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=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 struture: 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 autenticatedAttributes
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)|ERR_get_error(3)>, L<PKCS7_sign(3)|PKCS7_sign(3)>,
L<PKCS7_final(3)|PKCS7_final(3)>,
=head1 HISTORY
PPKCS7_sign_add_signer() was added to OpenSSL 1.0.0
=cut