=pod

=head1 NAME

PKCS7_sign_add_signer - add a signer PKCS7 signed data structure

=head1 SYNOPSIS

 #include <openssl/pkcs7.h>

 PKCS7_SIGNER_INFO *PKCS7_sign_add_signer(PKCS7 *p7, X509 *signcert,
                                          EVP_PKEY *pkey, const EVP_MD *md, int flags);


=head1 DESCRIPTION

PKCS7_sign_add_signer() adds a signer with certificate B<signcert> and private
key B<pkey> using message digest B<md> to a PKCS7 signed data structure
B<p7>.

The PKCS7 structure should be obtained from an initial call to PKCS7_sign()
with the flag B<PKCS7_PARTIAL> set or in the case or re-signing a valid PKCS7
signed data structure.

If the B<md> parameter is B<NULL> then the default digest for the public
key algorithm will be used.

Unless the B<PKCS7_REUSE_DIGEST> flag is set the returned PKCS7 structure
is not complete and must be finalized either by streaming (if applicable) or
a call to PKCS7_final().


=head1 NOTES

The main purpose of this function is to provide finer control over a PKCS#7
signed data structure where the simpler PKCS7_sign() function defaults are
not appropriate. For example if multiple signers or non default digest
algorithms are needed.

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

If B<PKCS7_REUSE_DIGEST> is set then an attempt is made to copy the content
digest value from the PKCS7 structure: to add a signer to an existing structure.
An error occurs if a matching digest value cannot be found to copy. The
returned PKCS7 structure will be valid and finalized when this flag is set.

If B<PKCS7_PARTIAL> is set in addition to B<PKCS7_REUSE_DIGEST> then the
B<PKCS7_SIGNER_INO> structure will not be finalized so additional attributes
can be added. In this case an explicit call to PKCS7_SIGNER_INFO_sign() is
needed to finalize it.

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 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.


PKCS7_sign_add_signers() returns an internal pointer to the PKCS7_SIGNER_INFO
structure just added, this can be used to set additional attributes
before it is finalized.

=head1 RETURN VALUES

PKCS7_sign_add_signers() returns an internal pointer to the PKCS7_SIGNER_INFO
structure just added or NULL if an error occurs.

=head1 SEE ALSO

L<ERR_get_error(3)>, L<PKCS7_sign(3)>,
L<PKCS7_final(3)>,

=head1 HISTORY

PPKCS7_sign_add_signer() was added to OpenSSL 1.0.0

=head1 COPYRIGHT

Copyright 2007-2016 The OpenSSL Project Authors. All Rights Reserved.

Licensed under the Apache License 2.0 (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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