PKCS7_sign_add_signer() adds a signer with certificate signcert and private
key pkey using message digest md to a PKCS7 signed data structure
The PKCS7 structure should be obtained from an initial call to PKCS7_sign()
with the flag PKCS7_PARTIAL set or in the case or re-signing a valid PKCS7
signed data structure.
If the md parameter is NULL then the default digest for the public
key algorithm will be used.
Unless the 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().
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 flags
If 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 PKCS7_PARTIAL is set in addition to PKCS7_REUSE_DIGEST then the
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 PKCS7_NOCERTS is set the signers certificate will not be included in the
PKCS7 structure, the signers certificate must still be supplied in the
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
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 PKCS7_NOATTR is set then no
authenticatedAttributes will be used. If 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.