=pod

=head1 NAME

ENGINE_get_DH, ENGINE_get_DSA,

ENGINE_by_id, ENGINE_get_cipher_engine, ENGINE_get_default_DH,

ENGINE_get_default_DSA,

ENGINE_get_default_RAND,

ENGINE_get_default_RSA, ENGINE_get_digest_engine, ENGINE_get_first,

ENGINE_get_last, ENGINE_get_next, ENGINE_get_prev, ENGINE_new,

ENGINE_get_ciphers, ENGINE_get_ctrl_function, ENGINE_get_digests,

ENGINE_get_destroy_function, ENGINE_get_finish_function,

ENGINE_get_init_function, ENGINE_get_load_privkey_function,

ENGINE_get_load_pubkey_function, ENGINE_load_private_key,

ENGINE_load_public_key, ENGINE_get_RAND, ENGINE_get_RSA, ENGINE_get_id,

ENGINE_get_name, ENGINE_get_cmd_defns, ENGINE_get_cipher,

ENGINE_get_digest, ENGINE_add, ENGINE_cmd_is_executable,

ENGINE_ctrl, ENGINE_ctrl_cmd, ENGINE_ctrl_cmd_string,

ENGINE_finish, ENGINE_free, ENGINE_get_flags, ENGINE_init,

ENGINE_register_DH, ENGINE_register_DSA,

ENGINE_register_RAND, ENGINE_register_RSA,

ENGINE_register_all_complete, ENGINE_register_ciphers,

ENGINE_register_complete, ENGINE_register_digests, ENGINE_remove,

ENGINE_set_DH, ENGINE_set_DSA,

ENGINE_set_RAND, ENGINE_set_RSA, ENGINE_set_ciphers,

ENGINE_set_cmd_defns, ENGINE_set_ctrl_function, ENGINE_set_default,

ENGINE_set_default_DH, ENGINE_set_default_DSA,

ENGINE_set_default_RAND, ENGINE_set_default_RSA,

ENGINE_set_default_ciphers, ENGINE_set_default_digests,

ENGINE_set_default_string, ENGINE_set_destroy_function,

ENGINE_set_digests, ENGINE_set_finish_function, ENGINE_set_flags,

ENGINE_set_id, ENGINE_set_init_function, ENGINE_set_load_privkey_function,

ENGINE_set_load_pubkey_function, ENGINE_set_name, ENGINE_up_ref,

ENGINE_get_table_flags, ENGINE_cleanup,

ENGINE_load_builtin_engines, ENGINE_register_all_DH,

ENGINE_register_all_DSA,

ENGINE_register_all_RAND,

ENGINE_register_all_RSA, ENGINE_register_all_ciphers,

ENGINE_register_all_digests, ENGINE_set_table_flags, ENGINE_unregister_DH,

ENGINE_unregister_DSA,

ENGINE_unregister_RAND, ENGINE_unregister_RSA, ENGINE_unregister_ciphers,

ENGINE_unregister_digests

- ENGINE cryptographic module support

=head1 SYNOPSIS

 #include <openssl/engine.h>

 ENGINE *ENGINE_get_first(void);

 ENGINE *ENGINE_get_last(void);

 ENGINE *ENGINE_get_next(ENGINE *e);

 ENGINE *ENGINE_get_prev(ENGINE *e);

 int ENGINE_add(ENGINE *e);

 int ENGINE_remove(ENGINE *e);

 ENGINE *ENGINE_by_id(const char *id);

 int ENGINE_init(ENGINE *e);

 int ENGINE_finish(ENGINE *e);

 void ENGINE_load_builtin_engines(void);

 ENGINE *ENGINE_get_default_RSA(void);

 ENGINE *ENGINE_get_default_DSA(void);

 ENGINE *ENGINE_get_default_DH(void);

 ENGINE *ENGINE_get_default_RAND(void);

 ENGINE *ENGINE_get_cipher_engine(int nid);

 ENGINE *ENGINE_get_digest_engine(int nid);

 int ENGINE_set_default_RSA(ENGINE *e);

 int ENGINE_set_default_DSA(ENGINE *e);

 int ENGINE_set_default_DH(ENGINE *e);

 int ENGINE_set_default_RAND(ENGINE *e);

 int ENGINE_set_default_ciphers(ENGINE *e);

 int ENGINE_set_default_digests(ENGINE *e);

 int ENGINE_set_default_string(ENGINE *e, const char *list);

 int ENGINE_set_default(ENGINE *e, unsigned int flags);

 unsigned int ENGINE_get_table_flags(void);

 void ENGINE_set_table_flags(unsigned int flags);

 int ENGINE_register_RSA(ENGINE *e);

 void ENGINE_unregister_RSA(ENGINE *e);

 void ENGINE_register_all_RSA(void);

 int ENGINE_register_DSA(ENGINE *e);

 void ENGINE_unregister_DSA(ENGINE *e);

 void ENGINE_register_all_DSA(void);

 int ENGINE_register_DH(ENGINE *e);

 void ENGINE_unregister_DH(ENGINE *e);

 void ENGINE_register_all_DH(void);

 int ENGINE_register_RAND(ENGINE *e);

 void ENGINE_unregister_RAND(ENGINE *e);

 void ENGINE_register_all_RAND(void);

 int ENGINE_register_ciphers(ENGINE *e);

 void ENGINE_unregister_ciphers(ENGINE *e);

 void ENGINE_register_all_ciphers(void);

 int ENGINE_register_digests(ENGINE *e);

 void ENGINE_unregister_digests(ENGINE *e);

 void ENGINE_register_all_digests(void);

 int ENGINE_register_complete(ENGINE *e);

 int ENGINE_register_all_complete(void);

 int ENGINE_ctrl(ENGINE *e, int cmd, long i, void *p, void (*f)(void));

 int ENGINE_cmd_is_executable(ENGINE *e, int cmd);

 int ENGINE_ctrl_cmd(ENGINE *e, const char *cmd_name,

                     long i, void *p, void (*f)(void), int cmd_optional);

 int ENGINE_ctrl_cmd_string(ENGINE *e, const char *cmd_name, const char *arg,

                            int cmd_optional);

 ENGINE *ENGINE_new(void);

 int ENGINE_free(ENGINE *e);

 int ENGINE_up_ref(ENGINE *e);

 int ENGINE_set_id(ENGINE *e, const char *id);

 int ENGINE_set_name(ENGINE *e, const char *name);

 int ENGINE_set_RSA(ENGINE *e, const RSA_METHOD *rsa_meth);

 int ENGINE_set_DSA(ENGINE *e, const DSA_METHOD *dsa_meth);

 int ENGINE_set_DH(ENGINE *e, const DH_METHOD *dh_meth);

 int ENGINE_set_RAND(ENGINE *e, const RAND_METHOD *rand_meth);

 int ENGINE_set_destroy_function(ENGINE *e, ENGINE_GEN_INT_FUNC_PTR destroy_f);

 int ENGINE_set_init_function(ENGINE *e, ENGINE_GEN_INT_FUNC_PTR init_f);

 int ENGINE_set_finish_function(ENGINE *e, ENGINE_GEN_INT_FUNC_PTR finish_f);

 int ENGINE_set_ctrl_function(ENGINE *e, ENGINE_CTRL_FUNC_PTR ctrl_f);

 int ENGINE_set_load_privkey_function(ENGINE *e, ENGINE_LOAD_KEY_PTR loadpriv_f);

 int ENGINE_set_load_pubkey_function(ENGINE *e, ENGINE_LOAD_KEY_PTR loadpub_f);

 int ENGINE_set_ciphers(ENGINE *e, ENGINE_CIPHERS_PTR f);

 int ENGINE_set_digests(ENGINE *e, ENGINE_DIGESTS_PTR f);

 int ENGINE_set_flags(ENGINE *e, int flags);

 int ENGINE_set_cmd_defns(ENGINE *e, const ENGINE_CMD_DEFN *defns);

 const char *ENGINE_get_id(const ENGINE *e);

 const char *ENGINE_get_name(const ENGINE *e);

 const RSA_METHOD *ENGINE_get_RSA(const ENGINE *e);

 const DSA_METHOD *ENGINE_get_DSA(const ENGINE *e);

 const DH_METHOD *ENGINE_get_DH(const ENGINE *e);

 const RAND_METHOD *ENGINE_get_RAND(const ENGINE *e);

 ENGINE_GEN_INT_FUNC_PTR ENGINE_get_destroy_function(const ENGINE *e);

 ENGINE_GEN_INT_FUNC_PTR ENGINE_get_init_function(const ENGINE *e);

 ENGINE_GEN_INT_FUNC_PTR ENGINE_get_finish_function(const ENGINE *e);

 ENGINE_CTRL_FUNC_PTR ENGINE_get_ctrl_function(const ENGINE *e);

 ENGINE_LOAD_KEY_PTR ENGINE_get_load_privkey_function(const ENGINE *e);

 ENGINE_LOAD_KEY_PTR ENGINE_get_load_pubkey_function(const ENGINE *e);

 ENGINE_CIPHERS_PTR ENGINE_get_ciphers(const ENGINE *e);

 ENGINE_DIGESTS_PTR ENGINE_get_digests(const ENGINE *e);

 const EVP_CIPHER *ENGINE_get_cipher(ENGINE *e, int nid);

 const EVP_MD *ENGINE_get_digest(ENGINE *e, int nid);

 int ENGINE_get_flags(const ENGINE *e);

 const ENGINE_CMD_DEFN *ENGINE_get_cmd_defns(const ENGINE *e);

 EVP_PKEY *ENGINE_load_private_key(ENGINE *e, const char *key_id,

                                   UI_METHOD *ui_method, void *callback_data);

 EVP_PKEY *ENGINE_load_public_key(ENGINE *e, const char *key_id,

                                  UI_METHOD *ui_method, void *callback_data);

Deprecated:

 #if OPENSSL_API_COMPAT < 0x10100000L

 void ENGINE_cleanup(void)

 #endif

=head1 DESCRIPTION

These functions create, manipulate, and use cryptographic modules in the

form of B<ENGINE> objects. These objects act as containers for

implementations of cryptographic algorithms, and support a

reference-counted mechanism to allow them to be dynamically loaded in and

out of the running application.

The cryptographic functionality that can be provided by an B<ENGINE>

implementation includes the following abstractions;

 RSA_METHOD - for providing alternative RSA implementations

 DSA_METHOD, DH_METHOD, RAND_METHOD, ECDH_METHOD, ECDSA_METHOD,

       - similarly for other OpenSSL APIs

 EVP_CIPHER - potentially multiple cipher algorithms (indexed by 'nid')

 EVP_DIGEST - potentially multiple hash algorithms (indexed by 'nid')

 key-loading - loading public and/or private EVP_PKEY keys

=head2 Reference counting and handles

Due to the modular nature of the ENGINE API, pointers to ENGINEs need to be

treated as handles - ie. not only as pointers, but also as references to

the underlying ENGINE object. Ie. one should obtain a new reference when

making copies of an ENGINE pointer if the copies will be used (and

released) independently.

ENGINE objects have two levels of reference-counting to match the way in

which the objects are used. At the most basic level, each ENGINE pointer is

inherently a B<structural> reference - a structural reference is required

to use the pointer value at all, as this kind of reference is a guarantee

that the structure can not be deallocated until the reference is released.

However, a structural reference provides no guarantee that the ENGINE is

initialised and able to use any of its cryptographic

implementations. Indeed it's quite possible that most ENGINEs will not

initialise at all in typical environments, as ENGINEs are typically used to

support specialised hardware. To use an ENGINE's functionality, you need a

B<functional> reference. This kind of reference can be considered a

specialised form of structural reference, because each functional reference

implicitly contains a structural reference as well - however to avoid

difficult-to-find programming bugs, it is recommended to treat the two

kinds of reference independently. If you have a functional reference to an

ENGINE, you have a guarantee that the ENGINE has been initialised and

is ready to perform cryptographic operations, and will remain initialised

until after you have released your reference.

I<Structural references>

This basic type of reference is used for instantiating new ENGINEs,

iterating across OpenSSL's internal linked-list of loaded

ENGINEs, reading information about an ENGINE, etc. Essentially a structural

reference is sufficient if you only need to query or manipulate the data of

an ENGINE implementation rather than use its functionality.

The ENGINE_new() function returns a structural reference to a new (empty)

ENGINE object. There are other ENGINE API functions that return structural

references such as; ENGINE_by_id(), ENGINE_get_first(), ENGINE_get_last(),

ENGINE_get_next(), ENGINE_get_prev(). All structural references should be

released by a corresponding to call to the ENGINE_free() function - the

ENGINE object itself will only actually be cleaned up and deallocated when

the last structural reference is released.

It should also be noted that many ENGINE API function calls that accept a

structural reference will internally obtain another reference - typically

this happens whenever the supplied ENGINE will be needed by OpenSSL after

the function has returned. Eg. the function to add a new ENGINE to

OpenSSL's internal list is ENGINE_add() - if this function returns success,

then OpenSSL will have stored a new structural reference internally so the

caller is still responsible for freeing their own reference with

ENGINE_free() when they are finished with it. In a similar way, some

functions will automatically release the structural reference passed to it

if part of the function's job is to do so. Eg. the ENGINE_get_next() and

ENGINE_get_prev() functions are used for iterating across the internal

ENGINE list - they will return a new structural reference to the next (or

previous) ENGINE in the list or NULL if at the end (or beginning) of the

list, but in either case the structural reference passed to the function is

released on behalf of the caller.

To clarify a particular function's handling of references, one should

always consult that function's documentation "man" page, or failing that

the openssl/engine.h header file includes some hints.

I<Functional references>

As mentioned, functional references exist when the cryptographic

functionality of an ENGINE is required to be available. A functional

reference can be obtained in one of two ways; from an existing structural

reference to the required ENGINE, or by asking OpenSSL for the default

operational ENGINE for a given cryptographic purpose.

To obtain a functional reference from an existing structural reference,

call the ENGINE_init() function. This returns zero if the ENGINE was not

already operational and couldn't be successfully initialised (eg. lack of

system drivers, no special hardware attached, etc), otherwise it will

return non-zero to indicate that the ENGINE is now operational and will

have allocated a new B<functional> reference to the ENGINE. All functional

references are released by calling ENGINE_finish() (which removes the

implicit structural reference as well).

The second way to get a functional reference is by asking OpenSSL for a

default implementation for a given task, eg. by ENGINE_get_default_RSA(),

ENGINE_get_default_cipher_engine(), etc. These are discussed in the next

section, though they are not usually required by application programmers as

they are used automatically when creating and using the relevant

algorithm-specific types in OpenSSL, such as RSA, DSA, EVP_CIPHER_CTX, etc.

=head2 Default implementations

For each supported abstraction, the ENGINE code maintains an internal table

of state to control which implementations are available for a given

abstraction and which should be used by default. These implementations are

registered in the tables and indexed by an 'nid' value, because

abstractions like EVP_CIPHER and EVP_DIGEST support many distinct

algorithms and modes, and ENGINEs can support arbitrarily many of them.

In the case of other abstractions like RSA, DSA, etc, there is only one

"algorithm" so all implementations implicitly register using the same 'nid'

index.

When a default ENGINE is requested for a given abstraction/algorithm/mode, (eg.

when calling RSA_new_method(NULL)), a "get_default" call will be made to the

ENGINE subsystem to process the corresponding state table and return a

functional reference to an initialised ENGINE whose implementation should be

used. If no ENGINE should (or can) be used, it will return NULL and the caller

will operate with a NULL ENGINE handle - this usually equates to using the

conventional software implementation. In the latter case, OpenSSL will from

then on behave the way it used to before the ENGINE API existed.

Each state table has a flag to note whether it has processed this

"get_default" query since the table was last modified, because to process

this question it must iterate across all the registered ENGINEs in the

table trying to initialise each of them in turn, in case one of them is

operational. If it returns a functional reference to an ENGINE, it will

also cache another reference to speed up processing future queries (without

needing to iterate across the table). Likewise, it will cache a NULL

response if no ENGINE was available so that future queries won't repeat the

same iteration unless the state table changes. This behaviour can also be

changed; if the ENGINE_TABLE_FLAG_NOINIT flag is set (using

ENGINE_set_table_flags()), no attempted initialisations will take place,

instead the only way for the state table to return a non-NULL ENGINE to the

"get_default" query will be if one is expressly set in the table. Eg.

ENGINE_set_default_RSA() does the same job as ENGINE_register_RSA() except

that it also sets the state table's cached response for the "get_default"

query. In the case of abstractions like EVP_CIPHER, where implementations are

indexed by 'nid', these flags and cached-responses are distinct for each 'nid'

value.

=head2 Application requirements

This section will explain the basic things an application programmer should

support to make the most useful elements of the ENGINE functionality

available to the user. The first thing to consider is whether the

programmer wishes to make alternative ENGINE modules available to the

application and user. OpenSSL maintains an internal linked list of

"visible" ENGINEs from which it has to operate - at start-up, this list is

empty and in fact if an application does not call any ENGINE API calls and

it uses static linking against openssl, then the resulting application

binary will not contain any alternative ENGINE code at all. So the first

consideration is whether any/all available ENGINE implementations should be

made visible to OpenSSL - this is controlled by calling the various "load"

functions.

The fact that ENGINEs are made visible to OpenSSL (and thus are linked into

the program and loaded into memory at run-time) does not mean they are

"registered" or called into use by OpenSSL automatically - that behaviour

is something for the application to control. Some applications

will want to allow the user to specify exactly which ENGINE they want used

if any is to be used at all. Others may prefer to load all support and have

OpenSSL automatically use at run-time any ENGINE that is able to

successfully initialise - ie. to assume that this corresponds to

acceleration hardware attached to the machine or some such thing. There are

probably numerous other ways in which applications may prefer to handle

things, so we will simply illustrate the consequences as they apply to a

couple of simple cases and leave developers to consider these and the

source code to openssl's builtin utilities as guides.

If no ENGINE API functions are called within an application, then OpenSSL

will not allocate any internal resources.  Prior to OpenSSL 1.1.0, however,

if any ENGINEs are loaded, even if not registered or used, it was necessary to

call ENGINE_cleanup() before the program exits.

I<Using a specific ENGINE implementation>

Here we'll assume an application has been configured by its user or admin

to want to use the "ACME" ENGINE if it is available in the version of

OpenSSL the application was compiled with. If it is available, it should be

used by default for all RSA, DSA, and symmetric cipher operations, otherwise

OpenSSL should use its builtin software as per usual. The following code

illustrates how to approach this;

 ENGINE *e;

 const char *engine_id = "ACME";

 ENGINE_load_builtin_engines();

 e = ENGINE_by_id(engine_id);

 if (!e)

     /* the engine isn't available */

     return;

 if (!ENGINE_init(e)) {

     /* the engine couldn't initialise, release 'e' */

     ENGINE_free(e);

     return;

 }

 if (!ENGINE_set_default_RSA(e))

     /*

      * This should only happen when 'e' can't initialise, but the previous

      * statement suggests it did.

      */

     abort();

 ENGINE_set_default_DSA(e);

 ENGINE_set_default_ciphers(e);

 /* Release the functional reference from ENGINE_init() */

 ENGINE_finish(e);

 /* Release the structural reference from ENGINE_by_id() */

 ENGINE_free(e);

I<Automatically using builtin ENGINE implementations>

Here we'll assume we want to load and register all ENGINE implementations

bundled with OpenSSL, such that for any cryptographic algorithm required by

OpenSSL - if there is an ENGINE that implements it and can be initialised,

it should be used. The following code illustrates how this can work;

 /* Load all bundled ENGINEs into memory and make them visible */

 ENGINE_load_builtin_engines();

 /* Register all of them for every algorithm they collectively implement */

 ENGINE_register_all_complete();

That's all that's required. Eg. the next time OpenSSL tries to set up an

RSA key, any bundled ENGINEs that implement RSA_METHOD will be passed to

ENGINE_init() and if any of those succeed, that ENGINE will be set as the

default for RSA use from then on.

=head2 Advanced configuration support

There is a mechanism supported by the ENGINE framework that allows each

ENGINE implementation to define an arbitrary set of configuration

"commands" and expose them to OpenSSL and any applications based on

OpenSSL. This mechanism is entirely based on the use of name-value pairs

and assumes ASCII input (no unicode or UTF for now!), so it is ideal if

applications want to provide a transparent way for users to provide

arbitrary configuration "directives" directly to such ENGINEs. It is also

possible for the application to dynamically interrogate the loaded ENGINE

implementations for the names, descriptions, and input flags of their

available "control commands", providing a more flexible configuration

scheme. However, if the user is expected to know which ENGINE device he/she

is using (in the case of specialised hardware, this goes without saying)

then applications may not need to concern themselves with discovering the

supported control commands and simply prefer to pass settings into ENGINEs

exactly as they are provided by the user.

Before illustrating how control commands work, it is worth mentioning what

they are typically used for. Broadly speaking there are two uses for

control commands; the first is to provide the necessary details to the

implementation (which may know nothing at all specific to the host system)

so that it can be initialised for use. This could include the path to any

driver or config files it needs to load, required network addresses,

smart-card identifiers, passwords to initialise protected devices,

logging information, etc etc. This class of commands typically needs to be

passed to an ENGINE B<before> attempting to initialise it, ie. before

calling ENGINE_init(). The other class of commands consist of settings or

operations that tweak certain behaviour or cause certain operations to take

place, and these commands may work either before or after ENGINE_init(), or

in some cases both. ENGINE implementations should provide indications of

this in the descriptions attached to builtin control commands and/or in

external product documentation.

I<Issuing control commands to an ENGINE>

Let's illustrate by example; a function for which the caller supplies the

name of the ENGINE it wishes to use, a table of string-pairs for use before

initialisation, and another table for use after initialisation. Note that

the string-pairs used for control commands consist of a command "name"

followed by the command "parameter" - the parameter could be NULL in some

cases but the name can not. This function should initialise the ENGINE

(issuing the "pre" commands beforehand and the "post" commands afterwards)

and set it as the default for everything except RAND and then return a

boolean success or failure.

 int generic_load_engine_fn(const char *engine_id,

                            const char **pre_cmds, int pre_num,

                            const char **post_cmds, int post_num)

 {

     ENGINE *e = ENGINE_by_id(engine_id);

     if (!e) return 0;

     while (pre_num--) {

         if (!ENGINE_ctrl_cmd_string(e, pre_cmds[0], pre_cmds[1], 0)) {

             fprintf(stderr, "Failed command (%s - %s:%s)\n", engine_id,

                     pre_cmds[0], pre_cmds[1] ? pre_cmds[1] : "(NULL)");

             ENGINE_free(e);

             return 0;

         }

         pre_cmds += 2;

     }

     if (!ENGINE_init(e)) {

         fprintf(stderr, "Failed initialisation\n");

         ENGINE_free(e);

         return 0;

     }

     /*

      * ENGINE_init() returned a functional reference, so free the structural

      * reference from ENGINE_by_id().

      */

     ENGINE_free(e);

     while (post_num--) {

         if (!ENGINE_ctrl_cmd_string(e, post_cmds[0], post_cmds[1], 0)) {

             fprintf(stderr, "Failed command (%s - %s:%s)\n", engine_id,

                     post_cmds[0], post_cmds[1] ? post_cmds[1] : "(NULL)");

             ENGINE_finish(e);

             return 0;

         }

         post_cmds += 2;

     }

     ENGINE_set_default(e, ENGINE_METHOD_ALL & ~ENGINE_METHOD_RAND);

     /* Success */

     return 1;

 }

Note that ENGINE_ctrl_cmd_string() accepts a boolean argument that can

relax the semantics of the function - if set non-zero it will only return

failure if the ENGINE supported the given command name but failed while

executing it, if the ENGINE doesn't support the command name it will simply

return success without doing anything. In this case we assume the user is

only supplying commands specific to the given ENGINE so we set this to

FALSE.

I<Discovering supported control commands>

It is possible to discover at run-time the names, numerical-ids, descriptions

and input parameters of the control commands supported by an ENGINE using a

structural reference. Note that some control commands are defined by OpenSSL

itself and it will intercept and handle these control commands on behalf of the

ENGINE, ie. the ENGINE's ctrl() handler is not used for the control command.

openssl/engine.h defines an index, ENGINE_CMD_BASE, that all control commands

implemented by ENGINEs should be numbered from. Any command value lower than

this symbol is considered a "generic" command is handled directly by the

OpenSSL core routines.

It is using these "core" control commands that one can discover the control

commands implemented by a given ENGINE, specifically the commands:

 ENGINE_HAS_CTRL_FUNCTION

 ENGINE_CTRL_GET_FIRST_CMD_TYPE

 ENGINE_CTRL_GET_NEXT_CMD_TYPE

 ENGINE_CTRL_GET_CMD_FROM_NAME

 ENGINE_CTRL_GET_NAME_LEN_FROM_CMD

 ENGINE_CTRL_GET_NAME_FROM_CMD

 ENGINE_CTRL_GET_DESC_LEN_FROM_CMD

 ENGINE_CTRL_GET_DESC_FROM_CMD

 ENGINE_CTRL_GET_CMD_FLAGS

Whilst these commands are automatically processed by the OpenSSL framework code,

they use various properties exposed by each ENGINE to process these

queries. An ENGINE has 3 properties it exposes that can affect how this behaves;

it can supply a ctrl() handler, it can specify ENGINE_FLAGS_MANUAL_CMD_CTRL in

the ENGINE's flags, and it can expose an array of control command descriptions.

If an ENGINE specifies the ENGINE_FLAGS_MANUAL_CMD_CTRL flag, then it will

simply pass all these "core" control commands directly to the ENGINE's ctrl()

handler (and thus, it must have supplied one), so it is up to the ENGINE to

reply to these "discovery" commands itself. If that flag is not set, then the

OpenSSL framework code will work with the following rules:

 if no ctrl() handler supplied;

     ENGINE_HAS_CTRL_FUNCTION returns FALSE (zero),

     all other commands fail.

 if a ctrl() handler was supplied but no array of control commands;

     ENGINE_HAS_CTRL_FUNCTION returns TRUE,

     all other commands fail.

 if a ctrl() handler and array of control commands was supplied;

     ENGINE_HAS_CTRL_FUNCTION returns TRUE,

     all other commands proceed processing ...

If the ENGINE's array of control commands is empty then all other commands will

fail, otherwise; ENGINE_CTRL_GET_FIRST_CMD_TYPE returns the identifier of

the first command supported by the ENGINE, ENGINE_GET_NEXT_CMD_TYPE takes the

identifier of a command supported by the ENGINE and returns the next command

identifier or fails if there are no more, ENGINE_CMD_FROM_NAME takes a string

name for a command and returns the corresponding identifier or fails if no such

command name exists, and the remaining commands take a command identifier and

return properties of the corresponding commands. All except

ENGINE_CTRL_GET_FLAGS return the string length of a command name or description,

or populate a supplied character buffer with a copy of the command name or

description. ENGINE_CTRL_GET_FLAGS returns a bitwise-OR'd mask of the following

possible values:

 ENGINE_CMD_FLAG_NUMERIC

 ENGINE_CMD_FLAG_STRING

 ENGINE_CMD_FLAG_NO_INPUT

 ENGINE_CMD_FLAG_INTERNAL

If the ENGINE_CMD_FLAG_INTERNAL flag is set, then any other flags are purely

informational to the caller - this flag will prevent the command being usable

for any higher-level ENGINE functions such as ENGINE_ctrl_cmd_string().

"INTERNAL" commands are not intended to be exposed to text-based configuration

by applications, administrations, users, etc. These can support arbitrary

operations via ENGINE_ctrl(), including passing to and/or from the control

commands data of any arbitrary type. These commands are supported in the

discovery mechanisms simply to allow applications to determine if an ENGINE

supports certain specific commands it might want to use (eg. application "foo"

might query various ENGINEs to see if they implement "FOO_GET_VENDOR_LOGO_GIF" -

and ENGINE could therefore decide whether or not to support this "foo"-specific

extension).

=head1 ENVIRONMENT

=over 4

=item B<OPENSSL_ENGINES>

The path to the engines directory.

Ignored in set-user-ID and set-group-ID programs.

=back

=head1 RETURN VALUES

ENGINE_get_first(), ENGINE_get_last(), ENGINE_get_next() and ENGINE_get_prev()

return a valid B<ENGINE> structure or NULL if an error occurred.

ENGINE_add() and ENGINE_remove() return 1 on success or 0 on error.

ENGINE_by_id() returns a valid B<ENGINE> structure or NULL if an error occurred.

ENGINE_init() and ENGINE_finish() return 1 on success or 0 on error.

All ENGINE_get_default_TYPE() functions, ENGINE_get_cipher_engine() and

ENGINE_get_digest_engine() return a valid B<ENGINE> structure on success or NULL

if an error occurred.

All ENGINE_set_default_TYPE() functions return 1 on success or 0 on error.

ENGINE_set_default() returns 1 on success or 0 on error.

ENGINE_get_table_flags() returns an unsigned integer value representing the

global table flags which are used to control the registration behaviour of

B<ENGINE> implementations.

All ENGINE_register_TYPE() functions return 1 on success or 0 on error.

ENGINE_register_complete() and ENGINE_register_all_complete() return 1 on success

or 0 on error.

ENGINE_ctrl() returns a positive value on success or others on error.

ENGINE_cmd_is_executable() returns 1 if B<cmd> is executable or 0 otherwise.

ENGINE_ctrl_cmd() and ENGINE_ctrl_cmd_string() return 1 on success or 0 on error.

ENGINE_new() returns a valid B<ENGINE> structure on success or NULL if an error

occurred.

ENGINE_free() returns 1 on success or 0 on error.

ENGINE_up_ref() returns 1 on success or 0 on error.

ENGINE_set_id() and ENGINE_set_name() return 1 on success or 0 on error.

All other B<ENGINE_set_*> functions return 1 on success or 0 on error.

ENGINE_get_id() and ENGINE_get_name() return a string representing the identifier

and the name of the ENGINE B<e> respectively.

ENGINE_get_RSA(), ENGINE_get_DSA(), ENGINE_get_DH() and ENGINE_get_RAND()

return corresponding method structures for each algorithms.

ENGINE_get_destroy_function(), ENGINE_get_init_function(),

ENGINE_get_finish_function(), ENGINE_get_ctrl_function(),

ENGINE_get_load_privkey_function(), ENGINE_get_load_pubkey_function(),

ENGINE_get_ciphers() and ENGINE_get_digests() return corresponding function

pointers of the callbacks.

ENGINE_get_cipher() returns a valid B<EVP_CIPHER> structure on success or NULL

if an error occurred.

ENGINE_get_digest() returns a valid B<EVP_MD> structure on success or NULL if an

error occurred.

ENGINE_get_flags() returns an integer representing the ENGINE flags which are

used to control various behaviours of an ENGINE.

ENGINE_get_cmd_defns() returns an B<ENGINE_CMD_DEFN> structure or NULL if it's

not set.

ENGINE_load_private_key() and ENGINE_load_public_key() return a valid B<EVP_PKEY>

structure on success or NULL if an error occurred.

=head1 SEE ALSO

L<OPENSSL_init_crypto(3)>, L<RSA_new_method(3)>, L<DSA_new(3)>, L<DH_new(3)>,

L<RAND_bytes(3)>, L<config(5)>

=head1 HISTORY

ENGINE_cleanup() was deprecated in OpenSSL 1.1.0 by the automatic cleanup

done by OPENSSL_cleanup()

and should not be used.

=head1 COPYRIGHT

Copyright 2002-2018 The OpenSSL Project Authors. All Rights Reserved.

Licensed under the OpenSSL license (the "License").  You may not use

this file except in compliance with the License.  You can obtain a copy

in the file LICENSE in the source distribution or at

L<https://www.openssl.org/source/license.html>.

=cut