=pod

=head1 NAME

PKCS7_sign - create a PKCS#7 signedData structure

=head1 SYNOPSIS

 #include <openssl/pkcs7.h>

 PKCS7 *PKCS7_sign(X509 *signcert, EVP_PKEY *pkey, STACK_OF(X509) *certs,

                   BIO *data, int flags);

=head1 DESCRIPTION

PKCS7_sign() creates and returns a PKCS#7 signedData structure. B<signcert> is

the certificate to sign with, B<pkey> is the corresponding private key.

B<certs> is an optional additional set of certificates to include in the PKCS#7

structure (for example any intermediate CAs in the chain).

The data to be signed is read from BIO B<data>.

B<flags> is an optional set of flags.

=head1 NOTES

Any of the following flags (ored together) can be passed in the B<flags>

parameter.

Many S/MIME clients expect the signed content to include valid MIME headers. If

the B<PKCS7_TEXT> flag is set MIME headers for type B<text/plain> are prepended

to the data.

If B<PKCS7_NOCERTS> is set the signer's certificate will not be included in the

PKCS7 structure, the signer's certificate must still be supplied in the

B<signcert> parameter though. This can reduce the size of the signature if the

signers certificate can be obtained by other means: for example a previously

signed message.

The data being signed is included in the PKCS7 structure, unless

B<PKCS7_DETACHED> is set in which case it is omitted. This is used for PKCS7

detached signatures which are used in S/MIME plaintext signed messages for

example.

Normally the supplied content is translated into MIME canonical format (as

required by the S/MIME specifications) if B<PKCS7_BINARY> is set no translation

occurs. This option should be used if the supplied data is in binary format

otherwise the translation will corrupt it.

The signedData structure includes several PKCS#7 authenticatedAttributes

including the signing time, the PKCS#7 content type and the supported list of

ciphers in an SMIMECapabilities attribute. If B<PKCS7_NOATTR> is set then no

authenticatedAttributes will be used. If B<PKCS7_NOSMIMECAP> is set then just

the SMIMECapabilities are omitted.

If present the SMIMECapabilities attribute indicates support for the following

algorithms: triple DES, 128 bit RC2, 64 bit RC2, DES and 40 bit RC2. If any of

these algorithms is disabled then it will not be included.

If the flags B<PKCS7_STREAM> is set then the returned B<PKCS7> structure is

just initialized ready to perform the signing operation. The signing is however

B<not> performed and the data to be signed is not read from the B<data>

parameter. Signing is deferred until after the data has been written. In this

way data can be signed in a single pass.

If the B<PKCS7_PARTIAL> flag is set a partial B<PKCS7> structure is output to

which additional signers and capabilities can be added before finalization.

=head1 NOTES

If the flag B<PKCS7_STREAM> is set the returned B<PKCS7> structure is B<not>

complete and outputting its contents via a function that does not properly

finalize the B<PKCS7> structure will give unpredictable results.

Several functions including SMIME_write_PKCS7(), i2d_PKCS7_bio_stream(),

PEM_write_bio_PKCS7_stream() finalize the structure. Alternatively finalization

can be performed by obtaining the streaming ASN1 B<BIO> directly using

BIO_new_PKCS7().

If a signer is specified it will use the default digest for the signing

algorithm. This is B<SHA1> for both RSA and DSA keys.

The B<certs>, B<signcert> and B<pkey> parameters can all be

B<NULL> if the B<PKCS7_PARTIAL> flag is set. One or more signers can be added

using the function PKCS7_sign_add_signer(). PKCS7_final() must also be

called to finalize the structure if streaming is not enabled. Alternative

signing digests can also be specified using this method.

If B<signcert> and B<pkey> are NULL then a certificates only

PKCS#7 structure is output.

In versions of OpenSSL before 1.0.0 the B<signcert> and B<pkey> parameters must

B<NOT> be NULL.

=head1 BUGS

Some advanced attributes such as counter signatures are not supported.

=head1 RETURN VALUES

PKCS7_sign() returns either a valid PKCS7 structure or NULL if an error

occurred.  The error can be obtained from ERR_get_error(3).

=head1 SEE ALSO

L<ERR_get_error(3)>, L<PKCS7_verify(3)>

=head1 HISTORY

The B<PKCS7_PARTIAL> flag, and the ability for B<certs>, B<signcert>,

and B<pkey> parameters to be B<NULL> were added in OpenSSL 1.0.0.

The B<PKCS7_STREAM> flag was added in OpenSSL 1.0.0.

=head1 COPYRIGHT

Copyright 2002-2016 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>.

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