CMS_add1_signer() adds a signer with certificate signcert and private
key pkey using message digest md to CMS_ContentInfo SignedData
The CMS_ContentInfo structure should be obtained from an initial call to
CMS_sign() with the flag CMS_PARTIAL set or in the case or re-signing a
valid CMS_ContentInfo SignedData structure.
If the md parameter is NULL then the default digest for the public
key algorithm will be used.
Unless the CMS_REUSE_DIGEST flag is set the returned CMS_ContentInfo
structure is not complete and must be finalized either by streaming (if
applicable) or a call to CMS_final().
The CMS_SignerInfo_sign() function will explicitly sign a CMS_SignerInfo
structure, its main use is when CMS_REUSE_DIGEST and CMS_PARTIAL flags
are both set.
The main purpose of CMS_add1_signer() is to provide finer control
over a CMS signed data structure where the simpler CMS_sign() function defaults
are not appropriate. For example if multiple signers or non default digest
algorithms are needed. New attributes can also be added using the returned
CMS_SignerInfo structure and the CMS attribute utility functions or the
CMS signed receipt request functions.
Any of the following flags (ored together) can be passed in the flags
If CMS_REUSE_DIGEST is set then an attempt is made to copy the content
digest value from the CMS_ContentInfo structure: to add a signer to an existing
structure. An error occurs if a matching digest value cannot be found to copy.
The returned CMS_ContentInfo structure will be valid and finalized when this
flag is set.
If CMS_PARTIAL is set in addition to CMS_REUSE_DIGEST then the
CMS_SignerInfo structure will not be finalized so additional attributes
can be added. In this case an explicit call to CMS_SignerInfo_sign() is
needed to finalize it.
If CMS_NOCERTS is set the signers certificate will not be included in the
CMS_ContentInfo 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 signed message.
The SignedData structure includes several CMS signedAttributes including the
signing time, the CMS content type and the supported list of ciphers in an
SMIMECapabilities attribute. If CMS_NOATTR is set then no signedAttributes
will be used. If CMS_NOSMIMECAP is set then just the SMIMECapabilities are
OpenSSL will by default identify signing certificates using issuer name
and serial number. If CMS_USE_KEYID is set it will use the subject key
identifier value instead. An error occurs if the signing certificate does not
have a subject key identifier extension.
If present the SMIMECapabilities attribute indicates support for the following
algorithms in preference order: 256 bit AES, Gost R3411-94, Gost 28147-89, 192
bit AES, 128 bit AES, triple DES, 128 bit RC2, 64 bit RC2, DES and 40 bit RC2.
If any of these algorithms is not available then it will not be included: for example the GOST algorithms will not be included if the GOST ENGINE is
CMS_add1_signer() returns an internal pointer to the CMS_SignerInfo
structure just added, this can be used to set additional attributes
before it is finalized.