be checked to test for a certain outcome of a function.  The manual only documents the error code part of an error value.  The error source is left unspecified and might be anything.

 -- Function: static inline gpgme_err_code_t gpgme_err_code (gpgme_error_t ERR)
     The static inline function gpgme_err_code returns the gpgme_err_code_t component of the error value ERR.  This function must be used to extract the error code from an error value in order to compare it with the GPG_ERR_* error code macros.

 -- Function: static inline gpgme_err_source_t gpgme_err_source (gpgme_error_t ERR)
     The static inline function gpgme_err_source returns the gpgme_err_source_t component of the error value ERR.  This function must be used to extract the error source from an error value in order to compare it with the GPG_ERR_SOURCE_* error source macros.

 -- Function: static inline gpgme_error_t gpgme_err_make (gpgme_err_source_t SOURCE, gpgme_err_code_t CODE)
     The static inline function gpgme_err_make returns the error value consisting of the error source SOURCE and the error code CODE.

     This function can be used in callback functions to construct an error value to return it to the library.

 -- Function: static inline gpgme_error_t gpgme_error (gpgme_err_code_t CODE)
     The static inline function gpgme_error returns the error value consisting of the default error source and the error code CODE.

     For GPGME applications, the default error source is GPG_ERR_SOURCE_USER_1.  You can define GPGME_ERR_SOURCE_DEFAULT before including gpgme.h to change this default.

     This function can be used in callback functions to construct an error value to return it to the library.

The libgpg-error library provides error codes for all system error numbers it knows about.  If ERR is an unknown error number, the error code GPG_ERR_UNKNOWN_ERRNO is used.  The following functions can be used to construct error values from system errnor numbers.

 -- Function: gpgme_error_t gpgme_err_make_from_errno (gpgme_err_source_t SOURCE, int ERR)
     The function gpgme_err_make_from_errno is like gpgme_err_make, but it takes a system error like errno instead of a gpgme_err_code_t error code.

 -- Function: gpgme_error_t gpgme_error_from_errno (int ERR)
     The function gpgme_error_from_errno is like gpgme_error, but it takes a system error like errno instead of a gpgme_err_code_t error code.

Sometimes you might want to map system error numbers to error codes directly, or map an error code representing a system error back to the system error number.  The following functions can be used to do that.

 -- Function: gpgme_err_code_t gpgme_err_code_from_errno (int ERR)
     The function gpgme_err_code_from_errno returns the error code for the system error ERR.  If ERR is not a known system error, the function returns GPG_ERR_UNKNOWN_ERRNO.

 -- Function: int gpgme_err_code_to_errno (gpgme_err_code_t ERR)
     The function gpgme_err_code_to_errno returns the system error for the error code ERR.  If ERR is not an error code representing a system error, or if this system error is not defined on this system, the function returns 0.

Error Sources
=============

The library libgpg-error defines an error source for every component of the GnuPG system.  The error source part of an error value is not well defined.  As such it is mainly useful to improve the diagnostic error message for the user.

If the error code part of an error value is 0, the whole error value will be 0.  In this case the error source part is of course GPG_ERR_SOURCE_UNKNOWN.

The list of error sources that might occur in applications using GPGME is:

GPG_ERR_SOURCE_UNKNOWN
     The error source is not known.  The value of this error source is 0.

GPG_ERR_SOURCE_GPGME
     The error source is GPGME itself.  This is the default for errors that occur in the GPGME library.

GPG_ERR_SOURCE_GPG
     The error source is GnuPG, which is the crypto engine used for the OpenPGP protocol.

GPG_ERR_SOURCE_GPGSM
     The error source is GPGSM, which is the crypto engine used for the CMS protocol.

GPG_ERR_SOURCE_GCRYPT
     The error source is libgcrypt, which is used by crypto engines to perform cryptographic operations.

GPG_ERR_SOURCE_GPGAGENT
     The error source is gpg-agent, which is used by crypto engines to perform operations with the secret key.

GPG_ERR_SOURCE_PINENTRY
     The error source is pinentry, which is used by gpg-agent to query the passphrase to unlock a secret key.

GPG_ERR_SOURCE_SCD
     The error source is the SmartCard Daemon, which is used by gpg-agent to delegate operations with the secret key to a SmartCard.

GPG_ERR_SOURCE_KEYBOX
     The error source is libkbx, a library used by the crypto engines to manage local keyrings.

GPG_ERR_SOURCE_USER_1

GPG_ERR_SOURCE_USER_2

GPG_ERR_SOURCE_USER_3

GPG_ERR_SOURCE_USER_4
     These error sources are not used by any GnuPG component and can be used by other software.  For example, applications using GPGME can use them to mark error values coming from callback handlers.  Thus GPG_ERR_SOURCE_USER_1 is the default for errors created with gpgme_error and gpgme_error_from_errno, unless you define GPGME_ERR_SOURCE_DEFAULT before including gpgme.h.

Error Codes
===========

The library libgpg-error defines many error values.  Most of them are not used by GPGME directly, but might be returned by GPGME because it received them from the crypto engine.  The below list only includes such error codes that have a specific meaning in GPGME, or which are so common that you should know about them.

GPG_ERR_EOF
     This value indicates the end of a list, buffer or file.

GPG_ERR_NO_ERROR
     This value indicates success.  The value of this error code is 0.  Also, it is guaranteed that an error value made from the error code 0 will be 0 itself (as a whole).  This means that the error source information is lost for this error code, however, as this error code indicates that no error occurred, this is generally not a problem.

GPG_ERR_GENERAL
     This value means that something went wrong, but either there is not enough information about the problem to return a more useful error value, or there is no separate error value for this type of problem.

GPG_ERR_ENOMEM
     This value means that an out-of-memory condition occurred.

GPG_ERR_E...
     System errors are mapped to GPG_ERR_FOO where FOO is the symbol for the system error.

GPG_ERR_INV_VALUE
     This value means that some user provided data was out of range.  This can also refer to objects.  For example, if an empty gpgme_data_t object was expected, but one containing data was provided, this error value is returned.

GPG_ERR_UNUSABLE_PUBKEY
     This value means that some recipients for a message were invalid.

GPG_ERR_UNUSABLE_SECKEY
     This value means that some signers were invalid.

GPG_ERR_NO_DATA
     This value means that a gpgme_data_t object which was expected to have content was found empty.

GPG_ERR_CONFLICT
     This value means that a conflict of some sort occurred.

GPG_ERR_NOT_IMPLEMENTED
     This value indicates that the specific function (or operation) is not implemented.  This error should never happen.  It can only occur if you use certain values or configuration options which do not work, but for which we think that they should work at some later time.

GPG_ERR_DECRYPT_FAILED
     This value indicates that a decryption operation was unsuccessful.

GPG_ERR_BAD_PASSPHRASE
     This value means that the user did not provide a correct passphrase when requested.

GPG_ERR_CANCELED
     This value means that the operation was canceled.

GPG_ERR_INV_ENGINE
     This value means that the engine that implements the desired protocol is currently not available.  This can either be because the sources were configured to exclude support for this engine, or because the engine is not installed properly.

GPG_ERR_AMBIGUOUS_NAME
     This value indicates that a user ID or other specifier did not specify a unique key.

GPG_ERR_WRONG_KEY_USAGE
     This value indicates that a key is not used appropriately.

GPG_ERR_CERT_REVOKED
     This value indicates that a key signature was revoced.

GPG_ERR_CERT_EXPIRED
     This value indicates that a key signature expired.

GPG_ERR_NO_CRL_KNOWN
     This value indicates that no certificate revocation list is known for the certificate.

GPG_ERR_NO_POLICY_MATCH
     This value indicates that a policy issue occurred.

GPG_ERR_NO_SECKEY
     This value indicates that no secret key for the user ID is available.

GPG_ERR_MISSING_CERT
     This value indicates that a key could not be imported because the issuer certificate is missing.

GPG_ERR_BAD_CERT_CHAIN
     This value indicates that a key could not be imported because its certificate chain is not good, for example it could be too long.

GPG_ERR_UNSUPPORTED_ALGORITHM
     This value means a verification failed because the cryptographic algorithm is not supported by the crypto backend.

GPG_ERR_BAD_SIGNATURE
     This value means a verification failed because the signature is bad.

GPG_ERR_NO_PUBKEY
     This value means a verification failed because the public key is not available.

GPG_ERR_USER_1

GPG_ERR_USER_2

...

GPG_ERR_USER_16
     These error codes are not used by any GnuPG component and can be freely used by other software.  Applications using GPGME might use them to mark specific errors returned by callback handlers if no suitable error codes (including the system errors) for these errors exist already.

Error Strings
=============

 -- Function: const char * gpgme_strerror (gpgme_error_t ERR)
     The function gpgme_strerror returns a pointer to a statically allocated string containing a description of the error code contained in the error value ERR.  This string can be used to output a diagnostic message to the user.

     This function is not thread safe.  Use gpgme_strerror_r in multi-threaded programs.

 -- Function: int gpgme_strerror_r (gpgme_error_t ERR, char *BUF, size_t BUFLEN)
     The function gpgme_strerror_r returns the error string for ERR in the user-supplied buffer BUF of size BUFLEN. This function is, in contrast to gpgme_strerror, thread-safe if a thread-safe strerror_r function is provided by the system. If the function succeeds, 0 is returned and BUF contains the string describing the error.  If the buffer was not large enough, ERANGE is returned and BUF contains as much of the beginning of the error string as fits into the buffer.

 -- Function: const char * gpgme_strsource (gpgme_error_t ERR)
     The function gpgme_strerror returns a pointer to a statically allocated string containing a description of the error source contained in the error value ERR.  This string can be used to output a diagnostic message to the user.

The following example illustrates the use of gpgme_strerror:

     gpgme_ctx_t ctx;
     gpgme_error_t err = gpgme_new (&ctx);
     if (err)
       {
         fprintf (stderr, "%s: creating GpgME context failed: %s: %s\n",
                  argv[0], gpgme_strsource (err), gpgme_strerror (err));
         exit (1);
       }

Exchanging Data
***************

A lot of data has to be exchanged between the user and the crypto engine, like plaintext messages, ciphertext, signatures and information about the keys.  The technical details about exchanging the data information are completely abstracted by GPGME. The user provides and receives the data via gpgme_data_t objects, regardless of the communication protocol between GPGME and the crypto engine in use.

 -- Data type: gpgme_data_t
     The gpgme_data_t type is a handle for a container for generic data, which is used by GPGME to exchange data with the user.

gpgme_data_t objects do not provide notifications on events. It is assumed that read and write operations are blocking until data is available.  If this is undesirable, the application must ensure that all GPGME data operations always have data available, for example by using memory buffers or files rather than pipes or sockets.  This might be relevant, for example, if the external event loop mechanism is used.

 -- Data type: gpgme_off_t
       SINCE: 1.4.1

     On POSIX platforms the gpgme_off_t type is an alias for off_t; it may be used interchangeable.  On Windows platforms gpgme_off_t is defined as a long (i.e. 32 bit) for 32 bit Windows and as a 64 bit signed integer for 64 bit Windows.

 -- Data type: gpgme_ssize_t
     The gpgme_ssize_t type is an alias for ssize_t.  It has only been introduced to overcome portability problems pertaining to the declaration of ssize_t by different toolchains.

Creating Data Buffers
=====================

Data objects can be based on memory, files, or callback functions provided by the user.  Not all operations are supported by all objects.

Memory Based Data Buffers
-------------------------

Memory based data objects store all data in allocated memory.  This is convenient, but only practical for an amount of data that is a fraction of the available physical memory.  The data has to be copied from its source and to its destination, which can often be avoided by using one of the other data object

 -- Function: gpgme_error_t gpgme_data_new (gpgme_data_t *DH)
     The function gpgme_data_new creates a new gpgme_data_t object and returns a handle for it in DH.  The data object is memory based and initially empty.

     The function returns the error code GPG_ERR_NO_ERROR if the data object was successfully created, GPG_ERR_INV_VALUE if DH is not a valid pointer, and GPG_ERR_ENOMEM if not enough memory is available.

 -- Function: gpgme_error_t gpgme_data_new_from_mem (gpgme_data_t *DH, const char *BUFFER, size_t SIZE, int COPY)
     The function gpgme_data_new_from_mem creates a new gpgme_data_t object and fills it with SIZE bytes starting from BUFFER.

     If COPY is not zero, a private copy of the data is made.  If COPY is zero, the data is taken from the specified buffer as needed, and the user has to ensure that the buffer remains valid for the whole life span of the data object.

     The function returns the error code GPG_ERR_NO_ERROR if the data object was successfully created, GPG_ERR_INV_VALUE if DH or BUFFER is not a valid pointer, and GPG_ERR_ENOMEM if not enough memory is available.

 -- Function: gpgme_error_t gpgme_data_new_from_file (gpgme_data_t *DH, const char *FILENAME, int COPY)
     The function gpgme_data_new_from_file creates a new gpgme_data_t object and fills it with the content of the file FILENAME.

     If COPY is not zero, the whole file is read in at initialization time and the file is not used anymore after that.  This is the only mode supported currently.  Later, a value of zero for COPY might cause all reads to be delayed until the data is needed, but this is not yet implemented.

     The function returns the error code GPG_ERR_NO_ERROR if the data object was successfully created, GPG_ERR_INV_VALUE if DH or FILENAME is not a valid pointer, GPG_ERR_NOT_IMPLEMENTED if CODE is zero, and GPG_ERR_ENOMEM if not enough memory is available.

 -- Function: gpgme_error_t gpgme_data_new_from_filepart (gpgme_data_t *DH, const char *FILENAME, FILE *FP, off_t OFFSET, size_t LENGTH)
     The function gpgme_data_new_from_filepart creates a new gpgme_data_t object and fills it with a part of the file specified by FILENAME or FP.

     Exactly one of FILENAME and FP must be non-zero, the other must be zero.  The argument that is not zero specifies the file from which LENGTH bytes are read into the data object, starting from OFFSET.

     The function returns the error code GPG_ERR_NO_ERROR if the data object was successfully created, GPG_ERR_INV_VALUE if DH and exactly one of FILENAME and FP is not a valid pointer, and GPG_ERR_ENOMEM if not enough memory is available.

File Based Data Buffers
-----------------------

File based data objects operate directly on file descriptors or streams.  Only a small amount of data is stored in core at any time, so the size of the data objects is not limited by GPGME.

 -- Function: gpgme_error_t gpgme_data_new_from_fd (gpgme_data_t *DH, int FD)
     The function gpgme_data_new_from_fd creates a new gpgme_data_t object and uses the file descriptor FD to read from (if used as an input data object) and write to (if used as an output data object).

     When using the data object as an input buffer, the function might read a bit more from the file descriptor than is actually needed by the crypto engine in the desired operation because of internal buffering.

     Note that GPGME assumes that the file descriptor is set to blocking mode.  Errors during I/O operations, except for EINTR, are usually fatal for crypto operations.

     The function returns the error code GPG_ERR_NO_ERROR if the data object was successfully created, and GPG_ERR_ENOMEM if not enough memory is available.

 -- Function: gpgme_error_t gpgme_data_new_from_stream (gpgme_data_t *DH, FILE *STREAM)
     The function gpgme_data_new_from_stream creates a new gpgme_data_t object and uses the I/O stream STREAM to read from (if used as an input data object) and write to (if used as an output data object).

     When using the data object as an input buffer, the function might read a bit more from the stream than is actually needed by the crypto engine in the desired operation because of internal buffering.

     Note that GPGME assumes that the stream is in blocking mode.  Errors during I/O operations, except for EINTR, are usually fatal for crypto operations.

     The function returns the error code GPG_ERR_NO_ERROR if the data object was successfully created, and GPG_ERR_ENOMEM if not enough memory is available.

Callback Based Data Buffers
---------------------------

If neither memory nor file based data objects are a good fit for your application, you can implement the functions a data object provides yourself and create a data object from these callback functions.

 -- Data type: ssize_t (*gpgme_data_read_cb_t) (void *HANDLE, void *BUFFER, size_t SIZE)
     The gpgme_data_read_cb_t type is the type of functions which GPGME calls if it wants to read data from a user-implemented data object.  The function should read up to SIZE bytes from the current read position into the space starting at BUFFER.  The HANDLE is provided by the user at data object creation time.

     Note that GPGME assumes that the read blocks until data is available. Errors during I/O operations, except for EINTR, are usually fatal for crypto operations.

     The function should return the number of bytes read, 0 on EOF, and -1 on error.  If an error occurs, ERRNO should be set to describe the type of the error.

 -- Data type: ssize_t (*gpgme_data_write_cb_t) (void *HANDLE, const void *BUFFER, size_t SIZE)
     The gpgme_data_write_cb_t type is the type of functions which GPGME calls if it wants to write data to a user-implemented data object.  The function should write up to SIZE bytes to the current write position from the space starting at BUFFER.  The HANDLE is provided by the user at data object creation time.

     Note that GPGME assumes that the write blocks until data is available. Errors during I/O operations, except for EINTR, are usually fatal for crypto operations.

     The function should return the number of bytes written, and -1 on error.  If an error occurs, ERRNO should be set to describe the type of the error.

 -- Data type: off_t (*gpgme_data_seek_cb_t) (void *HANDLE, off_t OFFSET, int WHENCE)
     The gpgme_data_seek_cb_t type is the type of functions which GPGME calls if it wants to change the current read/write position in a user-implemented data object, just like the lseek function.

     The function should return the new read/write position, and -1 on error.  If an error occurs, ERRNO should be set to describe the type of the error.

 -- Data type: void (*gpgme_data_release_cb_t) (void *HANDLE)
     The gpgme_data_release_cb_t type is the type of functions which GPGME calls if it wants to destroy a user-implemented data object.  The HANDLE is provided by the user at data object creation time.

 -- Data type: struct gpgme_data_cbs
     This structure is used to store the data callback interface functions described above.  It has the following members:

     gpgme_data_read_cb_t read
          This is the function called by GPGME to read data from the data object.  It is only required for input data object.

     gpgme_data_write_cb_t write
          This is the function called by GPGME to write data to the data object.  It is only required for output data object.

     gpgme_data_seek_cb_t seek
          This is the function called by GPGME to change the current read/write pointer in the data object (if available).  It is optional.

     gpgme_data_release_cb_t release
          This is the function called by GPGME to release a data object.  It is optional.

 -- Function: gpgme_error_t gpgme_data_new_from_cbs (gpgme_data_t *DH, gpgme_data_cbs_t CBS, void *HANDLE)
     The function gpgme_data_new_from_cbs creates a new gpgme_data_t object and uses the user-provided callback functions to operate on the data object.

     The handle HANDLE is passed as first argument to the callback functions.  This can be used to identify this data object.

     The function returns the error code GPG_ERR_NO_ERROR if the data object was successfully created, and GPG_ERR_ENOMEM if not enough memory is available.

Destroying Data Buffers
=======================

 -- Function: void gpgme_data_release (gpgme_data_t DH)
     The function gpgme_data_release destroys the data object with the handle DH.  It releases all associated resources that were not provided by the user in the first place.

 -- Function: char * gpgme_data_release_and_get_mem (gpgme_data_t DH, size_t *LENGTH)
     The function gpgme_data_release_and_get_mem is like gpgme_data_release, except that it returns the data buffer and its length that was provided by the object.

     The user has to release the buffer with gpgme_free.  In case the user provided the data buffer in non-copy mode, a copy will be made for this purpose.

     In case an error returns, or there is no suitable data buffer that can be returned to the user, the function will return NULL.  In any case, the data object DH is destroyed.

 -- Function: void gpgme_free (void *BUFFER)
       SINCE: 1.1.1

     The function gpgme_free releases the memory returned by gpgme_data_release_and_get_mem and gpgme_pubkey_algo_string.  It should be used instead of the system libraries free function in case different allocators are used by a program.  This is often the case if gpgme is used under Windows as a DLL.

Manipulating Data Buffers
=========================

Data buffers contain data and meta-data.  The following operations can be used to manipulate both.

Data Buffer I/O Operations
--------------------------

 -- Function: ssize_t gpgme_data_read (gpgme_data_t DH, void *BUFFER, size_t LENGTH)
     The function gpgme_data_read reads up to LENGTH bytes from the data object with the handle DH into the space starting at BUFFER.

     If no error occurs, the actual amount read is returned.  If the end of the data object is reached, the function returns 0.

     In all other cases, the function returns -1 and sets ERRNO.

 -- Function: ssize_t gpgme_data_write (gpgme_data_t DH, const void *BUFFER, size_t SIZE)
     The function gpgme_data_write writes up to SIZE bytes starting from BUFFER into the data object with the handle DH at the current write position.

     The function returns the number of bytes actually written, or -1 if an error occurs.  If an error occurs, ERRNO is set.

 -- Function: off_t gpgme_data_seek (gpgme_data_t DH, off_t OFFSET, int WHENCE)
     The function gpgme_data_seek changes the current read/write position.

     The WHENCE argument specifies how the OFFSET should be interpreted.  It must be one of the following symbolic constants:

     SEEK_SET
          Specifies that OFFSET is a count of characters from the beginning of the data object.

     SEEK_CUR
          Specifies that OFFSET is a count of characters from the current file position.  This count may be positive or negative.

     SEEK_END
          Specifies that OFFSET is a count of characters from the end of the data object.  A negative count specifies a position within the current extent of the data object; a positive count specifies a position past the current end.  If you set the position past the current end, and actually write data, you will extend the data object with zeros up to that position.

     If successful, the function returns the resulting file position, measured in bytes from the beginning of the data object.  You can use this feature together with SEEK_CUR to read the current read/write position.

     If the function fails, -1 is returned and ERRNO is set.

Data Buffer Meta-Data
---------------------

 -- Function: char * gpgme_data_get_file_name (gpgme_data_t DH)
       SINCE: 1.1.0

     The function gpgme_data_get_file_name returns a pointer to a string containing the file name associated with the data object.  The file name will be stored in the output when encrypting or signing the data and will be returned to the user when decrypting or verifying the output data.

     If no error occurs, the string containing the file name is returned. Otherwise, NULL will be returned.

 -- Function: gpgme_error_t gpgme_data_set_file_name (gpgme_data_t DH, const char *FILE_NAME)
       SINCE: 1.1.0

     The function gpgme_data_set_file_name sets the file name associated with the data object.  The file name will be stored in the output when encrypting or signing the data and will be returned to the user when decrypting or verifying the output data.

     The function returns the error code GPG_ERR_INV_VALUE if DH is not a valid pointer and GPG_ERR_ENOMEM if not enough memory is available.

 -- Data type: enum gpgme_data_encoding_t
     The gpgme_data_encoding_t type specifies the encoding of a gpgme_data_t object.  For input data objects, the encoding is useful to give the backend a hint on the type of data.  For output data objects, the encoding can specify the output data format on certain operations.  Please note that not all backends support all encodings on all operations.  The following data types are available:

     GPGME_DATA_ENCODING_NONE
          This specifies that the encoding is not known.  This is the default for a new data object.  The backend will try its best to detect the encoding automatically.

     GPGME_DATA_ENCODING_BINARY
          This specifies that the data is encoding in binary form; i.e. there is no special encoding.

     GPGME_DATA_ENCODING_BASE64
          This specifies that the data is encoded using the Base-64 encoding scheme as used by MIME and other protocols.

     GPGME_DATA_ENCODING_ARMOR
          This specifies that the data is encoded in an armored form as used by OpenPGP and PEM.

     GPGME_DATA_ENCODING_MIME
            SINCE: 1.7.0

          This specifies that the data is encoded as a MIME part.

     GPGME_DATA_ENCODING_URL
            SINCE: 1.2.0

          The data is a list of linefeed delimited URLs.  This is only useful with gpgme_op_import.

     GPGME_DATA_ENCODING_URL0
            SINCE: 1.2.0

          The data is a list of binary zero delimited URLs.  This is only useful with gpgme_op_import.

     GPGME_DATA_ENCODING_URLESC
            SINCE: 1.2.0

          The data is a list of linefeed delimited URLs with all control and space characters percent escaped.  This mode is is not yet implemented.

 -- Function: gpgme_data_encoding_t gpgme_data_get_encoding (gpgme_data_t DH)
     The function gpgme_data_get_encoding returns the encoding of the data object with the handle DH.  If DH is not a valid pointer (e.g. NULL) GPGME_DATA_ENCODING_NONE is returned.

 -- Function: gpgme_error_t gpgme_data_set_encoding (gpgme_data_t DH, gpgme_data_encoding_t ENC)
     The function gpgme_data_set_encoding changes the encoding of the data object with the handle DH to ENC.

 -- Function: gpgme_error_t gpgme_data_set_flag
     (gpgme_data_t DH,
     const char *NAME,
     const char *VALUE)

       SINCE: 1.7.0

     Some minor properties of the data object can be controlled with flags set by this function.  The properties are identified by the following values for NAME:

     size-hint
          The value is a decimal number with the length gpgme shall assume for this data object.  This is useful if the data is provided by callbacks or via file descriptors but the applications knows the total size of the data.  If this is set the OpenPGP engine may use this to decide on buffer allocation strategies and to provide a total value for its progress information.

     This function returns 0 on success.

Data Buffer Convenience Functions
---------------------------------

 -- Data type: enum gpgme_data_type_t
       SINCE: 1.4.3

     The gpgme_data_type_t type is used to return the detected type of the content of a data buffer.

GPGME_DATA_TYPE_INVALID
     This is returned by gpgme_data_identify if it was not possible to identify the data.  Reasons for this might be a non-seekable stream or a memory problem.  The value is 0.

GPGME_DATA_TYPE_UNKNOWN
     The type of the data is not known.

GPGME_DATA_TYPE_PGP_SIGNED
     The data is an OpenPGP signed message.  This may be a binary signature, a detached one or a cleartext signature.

GPGME_DATA_TYPE_PGP_ENCRYPTED
       SINCE: 1.7.0

     The data is an OpenPGP encrypted message.

GPGME_DATA_TYPE_PGP_SIGNATURE
       SINCE: 1.7.0

     The data is an OpenPGP detached signature.

GPGME_DATA_TYPE_PGP_OTHER
     This is a generic OpenPGP message.  In most cases this will be encrypted data.

GPGME_DATA_TYPE_PGP_KEY
     This is an OpenPGP key (private or public).

GPGME_DATA_TYPE_CMS_SIGNED
     This is a CMS signed message.

GPGME_DATA_TYPE_CMS_ENCRYPTED
     This is a CMS encrypted (enveloped data) message.

GPGME_DATA_TYPE_CMS_OTHER
     This is used for other CMS message types.

GPGME_DATA_TYPE_X509_CERT
     The data is a X.509 certificate

GPGME_DATA_TYPE_PKCS12
     The data is a PKCS#12 message.  This is commonly used to exchange private keys for X.509.

 -- Function: gpgme_data_type_t gpgme_data_identify (gpgme_data_t DH)
       SINCE: 1.4.3

     The function gpgme_data_identify returns the type of the data with the handle DH.  If it is not possible to perform the identification, the function returns zero (GPGME_DATA_TYPE_INVALID).  Note that depending on how the data object has been created the identification may not be possible or the data object may change its internal state (file pointer moved).  For file or memory based data object, the state should not change.

Contexts
********

All cryptographic operations in GPGME are performed within a context, which contains the internal state of the operation as well as configuration parameters.  By using several contexts you can run several cryptographic operations in parallel, with different configuration.

 -- Data type: gpgme_ctx_t
     The gpgme_ctx_t type is a handle for a GPGME context, which is used to hold the configuration, status and result of cryptographic operations.

Creating Contexts
=================

 -- Function: gpgme_error_t gpgme_new (gpgme_ctx_t *CTX)
     The function gpgme_new creates a new gpgme_ctx_t object and returns a handle for it in CTX.

     The function returns the error code GPG_ERR_NO_ERROR if the context was successfully created, GPG_ERR_INV_VALUE if CTX is not a valid pointer, and GPG_ERR_ENOMEM if not enough memory is available.  Also, it returns GPG_ERR_NOT_OPERATIONAL if gpgme_check_version was not called to initialize GPGME, and GPG_ERR_SELFTEST_FAILED if a selftest failed.  Currently, the only selftest is for Windows MingW32 targets to see if -mms-bitfields was used (as required).

Destroying Contexts
===================

 -- Function: void gpgme_release (gpgme_ctx_t CTX)
     The function gpgme_release destroys the context with the handle CTX and releases all associated resources.

Result Management
=================

The detailed result of an operation is returned in operation-specific structures such as gpgme_decrypt_result_t.  The corresponding retrieval functions such as gpgme_op_decrypt_result provide static access to the results after an operation completes.  Those structures shall be considered read-only and an application must not allocate such a structure on its own.  The following interfaces make it possible to detach a result structure from its associated context and give it a lifetime beyond that of the current operation or context.

 -- Function: void gpgme_result_ref (void *RESULT)
       SINCE: 1.2.0

     The function gpgme_result_ref acquires an additional reference for the result RESULT, which may be of any type gpgme_*_result_t.  As long as the user holds a reference, the result structure is guaranteed to be valid and unmodified.

 -- Function: void gpgme_result_unref (void *RESULT)
       SINCE: 1.2.0

     The function gpgme_result_unref releases a reference for the result RESULT.  If this was the last reference, the result structure will be destroyed and all resources associated to it will be released.

Note that a context may hold its own references to result structures, typically until the context is destroyed or the next operation is started.  In fact, these references are accessed through the gpgme_op_*_result functions.

Context Attributes
==================

Protocol Selection
------------------

 -- Function: gpgme_error_t gpgme_set_protocol (gpgme_ctx_t CTX, gpgme_protocol_t PROTO)
     The function gpgme_set_protocol sets the protocol used within the context CTX to PROTO.  All crypto operations will be performed by the crypto engine configured for that protocol. See Protocols and Engines.

     Setting the protocol with gpgme_set_protocol does not check if the crypto engine for that protocol is available and installed correctly.  See Engine Version Check.

     The function returns the error code GPG_ERR_NO_ERROR if the protocol could be set successfully, and GPG_ERR_INV_VALUE if PROTOCOL is not a valid protocol.

 -- Function: gpgme_protocol_t gpgme_get_protocol (gpgme_ctx_t CTX)
     The function gpgme_get_protocol retrieves the protocol currently use with the context CTX.

Crypto Engine
-------------

The following functions can be used to set and retrieve the configuration of the crypto engines of a specific context.  The default can also be retrieved without any particular context. See Engine Information.  The default can also be changed globally. See Engine Configuration.

 -- Function: gpgme_engine_info_t gpgme_ctx_get_engine_info (gpgme_ctx_t CTX)
       SINCE: 1.1.0

     The function gpgme_ctx_get_engine_info returns a linked list of engine info structures.  Each info structure describes the configuration of one configured backend, as used by the context CTX.

     The result is valid until the next invocation of gpgme_ctx_set_engine_info for this particular context.

     This function can not fail.

 -- Function: gpgme_error_t gpgme_ctx_set_engine_info (gpgme_ctx_t CTX, gpgme_protocol_t PROTO, const char *FILE_NAME, const char *HOME_DIR)
       SINCE: 1.1.0

     The function gpgme_ctx_set_engine_info changes the configuration of the crypto engine implementing the protocol PROTO for the context CTX.

     FILE_NAME is the file name of the executable program implementing this protocol, and HOME_DIR is the directory name of the configuration directory for this crypto engine.  If HOME_DIR is NULL, the engine's default will be used.

     Currently this function must be used before starting the first crypto operation.  It is unspecified if and when the changes will take effect if the function is called after starting the first operation on the context CTX.

     This function returns the error code GPG_ERR_NO_ERROR if successful, or an error code on failure.

How to tell the engine the sender.
----------------------------------

Some engines can make use of the sender’s address, for example to figure out the best user id in certain trust models.  For verification and signing of mails, it is thus suggested to let the engine know the sender ("From:") address.  GPGME provides two functions to accomplish that.  Note that the esoteric use of multiple "From:" addresses is not supported.

 -- Function: gpgme_error_t gpgme_set_sender
     (gpgme_ctx_t CTX,
     int ADDRESS)

       SINCE: 1.8.0

     The function gpgme_set_sender specifies the sender address for use in sign and verify operations.  ADDRESS is expected to be the ``addr-spec'' part of an address but my also be a complete mailbox address, in which case this function extracts the ``addr-spec'' from it.  Using NULL for ADDRESS clears the sender address.

     The function returns 0 on success or an error code on failure.  The most likely failure is that no valid ``addr-spec'' was found in ADDRESS.

 -- Function: const char * gpgme_get_sender
     (gpgme_ctx_t CTX)

       SINCE: 1.8.0

     The function gpgme_get_sender returns the current sender address from the context, or NULL if none was set.  The returned value is valid as long as the CTX is valid and gpgme_set_sender has not been called again.

ASCII Armor
-----------

 -- Function: void gpgme_set_armor (gpgme_ctx_t CTX, int YES)
     The function gpgme_set_armor specifies if the output should be ASCII armored.  By default, output is not ASCII armored.

     ASCII armored output is disabled if YES is zero, and enabled otherwise.

 -- Function: int gpgme_get_armor (gpgme_ctx_t CTX)
     The function gpgme_get_armor returns 1 if the output is ASCII armored, and 0 if it is not, or if CTX is not a valid pointer.

Text Mode
---------

 -- Function: void gpgme_set_textmode (gpgme_ctx_t CTX, int YES)
     The function gpgme_set_textmode specifies if canonical text mode should be used.  By default, text mode is not used.

     Text mode is for example used for the RFC2015 signatures; note that the updated RFC 3156 mandates that the mail user agent does some preparations so that text mode is not needed anymore.

     This option is only relevant to the OpenPGP crypto engine, and ignored by all other engines.

     Canonical text mode is disabled if YES is zero, and enabled otherwise.

 -- Function: int gpgme_get_textmode (gpgme_ctx_t CTX)
     The function gpgme_get_textmode returns 1 if canonical text mode is enabled, and 0 if it is not, or if CTX is not a valid pointer.

Offline Mode
------------

 -- Function: void gpgme_set_offline (gpgme_ctx_t CTX, int YES)
       SINCE: 1.6.0

     The function gpgme_set_offline specifies if offline mode should be used.  By default, offline mode is not used.

     The offline mode specifies if dirmngr should be used to do additional validation that might require connections to external services. (e.g. CRL / OCSP checks).

     Offline mode only affects the keylist mode GPGME_KEYLIST_MODE_VALIDATE and is only relevant to the CMS crypto engine. Offline mode is ignored otherwise.

     This option may be extended in the future to completely disable the use of dirmngr for any engine.

     Offline mode is disabled if YES is zero, and enabled otherwise.

 -- Function: int gpgme_get_offline (gpgme_ctx_t CTX)
       SINCE: 1.6.0

     The function gpgme_get_offline returns 1 if offline mode is enabled, and 0 if it is not, or if CTX is not a valid pointer.

Pinentry Mode
-------------

 -- Function: gpgme_error_t gpgme_set_pinentry_mode (gpgme_ctx_t CTX,
     gpgme_pinentry_mode_t MODE)

       SINCE: 1.4.0

     The function gpgme_set_pinentry_mode specifies the pinentry mode to be used.

     For GnuPG >= 2.1 this option is required to be set to GPGME_PINENTRY_MODE_LOOPBACK to enable the passphrase callback mechanism in GPGME through gpgme_set_passphrase_cb.

 -- Function: gpgme_pinentry_mode_t gpgme_get_pinentry_mode (gpgme_ctx_t CTX)
       SINCE: 1.4.0

     The function gpgme_get_pinenty_mode returns the mode set for the context.

 -- Data type: enum gpgme_pinentry_mode_t
       SINCE: 1.4.0

     The gpgme_minentry_mode_t type specifies the set of possible pinentry modes that are supported by GPGME if GnuPG >= 2.1 is used. The following modes are supported:

     GPGME_PINENTRY_MODE_DEFAULT
            SINCE: 1.4.0

          Use the default of the agent, which is ask.

     GPGME_PINENTRY_MODE_ASK
            SINCE: 1.4.0

          Force the use of the Pinentry.

     GPGME_PINENTRY_MODE_CANCEL
            SINCE: 1.4.0

          Emulate use of Pinentry's cancel button.

     GPGME_PINENTRY_MODE_ERROR
            SINCE: 1.4.0

          Return a Pinentry error No Pinentry.

     GPGME_PINENTRY_MODE_LOOPBACK
            SINCE: 1.4.0

          Redirect Pinentry queries to the caller. This enables the use of gpgme_set_passphrase_cb because pinentry queries are redirected to gpgme.

          Note: For 2.1.0 - 2.1.12 this mode requires allow-loopback-pinentry to be enabled in the gpg-agent.conf or an agent started with that option.

Included Certificates
---------------------

 -- Function: void gpgme_set_include_certs (gpgme_ctx_t CTX, int NR_OF_CERTS)
     The function gpgme_set_include_certs specifies how many certificates should be included in an S/MIME signed message.  By default, only the sender's certificate is included.  The possible values of NR_OF_CERTS are:

     GPGME_INCLUDE_CERTS_DEFAULT
            SINCE: 1.0.3

          Fall back to the default of the crypto backend.  This is the default for GPGME.

     -2
          Include all certificates except the root certificate.

     -1
          Include all certificates.

     0
          Include no certificates.

     1
          Include the sender's certificate only.

     n
          Include the first n certificates of the certificates path, starting from the sender's certificate.  The number n must be positive.

     Values of NR_OF_CERTS smaller than -2 are undefined.

     This option is only relevant to the CMS crypto engine, and ignored by all other engines.

 -- Function: int gpgme_get_include_certs (gpgme_ctx_t CTX)
     The function gpgme_get_include_certs returns the number of certificates to include into an S/MIME signed message.

Key Listing Mode
----------------

 -- Function: gpgme_error_t gpgme_set_keylist_mode (gpgme_ctx_t CTX, gpgme_keylist_mode_t MODE)
     The function gpgme_set_keylist_mode changes the default behaviour of the key listing functions.  The value in MODE is a bitwise-or combination of one or multiple of the following bit values:

     GPGME_KEYLIST_MODE_LOCAL
          The GPGME_KEYLIST_MODE_LOCAL symbol specifies that the local keyring should be searched for keys in the keylisting operation.  This is the default.

     GPGME_KEYLIST_MODE_EXTERN
          The GPGME_KEYLIST_MODE_EXTERN symbol specifies that an external source should be searched for keys in the keylisting operation.  The type of external source is dependent on the crypto engine used and whether it is combined with GPGME_KEYLIST_MODE_LOCAL.  For example, it can be a remote keyserver or LDAP certificate server.

     GPGME_KEYLIST_MODE_SIGS
          The GPGME_KEYLIST_MODE_SIGS symbol specifies that the key signatures should be included in the listed keys.

     GPGME_KEYLIST_MODE_SIG_NOTATIONS
            SINCE: 1.1.1

          The GPGME_KEYLIST_MODE_SIG_NOTATIONS symbol specifies that the signature notations on key signatures should be included in the listed keys.  This only works if GPGME_KEYLIST_MODE_SIGS is also enabled.

     GPGME_KEYLIST_MODE_WITH_TOFU
            SINCE: 1.7.0

          The GPGME_KEYLIST_MODE_WITH_TOFU symbol specifies that information pertaining to the TOFU trust model should be included in the listed keys.

     GPGME_KEYLIST_MODE_WITH_SECRET
            SINCE: 1.5.1

          The GPGME_KEYLIST_MODE_WITH_SECRET returns information about the presence of a corresponding secret key in a public key listing.  A public key listing with this mode is slower than a standard listing but can be used instead of a second run to list the secret keys.  This is only supported for GnuPG versions >= 2.1.

     GPGME_KEYLIST_MODE_EPHEMERAL
            SINCE: 1.2.0

          The GPGME_KEYLIST_MODE_EPHEMERAL symbol specifies that keys flagged as ephemeral are included in the listing.

     GPGME_KEYLIST_MODE_VALIDATE
            SINCE: 0.4.5

          The GPGME_KEYLIST_MODE_VALIDATE symbol specifies that the backend should do key or certificate validation and not just get the validity information from an internal cache.  This might be an expensive operation and is in general not useful.  Currently only implemented for the S/MIME backend and ignored for other backends.

     At least one of GPGME_KEYLIST_MODE_LOCAL and GPGME_KEYLIST_MODE_EXTERN must be specified.  For future binary compatibility, you should get the current mode with gpgme_get_keylist_mode and modify it by setting or clearing the appropriate bits, and then using that calculated value in the gpgme_set_keylisting_mode operation.  This will leave all other bits in the mode value intact (in particular those that are not used in the current version of the library).

     The function returns the error code GPG_ERR_NO_ERROR if the mode could be set correctly, and GPG_ERR_INV_VALUE if CTX is not a valid pointer or MODE is not a valid mode.

 -- Function: gpgme_keylist_mode_t gpgme_get_keylist_mode (gpgme_ctx_t CTX)
     The function gpgme_get_keylist_mode returns the current key listing mode of the context CTX.  This value can then be modified and used in a subsequent gpgme_set_keylist_mode operation to only affect the desired bits (and leave all others intact).

     The function returns 0 if CTX is not a valid pointer, and the current mode otherwise.  Note that 0 is not a valid mode value.

Passphrase Callback
-------------------

 -- Data type: gpgme_error_t (*gpgme_passphrase_cb_t)(void *HOOK, const char *UID_HINT, const char *PASSPHRASE_INFO, int PREV_WAS_BAD, int FD)
     The gpgme_passphrase_cb_t type is the type of functions usable as passphrase callback function.

     The argument UID_HINT might contain a string that gives an indication for which user ID the passphrase is required.  If this is not available, or not applicable (in the case of symmetric encryption, for example), UID_HINT will be NULL.

     The argument PASSPHRASE_INFO, if not NULL, will give further information about the context in which the passphrase is required.  This information is engine and operation specific.

     If this is the repeated attempt to get the passphrase, because previous attempts failed, then PREV_WAS_BAD is 1, otherwise it will be 0.

     The user must write the passphrase, followed by a newline character, to the file descriptor FD.  The function gpgme_io_writen should be used for the write operation.  Note that if the user returns 0 to indicate success, the user must at least write a newline character before returning from the callback.

     If an error occurs, return the corresponding gpgme_error_t value.  You can use the error code GPG_ERR_CANCELED to abort the operation.  Otherwise, return 0.

     Note: The passphrase_cb only works with GnuPG 1.x and 2.1.x and not with the 2.0.x series. See gpgme_set_pinentry_mode for more details on 2.1.x usage.

 -- Function: void gpgme_set_passphrase_cb (gpgme_ctx_t CTX, gpgme_passphrase_cb_t PASSFUNC, void *HOOK_VALUE)
     The function gpgme_set_passphrase_cb sets the function that is used when a passphrase needs to be provided by the user to PASSFUNC.  The function PASSFUNC needs to implemented by the user, and whenever it is called, it is called with its first argument being HOOK_VALUE.  By default, no passphrase callback function is set.

     Not all crypto engines require this callback to retrieve the passphrase.  It is better if the engine retrieves the passphrase from a trusted agent (a daemon process), rather than having each user to implement their own passphrase query.  Some engines do not even support an external passphrase callback at all, in this case the error code GPG_ERR_NOT_SUPPORTED is returned.

     For GnuPG >= 2.1 the pinentry mode has to be set to GPGME_PINENTRY_MODE_LOOPBACK to enable the passphrase callback. See gpgme_set_pinentry_mode.

     The user can disable the use of a passphrase callback function by calling gpgme_set_passphrase_cb with PASSFUNC being NULL.

 -- Function: void gpgme_get_passphrase_cb (gpgme_ctx_t CTX, gpgme_passphrase_cb_t *PASSFUNC, void **HOOK_VALUE)
     The function gpgme_get_passphrase_cb returns the function that is used when a passphrase needs to be provided by the user in *PASSFUNC, and the first argument for this function in *HOOK_VALUE.  If no passphrase callback is set, or CTX is not a valid pointer, NULL is returned in both variables.

     PASSFUNC or HOOK_VALUE can be NULL.  In this case, the corresponding value will not be returned.

Progress Meter Callback
-----------------------

 -- Data type: void (*gpgme_progress_cb_t)(void *HOOK, const char *WHAT, int TYPE, int CURRENT, int TOTAL)
     The gpgme_progress_cb_t type is the type of functions usable as progress callback function.

     The arguments are specific to the crypto engine.  More information about the progress information returned from the GnuPG engine can be found in the GnuPG source code in the file doc/DETAILS in the section PROGRESS.

 -- Function: void gpgme_set_progress_cb (gpgme_ctx_t CTX, gpgme_progress_cb_t PROGFUNC, void *HOOK_VALUE)
     The function gpgme_set_progress_cb sets the function that is used when progress information about a cryptographic operation is available.  The function PROGFUNC needs to implemented by the user, and whenever it is called, it is called with its first argument being HOOK_VALUE.  By default, no progress callback function is set.

     Setting a callback function allows an interactive program to display progress information about a long operation to the user.

     The user can disable the use of a progress callback function by calling gpgme_set_progress_cb with PROGFUNC being NULL.

 -- Function: void gpgme_get_progress_cb (gpgme_ctx_t CTX, gpgme_progress_cb_t *PROGFUNC, void **HOOK_VALUE)
     The function gpgme_get_progress_cb returns the function that is used to inform the user about the progress made in *PROGFUNC, and the first argument for this function in *HOOK_VALUE.  If no progress callback is set, or CTX is not a valid pointer, NULL is returned in both variables.

     PROGFUNC or HOOK_VALUE can be NULL.  In this case, the corresponding value will not be returned.

Status Message Callback
-----------------------

 -- Data type: gpgme_error_t (*gpgme_status_cb_t)(void *HOOK, const char *KEYWORD, const char *ARGS)
     The gpgme_status_cb_t type is the type of function usable as a status message callback function.

     The argument KEYWORD is the name of the status message while the ARGS argument contains any arguments for the status message.

     If an error occurs, return the corresponding gpgme_error_t value. Otherwise, return 0.

 -- Function: void gpgme_set_status_cb (gpgme_ctx_t CTX, gpgme_status_cb_t STATUSFUNC, void *HOOK_VALUE)
       SINCE: 1.6.0

     The function gpgme_set_status_cb sets the function that is used when a status message is received from gpg to STATUSFUNC. The function STATUSFUNC needs to be implemented by the user, and whenever it is called, it is called with its first argument being HOOK_VALUE.  By default, no status message callback function is set.

     The user can disable the use of a status message callback function by calling gpgme_set_status_cb with STATUSFUNC being NULL.

 -- Function: void gpgme_get_status_cb (gpgme_ctx_t CTX, gpgme_status_cb_t *STATUSFUNC, void **HOOK_VALUE)
       SINCE: 1.6.0

     The function gpgme_get_status_cb returns the function that is used to process status messages from gpg in *STATUSFUNC, and the first argument for this function in *HOOK_VALUE.  If no status message callback is set, or CTX is not a valid pointer, NULL is returned in both variables.

 -- Function: gpgme_error_t gpgme_set_ctx_flag
     (gpgme_ctx_t CTX,
     const char *NAME,
     const char *VALUE)

       SINCE: 1.7.0

     Some minor properties of the context can be controlled with flags set by this function.  The properties are identified by the following values for NAME:

     "redraw"
          This flag is normally not changed by the caller because GPGME sets and clears it automatically: The flag is cleared before an operation and set if an operation noticed that the engine has launched a Pinentry. A Curses based application may use this information to redraw the screen; for example:

                   err = gpgme_op_keylist_start (ctx, "foo@example.org", 0);
                   while (!err)
                     {
                       err = gpgme_op_keylist_next (ctx, &key);
                       if (err)
                         break;
                       show_key (key);
                       gpgme_key_release (key);
                     }
                   if ((s = gpgme_get_ctx_flag (ctx, "redraw")) && *s)
                     redraw_screen ();
                   gpgme_release (ctx);

     "full-status"
          Using a VALUE of "1" the status callback set by gpgme_set_status_cb returns all status lines with the exception of PROGRESS lines.  With the default of "0" the status callback is only called in certain situations.

     "raw-description"
          Setting the VALUE to "1" returns human readable strings in a raw format.  For example the non breaking space characters ("~") will not be removed from the description field of the gpgme_tofu_info_t object.

     "export-session-key"
          Using a VALUE of "1" specifies that the context should try to export the symmetric session key when decrypting data.  By default, or when using an empty string or "0" for VALUE, session keys are not exported.

     "override-session-key"
          The string given in VALUE is passed to the GnuPG engine to override the session key for decryption.  The format of that session key is specific to GnuPG and can be retrieved during a decrypt operation when the context flag "export-session-key" is enabled.  Please be aware that using this feature with GnuPG < 2.1.16 will leak the session key on many platforms via ps(1).

     "auto-key-retrieve"
          Setting the VALUE to "1" asks the backend to automatically retrieve a key for signature verification if possible.  Note that this option makes a "web bug" like behavior possible.  Keyserver or Web Key Directory operators can see which keys you request, so by sending you a message signed by a brand new key (which you naturally will not have on your local keyring), the operator can tell both your IP address and the time when you verified the signature.

     This function returns 0 on success.

 -- Function: const char * gpgme_get_ctx_flag
     (gpgme_ctx_t CTX,
     const char *NAME)

       SINCE: 1.8.0

     The value of flags settable by gpgme_set_ctx_flag can be retrieved by this function.  If NAME is unknown the function returns NULL.  For boolean flags an empty string is returned for False and the string "1" is returned for True; either atoi(3) or a test for an empty string can be used to get the boolean value.

Locale
------

A locale setting can be associated with a context.  This locale is passed to the crypto engine, and used for applications like the PIN entry, which is displayed to the user when entering a passphrase is required.

The default locale is used to initialize the locale setting of all contexts created afterwards.

 -- Function: gpgme_error_t gpgme_set_locale (gpgme_ctx_t CTX, int CATEGORY, const char *VALUE)
       SINCE: 0.4.3

     The function gpgme_set_locale sets the locale of the context CTX, or the default locale if CTX is a null pointer.

     The locale settings that should be changed are specified by CATEGORY.  Supported categories are LC_CTYPE, LC_MESSAGES, and LC_ALL, which is a wildcard you can use if you want to change all the categories at once.

     The value to be used for the locale setting is VALUE, which will be copied to GPGME's internal data structures.  VALUE can be a null pointer, which disables setting the locale, and will make PIN entry and other applications use their default setting, which is usually not what you want.

     Note that the settings are only used if the application runs on a text terminal, and that the settings should fit the configuration of the output terminal.  Normally, it is sufficient to initialize the default value at startup.

     The function returns an error if not enough memory is available.

Key Management
==============

Some of the cryptographic operations require that recipients or signers are specified.  This is always done by specifying the respective keys that should be used for the operation.  The following section describes how such keys can be selected and manipulated.

Key objects
-----------

The keys are represented in GPGME by structures which may only be read by the application but never be allocated or changed.  They are valid as long as the key object itself is valid.

 -- Data type: gpgme_key_t

     The gpgme_key_t type is a pointer to a key object.  It has the following members:

     gpgme_keylist_mode_t keylist_mode
            SINCE: 0.9.0

          The keylist mode that was active when the key was retrieved.

     unsigned int revoked : 1
          This is true if the key is revoked.

     unsigned int expired : 1
          This is true if the key is expired.

     unsigned int disabled : 1
          This is true if the key is disabled.

     unsigned int invalid : 1
          This is true if the key is invalid. This might have several reasons, for a example for the S/MIME backend, it will be set during key listings if the key could not be validated due to missing certificates or unmatched policies.

     unsigned int can_encrypt : 1
          This is true if the key (ie one of its subkeys) can be used for encryption.

     unsigned int can_sign : 1
          This is true if the key (ie one of its subkeys) can be used to create data signatures.

     unsigned int can_certify : 1
          This is true if the key (ie one of its subkeys) can be used to create key certificates.

     unsigned int can_authenticate : 1
            SINCE: 0.4.5

          This is true if the key (ie one of its subkeys) can be used for authentication.

     unsigned int is_qualified : 1
            SINCE: 1.1.0

          This is true if the key can be used for qualified signatures according to local government regulations.

     unsigned int secret : 1
          This is true if the key is a secret key.  Note, that this will always be true even if the corresponding subkey flag may be false (offline/stub keys).  This is only set if a listing of secret keys has been requested or if GPGME_KEYLIST_MODE_WITH_SECRET is active.

     unsigned int origin : 5
            SINCE: 1.8.0

          Reserved for the origin of this key.

     gpgme_protocol_t protocol
          This is the protocol supported by this key.

     char *issuer_serial
          If protocol is GPGME_PROTOCOL_CMS, then this is the issuer serial.

     char *issuer_name
          If protocol is GPGME_PROTOCOL_CMS, then this is the issuer name.

     char *chain_id
          If protocol is GPGME_PROTOCOL_CMS, then this is the chain ID, which can be used to built the certificate chain.

     gpgme_validity_t owner_trust
          If protocol is GPGME_PROTOCOL_OpenPGP, then this is the owner trust.

     gpgme_subkey_t subkeys
          This is a linked list with the subkeys of the key.  The first subkey in the list is the primary key and usually available.

     gpgme_user_id_t uids
          This is a linked list with the user IDs of the key.  The first user ID in the list is the main (or primary) user ID.

     char *fpr
            SINCE: 1.7.0

          This field gives the fingerprint of the primary key.  Note that this is a copy of the fingerprint of the first subkey.  For an incomplete key (for example from a verification result) a subkey may be missing but this field may be set nevertheless.

     unsigned long last_update
            SINCE: 1.8.0

          Reserved for the time of the last update of this key.

 -- Data type: gpgme_subkey_t
       SINCE: 1.5.0

     The gpgme_subkey_t type is a pointer to a subkey structure. Subkeys are one component of a gpgme_key_t object.  In fact, subkeys are those parts that contains the real information about the individual cryptographic keys that belong to the same key object.  One gpgme_key_t can contain several subkeys.  The first subkey in the linked list is also called the primary key.

     The subkey structure has the following members:

     gpgme_subkey_t next
          This is a pointer to the next subkey structure in the linked list, or NULL if this is the last element.

     unsigned int revoked : 1
          This is true if the subkey is revoked.

     unsigned int expired : 1
          This is true if the subkey is expired.

     unsigned int disabled : 1
          This is true if the subkey is disabled.

     unsigned int invalid : 1
          This is true if the subkey is invalid.

     unsigned int can_encrypt : 1
          This is true if the subkey can be used for encryption.

     unsigned int can_sign : 1
          This is true if the subkey can be used to create data signatures.

     unsigned int can_certify : 1
          This is true if the subkey can be used to create key certificates.

     unsigned int can_authenticate : 1
            SINCE: 0.4.5

          This is true if the subkey can be used for authentication.

     unsigned int is_qualified : 1
            SINCE: 1.1.0

          This is true if the subkey can be used for qualified signatures according to local government regulations.

     unsigned int is_de_vs : 1
            SINCE: 1.8.0

          This is true if the subkey complies with the rules for classified information in Germany at the restricted level (VS-NfD).  This are currently RSA keys of at least 2048 bits or ECDH/ECDSA keys using a Brainpool curve.

     unsigned int secret : 1
          This is true if the subkey is a secret key.  Note that it will be false if the key is actually a stub key; i.e. a secret key operation is currently not possible (offline-key).  This is only set if a listing of secret keys has been requested or if GPGME_KEYLIST_MODE_WITH_SECRET is active.

     gpgme_pubkey_algo_t pubkey_algo
          This is the public key algorithm supported by this subkey.

     unsigned int length
          This is the length of the subkey (in bits).

     char *keyid
          This is the key ID of the subkey in hexadecimal digits.

     char *fpr
          This is the fingerprint of the subkey in hexadecimal digits, if available.

     char *keygrip
            SINCE: 1.7.0

          The keygrip of the subkey in hex digit form or NULL if not availabale.

     long int timestamp
          This is the creation timestamp of the subkey.  This is -1 if the timestamp is invalid, and 0 if it is not available.

     long int expires
          This is the expiration timestamp of the subkey, or 0 if the subkey does not expire.

     unsigned int is_cardkey : 1
            SINCE: 1.2.0

          True if the secret key is stored on a smart card.

     char *card_number
            SINCE: 1.2.0

          The serial number of a smart card holding this key or NULL.

     char *curve
          For ECC algorithms the name of the curve.

 -- Data type: gpgme_user_id_t

     A user ID is a component of a gpgme_key_t object.  One key can have many user IDs.  The first one in the list is the main (or primary) user ID.

     The user ID structure has the following members.

     gpgme_user_id_t next
          This is a pointer to the next user ID structure in the linked list, or NULL if this is the last element.

     unsigned int revoked : 1
          This is true if the user ID is revoked.

     unsigned int invalid : 1
          This is true if the user ID is invalid.

     gpgme_validity_t validity
          This specifies the validity of the user ID.

     char *uid
          This is the user ID string.

     char *name
          This is the name component of uid, if available.

     char *comment
          This is the comment component of uid, if available.

     char *email
          This is the email component of uid, if available.

     char *address;
          The mail address (addr-spec from RFC-5322) of the user ID string. This is general the same as the email part of this structure but might be slightly different.  If no mail address is available NULL is stored.

     gpgme_tofu_info_t tofu
            SINCE: 1.7.0

          If not NULL information from the TOFU database pertaining to this user id.

     gpgme_key_sig_t signatures
          This is a linked list with the signatures on this user ID.

     unsigned int origin : 5
            SINCE: 1.8.0

          Reserved for the origin of this user ID.

     unsigned long last_update
            SINCE: 1.8.0

          Reserved for the time of the last update of this user ID.

 -- Data type: gpgme_tofu_info_t

       SINCE: 1.7.0

     The gpgme_tofu_info_t type is a pointer to a tofu info structure.  Tofu info structures are one component of a gpgme_user_id_t object, and provide information from the TOFU database pertaining to the user ID.

     The tofu info structure has the following members:

     gpgme_key_sig_t next
          This is a pointer to the next tofu info structure in the linked list, or NULL if this is the last element.

     unsigned int validity : 3
          This is the TOFU validity.  It can have the following values:

          0
               The value 0 indicates a conflict.

          1
               The value 1 indicates a key without history.

          2
               The value 2 indicates a key with too little history.

          3
               The value 3 indicates a key with enough history for basic trust.

          4
               The value 4 indicates a key with a lot of history.

     unsigned int policy : 4
          This is the TOFU policy, see gpgme_tofu_policy_t.

     unsigned short signcount
          This is the number of signatures seen for this binding (or USHRT_MAX if there are more than that).

     unsigned short encrcount
          This is the number of encryptions done with this binding (or USHRT_MAX if there are more than that).

     unsigned long signfirst
          Number of seconds since Epoch when the first signature was seen with this binding.

     unsigned long signlast
          Number of seconds since Epoch when the last signature was seen with this binding.

     unsigned long encrfirst
          Number of seconds since Epoch when the first encryption was done with this binding.

     unsigned long encrlast
          Number of seconds since Epoch when the last encryption was done with this binding.

     char *description
          A human-readable string summarizing the TOFU data (or NULL).

 -- Data type: gpgme_key_sig_t

     The gpgme_key_sig_t type is a pointer to a key signature structure. Key signatures are one component of a gpgme_key_t object, and validate user IDs on the key in the OpenPGP protocol.

     The signatures on a key are only available if the key was retrieved via a listing operation with the GPGME_KEYLIST_MODE_SIGS mode enabled, because it can be expensive to retrieve all signatures of a key.

     The signature notations on a key signature are only available if the key was retrieved via a listing operation with the GPGME_KEYLIST_MODE_SIG_NOTATIONS mode enabled, because it can be expensive to retrieve all signature notations.

     The key signature structure has the following members:

     gpgme_key_sig_t next
          This is a pointer to the next key signature structure in the linked list, or NULL if this is the last element.

     unsigned int revoked : 1
          This is true if the key signature is a revocation signature.

     unsigned int expired : 1
          This is true if the key signature is expired.

     unsigned int invalid : 1
          This is true if the key signature is invalid.

     unsigned int exportable : 1
          This is true if the key signature is exportable.

     gpgme_pubkey_algo_t pubkey_algo
          This is the public key algorithm used to create the signature.

     char *keyid
          This is the key ID of the key (in hexadecimal digits) used to create the signature.

     long int timestamp
          This is the creation timestamp of the key signature.  This is -1 if the timestamp is invalid, and 0 if it is not available.

     long int expires
          This is the expiration timestamp of the key signature, or 0 if the key signature does not expire.

     gpgme_error_t status
          This is the status of the signature and has the same meaning as the member of the same name in a gpgme_signature_t object.

     unsigned int sig_class
          This specifies the signature class of the key signature.  The meaning is specific to the crypto engine.

     char *uid
          This is the main user ID of the key used to create the signature.

     char *name
          This is the name component of uid, if available.

     char *comment
          This is the comment component of uid, if available.

     char *email
          This is the email component of uid, if available.

     gpgme_sig_notation_t notations
          This is a linked list with the notation data and policy URLs.

Listing Keys
------------

 -- Function: gpgme_error_t gpgme_op_keylist_start (gpgme_ctx_t CTX, const char *PATTERN, int SECRET_ONLY)

     The function gpgme_op_keylist_start initiates a key listing operation inside the context CTX.  It sets everything up so that subsequent invocations of gpgme_op_keylist_next return the keys in the list.

     If PATTERN is NULL, all available keys are returned. Otherwise, PATTERN contains an engine specific expression that is used to limit the list to all keys matching the pattern.  Note that the total length of the pattern is restricted to an engine-specific maximum (a couple of hundred characters are usually accepted).  The pattern should be used to restrict the search to a certain common name or user, not to list many specific keys at once by listing their fingerprints or key IDs.

     If SECRET_ONLY is not 0, the list is restricted to secret keys only.

     The context will be busy until either all keys are received (and gpgme_op_keylist_next returns GPG_ERR_EOF), or gpgme_op_keylist_end is called to finish the operation.

     The function returns the error code GPG_ERR_INV_VALUE if CTX is not a valid pointer, and passes through any errors that are reported by the crypto engine support routines.

 -- Function: gpgme_error_t gpgme_op_keylist_ext_start (gpgme_ctx_t CTX, const char *PATTERN[], int SECRET_ONLY, int RESERVED)

     The function gpgme_op_keylist_ext_start initiates an extended key listing operation inside the context CTX.  It sets everything up so that subsequent invocations of gpgme_op_keylist_next return the keys in the list.

     If PATTERN or *PATTERN is NULL, all available keys are returned.  Otherwise, PATTERN is a NULL terminated array of strings that are used to limit the list to all keys matching at least one of the patterns verbatim.  Note that the total length of all patterns is restricted to an engine-specific maximum (the exact limit also depends on the number of patterns and amount of quoting required, but a couple of hundred characters are usually accepted). Patterns should be used to restrict the search to a certain common name or user, not to list many specific keys at once by listing their fingerprints or key IDs.

     If SECRET_ONLY is not 0, the list is restricted to secret keys only.

     The value of RESERVED must be 0.

     The context will be busy until either all keys are received (and gpgme_op_keylist_next returns GPG_ERR_EOF), or gpgme_op_keylist_end is called to finish the operation.

     The function returns the error code GPG_ERR_INV_VALUE if CTX is not a valid pointer, and passes through any errors that are reported by the crypto engine support routines.

 -- Function: gpgme_error_t gpgme_op_keylist_from_data_start
     (gpgme_ctx_t CTX,
     gpgme_data_t DATA,
     int RESERVED)

       SINCE: 1.8.0

     The function gpgme_op_keylist_from_data_start initiates a key listing operation inside the context CTX.  In contrast to the other key listing operation the keys are read from the supplied DATA and not from the local key database.  The keys are also not imported into the local key database.  The function sets everything up so that subsequent invocations of gpgme_op_keylist_next return the keys from DATA.

     The value of RESERVED must be 0.

     This function requires at least GnuPG version 2.1.14 and currently works only with OpenPGP keys.

     The context will be busy until either all keys are received (and gpgme_op_keylist_next returns GPG_ERR_EOF), or gpgme_op_keylist_end is called to finish the operation. While the context is busy DATA may not be released.

     The function returns the error code GPG_ERR_INV_VALUE if CTX is not a valid pointer, and passes through any errors that are reported by the crypto engine support routines.

 -- Function: gpgme_error_t gpgme_op_keylist_next (gpgme_ctx_t CTX, gpgme_key_t *R_KEY)

     The function gpgme_op_keylist_next returns the next key in the list created by a previous gpgme_op_keylist_start operation in the context CTX.  The key will have one reference for the user. See Manipulating Keys.

     This is the only way to get at gpgme_key_t objects in GPGME.

     If the last key in the list has already been returned, gpgme_op_keylist_next returns GPG_ERR_EOF.

     The function returns the error code GPG_ERR_INV_VALUE if CTX or R_KEY is not a valid pointer, and GPG_ERR_ENOMEM if there is not enough memory for the operation.

 -- Function: gpgme_error_t gpgme_op_keylist_end (gpgme_ctx_t CTX)

     The function gpgme_op_keylist_end ends a pending key list operation in the context CTX.

     After the operation completed successfully, the result of the key listing operation can be retrieved with gpgme_op_keylist_result.

     The function returns the error code GPG_ERR_INV_VALUE if CTX is not a valid pointer, and GPG_ERR_ENOMEM if at some time during the operation there was not enough memory available.

The following example illustrates how all keys containing a certain string (g10code) can be listed with their key ID and the name and email address of the main user ID:

     gpgme_ctx_t ctx;
     gpgme_key_t key;
     gpgme_error_t err = gpgme_new (&ctx);

     if (!err)
       {
         err = gpgme_op_keylist_start (ctx, "g10code", 0);
         while (!err)
           {
             err = gpgme_op_keylist_next (ctx, &key);
             if (err)
               break;
             printf ("%s:", key->subkeys->keyid);
             if (key->uids && key->uids->name)
               printf (" %s", key->uids->name);
             if (key->uids && key->uids->email)
               printf (" <%s>", key->uids->email);
             putchar ('\n');
             gpgme_key_release (key);
           }
         gpgme_release (ctx);
       }
     if (gpg_err_code (err) != GPG_ERR_EOF)
       {
         fprintf (stderr, "can not list keys: %s\n", gpgme_strerror (err));
         exit (1);
       }

 -- Data type: gpgme_keylist_result_t
     This is a pointer to a structure used to store the result of a gpgme_op_keylist_* operation.  After successfully ending a key listing operation, you can retrieve the pointer to the result with gpgme_op_keylist_result.  The structure contains the following member:

     unsigned int truncated : 1
          This is true if the crypto backend had to truncate the result, and less than the desired keys could be listed.

 -- Function: gpgme_keylist_result_t gpgme_op_keylist_result (gpgme_ctx_t CTX)
     The function gpgme_op_keylist_result returns a gpgme_keylist_result_t pointer to a structure holding the result of a gpgme_op_keylist_* operation.  The pointer is only valid if the last operation on the context was a key listing operation, and if this operation finished successfully.  The returned pointer is only valid until the next operation is started on the context.

In a simple program, for which a blocking operation is acceptable, the following function can be used to retrieve a single key.

 -- Function: gpgme_error_t gpgme_get_key (gpgme_ctx_t CTX, const char *FPR, gpgme_key_t *R_KEY, int SECRET)
     The function gpgme_get_key gets the key with the fingerprint (or key ID) FPR from the crypto backend and return it in R_KEY.  If SECRET is true, get the secret key.  The currently active keylist mode is used to retrieve the key.  The key will have one reference for the user.

     If the key is not found in the keyring, gpgme_get_key returns the error code GPG_ERR_EOF and *R_KEY will be set to NULL.

     The function returns the error code GPG_ERR_INV_VALUE if CTX or R_KEY is not a valid pointer or FPR is not a fingerprint or key ID, GPG_ERR_AMBIGUOUS_NAME if the key ID was not a unique specifier for a key, and GPG_ERR_ENOMEM if at some time during the operation there was not enough memory available.

Information About Keys
----------------------

Please see the beginning of this section for more information about gpgme_key_t objects.

 -- Data type: gpgme_validity_t
     The gpgme_validity_t type is used to specify the validity of a user ID in a key.  The following validities are defined:

     GPGME_VALIDITY_UNKNOWN
          The user ID is of unknown validity.  The string representation of this validity is ``?''.

     GPGME_VALIDITY_UNDEFINED
          The validity of the user ID is undefined.  The string representation of this validity is ``q''.

     GPGME_VALIDITY_NEVER
          The user ID is never valid.  The string representation of this validity is ``n''.

     GPGME_VALIDITY_MARGINAL
          The user ID is marginally valid.  The string representation of this validity is ``m''.

     GPGME_VALIDITY_FULL
          The user ID is fully valid.  The string representation of this validity is ``f''.

     GPGME_VALIDITY_ULTIMATE
          The user ID is ultimately valid.  The string representation of this validity is ``u''.

Manipulating Keys
-----------------

 -- Function: void gpgme_key_ref (gpgme_key_t KEY)
     The function gpgme_key_ref acquires an additional reference for the key KEY.

 -- Function: void gpgme_key_unref (gpgme_key_t KEY)
     The function gpgme_key_unref releases a reference for the key KEY.  If this was the last reference, the key will be destroyed and all resources associated to it will be released.

Generating Keys
---------------

GPGME provides a set of functions to create public key pairs.  Most of these functions require the use of GnuPG 2.1 and later; for older GnuPG versions the gpgme_op_genkey function can be used. Existing code which wants to update to the new functions or new code which shall supports older GnuPG versions may try the new functions first and provide a fallback to the old function if the error code GPG_ERR_NOT_SUPPORTED is received.

 -- Function: gpgme_error_t gpgme_op_createkey
     (gpgme_ctx_t CTX,
     const char *USERID,
     const char *ALGO,
     unsigned long RESERVED,
     unsigned long EXPIRES,
     gpgme_key_t EXTRAKEY,
     unsigned int FLAGS);

       SINCE: 1.7.0

     The function gpgme_op_createkey generates a new key for the procotol active in the context CTX.  As of now this function does only work for OpenPGP and requires at least version 2.1.13 of GnuPG.

     USERID is commonly the mail address associated with the key. GPGME does not require a specificy syntax but if more than a mail address is given, RFC-822 style format is suggested.  The value is expected to be in UTF-8 encoding (i.e. no IDN encoding for mail addresses).  This is a required parameter.

     ALGO specifies the algorithm for the new key (actually a keypair of public and private key).  For a list of supported algorithms, see the GnuPG manual.  If ALGO is NULL or the string "default", the key is generated using the default algorithm of the engine.  If the string "future-default" is used the engine may use an algorithm which is planned to be the default in a future release of the engine; however existing implementation of the protocol may not be able to already handle such future algorithms.  For the OpenPGP protocol, the specification of a default algorithm, without requesting a non-default usage via FLAGS, triggers the creation of a primary key plus a secondary key (subkey).

     RESERVED must be set to zero.

     EXPIRES specifies the expiration time in seconds.  If you supply 0, a reasonable expiration time is chosen.  Use the flag GPGME_CREATE_NOEXPIRE to create keys that do not expire.  Note that this parameter takes an unsigned long value and not a time_t to avoid problems on systems which use a signed 32 bit time_t.  Note further that the OpenPGP protocol uses 32 bit values for timestamps and thus can only encode dates up to the year 2106.

     EXTRAKEY is currently not used and must be set to NULL. A future version of GPGME may use this parameter to create X.509 keys.

     FLAGS can be set to the bit-wise OR of the following flags:

     GPGME_CREATE_SIGN
     GPGME_CREATE_ENCR
     GPGME_CREATE_CERT
     GPGME_CREATE_AUTH
            SINCE: 1.7.0

          Do not create the key with the default capabilities (key usage) of the requested algorithm but use those explicitly given by these flags: ``signing'', ``encryption'', ``certification'', or ``authentication''. The allowed combinations depend on the algorithm.

          If any of these flags are set and a default algorithm has been selected only one key is created in the case of the OpenPGP protocol.

     GPGME_CREATE_NOPASSWD
            SINCE: 1.7.0

          Request generation of the key without password protection.

     GPGME_CREATE_SELFSIGNED
            SINCE: 1.7.0

          For an X.509 key do not create a CSR but a self-signed certificate. This has not yet been implemented.

     GPGME_CREATE_NOSTORE
            SINCE: 1.7.0

          Do not store the created key in the local key database. This has not yet been implemented.

     GPGME_CREATE_WANTPUB
     GPGME_CREATE_WANTSEC
            SINCE: 1.7.0

          Return the public or secret key as part of the result structure. This has not yet been implemented.

     GPGME_CREATE_FORCE
            SINCE: 1.7.0

          The engine does not allow the creation of a key with a user ID already existing in the local key database.  This flag can be used to override this check.

     GPGME_CREATE_NOEXPIRE
            SINCE: 1.8.0

          Request generation of keys that do not expire.

     After the operation completed successfully, information about the created key can be retrieved with gpgme_op_genkey_result.

     The function returns zero on success, GPG_ERR_NOT_SUPPORTED if the engine does not support the command, or a bunch of other error codes.

 -- Function: gpgme_error_t gpgme_op_createkey_start
     (gpgme_ctx_t CTX,
     const char *USERID,
     const char *ALGO,
     unsigned long RESERVED,
     unsigned long EXPIRES,
     gpgme_key_t EXTRAKEY,
     unsigned int FLAGS);

       SINCE: 1.7.0

     The function gpgme_op_createkey_start initiates a gpgme_op_createkey operation; see there for details.  It must be completed by calling gpgme_wait on the context. See Waiting For Completion.

 -- Function: gpgme_error_t gpgme_op_createsubkey
     (gpgme_ctx_t CTX,
     gpgme_key_t KEY,
     const char *ALGO,
     unsigned long RESERVED,
     unsigned long EXPIRES,
     unsigned int FLAGS);

       SINCE: 1.7.0

     The function gpgme_op_createsubkey creates and adds a new subkey to the primary OpenPGP key given by KEY.  The only allowed protocol in CTX is GPGME_PROTOCOL_OPENPGP. Subkeys (aka secondary keys) are a concept in the OpenPGP protocol to bind several keys to a primary key.  As of now this function requires at least version 2.1.13 of GnuPG.

     KEY specifies the key to operate on.

     ALGO specifies the algorithm for the new subkey.  For a list of supported algorithms, see the GnuPG manual.  If ALGO is NULL or the string "default", the subkey is generated using the default algorithm for an encryption subkey of the engine.  If the string "future-default" is used the engine may use an encryption algorithm which is planned to be the default in a future release of the engine; however existing implementation of the protocol may not be able to already handle such future algorithms.

     RESERVED must be set to zero.

     EXPIRES specifies the expiration time in seconds.  If you supply 0, a reasonable expiration time is chosen.  Use the flag GPGME_CREATE_NOEXPIRE to create keys that do not expire.  Note that this parameter takes an unsigned long value and not a time_t to avoid problems on systems which use a signed 32 bit time_t.  Note further that the OpenPGP protocol uses 32 bit values for timestamps and thus can only encode dates up to the year 2106.

     FLAGS takes the same values as described above for gpgme_op_createkey.

     After the operation completed successfully, information about the created key can be retrieved with gpgme_op_genkey_result.

     The function returns zero on success, GPG_ERR_NOT_SUPPORTED if the engine does not support the command, or a bunch of other error codes.

 -- Function: gpgme_error_t gpgme_op_createsubkey_start
     (gpgme_ctx_t CTX,
     gpgme_key_t KEY,
     const char *ALGO,
     unsigned long RESERVED,
     unsigned long EXPIRES,
     unsigned int FLAGS);

       SINCE: 1.7.0

     The function gpgme_op_createsubkey_start initiates a gpgme_op_createsubkey operation; see there for details.  It must be completed by calling gpgme_wait on the context. See Waiting For Completion.

 -- Function: gpgme_error_t gpgme_op_adduid
     (gpgme_ctx_t CTX,
     gpgme_key_t KEY,
     const char *USERID,
     unsigned int FLAGS);

       SINCE: 1.7.0

     The function gpgme_op_adduid adds a new user ID to the OpenPGP key given by KEY.  Adding additional user IDs after key creation is a feature of the OpenPGP protocol and thus the protocol for the context CTX must be set to OpenPGP.  As of now this function requires at least version 2.1.13 of GnuPG.

     KEY specifies the key to operate on.

     USERID is the user ID to add to the key.  A user ID is commonly the mail address to be associated with the key.  GPGME does not require a specificy syntax but if more than a mail address is given, RFC-822 style format is suggested.  The value is expected to be in UTF-8 encoding (i.e. no IDN encoding for mail addresses).  This is a required parameter.

     FLAGS are currently not used and must be set to zero.

     The function returns zero on success, GPG_ERR_NOT_SUPPORTED if the engine does not support the command, or a bunch of other error codes.

 -- Function: gpgme_error_t gpgme_op_adduid_start
     (gpgme_ctx_t CTX,
     gpgme_key_t KEY,
     const char *USERID,
     unsigned int FLAGS);

       SINCE: 1.7.0

     The function gpgme_op_adduid_start initiates a gpgme_op_adduid operation; see there for details.  It must be completed by calling gpgme_wait on the context. See Waiting For Completion.

 -- Function: gpgme_error_t gpgme_op_revuid
     (gpgme_ctx_t CTX,
     gpgme_key_t KEY,
     const char *USERID,
     unsigned int FLAGS);

       SINCE: 1.7.0

     The function gpgme_op_revuid revokes a user ID from the OpenPGP key given by KEY.  Revoking user IDs after key creation is a feature of the OpenPGP protocol and thus the protocol for the context CTX must be set to OpenPGP.  As of now this function requires at least version 2.1.13 of GnuPG.

     KEY specifies the key to operate on.

     USERID is the user ID to be revoked from the key.  The user ID must be given verbatim because the engine does an exact and case sensitive match.  Thus the uid field from the user ID object (gpgme_user_id_t) is to be used.  This is a required parameter.

     FLAGS are currently not used and must be set to zero.

     Note that the engine won't allow to revoke the last valid user ID.  To change a user ID is better to first add the new user ID, then revoke the old one, and finally publish the key.

     The function returns zero on success, GPG_ERR_NOT_SUPPORTED if the engine does not support the command, or a bunch of other error codes.

 -- Function: gpgme_error_t gpgme_op_revuid_start
     (gpgme_ctx_t CTX,
     gpgme_key_t KEY,
     const char *USERID,
     unsigned int FLAGS);

       SINCE: 1.7.0

     The function gpgme_op_revuid_start initiates a gpgme_op_revuid operation; see there for details.  It must be completed by calling gpgme_wait on the context. See Waiting For Completion.

 -- Function: gpgme_error_t gpgme_op_set_ui_flag
     (gpgme_ctx_t CTX,
     gpgme_key_t KEY,
     const char *USERID,
     cons char * NAME,
     cons char * VALUE);

       SINCE: 1.8.0

     The function gpgme_op_set_uid_flag is used to set flags on a user ID from the OpenPGP key given by KEY.  Setting flags on user IDs after key creation is a feature of the OpenPGP protocol and thus the protocol for the context CTX must be set to OpenPGP.

     KEY specifies the key to operate on.  This parameters is required.

     USERID is the user ID of the key to be manipulated.  This user ID must be given verbatim because the engine does an exact and case sensitive match.  Thus the uid field from the user ID object (gpgme_user_id_t) is to be used.  This is a required parameter.

     NAME names the flag which is to be changed.  The only currently supported flag is:

     primary
          This sets the primary key flag on the given user ID.  All other primary key flag on other user IDs are removed.  VALUE must be given as NULL.  For technical reasons this functions bumps the creation timestamp of all affected self-signatures up by one second. At least GnuPG version 2.1.20 is required.

     The function returns zero on success, GPG_ERR_NOT_SUPPORTED if the engine does not support the command, or a bunch of other error codes.

 -- Function: gpgme_error_t gpgme_op_set_uid_flag_start
     (gpgme_ctx_t CTX,
     gpgme_key_t KEY,
     const char *USERID,
     cons char * NAME,
     cons char * VALUE);

       SINCE: 1.8.0

     The function gpgme_op_set_uid_flag_start initiates a gpgme_op_set_uid_flag operation; see there for details.  It must be completed by calling gpgme_wait on the context. See Waiting For Completion.

 -- Function: gpgme_error_t gpgme_op_genkey
     (gpgme_ctx_t CTX,
     const char *PARMS,
     gpgme_data_t PUBLIC,
     gpgme_data_t SECRET)

     The function gpgme_op_genkey generates a new key pair in the context CTX.  The meaning of PUBLIC and SECRET depends on the crypto backend.

     GPG does not support PUBLIC and SECRET, they should be NULL.  GnuPG will generate a key pair and add it to the standard key ring.  The fingerprint of the generated key is available with gpgme_op_genkey_result.

     GpgSM requires PUBLIC to be a writable data object.  GpgSM will generate a secret key (which will be stored by gpg-agent, and return a certificate request in PUBLIC, which then needs to be signed by the certification authority and imported before it can be used.  GpgSM does not make the fingerprint available.

     The argument PARMS specifies parameters for the key in an string that looks something like XML.  The details about the format of PARMS are specific to the crypto engine used by CTX.  The first line of the parameters must be <GnupgKeyParams format="internal"> and the last line must be </GnupgKeyParams>.  Every line in between the first and last lines is treated as a Header: Value pair.  In particular, no XML escaping is necessary if you need to include the characters <, >, or &.

     Here is an example for GnuPG as the crypto engine (all parameters of OpenPGP key generation are documented in the GPG manual):

          <GnupgKeyParms format="internal">
          Key-Type: default
          Subkey-Type: default
          Name-Real: Joe Tester
          Name-Comment: with stupid passphrase
          Name-Email: joe@foo.bar
          Expire-Date: 0
          Passphrase: abc
          </GnupgKeyParms>

     Here is an example for GpgSM as the crypto engine (all parameters of OpenPGP key generation are documented in the GPGSM manual):

          <GnupgKeyParms format="internal">
          Key-Type: RSA
          Key-Length: 1024
          Name-DN: C=de,O=g10 code,OU=Testlab,CN=Joe 2 Tester
          Name-Email: joe@foo.bar
          </GnupgKeyParms>

     Strings should be given in UTF-8 encoding.  The only format supported for now is ``internal''.  The content of the GnupgKeyParms container is passed verbatim to the crypto backend.  Control statements are not allowed.

     After the operation completed successfully, the result can be retrieved with gpgme_op_genkey_result.

     The function returns the error code GPG_ERR_NO_ERROR if the operation could be started successfully, GPG_ERR_INV_VALUE if PARMS is not a well-formed string (e.g. does not have the expected tag-like headers and footers), GPG_ERR_NOT_SUPPORTED if PUBLIC or SECRET is not valid, and GPG_ERR_GENERAL if no key was created by the backend.

 -- Function: gpgme_error_t gpgme_op_genkey_start (gpgme_ctx_t CTX, const char *PARMS, gpgme_data_t PUBLIC, gpgme_data_t SECRET)

     The function gpgme_op_genkey_start initiates a gpgme_op_genkey operation.  It can be completed by calling gpgme_wait on the context.  See Waiting For Completion.

     The function returns the error code GPG_ERR_NO_ERROR if the operation could be started successfully, GPG_ERR_INV_VALUE if PARMS is not a valid XML string, and GPG_ERR_NOT_SUPPORTED if PUBLIC or SECRET is not NULL.

 -- Data type: gpgme_genkey_result_t

     This is a pointer to a structure used to store the result of a gpgme_op_genkey operation.  After successfully generating a key, you can retrieve the pointer to the result with gpgme_op_genkey_result.  The structure contains the following members:

     unsigned int primary : 1
          This flag is set to 1 if a primary key was created and to 0 if not.

     unsigned int sub : 1
          This flag is set to 1 if a subkey was created and to 0 if not.

     unsigned int uid : 1
          This flag is set to 1 if a user ID was created and to 0 if not.

     char *fpr
          This is the fingerprint of the key that was created.  If both a primary and a subkey were generated, the fingerprint of the primary key will be returned.  If the crypto engine does not provide the fingerprint, fpr will be a null pointer.

     gpgme_data_t pubkey
            SINCE: 1.7.0

          This will eventually be used to return the public key.  It is currently not used.

     gpgme_data_t seckey
            SINCE: 1.7.0

          This will eventually be used to return the secret key.  It is currently not used.

 -- Function: gpgme_genkey_result_t gpgme_op_genkey_result (gpgme_ctx_t CTX)

     The function gpgme_op_genkey_result returns a gpgme_genkey_result_t pointer to a structure holding the result of a gpgme_op_genkey operation.  The pointer is only valid if the last operation on the context was a gpgme_op_genkey or gpgme_op_genkey_start operation, and if this operation finished successfully.  The returned pointer is only valid until the next operation is started on the context.

Signing Keys
------------

Key signatures are a unique concept of the OpenPGP protocol.  They can be used to certify the validity of a key and are used to create the Web-of-Trust (WoT).  Instead of using the gpgme_op_interact function along with a finite state machine, GPGME provides a convenient function to create key signatures when using modern GnuPG versions.

 -- Function: gpgme_error_t gpgme_op_keysign
     (gpgme_ctx_t CTX,
     gpgme_key_t KEY,
     const char *USERID,
     unsigned long EXPIRES,
     unsigned int FLAGS);

       SINCE: 1.7.0

     The function gpgme_op_keysign adds a new key signature to the public key KEY.   This function requires at least version 2.1.12 of GnuPG.

     CTX is the usual context which describes the protocol to use (which must be OpenPGP) and has also the list of signer keys to be used for the signature.  The common case is to use the default key for signing other keys.  If another key or more than one key shall be used for a key signature, gpgme_signers_add can be used. See Selecting Signers.

     KEY specifies the key to operate on.

     USERID selects the user ID or user IDs to be signed.  If USERID is set to NULL all valid user IDs are signed.  The user ID must be given verbatim because the engine does an exact and case sensitive match.  Thus the uid field from the user ID object (gpgme_user_id_t) is to be used.  To select more than one user ID put them all into one string separated by linefeeds characters (\n) and set the flag GPGME_KEYSIGN_LFSEP.

     EXPIRES specifies the expiration time of the new signature in seconds.  The common case is to use 0 to not set an expiration date. However, if the configuration of the engine defines a default expiration for key signatures, that is still used unless the flag GPGME_KEYSIGN_NOEXPIRE is used.  Note that this parameter takes an unsigned long value and not a time_t to avoid problems on systems which use a signed 32 bit time_t.  Note further that the OpenPGP protocol uses 32 bit values for timestamps and thus can only encode dates up to the year 2106.

     FLAGS can be set to the bit-wise OR of the following flags:

     GPGME_KEYSIGN_LOCAL
            SINCE: 1.7.0

          Instead of creating an exportable key signature, create a key signature which is is marked as non-exportable.

     GPGME_KEYSIGN_LFSEP
            SINCE: 1.7.0

          Although linefeeds are uncommon in user IDs this flag is required to explicitly declare that USERID may contain several linefeed separated user IDs.

     GPGME_KEYSIGN_NOEXPIRE
          Force the creation of a key signature without an expiration date.  This overrides EXPIRE and any local configuration of the engine.

     The function returns zero on success, GPG_ERR_NOT_SUPPORTED if the engine does not support the command, or a bunch of other error codes.

 -- Function: gpgme_error_t gpgme_op_keysign_start
     (gpgme_ctx_t CTX,
     gpgme_key_t KEY,
     const char *USERID,
     unsigned long EXPIRES,
     unsigned int FLAGS);

       SINCE: 1.7.0

     The function gpgme_op_keysign_start initiates a gpgme_op_keysign operation; see there for details.  It must be completed by calling gpgme_wait on the context. See Waiting For Completion.

Exporting Keys
--------------

Exporting keys means the same as running gpg with the command --export.  However, a mode flag can be used to change the way the export works.  The available mode flags are described below, they may be or-ed together.

GPGME_EXPORT_MODE_EXTERN
     If this bit is set, the output is send directly to the default keyserver. This is currently only allowed for OpenPGP keys.  It is good practise to not send more than a few dozens key to a keyserver at one time.  Using this flag requires that the KEYDATA argument of the export function is set to NULL.

GPGME_EXPORT_MODE_MINIMAL
       SINCE: 1.3.1

     If this bit is set, the smallest possible key is exported.  For OpenPGP keys it removes all signatures except for the latest self-signatures. For X.509 keys it has no effect.

GPGME_EXPORT_MODE_SECRET
       SINCE: 1.6.0

     Instead of exporting the public key, the secret key is exported.  This may not be combined with GPGME_EXPORT_MODE_EXTERN.  For X.509 the export format is PKCS#8.

GPGME_EXPORT_MODE_RAW
       SINCE: 1.6.0

     If this flag is used with GPGME_EXPORT_MODE_SECRET for an X.509 key the export format will be changed to PKCS#1.  This flag may not be used with OpenPGP.

GPGME_EXPORT_MODE_PKCS12
       SINCE: 1.6.0

     If this flag is used with GPGME_EXPORT_MODE_SECRET for an X.509 key the export format will be changed to PKCS#12 which also includes the certificate.  This flag may not be used with OpenPGP.

 -- Function: gpgme_error_t gpgme_op_export (gpgme_ctx_t CTX, const char *PATTERN, gpgme_export_mode_t MODE, gpgme_data_t KEYDATA)
     The function gpgme_op_export extracts public keys and returns them in the data buffer KEYDATA.  The output format of the key data returned is determined by the ASCII armor attribute set for the context CTX, or, if that is not set, by the encoding specified for KEYDATA.

     If PATTERN is NULL, all available keys are returned. Otherwise, PATTERN contains an engine specific expression that is used to limit the list to all keys matching the pattern.

     MODE is usually 0; other values are described above.

     The function returns the error code GPG_ERR_NO_ERROR if the operation completed successfully, GPG_ERR_INV_VALUE if KEYDATA is not a valid empty data buffer, and passes through any errors that are reported by the crypto engine support routines.

 -- Function: gpgme_error_t gpgme_op_export_start (gpgme_ctx_t CTX, const char *PATTERN, gpgme_export_mode_t MODE, gpgme_data_t KEYDATA)
     The function gpgme_op_export_start initiates a gpgme_op_export operation.  It can be completed by calling gpgme_wait on the context.  See Waiting For Completion.

     The function returns the error code GPG_ERR_NO_ERROR if the operation could be started successfully, and GPG_ERR_INV_VALUE if KEYDATA is not a valid empty data buffer.

 -- Function: gpgme_error_t gpgme_op_export_ext (gpgme_ctx_t CTX, const char *PATTERN[], gpgme_export_mode_t MODE, gpgme_data_t KEYDATA)
     The function gpgme_op_export extracts public keys and returns them in the data buffer KEYDATA.  The output format of the key data returned is determined by the ASCII armor attribute set for the context CTX, or, if that is not set, by the encoding specified for KEYDATA.

     If PATTERN or *PATTERN is NULL, all available keys are returned.  Otherwise, PATTERN is a NULL terminated array of strings that are used to limit the list to all keys matching at least one of the patterns verbatim.

     MODE is usually 0; other values are described above.

     The function returns the error code GPG_ERR_NO_ERROR if the operation completed successfully, GPG_ERR_INV_VALUE if KEYDATA is not a valid empty data buffer, and passes through any errors that are reported by the crypto engine support routines.

 -- Function: gpgme_error_t gpgme_op_export_ext_start (gpgme_ctx_t CTX, const char *PATTERN[], gpgme_export_mode_t MODE, gpgme_data_t KEYDATA)
     The function gpgme_op_export_ext_start initiates a gpgme_op_export_ext operation.  It can be completed by calling gpgme_wait on the context.  See Waiting For Completion.

     The function returns the error code GPG_ERR_NO_ERROR if the operation could be started successfully, and GPG_ERR_INV_VALUE if KEYDATA is not a valid empty data buffer.

 -- Function: gpgme_error_t gpgme_op_export_keys (gpgme_ctx_t CTX, gpgme_key_t keys[], gpgme_export_mode_t MODE, gpgme_data_t KEYDATA)
       SINCE: 1.2.0

     The function gpgme_op_export_keys extracts public keys and returns them in the data buffer KEYDATA.  The output format of the key data returned is determined by the ASCII armor attribute set for the context CTX, or, if that is not set, by the encoding specified for KEYDATA.

     The keys to export are taken form the NULL terminated array KEYS.  Only keys of the currently selected protocol of CTX which do have a fingerprint set are considered for export. Other keys specified by the KEYS are ignored.  In particular OpenPGP keys retrieved via an external key listing are not included.

     MODE is usually 0; other values are described above.

     The function returns the error code GPG_ERR_NO_ERROR if the operation completed successfully, GPG_ERR_INV_VALUE if KEYDATA is not a valid empty data buffer, GPG_ERR_NO_DATA if no useful keys are in KEYS and passes through any errors that are reported by the crypto engine support routines.

 -- Function: gpgme_error_t gpgme_op_export_keys_start (gpgme_ctx_t CTX, gpgme_key_t KEYS[], gpgme_export_mode_t MODE, gpgme_data_t KEYDATA)
       SINCE: 1.2.0

     The function gpgme_op_export_keys_start initiates a gpgme_op_export_ext operation.  It can be completed by calling gpgme_wait on the context.  See Waiting For Completion.

     The function returns the error code GPG_ERR_NO_ERROR if the operation could be started successfully, and GPG_ERR_INV_VALUE if KEYDATA is not a valid empty data buffer, GPG_ERR_NO_DATA if no useful keys are in KEYS and passes through any errors that are reported by the crypto engine support routines.

Importing Keys
--------------

Importing keys means the same as running gpg with the command --import.

 -- Function: gpgme_error_t gpgme_op_import (gpgme_ctx_t CTX, gpgme_data_t KEYDATA)
     The function gpgme_op_import adds the keys in the data buffer KEYDATA to the key ring of the crypto engine used by CTX. The format of KEYDATA can be ASCII armored, for example, but the details are specific to the crypto engine.

     After the operation completed successfully, the result can be retrieved with gpgme_op_import_result.

     The function returns the error code GPG_ERR_NO_ERROR if the import was completed successfully, GPG_ERR_INV_VALUE if KEYDATA if CTX or KEYDATA is not a valid pointer, and GPG_ERR_NO_DATA if KEYDATA is an empty data buffer.

 -- Function: gpgme_error_t gpgme_op_import_start (gpgme_ctx_t CTX, gpgme_data_t KEYDATA)
     The function gpgme_op_import_start initiates a gpgme_op_import operation.  It can be completed by calling gpgme_wait on the context.  See Waiting For Completion.

     The function returns the error code GPG_ERR_NO_ERROR if the import could be started successfully, GPG_ERR_INV_VALUE if CTX or KEYDATA is not a valid pointer, and GPG_ERR_NO_DATA if KEYDATA is an empty data buffer.

 -- Function: gpgme_error_t gpgme_op_import_keys (gpgme_ctx_t CTX, gpgme_key_t *KEYS)
       SINCE: 1.2.0

     The function gpgme_op_import_keys adds the keys described by the NULL terminated array KEYS to the key ring of the crypto engine used by CTX.  It is used to actually import and make keys permanent which have been retrieved from an external source (i.e. using GPGME_KEYLIST_MODE_EXTERN) earlier.  The external keylisting must have been made with the same context configuration (in particular the same home directory).   (Thus it is a replacement for the usual workaround of exporting and then importing a key to make an X.509 key permanent.)  Note that for OpenPGP this may require another access to the keyserver over the network.

     Only keys of the currently selected protocol of CTX are considered for import.  Other keys specified by the KEYS are ignored.  As of now all considered keys must have been retrieved using the same method, i.e. the used key listing mode must be identical.

     After the operation completed successfully, the result can be retrieved with gpgme_op_import_result.

     To move keys from one home directory to another, export and import the keydata using gpgme_op_export and gpgme_op_import.

     The function returns the error code GPG_ERR_NO_ERROR if the import was completed successfully, GPG_ERR_INV_VALUE if CTX is not a valid pointer, GPG_ERR_CONFLICT if the key listing mode does not match, and GPG_ERR_NO_DATA if no keys are considered for export.

 -- Function: gpgme_error_t gpgme_op_import_keys_start (gpgme_ctx_t CTX, gpgme_key_t *KEYS)
       SINCE: 1.2.0

     The function gpgme_op_import_keys_start initiates a gpgme_op_import_keys operation.  It can be completed by calling gpgme_wait on the context.  See Waiting For Completion.

     The function returns the error code GPG_ERR_NO_ERROR if the import was completed successfully, GPG_ERR_INV_VALUE if KEYDATA if CTX or KEYDATA is not a valid pointer, GPG_ERR_CONFLICT if the key listing mode does not match, and GPG_ERR_NO_DATA if no keys are considered for export.

 -- Data type: gpgme_import_status_t
     This is a pointer to a structure used to store a part of the result of a gpgme_op_import operation.  For each considered key one status is added that contains information about the result of the import.  The structure contains the following members:

     gpgme_import_status_t next
          This is a pointer to the next status structure in the linked list, or NULL if this is the last element.

     char *fpr
          This is the fingerprint of the key that was considered.

     gpgme_error_t result
          If the import was not successful, this is the error value that caused the import to fail.  Otherwise the error code is GPG_ERR_NO_ERROR.

     unsigned int status
          This is a bit-wise OR of the following flags that give more information about what part of the key was imported.  If the key was already known, this might be 0.

          GPGME_IMPORT_NEW
               The key was new.

          GPGME_IMPORT_UID
               The key contained new user IDs.

          GPGME_IMPORT_SIG
               The key contained new signatures.

          GPGME_IMPORT_SUBKEY
               The key contained new sub keys.

          GPGME_IMPORT_SECRET
               The key contained a secret key.

 -- Data type: gpgme_import_result_t
     This is a pointer to a structure used to store the result of a gpgme_op_import operation.  After a successful import operation, you can retrieve the pointer to the result with gpgme_op_import_result.  The structure contains the following members:

     int considered
          The total number of considered keys.

     int no_user_id
          The number of keys without user ID.

     int imported
          The total number of imported keys.

     int imported_rsa
          The number of imported RSA keys.

     int unchanged
          The number of unchanged keys.

     int new_user_ids
          The number of new user IDs.

     int new_sub_keys
          The number of new sub keys.

     int new_signatures
          The number of new signatures.

     int new_revocations
          The number of new revocations.

     int secret_read
          The total number of secret keys read.

     int secret_imported
          The number of imported secret keys.

     int secret_unchanged
          The number of unchanged secret keys.

     int not_imported
          The number of keys not imported.

     gpgme_import_status_t imports
          A list of gpgme_import_status_t objects which contain more information about the keys for which an import was attempted.

 -- Function: gpgme_import_result_t gpgme_op_import_result (gpgme_ctx_t CTX)
     The function gpgme_op_import_result returns a gpgme_import_result_t pointer to a structure holding the result of a gpgme_op_import operation.  The pointer is only valid if the last operation on the context was a gpgme_op_import or gpgme_op_import_start operation, and if this operation finished successfully.  The returned pointer is only valid until the next operation is started on the context.

Deleting Keys
-------------

 -- Function: gpgme_error_t gpgme_op_delete_ext (gpgme_ctx_t CTX, const gpgme_key_t KEY, unsigned int FLAGS)
       SINCE: 1.9.1

     The function gpgme_op_delete_ext deletes the key KEY from the key ring of the crypto engine used by CTX.

     FLAGS can be set to the bit-wise OR of the following flags:

     GPGME_DELETE_ALLOW_SECRET
            SINCE: 1.9.1

          If not set, only public keys are deleted. If set, secret keys are deleted as well, if that is supported.

     GPGME_DELETE_FORCE
            SINCE: 1.9.1

          If set, the user is not asked to confirm the deletion.

     The function returns the error code GPG_ERR_NO_ERROR if the key was deleted successfully, GPG_ERR_INV_VALUE if CTX or KEY is not a valid pointer, GPG_ERR_NO_PUBKEY if KEY could not be found in the keyring, GPG_ERR_AMBIGUOUS_NAME if the key was not specified unambiguously, and GPG_ERR_CONFLICT if the secret key for KEY is available, but ALLOW_SECRET is zero.

 -- Function: gpgme_error_t gpgme_op_delete_ext_start (gpgme_ctx_t CTX, const gpgme_key_t KEY, unsigned int FLAGS)
       SINCE: 1.9.1

     The function gpgme_op_delete_ext_start initiates a gpgme_op_delete operation.  It can be completed by calling gpgme_wait on the context.  See Waiting For Completion.

     The function returns the error code GPG_ERR_NO_ERROR if the operation was started successfully, and GPG_ERR_INV_VALUE if CTX or KEY is not a valid pointer.

The following functions allow only to use one particular flag.

 -- Function: gpgme_error_t gpgme_op_delete (gpgme_ctx_t CTX, const gpgme_key_t KEY, int ALLOW_SECRET)
     Similar to gpgme_op_delete_ext, but only the flag GPGME_DELETE_ALLOW_SECRET can be provided.

 -- Function: gpgme_error_t gpgme_op_delete_start (gpgme_ctx_t CTX, const gpgme_key_t KEY, int ALLOW_SECRET)
     Similar to gpgme_op_delete_ext_start, but only the flag GPGME_DELETE_ALLOW_SECRET can be provided.

Changing Passphrases
--------------------

 -- Function: gpgme_error_t gpgme_op_passwd
     (gpgme_ctx_t CTX,
     const gpgme_key_t KEY,
     unsigned int FLAGS)

       SINCE: 1.3.0

     The function gpgme_op_passwd changes the passphrase of the private key associated with KEY.  The only allowed value for FLAGS is 0.  The backend engine will usually popup a window to ask for the old and the new passphrase.  Thus this function is not useful in a server application (where passphrases are not required anyway).

     Note that old gpg engines (before version 2.0.15) do not support this command and will silently ignore it.

 -- Function: gpgme_error_t gpgme_op_passwd_start
     (gpgme_ctx_t CTX,
     const gpgme_key_t KEY,
     unsigned int FLAGS)

       SINCE: 1.3.0

     The function gpgme_op_passwd_start initiates a gpgme_op_passwd operation.    It can be completed by calling gpgme_wait on the context.  See Waiting For Completion.

     The function returns 0 if the operation was started successfully, and an error code if one of the arguments is not valid or the oepration could not be started.

Changing TOFU Data
------------------

The OpenPGP engine features a Trust-On-First-Use (TOFU) key validation model.  For resolving conflicts it is necessary to declare the policy for a key.  See the GnuPG manual for details on the TOFU implementation.

 -- Data type: enum gpgme_tofu_policy_t
       SINCE: 1.7.0

     The gpgme_tofu_policy_t type specifies the set of possible policy values that are supported by GPGME:

     GPGME_TOFU_POLICY_AUTO
          Set the policy to ``auto''.

     GPGME_TOFU_POLICY_GOOD
          Set the policy to ``good''.

     GPGME_TOFU_POLICY_BAD
          Set the policy to ``bad''.

     GPGME_TOFU_POLICY_ASK
          Set the policy to ``ask''.

     GPGME_TOFU_POLICY_UNKNOWN
          Set the policy to ``unknown''.

To change the policy for a key the following functions can be used:

 -- Function: gpgme_error_t gpgme_op_tofu_policy
     (gpgme_ctx_t CTX,
     const gpgme_key_t KEY,
     gpgme_tofu_policy_t POLICY)

       SINCE: 1.7.0

     The function gpgme_op_tofu_policy changes the TOFU policy of KEY.  The valid values for POLICY are listed above.  As of now this function does only work for OpenPGP and requires at least version 2.1.10 of GnuPG.

     The function returns zero on success, GPG_ERR_NOT_SUPPORTED if the engine does not support the command, or a bunch of other error codes.

 -- Function: gpgme_error_t gpgme_op_tofu_policy_start
     (gpgme_ctx_t CTX,
     const gpgme_key_t KEY,
     gpgme_tofu_policy_t POLICY)

       SINCE: 1.7.0

     The function gpgme_op_tofu_policy_start initiates a gpgme_op_tofu_policy operation.    It can be completed by calling gpgme_wait on the context.  See Waiting For Completion.

     The function returns 0 if the operation was started successfully, and an error code if one of the arguments is not valid or the oepration could not be started.

Advanced Key Editing
--------------------

 -- Data type: gpgme_error_t (*gpgme_interact_cb_t)
     (void *HANDLE,
     const char *STATUS,
     const char *ARGS,
     int FD)

       SINCE: 1.7.0

     The gpgme_interact_cb_t type is the type of functions which GPGME calls if it a key interact operation is on-going.  The status keyword STATUS and the argument line ARGS are passed through by GPGME from the crypto engine.  An empty string represents EOF.  The file descriptor FD is -1 for normal status messages.  If STATUS indicates a command rather than a status message, the response to the command should be written to FD.  The HANDLE is provided by the user at start of operation.

     The function should return GPG_ERR_FALSE if it did not handle the status code, 0 for success, or any other error value.

 -- Function: gpgme_error_t gpgme_op_interact (gpgme_ctx_t CTX,
     gpgme_key_t KEY,
     unsigned int FLAGS,
     gpgme_interact_cb_t FNC,
     void *HANDLE,
     gpgme_data_t OUT)

       SINCE: 1.7.0

     The function gpgme_op_interact processes the key KEY interactively, using the interact callback function FNC with the handle HANDLE.  The callback is invoked for every status and command request from the crypto engine.  The output of the crypto engine is written to the data object OUT.

     Note that the protocol between the callback function and the crypto engine is specific to the crypto engine and no further support in implementing this protocol correctly is provided by GPGME.

     FLAGS modifies the behaviour of the function; the only defined bit value is:

     GPGME_INTERACT_CARD
            SINCE: 1.7.0

          This is used for smartcard based keys and uses gpg’s --card-edit command.

     The function returns 0 if the edit operation completes successfully, GPG_ERR_INV_VALUE if CTX or KEY is not a valid pointer, and any error returned by the crypto engine or the edit callback handler.

 -- Function: gpgme_error_t gpgme_op_interact_start (gpgme_ctx_t CTX,
     gpgme_key_t KEY,
     unsigned int FLAGS,
     gpgme_interact_cb_t FNC,
     void *HANDLE,
     gpgme_data_t OUT)

       SINCE: 1.7.0

     The function gpgme_op_interact_start initiates a gpgme_op_interact operation.  It can be completed by calling gpgme_wait on the context.  See Waiting For Completion.

     The function returns 0 if the operation was started successfully, and GPG_ERR_INV_VALUE if CTX or KEY is not a valid pointer.

Trust Item Management
=====================

Caution: The trust items interface is experimental.

 -- Data type: gpgme_trust_item_t
     The gpgme_trust_item_t type is a pointer to a trust item object. It has the following members:

     char *keyid
          This is a string describing the key to which this trust items belongs.

     int type
          This is the type of the trust item.  A value of 1 refers to a key, a value of 2 refers to a user ID.

     int level
          This is the trust level.

     char *owner_trust
          The owner trust if type is 1.

     char *validity
          The calculated validity.

     char *name
          The user name if type is 2.

Listing Trust Items
-------------------

 -- Function: gpgme_error_t gpgme_op_trustlist_start (gpgme_ctx_t CTX, const char *PATTERN, int MAX_LEVEL)
     The function gpgme_op_trustlist_start initiates a trust item listing operation inside the context CTX.  It sets everything up so that subsequent invocations of gpgme_op_trustlist_next return the trust items in the list.

     The string PATTERN contains an engine specific expression that is used to limit the list to all trust items matching the pattern.  It can not be the empty string.

     The argument MAX_LEVEL is currently ignored.

     The context will be busy until either all trust items are received (and gpgme_op_trustlist_next returns GPG_ERR_EOF), or gpgme_op_trustlist_end is called to finish the operation.

     The function returns the error code GPG_ERR_INV_VALUE if CTX is not a valid pointer, and passes through any errors that are reported by the crypto engine support routines.

 -- Function: gpgme_error_t gpgme_op_trustlist_next (gpgme_ctx_t CTX, gpgme_trust_item_t *R_ITEM)
     The function gpgme_op_trustlist_next returns the next trust item in the list created by a previous gpgme_op_trustlist_start operation in the context CTX.  The trust item can be destroyed with gpgme_trust_item_release.  See Manipulating Trust Items.

     This is the only way to get at gpgme_trust_item_t objects in GPGME.

     If the last trust item in the list has already been returned, gpgme_op_trustlist_next returns GPG_ERR_EOF.

     The function returns the error code GPG_ERR_INV_VALUE if CTX or R_ITEM is not a valid pointer, and GPG_ERR_ENOMEM if there is not enough memory for the operation.

 -- Function: gpgme_error_t gpgme_op_trustlist_end (gpgme_ctx_t CTX)
     The function gpgme_op_trustlist_end ends a pending trust list operation in the context CTX.

     The function returns the error code GPG_ERR_INV_VALUE if CTX is not a valid pointer, and GPG_ERR_ENOMEM if at some time during the operation there was not enough memory available.

Manipulating Trust Items
------------------------

 -- Function: void gpgme_trust_item_ref (gpgme_trust_item_t ITEM)
     The function gpgme_trust_item_ref acquires an additional reference for the trust item ITEM.

 -- Function: void gpgme_trust_item_unref (gpgme_trust_item_t ITEM)
     The function gpgme_trust_item_unref releases a reference for the trust item ITEM.  If this was the last reference, the trust item will be destroyed and all resources associated to it will be released.

Crypto Operations
=================

Sometimes, the result of a crypto operation returns a list of invalid keys encountered in processing the request.  The following structure is used to hold information about such a key.

 -- Data type: gpgme_invalid_key_t
     This is a pointer to a structure used to store a part of the result of a crypto operation which takes user IDs as one input parameter.  The structure contains the following members:

     gpgme_invalid_key_t next
          This is a pointer to the next invalid key structure in the linked list, or NULL if this is the last element.

     char *fpr
          The fingerprint or key ID of the invalid key encountered.

     gpgme_error_t reason
          An error code describing the reason why the key was found invalid.

Decrypt
-------

 -- Function: gpgme_error_t gpgme_op_decrypt (gpgme_ctx_t CTX, gpgme_data_t CIPHER, gpgme_data_t PLAIN)
     The function gpgme_op_decrypt decrypts the ciphertext in the data object CIPHER and stores it into the data object PLAIN.

     The function returns the error code GPG_ERR_NO_ERROR if the ciphertext could be decrypted successfully, GPG_ERR_INV_VALUE if CTX, CIPHER or PLAIN is not a valid pointer, GPG_ERR_NO_DATA if CIPHER does not contain any data to decrypt, GPG_ERR_DECRYPT_FAILED if CIPHER is not a valid cipher text, GPG_ERR_BAD_PASSPHRASE if the passphrase for the secret key could not be retrieved, and passes through some errors that are reported by the crypto engine support routines.

 -- Function: gpgme_error_t gpgme_op_decrypt_start (gpgme_ctx_t CTX, gpgme_data_t CIPHER, gpgme_data_t PLAIN)
     The function gpgme_op_decrypt_start initiates a gpgme_op_decrypt operation.  It can be completed by calling gpgme_wait on the context.  See Waiting For Completion.

     The function returns the error code GPG_ERR_NO_ERROR if the operation could be started successfully, and GPG_ERR_INV_VALUE if CIPHER or PLAIN is not a valid pointer.

 -- Function: gpgme_error_t gpgme_op_decrypt_ext (
     gpgme_ctx_t CTX,
     gpgme_decrypt_flags_t FLAGS,
     gpgme_data_t CIPHER,
     gpgme_data_t PLAIN)

       SINCE: 1.8.0

     The function gpgme_op_decrypt_ext is the same as gpgme_op_decrypt_ext but has an additional argument FLAGS.  If FLAGS is 0 both function behave identically.

     The value in FLAGS is a bitwise-or combination of one or multiple of the following bit values:

     GPGME_DECRYPT_VERIFY
            SINCE: 1.8.0

          The GPGME_DECRYPT_VERIFY symbol specifies that this function shall exacty act as gpgme_op_decrypt_verify.

     GPGME_DECRYPT_UNWRAP
            SINCE: 1.8.0

          The GPGME_DECRYPT_UNWRAP symbol specifies that the output shall be an OpenPGP message with only the encryption layer removed.  This requires GnuPG 2.1.12 and works only for OpenPGP.  This is the counterpart to GPGME_ENCRYPT_WRAP.

     The function returns the error codes as descriped for gpgme_op_decrypt respective gpgme_op_encrypt.

 -- Function: gpgme_error_t gpgme_op_decrypt_ext_start (
     gpgme_ctx_t CTX,
     gpgme_decrypt_flags_t FLAGS,
     gpgme_data_t CIPHER,
     gpgme_data_t PLAIN)

       SINCE: 1.8.0

     The function gpgme_op_decrypt_ext_start initiates a gpgme_op_decrypt_ext operation.  It can be completed by calling gpgme_wait on the context.  See Waiting For Completion.

     The function returns the error code GPG_ERR_NO_ERROR if the operation could be started successfully, and GPG_ERR_INV_VALUE if CIPHER or PLAIN is not a valid pointer.

 -- Data type: gpgme_recipient_t
       SINCE: 1.1.0

     This is a pointer to a structure used to store information about the recipient of an encrypted text which is decrypted in a gpgme_op_decrypt operation.  This information (except for the status field) is even available before the operation finished successfully, for example in a passphrase callback.  The structure contains the following members:

     gpgme_recipient_t next
          This is a pointer to the next recipient structure in the linked list, or NULL if this is the last element.

     gpgme_pubkey_algo_t
          The public key algorithm used in the encryption.

     char *keyid
          This is the key ID of the key (in hexadecimal digits) used as recipient.

     gpgme_error_t status
          This is an error number with the error code GPG_ERR_NO_SECKEY if the secret key for this recipient is not available, and 0 otherwise.

 -- Data type: gpgme_decrypt_result_t
     This is a pointer to a structure used to store the result of a gpgme_op_decrypt operation.  After successfully decrypting data, you can retrieve the pointer to the result with gpgme_op_decrypt_result.  As with all result structures, it this structure shall be considered read-only and an application must not allocated such a strucure on its own.  The structure contains the following members:

     char *unsupported_algorithm
          If an unsupported algorithm was encountered, this string describes the algorithm that is not supported.

     unsigned int wrong_key_usage : 1
            SINCE: 0.9.0

          This is true if the key was not used according to its policy.

     gpgme_recipient_t recipients
            SINCE: 1.1.0

          This is a linked list of recipients to which this message was encrypted.

     char *file_name
          This is the filename of the original plaintext message file if it is known, otherwise this is a null pointer.

     char *session_key
            SINCE: 1.8.0

          A textual representation (nul-terminated string) of the session key used in symmetric encryption of the message, if the context has been set to export session keys (see gpgme_set_ctx_flag, "export-session-key"), and a session key was available for the most recent decryption operation.  Otherwise, this is a null pointer.

          You must not try to access this member of the struct unless gpgme_set_ctx_flag (ctx, "export-session-key") returns success or gpgme_get_ctx_flag (ctx, "export-session-key") returns true (non-empty string).

 -- Function: gpgme_decrypt_result_t gpgme_op_decrypt_result (gpgme_ctx_t CTX)
     The function gpgme_op_decrypt_result returns a gpgme_decrypt_result_t pointer to a structure holding the result of a gpgme_op_decrypt operation.  The pointer is only valid if the last operation on the context was a gpgme_op_decrypt or gpgme_op_decrypt_start operation. If the operation failed this might be a NULL pointer.  The returned pointer is only valid until the next operation is started on the context.

Verify
------

 -- Function: gpgme_error_t gpgme_op_verify (gpgme_ctx_t CTX, gpgme_data_t SIG, gpgme_data_t SIGNED_TEXT, gpgme_data_t PLAIN)
     The function gpgme_op_verify verifies that the signature in the data object SIG is a valid signature.  If SIG is a detached signature, then the signed text should be provided in SIGNED_TEXT and PLAIN should be a null pointer. Otherwise, if SIG is a normal (or cleartext) signature, SIGNED_TEXT should be a null pointer and PLAIN should be a writable data object that will contain the plaintext after successful verification.

     The results of the individual signature verifications can be retrieved with gpgme_op_verify_result.

     The function returns the error code GPG_ERR_NO_ERROR if the operation could be completed successfully, GPG_ERR_INV_VALUE if CTX, SIG or PLAIN is not a valid pointer, GPG_ERR_NO_DATA if SIG does not contain any data to verify, and passes through any errors that are reported by the crypto engine support routines.

 -- Function: gpgme_error_t gpgme_op_verify_start (gpgme_ctx_t CTX, gpgme_data_t SIG, gpgme_data_t SIGNED_TEXT, gpgme_data_t PLAIN)
     The function gpgme_op_verify_start initiates a gpgme_op_verify operation.  It can be completed by calling gpgme_wait on the context.  See Waiting For Completion.

     The function returns the error code GPG_ERR_NO_ERROR if the operation could be started successfully, GPG_ERR_INV_VALUE if CTX, SIG or PLAIN is not a valid pointer, and GPG_ERR_NO_DATA if SIG or PLAIN does not contain any data to verify.

 -- Data type: gpgme_sig_notation_t
     This is a pointer to a structure used to store a part of the result of a gpgme_op_verify operation.  The structure contains the following members:

     gpgme_sig_notation_t next
          This is a pointer to the next new signature notation structure in the linked list, or NULL if this is the last element.

     char *name
          The name of the notation field.  If this is NULL, then the member value will contain a policy URL.

     int name_len
          The length of the name field.  For strings the length is counted without the trailing binary zero.

     char *value
          The value of the notation field.  If name is NULL, then this is a policy URL.

     int value_len
          The length of the value field.  For strings the length is counted without the trailing binary zero.

     gpgme_sig_notation_flags_t flags
          The accumulated flags field.  This field contains the flags associated with the notation data in an accumulated form which can be used as an argument to the function gpgme_sig_notation_add.  The value flags is a bitwise-or combination of one or multiple of the following bit values:

          GPGME_SIG_NOTATION_HUMAN_READABLE
                 SINCE: 1.1.0

               The GPGME_SIG_NOTATION_HUMAN_READABLE symbol specifies that the notation data is in human readable form

          GPGME_SIG_NOTATION_CRITICAL
                 SINCE: 1.1.0

               The GPGME_SIG_NOTATION_CRITICAL symbol specifies that the notation data is critical.

     unsigned int human_readable : 1
          This is true if the GPGME_SIG_NOTATION_HUMAN_READABLE flag is set and false otherwise.  This flag is only valid for notation data, not for policy URLs.

     unsigned int critical : 1
          This is true if the GPGME_SIG_NOTATION_CRITICAL flag is set and false otherwise.  This flag is valid for notation data and policy URLs.

 -- Data type: gpgme_signature_t
     This is a pointer to a structure used to store a part of the result of a gpgme_op_verify operation.  The structure contains the following members:

     gpgme_signature_t next
          This is a pointer to the next new signature structure in the linked list, or NULL if this is the last element.

     gpgme_sigsum_t summary
          This is a bit vector giving a summary of the signature status.  It provides an easy interface to a defined semantic of the signature status.  Checking just one bit is sufficient to see whether a signature is valid without any restrictions. This means that you can check for GPGME_SIGSUM_VALID like this:

               if ((sig.summary & GPGME_SIGSUM_VALID))
               {
                  ..do stuff if valid..
               }
               else
               {
                  ..do stuff if not fully valid..
               }

          The defined bits are:

          GPGME_SIGSUM_VALID
               The signature is fully valid.

          GPGME_SIGSUM_GREEN
               The signature is good but one might want to display some extra information.  Check the other bits.

          GPGME_SIGSUM_RED
               The signature is bad. It might be useful to check other bits and display more information, i.e. a revoked certificate might not render a signature invalid when the message was received prior to the cause for the revocation.

          GPGME_SIGSUM_KEY_REVOKED
               The key or at least one certificate has been revoked.

          GPGME_SIGSUM_KEY_EXPIRED
               The key or one of the certificates has expired. It is probably a good idea to display the date of the expiration.

          GPGME_SIGSUM_SIG_EXPIRED
               The signature has expired.

          GPGME_SIGSUM_KEY_MISSING
               Can't verify due to a missing key or certificate.

          GPGME_SIGSUM_CRL_MISSING
               The CRL (or an equivalent mechanism) is not available.

          GPGME_SIGSUM_CRL_TOO_OLD
               Available CRL is too old.

          GPGME_SIGSUM_BAD_POLICY
               A policy requirement was not met.

          GPGME_SIGSUM_SYS_ERROR
               A system error occured.

          GPGME_SIGSUM_TOFU_CONFLICT
               A TOFU conflict was detected.

     char *fpr
          This is the fingerprint or key ID of the signature.

     gpgme_error_t status
          This is the status of the signature.  In particular, the following status codes are of interest:

          GPG_ERR_NO_ERROR
               This status indicates that the signature is valid.  For the combined result this status means that all signatures are valid.

          GPG_ERR_SIG_EXPIRED
               This status indicates that the signature is valid but expired.  For the combined result this status means that all signatures are valid and expired.

          GPG_ERR_KEY_EXPIRED
               This status indicates that the signature is valid but the key used to verify the signature has expired.  For the combined result this status means that all signatures are valid and all keys are expired.

          GPG_ERR_CERT_REVOKED
               This status indicates that the signature is valid but the key used to verify the signature has been revoked.  For the combined result this status means that all signatures are valid and all keys are revoked.

          GPG_ERR_BAD_SIGNATURE
               This status indicates that the signature is invalid.  For the combined result this status means that all signatures are invalid.

          GPG_ERR_NO_PUBKEY
               This status indicates that the signature could not be verified due to a missing key.  For the combined result this status means that all signatures could not be checked due to missing keys.

          GPG_ERR_GENERAL
               This status indicates that there was some other error which prevented the signature verification.

     gpgme_sig_notation_t notations
          This is a linked list with the notation data and policy URLs.

     unsigned long timestamp
          The creation timestamp of this signature.

     unsigned long exp_timestamp
          The expiration timestamp of this signature, or 0 if the signature does not expire.

     unsigned int wrong_key_usage : 1
          This is true if the key was not used according to its policy.

     unsigned int pka_trust : 2
          This is set to the trust information gained by means of the PKA system. Values are:

          0
               No PKA information available or verification not possible.

          1
               PKA verification failed.

          2
               PKA verification succeeded.

          3
               Reserved for future use.

          Depending on the configuration of the engine, this metric may also be reflected by the validity of the signature.

     unsigned int chain_model : 1
            SINCE: 1.1.6

          This is true if the validity of the signature has been checked using the chain model.  In the chain model the time the signature has been created must be within the validity period of the certificate and the time the certificate itself has been created must be within the validity period of the issuing certificate.  In contrast the default validation model checks the validity of signature as well at the entire certificate chain at the current time.

     gpgme_validity_t validity
          The validity of the signature.

     gpgme_error_t validity_reason
          If a signature is not valid, this provides a reason why.

     gpgme_pubkey_algo_t
          The public key algorithm used to create this signature.

     gpgme_hash_algo_t
          The hash algorithm used to create this signature.

     char *pka_address
          The mailbox from the PKA information or NULL.

     gpgme_key_t key
            SINCE: 1.7.0

          An object describing the key used to create the signature.  This key object may be incomplete in that it only conveys information availabale directly with a signature.  It may also be NULL if such information is not readily available.

 -- Data type: gpgme_verify_result_t
     This is a pointer to a structure used to store the result of a gpgme_op_verify operation.  After verifying a signature, you can retrieve the pointer to the result with gpgme_op_verify_result.  If the operation failed this might be a NULL pointer.  The structure contains the following member:

     gpgme_signature_t signatures
          A linked list with information about all signatures for which a verification was attempted.

     char *file_name
          This is the filename of the original plaintext message file if it is known, otherwise this is a null pointer.

 -- Function: gpgme_verify_result_t gpgme_op_verify_result (gpgme_ctx_t CTX)
     The function gpgme_op_verify_result returns a gpgme_verify_result_t pointer to a structure holding the result of a gpgme_op_verify operation.  The pointer is only valid if the last operation on the context was a gpgme_op_verify, gpgme_op_verify_start, gpgme_op_decrypt_verify or gpgme_op_decrypt_verify_start operation, and if this operation finished successfully (for gpgme_op_decrypt_verify and gpgme_op_decrypt_verify_start, the error code GPG_ERR_NO_DATA counts as successful in this context).  The returned pointer is only valid until the next operation is started on the context.

Decrypt and Verify
------------------

 -- Function: gpgme_error_t gpgme_op_decrypt_verify (gpgme_ctx_t CTX, gpgme_data_t CIPHER, gpgme_data_t PLAIN)
     The function gpgme_op_decrypt_verify decrypts the ciphertext in the data object CIPHER and stores it into the data object PLAIN.  If CIPHER contains signatures, they will be verified.

     After the operation completed, gpgme_op_decrypt_result and gpgme_op_verify_result can be used to retrieve more information about the signatures.

     If the error code GPG_ERR_NO_DATA is returned, CIPHER does not contain any data to decrypt.  However, it might still be signed.  The information about detected signatures is available with gpgme_op_verify_result in this case.

     The function returns the error code GPG_ERR_NO_ERROR if the ciphertext could be decrypted successfully, GPG_ERR_INV_VALUE if CTX, CIPHER or PLAIN is not a valid pointer, GPG_ERR_NO_DATA if CIPHER does not contain any data to decrypt, GPG_ERR_DECRYPT_FAILED if CIPHER is not a valid cipher text, GPG_ERR_BAD_PASSPHRASE if the passphrase for the secret key could not be retrieved, and passes through any errors that are reported by the crypto engine support routines.

 -- Function: gpgme_error_t gpgme_op_decrypt_verify_start (gpgme_ctx_t CTX, gpgme_data_t CIPHER, gpgme_data_t PLAIN)
     The function gpgme_op_decrypt_verify_start initiates a gpgme_op_decrypt_verify operation.  It can be completed by calling gpgme_wait on the context.  See Waiting For Completion.

     The function returns the error code GPG_ERR_NO_ERROR if the operation could be started successfully, GPG_ERR_INV_VALUE if CTX, CIPHER, PLAIN or R_STAT is not a valid pointer, and GPG_ERR_NO_DATA if CIPHER does not contain any data to decrypt.

When processing mails it is sometimes useful to extract the actual mail address (the ``addr-spec'') from a string.  GPGME provides this helper function which uses the same semantics as the internal functions in GPGME and GnuPG:

 -- Function: char * gpgme_addrspec_from_uid (const char *UID)

       SINCE: 1.7.1

     Return the mail address (called ``addr-spec'' in RFC-5322) from the string UID which is assumed to be a user id (called ``address'' in RFC-5322).  All plain ASCII characters (i.e. those with bit 7 cleared) in the result are converted to lowercase.  Caller must free the result using gpgme_free.  Returns NULL if no valid address was found (in which case ERRNO is set to EINVAL) or for other errors.

Sign
----

A signature can contain signatures by one or more keys.  The set of keys used to create a signatures is contained in a context, and is applied to all following signing operations in this context (until the set is changed).

Selecting Signers
.................

The key or the keys used to create a signature are stored in the context.  The following functions can be used to manipulate this list. If no signer has been set into the context a default key is used for signing.

 -- Function: void gpgme_signers_clear (gpgme_ctx_t CTX)
     The function gpgme_signers_clear releases a reference for each key on the signers list and removes the list of signers from the context CTX.

     Every context starts with an empty list.

 -- Function: gpgme_error_t gpgme_signers_add (gpgme_ctx_t CTX, const gpgme_key_t KEY)
     The function gpgme_signers_add adds the key KEY to the list of signers in the context CTX.

     Calling this function acquires an additional reference for the key.

 -- Function: unsigned int gpgme_signers_count (const gpgme_ctx_t CTX)
       SINCE: 1.4.3

     The function gpgme_signers_count returns the number of signer keys in the context CTX.

 -- Function: gpgme_key_t gpgme_signers_enum (const gpgme_ctx_t CTX, int SEQ)
     The function gpgme_signers_enum returns the SEQth key in the list of signers in the context CTX.  An additional reference is acquired for the user.

     If SEQ is out of range, NULL is returned.

Creating a Signature
....................

 -- Data type: enum gpgme_sig_mode_t
     The gpgme_sig_mode_t type is used to specify the desired type of a signature.  The following modes are available:

     GPGME_SIG_MODE_NORMAL
          A normal signature is made, the output includes the plaintext and the signature.

     GPGME_SIG_MODE_DETACH
          A detached signature is made.

     GPGME_SIG_MODE_CLEAR
          A clear text signature is made.  The ASCII armor and text mode settings of the context are ignored.

 -- Function: gpgme_error_t gpgme_op_sign (gpgme_ctx_t CTX, gpgme_data_t PLAIN, gpgme_data_t SIG, gpgme_sig_mode_t MODE)
     The function gpgme_op_sign creates a signature for the text in the data object PLAIN and returns it in the data object SIG.  The type of the signature created is determined by the ASCII armor (or, if that is not set, by the encoding specified for SIG), the text mode attributes set for the context CTX and the requested signature mode MODE.

     After the operation completed successfully, the result can be retrieved with gpgme_op_sign_result.

     If an S/MIME signed message is created using the CMS crypto engine, the number of certificates to include in the message can be specified with gpgme_set_include_certs.  See Included Certificates.

     The function returns the error code GPG_ERR_NO_ERROR if the signature could be created successfully, GPG_ERR_INV_VALUE if CTX, PLAIN or SIG is not a valid pointer, GPG_ERR_NO_DATA if the signature could not be created, GPG_ERR_BAD_PASSPHRASE if the passphrase for the secret key could not be retrieved, GPG_ERR_UNUSABLE_SECKEY if there are invalid signers, and passes through any errors that are reported by the crypto engine support routines.

 -- Function: gpgme_error_t gpgme_op_sign_start (gpgme_ctx_t CTX, gpgme_data_t PLAIN, gpgme_data_t SIG, gpgme_sig_mode_t MODE)
     The function gpgme_op_sign_start initiates a gpgme_op_sign operation.  It can be completed by calling gpgme_wait on the context.  See Waiting For Completion.

     The function returns the error code GPG_ERR_NO_ERROR if the operation could be started successfully, and GPG_ERR_INV_VALUE if CTX, PLAIN or SIG is not a valid pointer.

 -- Data type: gpgme_new_signature_t
     This is a pointer to a structure used to store a part of the result of a gpgme_op_sign operation.  The structure contains the following members:

     gpgme_new_signature_t next
          This is a pointer to the next new signature structure in the linked list, or NULL if this is the last element.

     gpgme_sig_mode_t type
          The type of this signature.

     gpgme_pubkey_algo_t pubkey_algo
          The public key algorithm used to create this signature.

     gpgme_hash_algo_t hash_algo
          The hash algorithm used to create this signature.

     unsigned int sig_class
          The signature class of this signature.

     long int timestamp
          The creation timestamp of this signature.

     char *fpr
          The fingerprint of the key which was used to create this signature.

 -- Data type: gpgme_sign_result_t
     This is a pointer to a structure used to store the result of a gpgme_op_sign operation.  After successfully generating a signature, you can retrieve the pointer to the result with gpgme_op_sign_result.  The structure contains the following members:

     gpgme_invalid_key_t invalid_signers
          A linked list with information about all invalid keys for which a signature could not be created.

     gpgme_new_signature_t signatures
          A linked list with information about all signatures created.

 -- Function: gpgme_sign_result_t gpgme_op_sign_result (gpgme_ctx_t CTX)
     The function gpgme_op_sign_result returns a gpgme_sign_result_t pointer to a structure holding the result of a gpgme_op_sign operation.  The pointer is only valid if the last operation on the context was a gpgme_op_sign, gpgme_op_sign_start, gpgme_op_encrypt_sign or gpgme_op_encrypt_sign_start operation.  If that operation failed, the function might return a NULL pointer. The returned pointer is only valid until the next operation is started on the context.

Signature Notation Data
.......................

Using the following functions, you can attach arbitrary notation data to a signature.  This information is then available to the user when the signature is verified.

 -- Function: void gpgme_sig_notation_clear (gpgme_ctx_t CTX)
       SINCE: 1.1.0

     The function gpgme_sig_notation_clear removes the notation data from the context CTX.  Subsequent signing operations from this context will not include any notation data.

     Every context starts with an empty notation data list.

 -- Function: gpgme_error_t gpgme_sig_notation_add (gpgme_ctx_t CTX, const char *NAME, const char *VALUE, gpgme_sig_notation_flags_t FLAGS)
       SINCE: 1.1.0

     The function gpgme_sig_notation_add adds the notation data with the name NAME and the value VALUE to the context CTX.

     Subsequent signing operations will include this notation data, as well as any other notation data that was added since the creation of the context or the last gpgme_sig_notation_clear operation.

     The arguments NAME and VALUE must be NUL-terminated strings in human-readable form.  The flag GPGME_SIG_NOTATION_HUMAN_READABLE is implied (non-human-readable notation data is currently not supported).  The strings must be in UTF-8 encoding.

     If NAME is NULL, then VALUE should be a policy URL.

     The function gpgme_sig_notation_add returns the error code GPG_ERR_NO_ERROR if the notation data could be added successfully, GPG_ERR_INV_VALUE if CTX is not a valid pointer, or if NAME, VALUE and FLAGS are an invalid combination.  The function also passes through any errors that are reported by the crypto engine support routines.

 -- Function: gpgme_sig_notation_t gpgme_sig_notation_get (const gpgme_ctx_t CTX)
       SINCE: 1.1.0

     The function gpgme_sig_notation_get returns the linked list of notation data structures that are contained in the context CTX.

     If CTX is not a valid pointer, or there is no notation data added for this context, NULL is returned.

Encrypt
-------

One plaintext can be encrypted for several recipients at the same time.  The list of recipients is created independently of any context, and then passed to the encryption operation.

Encrypting a Plaintext
......................

 -- Function: gpgme_error_t gpgme_op_encrypt (gpgme_ctx_t CTX, gpgme_key_t RECP[], gpgme_encrypt_flags_t FLAGS, gpgme_data_t PLAIN, gpgme_data_t CIPHER)
     The function gpgme_op_encrypt encrypts the plaintext in the data object PLAIN for the recipients RECP and stores the ciphertext in the data object CIPHER.  The type of the ciphertext created is determined by the ASCII armor (or, if that is not set, by the encoding specified for CIPHER) and the text mode attributes set for the context CTX.

     RECP must be a NULL-terminated array of keys.  The user must keep references for all keys during the whole duration of the call (but see gpgme_op_encrypt_start for the requirements with the asynchronous variant).

     The value in FLAGS is a bitwise-or combination of one or multiple of the following bit values:

     GPGME_ENCRYPT_ALWAYS_TRUST
          The GPGME_ENCRYPT_ALWAYS_TRUST symbol specifies that all the recipients in RECP should be trusted, even if the keys do not have a high enough validity in the keyring.  This flag should be used with care; in general it is not a good idea to use any untrusted keys.

     GPGME_ENCRYPT_NO_ENCRYPT_TO
            SINCE: 1.2.0

          The GPGME_ENCRYPT_NO_ENCRYPT_TO symbol specifies that no default or hidden default recipients as configured in the crypto backend should be included.  This can be useful for managing different user profiles.

     GPGME_ENCRYPT_NO_COMPRESS
            SINCE: 1.5.0

          The GPGME_ENCRYPT_NO_COMPRESS symbol specifies that the plaintext shall not be compressed before it is encrypted.  This is in some cases useful if the length of the encrypted message may reveal information about the plaintext.

     GPGME_ENCRYPT_PREPARE
     GPGME_ENCRYPT_EXPECT_SIGN
          The GPGME_ENCRYPT_PREPARE symbol is used with the UI Server protocol to prepare an encryption (i.e. sending the PREP_ENCRYPT command).  With the GPGME_ENCRYPT_EXPECT_SIGN symbol the UI Server is advised to also expect a sign command.

     GPGME_ENCRYPT_SYMMETRIC
            SINCE: 1.7.0

          The GPGME_ENCRYPT_SYMMETRIC symbol specifies that the output should be additionally encrypted symmetrically even if recipients are provided. This feature is only supported for for the OpenPGP crypto engine.

     GPGME_ENCRYPT_THROW_KEYIDS
            SINCE: 1.8.0

          The GPGME_ENCRYPT_THROW_KEYIDS symbols requests that the identifiers for the decrption keys are not included in the ciphertext. On the receiving side, the use of this flag may slow down the decryption process because all available secret keys must be tried. This flag is only honored for OpenPGP encryption.

     GPGME_ENCRYPT_WRAP
            SINCE: 1.8.0

          The GPGME_ENCRYPT_WRAP symbol specifies that the input is an OpenPGP message and not a plain data.  This is the counterpart to GPGME_DECRYPT_UNWRAP.

     If GPG_ERR_UNUSABLE_PUBKEY is returned, some recipients in RECP are invalid, but not all.  In this case the plaintext might be encrypted for all valid recipients and returned in CIPHER (if this happens depends on the crypto engine).  More information about the invalid recipients is available with gpgme_op_encrypt_result.

     If RECP is NULL, symmetric rather than public key encryption is performed.  Symmetrically encrypted cipher text can be deciphered with gpgme_op_decrypt.  Note that in this case the crypto backend needs to retrieve a passphrase from the user. Symmetric encryption is currently only supported for the OpenPGP crypto backend.

     The function returns the error code GPG_ERR_NO_ERROR if the ciphertext could be created successfully, GPG_ERR_INV_VALUE if CTX, RECP, PLAIN or CIPHER is not a valid pointer, GPG_ERR_UNUSABLE_PUBKEY if RECP contains some invalid recipients, GPG_ERR_BAD_PASSPHRASE if the passphrase for the symmetric key could not be retrieved, and passes through any errors that are reported by the crypto engine support routines.

 -- Function: gpgme_error_t gpgme_op_encrypt_start (gpgme_ctx_t CTX, gpgme_key_t RECP[], gpgme_encrypt_flags_t FLAGS, gpgme_data_t PLAIN, gpgme_data_t CIPHER)
     The function gpgme_op_encrypt_start initiates a gpgme_op_encrypt operation.  It can be completed by calling gpgme_wait on the context.  See Waiting For Completion.

     References to the keys only need to be held for the duration of this call.  The user can release its references to the keys after this function returns, even if the operation is not yet finished.

     The function returns the error code GPG_ERR_NO_ERROR if the operation could be started successfully, GPG_ERR_INV_VALUE if CTX, RSET, PLAIN or CIPHER is not a valid pointer, and GPG_ERR_UNUSABLE_PUBKEY if RSET does not contain any valid recipients.

 -- Data type: gpgme_encrypt_result_t
     This is a pointer to a structure used to store the result of a gpgme_op_encrypt operation.  After successfully encrypting data, you can retrieve the pointer to the result with gpgme_op_encrypt_result.  The structure contains the following members:

     gpgme_invalid_key_t invalid_recipients
          A linked list with information about all invalid keys for which the data could not be encrypted.

 -- Function: gpgme_encrypt_result_t gpgme_op_encrypt_result (gpgme_ctx_t CTX)
     The function gpgme_op_encrypt_result returns a gpgme_encrypt_result_t pointer to a structure holding the result of a gpgme_op_encrypt operation.  The pointer is only valid if the last operation on the context was a gpgme_op_encrypt, gpgme_op_encrypt_start, gpgme_op_sign or gpgme_op_sign_start operation.  If this operation failed, this might be a NULL pointer.  The returned pointer is only valid until the next operation is started on the context.

 -- Function: gpgme_error_t gpgme_op_encrypt_sign (gpgme_ctx_t CTX, gpgme_key_t RECP[], gpgme_encrypt_flags_t FLAGS, gpgme_data_t PLAIN, gpgme_data_t CIPHER)
     The function gpgme_op_encrypt_sign does a combined encrypt and sign operation.  It is used like gpgme_op_encrypt, but the ciphertext also contains signatures for the signers listed in CTX.

     The combined encrypt and sign operation is currently only available for the OpenPGP crypto engine.

 -- Function: gpgme_error_t gpgme_op_encrypt_sign_start (gpgme_ctx_t CTX, gpgme_key_t RECP[], gpgme_encrypt_flags_t FLAGS, gpgme_data_t PLAIN, gpgme_data_t CIPHER)
     The function gpgme_op_encrypt_sign_start initiates a gpgme_op_encrypt_sign operation.  It can be completed by calling gpgme_wait on the context.  See Waiting For Completion.

     The function returns the error code GPG_ERR_NO_ERROR if the operation could be started successfully, and GPG_ERR_INV_VALUE if CTX, RSET, PLAIN or CIPHER is not a valid pointer.

Miscellaneous operations
========================

Here are some support functions which are sometimes useful.

Running other Programs
----------------------

GPGME features an internal subsystem to run the actual backend engines.  Along with data abstraction object this subsystem can be used to run arbitrary simple programs which even need not be related to cryptographic features.  It may for example be used to run tools which are part of the GnuPG system but are not directly accessible with the GPGME API.

 -- Function: gpgme_error_t gpgme_op_spawn
     (gpgme_ctx_t CTX, const char *FILE,
     const char *ARGV[], gpgme_data_t DATAIN,
     gpgme_data_t DATAOUT, gpgme_data_t DATAERR,
     unsigned int FLAGS)

       SINCE: 1.5.0

     The function gpgme_op_spawn runs the program FILE with the arguments taken from the NULL terminated array ARGV.  If no arguments are required ARGV may be given as NULL.  In the latter case or if argv[0] is the empty string, GPGME uses the basename of FILE for argv[0].  The file descriptors stdin, stdout, and stderr are connected to the data objects DATAIN, DATAOUT, and DATAERR.  If NULL is passed for one of these data objects the corresponding file descriptor is connected to /dev/null.

     The value in FLAGS is a bitwise-or combination of one or multiple of the following bit values:

     GPGME_SPAWN_DETACHED
            SINCE: 1.5.0

          Under Windows this flag inhibits the allocation of a new console for the program.  This is useful for a GUI application which needs to call a command line helper tool.

     GPGME_SPAWN_ALLOW_SET_FG
            SINCE: 1.5.0

          Under Windows this flag allows the called program to put itself into the foreground.

 -- Function: gpgme_error_t gpgme_op_spawn_start
     (gpgme_ctx_t CTX, const char *FILE,
     const char *ARGV[], gpgme_data_t DATAIN,
     gpgme_data_t DATAOUT, gpgme_data_t DATAERR,
     unsigned int FLAGS)

       SINCE: 1.5.0

     This is the asynchronous variant of gpgme_op_spawn.

Using the Assuan protocol
-------------------------

The Assuan protocol can be used to talk to arbitrary Assuan servers. By default it is connected to the GnuPG agent, but it may be connected to arbitrary servers by using gpgme_ctx_set_engine_info, passing the location of the servers socket as FILE_NAME argument, and an empty string as HOME_DIR argument.

The Assuan protocol functions use three kinds of callbacks to transfer data:

 -- Data type: gpgme_error_t (*gpgme_assuan_data_cb_t)
     (void *OPAQUE, const void *DATA,
     size_t DATALEN)

       SINCE: 1.2.0

     This callback receives any data sent by the server.  OPAQUE is the pointer passed to gpgme_op_assuan_transact_start, DATA of length DATALEN refers to the data sent.

 -- Data type: gpgme_error_t (*gpgme_assuan_inquire_cb_t)
     (void *OPAQUE, const char *NAME,
     const char *ARGS, gpgme_data_t *R_DATA)

       SINCE: 1.2.0

     This callback is used to provide additional data to the Assuan server. OPAQUE is the pointer passed to gpgme_op_assuan_transact_start, NAME and ARGS specify what kind of data the server requested, and R_DATA is used to return the actual data.

     Note: Returning data is currently not implemented in GPGME.

 -- Data type: gpgme_error_t (*gpgme_assuan_status_cb_t)
     (void *OPAQUE, const char *STATUS,
     const char *ARGS)

       SINCE: 1.2.0

     This callback receives any status lines sent by the server. OPAQUE is the pointer passed to gpgme_op_assuan_transact_start, STATUS and ARGS denote the status update sent.

 -- Function: gpgme_error_t gpgme_op_assuan_transact_start
     (gpgme_ctx_t CTX, const char *COMMAND,
     gpgme_assuan_data_cb_t DATA_CB,
     void * DATA_CB_VALUE,
     gpgme_assuan_inquire_cb_t INQUIRE_CB,
     void * INQUIRE_CB_VALUE,
     gpgme_assuan_status_cb_t STATUS_CB,
     void * STATUS_CB_VALUE)

       SINCE: 1.2.0

     Send the Assuan COMMAND and return results via the callbacks. Any callback may be NULL.  The result of the operation may be retrieved using gpgme_wait_ext.

     Asynchronous variant.

 -- Function: gpgme_error_t gpgme_op_assuan_transact_ext
     (gpgme_ctx_t CTX, const char *COMMAND,
     gpgme_assuan_data_cb_t DATA_CB,
     void * DATA_CB_VALUE,
     gpgme_assuan_inquire_cb_t INQUIRE_CB,
     void * INQUIRE_CB_VALUE,
     gpgme_assuan_status_cb_t STATUS_CB,
     void * STATUS_CB_VALUE,
     gpgme_error_t *OP_ERR)

     Send the Assuan COMMAND and return results via the callbacks. The result of the operation is returned in OP_ERR.

     Synchronous variant.

How to check for software updates
---------------------------------

The GnuPG Project operates a server to query the current versions of software packages related to GnuPG.  GPGME can be used to access this online database and check whether a new version of a software package is available.

 -- Data type: gpgme_query_swdb_result_t
       SINCE: 1.8.0

     This is a pointer to a structure used to store the result of a gpgme_op_query_swdb operation.  After success full call to that function, you can retrieve the pointer to the result with gpgme_op_query_swdb_result.  The structure contains the following member:

     name
          This is the name of the package.

     iversion
          The currently installed version or an empty string.  This value is either a copy of the argument given to gpgme_op_query_swdb or the version of the installed software as figured out by GPGME or GnuPG.

     created
          This gives the date the file with the list of version numbers has originally be created by the GnuPG project.

     retrieved
          This gives the date the file was downloaded.

     warning
          If this flag is set either an error has occurred or some of the information in this structure are not properly set.  For example if the version number of the installed software could not be figured out, the update flag may not reflect a required update status.

     update
          If this flag is set an update of the software is available.

     urgent
          If this flag is set an available update is important.

     noinfo
          If this flag is set, no valid information could be retrieved.

     unknown
          If this flag is set the given name is not known.

     tooold
          If this flag is set the available information is not fresh enough.

     error
          If this flag is set some other error has occurred.

     version
          The version string of the latest released version.

     reldate
          The release date of the latest released version.

 -- Function: gpgme_error_t gpgme_op_query_swdb
     (gpgme_ctx_t CTX,
     const char *NAME,
     const char *IVERSION,
     gpgme_data_t RESERVED)

       SINCE: 1.8.0

     Query the software version database for software package NAME and check against the installed version given by IVERSION.  If IVERSION is given as NULL a check is only done if GPGME can figure out the version by itself (for example when using "gpgme" or "gnupg").  If NULL is used for NAME the current gpgme version is checked.  RESERVED must be set to 0.

 -- Function: gpgme_query_swdb_result_t gpgme_op_query_swdb_result
     (gpgme_ctx_t CTX)

       SINCE: 1.8.0

     The function gpgme_op_query_swdb_result returns a gpgme_query_swdb_result_t pointer to a structure holding the result of a gpgme_op_query_swdb operation.  The pointer is only valid if the last operation on the context was a successful call to gpgme_op_query_swdb.  If that call failed, the result might be a NULL pointer.  The returned pointer is only valid until the next operation is started on the context CTX.

Here is an example on how to check whether GnuPG is current:

     #include <gpgme.h>

     int
     main (void)
     {
       gpg_error_t err;
       gpgme_ctx_t ctx;
       gpgme_query_swdb_result_t result;

       gpgme_check_version (NULL);
       err = gpgme_new (&ctx);
       if (err)
         fprintf (stderr, "error creating context: %s\n", gpg_strerror (err));
       else
         {
           gpgme_set_protocol (ctx, GPGME_PROTOCOL_GPGCONF);

           err = gpgme_op_query_swdb (ctx, "gnupg", NULL, 0);
           if (err)
             fprintf (stderr, "error querying swdb: %s\n", gpg_strerror (err));
           else
             {
               result = gpgme_op_query_swdb_result (ctx);
               if (!result)
                 fprintf (stderr, "error querying swdb\n");
               if (!result->warning && !result->update)
                 printf ("GnuPG version %s is current\n",
                         result->iversion);
               else if (!result->warning && result->update)
                 printf ("GnuPG version %s can be updated to %s\n",
                         result->iversion, result->version);
               else
                 fprintf (stderr, "error finding the update status\n");
             }
           gpgme_release (ctx);
         }
       return 0;
     }

Run Control
===========

GPGME supports running operations synchronously and asynchronously.  You can use asynchronous operation to set up a context up to initiating the desired operation, but delay performing it to a later point.

Furthermore, you can use an external event loop to control exactly when GPGME runs.  This ensures that GPGME only runs when necessary and also prevents it from blocking for a long time.

Waiting For Completion
----------------------

 -- Function: gpgme_ctx_t gpgme_wait (gpgme_ctx_t CTX, gpgme_error_t *STATUS, int HANG)
     The function gpgme_wait continues the pending operation within the context CTX.  In particular, it ensures the data exchange between GPGME and the crypto backend and watches over the run time status of the backend process.

     If HANG is true, the function does not return until the operation is completed or cancelled.  Otherwise the function will not block for a long time.

     The error status of the finished operation is returned in STATUS if gpgme_wait does not return NULL.

     The CTX argument can be NULL.  In that case, gpgme_wait waits for any context to complete its operation.

     gpgme_wait can be used only in conjunction with any context that has a pending operation initiated with one of the gpgme_op_*_start functions except gpgme_op_keylist_start and gpgme_op_trustlist_start (for which you should use the corresponding gpgme_op_*_next functions).  If CTX is NULL, all of such contexts are waited upon and possibly returned.  Synchronous operations running in parallel, as well as key and trust item list operations, do not affect gpgme_wait.

     In a multi-threaded environment, only one thread should ever call gpgme_wait at any time, irregardless if CTX is specified or not.  This means that all calls to this function should be fully synchronized by locking primitives.  It is safe to start asynchronous operations while a thread is running in gpgme_wait.

     The function returns the CTX of the context which has finished the operation.  If HANG is false, and the timeout expires, NULL is returned and *status will be set to 0.  If an error occurs, NULL is returned and the error is returned in *status.

Using External Event Loops
--------------------------

GPGME hides the complexity of the communication between the library and the crypto engine.  The price of this convenience is that the calling thread can block arbitrary long waiting for the data returned by the crypto engine.  In single-threaded programs, in particular if they are interactive, this is an unwanted side-effect. OTOH, if gpgme_wait is used without the HANG option being enabled, it might be called unnecessarily often, wasting CPU time that could be used otherwise.

The I/O callback interface described in this section lets the user take control over what happens when.  GPGME will provide the user with the file descriptors that should be monitored, and the callback functions that should be invoked when a file descriptor is ready for reading or writing.  It is then the user's responsibility to decide when to check the file descriptors and when to invoke the callback functions.  Usually this is done in an event loop, that also checks for events in other parts of the program.  If the callback functions are only called when the file descriptors are ready, GPGME will never block.  This gives the user more control over the program flow, and allows to perform other tasks when GPGME would block otherwise.

By using this advanced mechanism, GPGME can be integrated smoothly into GUI toolkits like GTK+ even for single-threaded programs.

I/O Callback Interface
......................

 -- Data type: gpgme_error_t (*gpgme_io_cb_t) (void *DATA, int FD)
     The gpgme_io_cb_t type is the type of functions which GPGME wants to register as I/O callback handlers using the gpgme_register_io_cb_t functions provided by the user.

     DATA and FD are provided by GPGME when the I/O callback handler is registered, and should be passed through to the handler when it is invoked by the user because it noticed activity on the file descriptor FD.

     The callback handler always returns 0, but you should consider the return value to be reserved for later use.

 -- Data type: gpgme_error_t (*gpgme_register_io_cb_t) (void *DATA, int FD, int DIR, gpgme_io_cb_t FNC, void *FNC_DATA, void **TAG)
     The gpgme_register_io_cb_t type is the type of functions which can be called by GPGME to register an I/O callback function FNC for the file descriptor FD with the user. FNC_DATA should be passed as the first argument to FNC when the handler is invoked (the second argument should be FD). If DIR is 0, FNC should be called by the user when FD is ready for writing.  If DIR is 1, FNC should be called when FD is ready for reading.

     DATA was provided by the user when registering the gpgme_register_io_cb_t function with GPGME and will always be passed as the first argument when registering a callback function. For example, the user can use this to determine the event loop to which the file descriptor should be added.

     GPGME will call this function when a crypto operation is initiated in a context for which the user has registered I/O callback handler functions with gpgme_set_io_cbs.  It can also call this function when it is in an I/O callback handler for a file descriptor associated to this context.

     The user should return a unique handle in TAG identifying this I/O callback registration, which will be passed to the gpgme_register_io_cb_t function without interpretation when the file descriptor should not be monitored anymore.

 -- Data type: void (*gpgme_remove_io_cb_t) (void *TAG)
     The gpgme_remove_io_cb_t type is the type of functions which can be called by GPGME to remove an I/O callback handler that was registered before.  TAG is the handle that was returned by the gpgme_register_io_cb_t for this I/O callback.

     GPGME can call this function when a crypto operation is in an I/O callback.  It will also call this function when the context is destroyed while an operation is pending.

 -- Data type: enum gpgme_event_io_t
     The gpgme_event_io_t type specifies the type of an event that is reported to the user by GPGME as a consequence of an I/O operation.  The following events are defined:

     GPGME_EVENT_START
          The operation is fully initialized now, and you can start to run the registered I/O callback handlers now.  Note that registered I/O callback handlers must not be run before this event is signalled. TYPE_DATA is NULL and reserved for later use.

     GPGME_EVENT_DONE
          The operation is finished, the last I/O callback for this operation was removed.  The accompanying TYPE_DATA points to a struct gpgme_io_event_done_data variable that contains the status of the operation that finished.  This event is signalled after the last I/O callback has been removed.

     GPGME_EVENT_NEXT_KEY
          In a gpgme_op_keylist_start operation, the next key was received from the crypto engine.  The accompanying TYPE_DATA is a gpgme_key_t variable that contains the key with one reference for the user.

     GPGME_EVENT_NEXT_TRUSTITEM
          In a gpgme_op_trustlist_start operation, the next trust item was received from the crypto engine.  The accompanying TYPE_DATA is a gpgme_trust_item_t variable that contains the trust item with one reference for the user.

 -- Data type: void (*gpgme_event_io_cb_t) (void *DATA, gpgme_event_io_t TYPE, void *TYPE_DATA)
     The gpgme_event_io_cb_t type is the type of functions which can be called by GPGME to signal an event for an operation running in a context which has I/O callback functions registered by the user.

     DATA was provided by the user when registering the gpgme_event_io_cb_t function with GPGME and will always be passed as the first argument when registering a callback function. For example, the user can use this to determine the context in which this event has occurred.

     TYPE will specify the type of event that has occurred. TYPE_DATA specifies the event further, as described in the above list of possible gpgme_event_io_t types.

     GPGME can call this function in an I/O callback handler.

Registering I/O Callbacks
.........................

 -- Data type: struct gpgme_io_cbs
     This structure is used to store the I/O callback interface functions described in the previous section.  It has the following members:

     gpgme_register_io_cb_t add
          This is the function called by GPGME to register an I/O callback handler.  It must be specified.

     void *add_priv
          This is passed as the first argument to the add function when it is called by GPGME.  For example, it can be used to determine the event loop to which the file descriptor should be added.

     gpgme_remove_io_cb_t remove
          This is the function called by GPGME to remove an I/O callback handler.  It must be specified.

     gpgme_event_io_cb_t event
          This is the function called by GPGME to signal an event for an operation.  It must be specified, because at least the start event must be processed.

     void *event_priv
          This is passed as the first argument to the event function when it is called by GPGME.  For example, it can be used to determine the context in which the event has occurred.

 -- Function: void gpgme_set_io_cbs (gpgme_ctx_t CTX, struct gpgme_io_cbs *IO_CBS)
     The function gpgme_set_io_cbs enables the I/O callback interface for the context CTX.  The I/O callback functions are specified by IO_CBS.

     If IO_CBS->add is NULL, the I/O callback interface is disabled for the context, and normal operation is restored.

 -- Function: void gpgme_get_io_cbs (gpgme_ctx_t CTX, struct gpgme_io_cbs *IO_CBS)
     The function gpgme_get_io_cbs returns the I/O callback functions set with gpgme_set_io_cbs in IO_CBS.

I/O Callback Example
....................

To actually use an external event loop, you have to implement the I/O callback functions that are used by GPGME to register and unregister file descriptors.  Furthermore, you have to actually monitor these file descriptors for activity and call the appropriate I/O callbacks.

The following example illustrates how to do that.  The example uses locking to show in which way the callbacks and the event loop can run concurrently.  For the event loop, we use a fixed array.  For a real-world implementation, you should use a dynamically sized structure because the number of file descriptors needed for a crypto operation in GPGME is not predictable.

     #include <assert.h>
     #include <errno.h>
     #include <stdlib.h>
     #include <pthread.h>
     #include <sys/types.h>
     #include <gpgme.h>

     /* The following structure holds the result of a crypto operation.  */
     struct op_result
     {
       int done;
       gpgme_error_t err;
     };

     /* The following structure holds the data associated with one I/O
     callback.  */
     struct one_fd
     {
       int fd;
       int dir;
       gpgme_io_cb_t fnc;
       void *fnc_data;
       void *loop;
     };

     struct event_loop
     {
       pthread_mutex_t lock;
     #define MAX_FDS 32
       /* Unused slots are marked with FD being -1.  */
       struct one_fd fds[MAX_FDS];
     };

The following functions implement the I/O callback interface.

     gpgme_error_t
     add_io_cb (void *data, int fd, int dir, gpgme_io_cb_t fnc, void *fnc_data,
     	   void **r_tag)
     {
       struct event_loop *loop = data;
       struct one_fd *fds = loop->fds;
       int i;

       pthread_mutex_lock (&loop->lock);
       for (i = 0; i < MAX_FDS; i++)
         {
           if (fds[i].fd == -1)
     	{
     	  fds[i].fd = fd;
     	  fds[i].dir = dir;
     	  fds[i].fnc = fnc;
     	  fds[i].fnc_data = fnc_data;
     	  fds[i].loop = loop;
     	  break;
     	}
         }
       pthread_mutex_unlock (&loop->lock);
       if (i == MAX_FDS)
         return gpg_error (GPG_ERR_GENERAL);
       *r_tag = &fds[i];
       return 0;
     }

     void
     remove_io_cb (void *tag)
     {
       struct one_fd *fd = tag;
       struct event_loop *loop = fd->loop;

       pthread_mutex_lock (&loop->lock);
       fd->fd = -1;
       pthread_mutex_unlock (&loop->lock);
     }

     void
     event_io_cb (void *data, gpgme_event_io_t type, void *type_data)
     {
       struct op_result *result = data;

       /* We don't support list operations here.  */
       if (type == GPGME_EVENT_DONE)
         {
           result->done = 1;
           result->err = *type_data;
         }
     }

The final missing piece is the event loop, which will be presented next.  We only support waiting for the success of a single operation.

     int
     do_select (struct event_loop *loop)
     {
       fd_set rfds;
       fd_set wfds;
       int i, n;
       int any = 0;
       struct timeval tv;
       struct one_fd *fdlist = loop->fds;

       pthread_mutex_lock (&loop->lock);
       FD_ZERO (&rfds);
       FD_ZERO (&wfds);
       for (i = 0; i < MAX_FDS; i++)
         if (fdlist[i].fd != -1)
           FD_SET (fdlist[i].fd, fdlist[i].dir ? &rfds : &wfds);
       pthread_mutex_unlock (&loop->lock);

       tv.tv_sec = 0;
       tv.tv_usec = 1000;

       do
         {
           n = select (FD_SETSIZE, &rfds, &wfds, NULL, &tv);
         }
       while (n < 0 && errno == EINTR);

       if (n < 0)
         return n;	/* Error or timeout.  */

       pthread_mutex_lock (&loop->lock);
       for (i = 0; i < MAX_FDS && n; i++)
         {
           if (fdlist[i].fd != -1)
     	{
     	  if (FD_ISSET (fdlist[i].fd, fdlist[i].dir ? &rfds : &wfds))
     	    {
     	      assert (n);
     	      n--;
     	      any = 1;
                   /* The I/O callback handler can register/remove callbacks,
                      so we have to unlock the file descriptor list.  */
                   pthread_mutex_unlock (&loop->lock);
     	      (*fdlist[i].fnc) (fdlist[i].fnc_data, fdlist[i].fd);
                   pthread_mutex_lock (&loop->lock);
     	    }
     	}
         }
       pthread_mutex_unlock (&loop->lock);
       return any;
     }

     void
     wait_for_op (struct event_loop *loop, struct op_result *result)
     {
       int ret;

       do
         {
           ret = do_select (loop);
         }
       while (ret >= 0 && !result->done);
     }

The main function shows how to put it all together.

     int
     main (int argc, char *argv[])
     {
       struct event_loop loop;
       struct op_result result;
       gpgme_ctx_t ctx;
       gpgme_error_t err;
       gpgme_data_t sig, text;
       int i;
       pthread_mutexattr_t attr;
       struct gpgme_io_cbs io_cbs =
       {
         add_io_cb,
         &loop,
         remove_io_cb,
         event_io_cb,
         &result
       };

       init_gpgme ();

       /* Initialize the loop structure.  */

       /* The mutex must be recursive, since remove_io_cb (which acquires a
          lock) can be called while holding a lock acquired in do_select.  */
       pthread_mutexattr_init (&attr);
       pthread_mutexattr_settype (&attr, PTHREAD_MUTEX_RECURSIVE);
       pthread_mutex_init (&loop.lock, &attr);
       pthread_mutexattr_destroy (&attr);

       for (i = 0; i < MAX_FDS; i++)
         loop.fds[i].fd = -1;

       /* Initialize the result structure.  */
       result.done = 0;

       err = gpgme_data_new_from_file (&sig, "signature", 1);
       if (!err)
         err = gpgme_data_new_from_file (&text, "text", 1);
       if (!err)
         err = gpgme_new (&ctx);
       if (!err)
         {
            gpgme_set_io_cbs (ctx, &io_cbs);
            err = gpgme_op_verify_start (ctx, sig, text, NULL);
         }
       if (err)
         {
           fprintf (stderr, "gpgme error: %s: %s\n",
                    gpgme_strsource (err), gpgme_strerror (err));
           exit (1);
         }

       wait_for_op (&loop, &result);
       if (!result.done)
         {
           fprintf (stderr, "select error\n");
           exit (1);
         }
       if (!result.err)
         {
           fprintf (stderr, "verification failed: %s: %s\n",
                    gpgme_strsource (result.err), gpgme_strerror (result.err));
           exit (1);
         }
       /* Evaluate verify result.  */
       ...
       return 0;
     }

I/O Callback Example GTK+
.........................

The I/O callback interface can be used to integrate GPGME with the GTK+ event loop.  The following code snippets shows how this can be done using the appropriate register and remove I/O callback functions.  In this example, the private data of the register I/O callback function is unused.  The event notifications is missing because it does not require any GTK+ specific setup.

     #include <gtk/gtk.h>

     struct my_gpgme_io_cb
     {
       gpgme_io_cb_t fnc;
       void *fnc_data;
       guint input_handler_id
     };

     void
     my_gpgme_io_cb (gpointer data, gint source, GdkInputCondition condition)
     {
       struct my_gpgme_io_cb *iocb = data;
       (*(iocb->fnc)) (iocb->data, source);
     }

     void
     my_gpgme_remove_io_cb (void *data)
     {
       struct my_gpgme_io_cb *iocb = data;
       gtk_input_remove (data->input_handler_id);
     }

     void
     my_gpgme_register_io_callback (void *data, int fd, int dir, gpgme_io_cb_t fnc,
                                    void *fnc_data, void **tag)
     {
       struct my_gpgme_io_cb *iocb = g_malloc (sizeof (struct my_gpgme_io_cb));
       iocb->fnc = fnc;
       iocb->data = fnc_data;
       iocb->input_handler_id = gtk_input_add_full (fd, dir
                                                        ? GDK_INPUT_READ
                                                        : GDK_INPUT_WRITE,
                                                    my_gpgme_io_callback,
                                                    0, iocb, NULL);
       *tag = iocb;
       return 0;
     }

I/O Callback Example GDK
........................

The I/O callback interface can also be used to integrate GPGME with the GDK event loop.  The following code snippets shows how this can be done using the appropriate register and remove I/O callback functions.  In this example, the private data of the register I/O callback function is unused.  The event notifications is missing because it does not require any GDK specific setup.

It is very similar to the GTK+ example in the previous section.

     #include <gdk/gdk.h>

     struct my_gpgme_io_cb
     {
       gpgme_io_cb_t fnc;
       void *fnc_data;
       gint tag;
     };

     void
     my_gpgme_io_cb (gpointer data, gint source, GdkInputCondition condition)
     {
       struct my_gpgme_io_cb *iocb = data;
       (*(iocb->fnc)) (iocb->data, source);
     }

     void
     my_gpgme_remove_io_cb (void *data)
     {
       struct my_gpgme_io_cb *iocb = data;
       gdk_input_remove (data->tag);
     }

     void
     my_gpgme_register_io_callback (void *data, int fd, int dir, gpgme_io_cb_t fnc,
                                    void *fnc_data, void **tag)
     {
       struct my_gpgme_io_cb *iocb = g_malloc (sizeof (struct my_gpgme_io_cb));
       iocb->fnc = fnc;
       iocb->data = fnc_data;
       iocb->tag = gtk_input_add_full (fd, dir ? GDK_INPUT_READ : GDK_INPUT_WRITE,
                                       my_gpgme_io_callback, iocb, NULL);
       *tag = iocb;
       return 0;
     }

I/O Callback Example Qt
.......................

The I/O callback interface can also be used to integrate GPGME with the Qt event loop.  The following code snippets show how this can be done using the appropriate register and remove I/O callback functions.  In this example, the private data of the register I/O callback function is unused.  The event notifications is missing because it does not require any Qt specific setup.

     #include <qsocketnotifier.h>
     #include <qapplication.h>

     struct IOCB {
       IOCB( GpgmeIOCb f, void * d, QSocketNotifier * n )
         : func( f ), data( d ), notifier( n ) {}
       GpgmeIOCb func;
       void * data;
       QSocketNotifier * notifier;
     }

     class MyApp : public QApplication {

       // ...

       static void registerGpgmeIOCallback( void * data, int fd, int dir,
                                            GpgmeIOCb func, void * func_data,
                                            void ** tag ) {
         QSocketNotifier * n =
           new QSocketNotifier( fd, dir ? QSocketNotifier::Read
                                        : QSocketNotifier::Write );
         connect( n, SIGNAL(activated(int)),
                  qApp, SLOT(slotGpgmeIOCallback(int)) );
         qApp->mIOCBs.push_back( IOCB( func, func_data, n ) );
         *tag = (void*)n;
       }

       static void removeGpgmeIOCallback( void * tag ) {
         if ( !tag ) return;
         QSocketNotifier * n = static_cast<QSocketNotifier*>( tag );
         for ( QValueList<IOCB>::iterator it = qApp->mIOCBs.begin() ;
               it != qApp->mIOCBs.end() ; ++it )
           if ( it->notifier == n ) {
             delete it->notifier;
             qApp->mIOCBs.erase( it );
             return;
           }
       }

     public slots:
       void slotGpgmeIOCallback( int fd ) {
         for ( QValueList<IOCB>::const_iterator it = mIOCBs.begin() ;
               it != mIOCBs.end() ; ++it )
           if ( it->notifier && it->notifier->socket() == fd )
             (*(it->func)) ( it->func_data, fd );
       }

       // ...

     private:
       QValueList<IOCB> mIOCBs;
       // ...
     };

Cancellation
------------

Sometimes you do not want to wait for an operation to finish. GPGME provides two different functions to achieve that.  The function gpgme_cancel takes effect immediately.  When it returns, the operation is effectively canceled.  However, it has some limitations and can not be used with synchronous operations.  In contrast, the function gpgme_cancel_async can be used with any context and from any thread, but it is not guaranteed to take effect immediately.  Instead, cancellation occurs at the next possible time (typically the next time I/O occurs in the target context).

 -- Function: gpgme_ctx_t gpgme_cancel (gpgme_ctx_t CTX)
       SINCE: 0.4.5

     The function gpgme_cancel attempts to cancel a pending operation in the context CTX.  This only works if you use the global event loop or your own event loop.

     If you use the global event loop, you must not call gpgme_wait during cancellation.  After successful cancellation, you can call gpgme_wait (optionally waiting on CTX), and the context CTX will appear as if it had finished with the error code GPG_ERR_CANCEL.

     If you use an external event loop, you must ensure that no I/O callbacks are invoked for this context (for example by halting the event loop).  On successful cancellation, all registered I/O callbacks for this context will be unregistered, and a GPGME_EVENT_DONE event with the error code GPG_ERR_CANCEL will be signalled.

     The function returns an error code if the cancellation failed (in this case the state of CTX is not modified).

 -- Function: gpgme_ctx_t gpgme_cancel_async (gpgme_ctx_t CTX)
       SINCE: 1.1.7

     The function gpgme_cancel_async attempts to cancel a pending operation in the context CTX.  This can be called by any thread at any time after starting an operation on the context, but will not take effect immediately.  The actual cancellation happens at the next time GPGME processes I/O in that context.

     The function returns an error code if the cancellation failed (in this case the state of CTX is not modified).

     [file uiserver.texi not included]

How to solve problems
*********************

Everyone knows that software often does not do what it should do and thus there is a need to track down problems.  This is in particular true for applications using a complex library like GPGME and of course also for the library itself.  Here we give a few hints on how to solve such problems.

First of all you should make sure that the keys you want to use are installed in the GnuPG engine and are usable.  Thus the first test is to run the desired operation using gpg or gpgsm on the command line.  If you can't figure out why things don't work, you may use GPGME's built in trace feature.  This feature is either enabled using the environment variable GPGME_DEBUG or, if this is not possible, by calling the function gpgme_set_global_flag.  The value is the trace level and an optional file name.  If no file name is given the trace output is printed to stderr.

For example

     GPGME_DEBUG=9:/home/user/mygpgme.log

(Note that under Windows you use a semicolon in place of the colon to separate the fields.)

A trace level of 9 is pretty verbose and thus you may want to start off with a lower level.  The exact definition of the trace levels and the output format may change with any release; you need to check the source code for details.  In any case the trace log should be helpful to understand what is going going on.  Warning: The trace log may reveal sensitive details like passphrases or other data you use in your application.  If you are asked to send a log file, make sure that you run your tests only with play data.

Deprecated Functions
********************

For backward compatibility GPGME has a number of functions, data types and constants which are deprecated and should not be used anymore.  We document here those which are really old to help understanding old code and to allow migration to their modern counterparts.

Warning: These interfaces will be removed in a future version of GPGME.

 -- Function: void gpgme_key_release (gpgme_key_t KEY)
     The function gpgme_key_release is equivalent to gpgme_key_unref.

 -- Function: void gpgme_trust_item_release (gpgme_trust_item_t ITEM)
     The function gpgme_trust_item_release is an alias for gpgme_trust_item_unref.

 -- Function: gpgme_error_t gpgme_op_import_ext (gpgme_ctx_t CTX, gpgme_data_t KEYDATA, int *NR)
       SINCE: 0.3.9

     The function gpgme_op_import_ext is equivalent to:

            gpgme_error_t err = gpgme_op_import (ctx, keydata);
            if (!err)
              {
                gpgme_import_result_t result = gpgme_op_import_result (ctx);
                *nr = result->considered;
              }

 -- Data type: gpgme_error_t (*gpgme_edit_cb_t)
     (void *HANDLE,
     gpgme_status_code_t STATUS,
     const char *ARGS,
     int FD)
     The gpgme_edit_cb_t type is the type of functions which GPGME calls if it a key edit operation is on-going.  The status code STATUS and the argument line ARGS are passed through by GPGME from the crypto engine.  The file descriptor FD is -1 for normal status messages.  If STATUS indicates a command rather than a status message, the response to the command should be written to FD.  The HANDLE is provided by the user at start of operation.

     The function should return GPG_ERR_FALSE if it did not handle the status code, 0 for success, or any other error value.

 -- Function: gpgme_error_t gpgme_op_edit (gpgme_ctx_t CTX, gpgme_key_t KEY, gpgme_edit_cb_t FNC, void *HANDLE, gpgme_data_t OUT)
       SINCE: 0.3.9

     Note: This function is deprecated, please use gpgme_op_interact instead.

     The function gpgme_op_edit processes the key KEY interactively, using the edit callback function FNC with the handle HANDLE.  The callback is invoked for every status and command request from the crypto engine.  The output of the crypto engine is written to the data object OUT.

     Note that the protocol between the callback function and the crypto engine is specific to the crypto engine and no further support in implementing this protocol correctly is provided by GPGME.

     The function returns the error code GPG_ERR_NO_ERROR if the edit operation completes successfully, GPG_ERR_INV_VALUE if CTX or KEY is not a valid pointer, and any error returned by the crypto engine or the edit callback handler.

 -- Function: gpgme_error_t gpgme_op_edit_start (gpgme_ctx_t CTX, gpgme_key_t KEY, gpgme_edit_cb_t FNC, void *HANDLE, gpgme_data_t OUT)

       SINCE: 0.3.9

     Note: This function is deprecated, please use gpgme_op_interact_start instead.

     The function gpgme_op_edit_start initiates a gpgme_op_edit operation.  It can be completed by calling gpgme_wait on the context.  See Waiting For Completion.

     The function returns the error code GPG_ERR_NO_ERROR if the operation was started successfully, and GPG_ERR_INV_VALUE if CTX or KEY is not a valid pointer.

 -- Function: gpgme_error_t gpgme_op_card_edit (gpgme_ctx_t CTX, gpgme_key_t KEY, gpgme_edit_cb_t FNC, void *HANDLE, gpgme_data_t OUT)
     Note: This function is deprecated, please use gpgme_op_interact with the flag GPGME_INTERACT_CARD instead.

     The function gpgme_op_card_edit is analogous to gpgme_op_edit, but should be used to process the smart card corresponding to the key KEY.

 -- Function: gpgme_error_t gpgme_op_card_edit_start (gpgme_ctx_t CTX, gpgme_key_t KEY, gpgme_edit_cb_t FNC, void *HANDLE, gpgme_data_t OUT)
     Note: This function is deprecated, please use gpgme_op_interact_start with the flag GPGME_INTERACT_CARD instead.

     The function gpgme_op_card_edit_start initiates a gpgme_op_card_edit operation.  It can be completed by calling gpgme_wait on the context.  See Waiting For Completion.

     The function returns the error code GPG_ERR_NO_ERROR if the operation was started successfully, and GPG_ERR_INV_VALUE if CTX or KEY is not a valid pointer.

 -- Function: gpgme_error_t gpgme_data_new_with_read_cb (gpgme_data_t *DH, int (*READFUNC) (void *HOOK, char *BUFFER, size_t COUNT, size_t *NREAD), void *HOOK_VALUE)
     The function gpgme_data_new_with_read_cb creates a new gpgme_data_t object and uses the callback function READFUNC to retrieve the data on demand.  As the callback function can supply the data in any way it wants, this is the most flexible data type GPGME provides.  However, it can not be used to write data.

     The callback function receives HOOK_VALUE as its first argument whenever it is invoked.  It should return up to COUNT bytes in BUFFER, and return the number of bytes actually read in NREAD.  It may return 0 in NREAD if no data is currently available.  To indicate EOF the function should return with an error code of -1 and set NREAD to 0.  The callback function may support to reset its internal read pointer if it is invoked with BUFFER and NREAD being NULL and COUNT being 0.

     The function returns the error code GPG_ERR_NO_ERROR if the data object was successfully created, GPG_ERR_INV_VALUE if DH or READFUNC is not a valid pointer, and GPG_ERR_ENOMEM if not enough memory is available.

 -- Function: gpgme_error_t gpgme_data_rewind (gpgme_data_t DH)
     The function gpgme_data_rewind is equivalent to:

            return (gpgme_data_seek (dh, 0, SEEK_SET) == -1)
              ? gpgme_error_from_errno (errno) : 0;

 -- Data type: gpgme_attr_t
     The gpgme_attr_t type is used to specify a key or trust item attribute.  The following attributes are defined:

     GPGME_ATTR_KEYID
          This is the key ID of a sub key.  It is representable as a string.

          For trust items, the trust item refers to the key with this ID.

     GPGME_ATTR_FPR
          This is the fingerprint of a sub key.  It is representable as a string.

     GPGME_ATTR_ALGO
          This is the crypto algorithm for which the sub key can be used.  It is representable as a string and as a number.  The numbers correspond to the enum gcry_pk_algos values in the gcrypt library.

     GPGME_ATTR_LEN
          This is the key length of a sub key.  It is representable as a number.

     GPGME_ATTR_CREATED
          This is the timestamp at creation time of a sub key.  It is representable as a number.

     GPGME_ATTR_EXPIRE
          This is the expiration time of a sub key.  It is representable as a number.

     GPGME_ATTR_OTRUST
          XXX FIXME  (also for trust items)

     GPGME_ATTR_USERID
          This is a user ID.  There can be more than one user IDs in a GPGME_KEY_T object.  The first one (with index 0) is the primary user ID.  The user ID is representable as a number.

          For trust items, this is the user ID associated with this trust item.

     GPGME_ATTR_NAME
          This is the name belonging to a user ID.  It is representable as a string.

     GPGME_ATTR_EMAIL
          This is the email address belonging to a user ID.  It is representable as a string.

     GPGME_ATTR_COMMENT
          This is the comment belonging to a user ID.  It is representable as a string.

     GPGME_ATTR_VALIDITY
          This is the validity belonging to a user ID.  It is representable as a string and as a number.  See below for a list of available validities.

          For trust items, this is the validity that is associated with this trust item.

     GPGME_ATTR_UID_REVOKED
          This specifies if a user ID is revoked.  It is representable as a number, and is 1 if the user ID is revoked, and 0 otherwise.

     GPGME_ATTR_UID_INVALID
          This specifies if a user ID is invalid.  It is representable as a number, and is 1 if the user ID is invalid, and 0 otherwise.

     GPGME_ATTR_LEVEL
          This is the trust level of a trust item.

     GPGME_ATTR_TYPE
          This returns information about the type of key.  For the string function this will eother be "PGP" or "X.509".  The integer function returns 0 for PGP and 1 for X.509.  It is also used for the type of a trust item.

     GPGME_ATTR_IS_SECRET
          This specifies if the key is a secret key.  It is representable as a number, and is 1 if the key is revoked, and 0 otherwise.

     GPGME_ATTR_KEY_REVOKED
          This specifies if a sub key is revoked.  It is representable as a number, and is 1 if the key is revoked, and 0 otherwise.

     GPGME_ATTR_KEY_INVALID
          This specifies if a sub key is invalid.  It is representable as a number, and is 1 if the key is invalid, and 0 otherwise.

     GPGME_ATTR_KEY_EXPIRED
          This specifies if a sub key is expired.  It is representable as a number, and is 1 if the key is expired, and 0 otherwise.

     GPGME_ATTR_KEY_DISABLED
          This specifies if a sub key is disabled.  It is representable as a number, and is 1 if the key is disabled, and 0 otherwise.

     GPGME_ATTR_KEY_CAPS
          This is a description of the capabilities of a sub key.  It is representable as a string.  The string contains the letter ``e'' if the key can be used for encryption, ``s'' if the key can be used for signatures, and ``c'' if the key can be used for certifications.

     GPGME_ATTR_CAN_ENCRYPT
          This specifies if a sub key can be used for encryption.  It is representable as a number, and is 1 if the sub key can be used for encryption, and 0 otherwise.

     GPGME_ATTR_CAN_SIGN
          This specifies if a sub key can be used to create data signatures.  It is representable as a number, and is 1 if the sub key can be used for signatures, and 0 otherwise.

     GPGME_ATTR_CAN_CERTIFY
          This specifies if a sub key can be used to create key certificates. It is representable as a number, and is 1 if the sub key can be used for certifications, and 0 otherwise.

     GPGME_ATTR_SERIAL
          The X.509 issuer serial attribute of the key.  It is representable as a string.

     GPGME_ATTR_ISSUE
          The X.509 issuer name attribute of the key.  It is representable as a string.

     GPGME_ATTR_CHAINID
          The X.509 chain ID can be used to build the certification chain.  It is representable as a string.

 -- Function: const char * gpgme_key_get_string_attr (gpgme_key_t KEY, gpgme_attr_t WHAT, const void *RESERVED, int IDX)
     The function gpgme_key_get_string_attr returns the value of the string-representable attribute WHAT of key KEY.  If the attribute is an attribute of a sub key or an user ID, IDX specifies the sub key or user ID of which the attribute value is returned.  The argument RESERVED is reserved for later use and should be NULL.

     The string returned is only valid as long as the key is valid.

     The function returns 0 if an attribute can't be returned as a string, KEY is not a valid pointer, IDX out of range, or RESERVED not NULL.

 -- Function: unsigned long gpgme_key_get_ulong_attr (gpgme_key_t KEY, gpgme_attr_t WHAT, const void *RESERVED, int IDX)
     The function gpgme_key_get_ulong_attr returns the value of the number-representable attribute WHAT of key KEY.  If the attribute is an attribute of a sub key or an user ID, IDX specifies the sub key or user ID of which the attribute value is returned.  The argument RESERVED is reserved for later use and should be NULL.

     The function returns 0 if the attribute can't be returned as a number, KEY is not a valid pointer, IDX out of range, or RESERVED not NULL.

The signatures on a key are only available if the key was retrieved via a listing operation with the GPGME_KEYLIST_MODE_SIGS mode enabled, because it is expensive to retrieve all signatures of a key.

So, before using the below interfaces to retrieve the signatures on a key, you have to make sure that the key was listed with signatures enabled.  One convenient, but blocking, way to do this is to use the function gpgme_get_key.

 -- Data type: gpgme_attr_t
     The gpgme_attr_t type is used to specify a key signature attribute.  The following attributes are defined:

     GPGME_ATTR_KEYID
          This is the key ID of the key which was used for the signature.  It is representable as a string.

     GPGME_ATTR_ALGO
          This is the crypto algorithm used to create the signature.  It is representable as a string and as a number.  The numbers correspond to the enum gcry_pk_algos values in the gcrypt library.

     GPGME_ATTR_CREATED
          This is the timestamp at creation time of the signature.  It is representable as a number.

     GPGME_ATTR_EXPIRE
          This is the expiration time of the signature.  It is representable as a number.

     GPGME_ATTR_USERID
          This is the user ID associated with the signing key.  The user ID is representable as a number.

     GPGME_ATTR_NAME
          This is the name belonging to a user ID.  It is representable as a string.

     GPGME_ATTR_EMAIL
          This is the email address belonging to a user ID.  It is representable as a string.

     GPGME_ATTR_COMMENT
          This is the comment belonging to a user ID.  It is representable as a string.

     GPGME_ATTR_KEY_REVOKED
          This specifies if a key signature is a revocation signature.  It is representable as a number, and is 1 if the key is revoked, and 0 otherwise.

     GPGME_ATTR_SIG_CLASS
          This specifies the signature class of a key signature.  It is representable as a number.  The meaning is specific to the crypto engine.

     GPGME_ATTR_SIG_CLASS
          This specifies the signature class of a key signature.  It is representable as a number.  The meaning is specific to the crypto engine.

     GPGME_ATTR_SIG_STATUS
          This is the same value as returned by gpgme_get_sig_status.

 -- Function: const char * gpgme_key_sig_get_string_attr (gpgme_key_t KEY, int UID_IDX, gpgme_attr_t WHAT, const void *RESERVED, int IDX)
     The function gpgme_key_sig_get_string_attr returns the value of the string-representable attribute WHAT of the signature IDX on the user ID UID_IDX in the key KEY.  The argument RESERVED is reserved for later use and should be NULL.

     The string returned is only valid as long as the key is valid.

     The function returns 0 if an attribute can't be returned as a string, KEY is not a valid pointer, UID_IDX or IDX out of range, or RESERVED not NULL.

 -- Function: unsigned long gpgme_key_sig_get_ulong_attr (gpgme_key_t KEY, int UID_IDX, gpgme_attr_t WHAT, const void *RESERVED, int IDX)
     The function gpgme_key_sig_get_ulong_attr returns the value of the number-representable attribute WHAT of the signature IDX on the user ID UID_IDX in the key KEY.  The argument RESERVED is reserved for later use and should be NULL.

     The function returns 0 if an attribute can't be returned as a string, KEY is not a valid pointer, UID_IDX or IDX out of range, or RESERVED not NULL.

Trust items have attributes which can be queried using the interfaces below.  The attribute identifiers are shared with those for key attributes.  See Information About Keys.

 -- Function: const char * gpgme_trust_item_get_string_attr (gpgme_trust_item_t ITEM, gpgme_attr_t WHAT, const void *RESERVED, int IDX)
     The function gpgme_trust_item_get_string_attr returns the value of the string-representable attribute WHAT of trust item ITEM.  The arguments IDX and RESERVED are reserved for later use and should be 0 and NULL respectively.

     The string returned is only valid as long as the key is valid.

     The function returns 0 if an attribute can't be returned as a string, KEY is not a valid pointer, IDX out of range, or RESERVED not NULL.

 -- Function: int gpgme_trust_item_get_int_attr (gpgme_trust_item_t ITEM, gpgme_attr_t WHAT, const void *RESERVED, int IDX)
     The function gpgme_trust_item_get_int_attr returns the value of the number-representable attribute WHAT of trust item ITEM.  If the attribute occurs more than once in the trust item, the index is specified by IDX.  However, currently no such attribute exists, so IDX should be 0.  The argument RESERVED is reserved for later use and should be NULL.

     The function returns 0 if the attribute can't be returned as a number, KEY is not a valid pointer, IDX out of range, or RESERVED not NULL.

 -- Data type: enum gpgme_sig_stat_t
     The gpgme_sig_stat_t type holds the result of a signature check, or the combined result of all signatures.  The following results are possible:

     GPGME_SIG_STAT_NONE
          This status should not occur in normal operation.

     GPGME_SIG_STAT_GOOD
          This status indicates that the signature is valid.  For the combined result this status means that all signatures are valid.

     GPGME_SIG_STAT_GOOD_EXP
          This status indicates that the signature is valid but expired.  For the combined result this status means that all signatures are valid and expired.

     GPGME_SIG_STAT_GOOD_EXPKEY
          This status indicates that the signature is valid but the key used to verify the signature has expired.  For the combined result this status means that all signatures are valid and all keys are expired.

     GPGME_SIG_STAT_BAD
          This status indicates that the signature is invalid.  For the combined result this status means that all signatures are invalid.

     GPGME_SIG_STAT_NOKEY
          This status indicates that the signature could not be verified due to a missing key.  For the combined result this status means that all signatures could not be checked due to missing keys.

     GPGME_SIG_STAT_NOSIG
          This status indicates that the signature data provided was not a real signature.

     GPGME_SIG_STAT_ERROR
          This status indicates that there was some other error which prevented the signature verification.

     GPGME_SIG_STAT_DIFF
          For the combined result this status means that at least two signatures have a different status.  You can get each key's status with gpgme_get_sig_status.

 -- Function: const char * gpgme_get_sig_status (gpgme_ctx_t CTX, int IDX, gpgme_sig_stat_t *R_STAT, time_t *R_CREATED)
     The function gpgme_get_sig_status is equivalent to:

            gpgme_verify_result_t result;
            gpgme_signature_t sig;

            result = gpgme_op_verify_result (ctx);
            sig = result->signatures;

            while (sig && idx)
              {
                sig = sig->next;
                idx--;
              }
            if (!sig || idx)
              return NULL;

            if (r_stat)
              {
                switch (gpg_err_code (sig->status))
          	{
          	case GPG_ERR_NO_ERROR:
          	  *r_stat = GPGME_SIG_STAT_GOOD;
          	  break;

          	case GPG_ERR_BAD_SIGNATURE:
          	  *r_stat = GPGME_SIG_STAT_BAD;
          	  break;

          	case GPG_ERR_NO_PUBKEY:
          	  *r_stat = GPGME_SIG_STAT_NOKEY;
          	  break;

          	case GPG_ERR_NO_DATA:
          	  *r_stat = GPGME_SIG_STAT_NOSIG;
          	  break;

          	case GPG_ERR_SIG_EXPIRED:
          	  *r_stat = GPGME_SIG_STAT_GOOD_EXP;
          	  break;

          	case GPG_ERR_KEY_EXPIRED:
          	  *r_stat = GPGME_SIG_STAT_GOOD_EXPKEY;
          	  break;

          	default:
          	  *r_stat = GPGME_SIG_STAT_ERROR;
          	  break;
          	}
              }
            if (r_created)
              *r_created = sig->timestamp;
            return sig->fpr;

 -- Function: const char * gpgme_get_sig_string_attr (gpgme_ctx_t CTX, int IDX, gpgme_attr_t WHAT, int WHATIDX)
     The function gpgme_get_sig_string_attr is equivalent to:

            gpgme_verify_result_t result;
            gpgme_signature_t sig;

            result = gpgme_op_verify_result (ctx);
            sig = result->signatures;

            while (sig && idx)
              {
                sig = sig->next;
                idx--;
              }
            if (!sig || idx)
              return NULL;

            switch (what)
              {
              case GPGME_ATTR_FPR:
                return sig->fpr;

              case GPGME_ATTR_ERRTOK:
                if (whatidx == 1)
                  return sig->wrong_key_usage ? "Wrong_Key_Usage" : "";
                else
          	return "";
              default:
                break;
              }

            return NULL;

 -- Function: const char * gpgme_get_sig_ulong_attr (gpgme_ctx_t CTX, int IDX, gpgme_attr_t WAHT, int WHATIDX)
     The function gpgme_get_sig_ulong_attr is equivalent to:

            gpgme_verify_result_t result;
            gpgme_signature_t sig;

            result = gpgme_op_verify_result (ctx);
            sig = result->signatures;

            while (sig && idx)
              {
                sig = sig->next;
                idx--;
              }
            if (!sig || idx)
              return 0;

            switch (what)
              {
              case GPGME_ATTR_CREATED:
                return sig->timestamp;

              case GPGME_ATTR_EXPIRE:
                return sig->exp_timestamp;

              case GPGME_ATTR_VALIDITY:
                return (unsigned long) sig->validity;

              case GPGME_ATTR_SIG_STATUS:
                switch (sig->status)
          	{
          	case GPG_ERR_NO_ERROR:
          	  return GPGME_SIG_STAT_GOOD;

          	case GPG_ERR_BAD_SIGNATURE:
          	  return GPGME_SIG_STAT_BAD;

          	case GPG_ERR_NO_PUBKEY:
          	  return GPGME_SIG_STAT_NOKEY;

          	case GPG_ERR_NO_DATA:
          	  return GPGME_SIG_STAT_NOSIG;

          	case GPG_ERR_SIG_EXPIRED:
          	  return GPGME_SIG_STAT_GOOD_EXP;

          	case GPG_ERR_KEY_EXPIRED:
          	  return GPGME_SIG_STAT_GOOD_EXPKEY;

          	default:
          	  return GPGME_SIG_STAT_ERROR;
          	}

              case GPGME_ATTR_SIG_SUMMARY:
                return sig->summary;

              default:
                break;
              }
            return 0;

 -- Function: const char * gpgme_get_sig_key (gpgme_ctx_t CTX, int IDX, gpgme_key_t *R_KEY)
     The function gpgme_get_sig_key is equivalent to:

            gpgme_verify_result_t result;
            gpgme_signature_t sig;

            result = gpgme_op_verify_result (ctx);
            sig = result->signatures;

            while (sig && idx)
              {
                sig = sig->next;
                idx--;
              }
            if (!sig || idx)
              return gpg_error (GPG_ERR_EOF);

            return gpgme_get_key (ctx, sig->fpr, r_key, 0);

     [file lesser.texi not included]

     [file gpl.texi not included]