#include <openssl/pkcs7.h> PKCS7_SIGNER_INFO *PKCS7_sign_add_signer(PKCS7 *p7, X509 *signcert, EVP_PKEY *pkey, const EVP_MD *md, int flags);
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.
Any of the following flags (ored together) can be passed in the flags parameter.
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 signer's certificate will not be included in the PKCS7 structure, the signer's 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 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.