CMS_ADD1_SIGNER(3)

NAME
CMS_add1_signer, CMS_SignerInfo_sign - add a signer to a CMS_ContentInfo signed data structure

SYNOPSIS
 #include <openssl/cms.h>

 CMS_SignerInfo *CMS_add1_signer(CMS_ContentInfo *cms, X509 *signcert,
                                 EVP_PKEY *pkey, const EVP_MD *md,
                                 unsigned int flags);

 int CMS_SignerInfo_sign(CMS_SignerInfo *si);

DESCRIPTION
CMS_add1_signer() adds a signer with certificate signcert and private key pkey using message digest md to CMS_ContentInfo SignedData structure cms.

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 explicitly signs a CMS_SignerInfo structure, its main use is when the CMS_REUSE_DIGEST and CMS_PARTIAL flags are both set.

NOTES
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 parameter.

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 signer's certificate will not be included in the CMS_ContentInfo 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 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 at all. If CMS_NOSMIMECAP is set then the SMIMECapabilities will be omitted. If CMS_NO_SIGNING_TIME is set then the signing time will be omitted.

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.

Note that, in the case signedAttributes are not used, for some hash-less signing schemes the given hash md will be ignored and a hash required by the signing scheme will be used. This is the case for EdDSA (RFC 8419). For SLH-DSA (RFC 9814) and ML-DSA (RFC 9882), the scheme-suggested hash will only be used if md is NULL.

Signing with Ed448 is currently not supported for the case of signed-data with signedAttributes due to missing support for id-shake256-len (RFC 8419; sec 3.1).

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.

RETURN VALUES
CMS_add1_signer() returns an internal pointer to the CMS_SignerInfo structure just added or NULL if an error occurs.

CMS_SignerInfo_sign() returns 1 on success, 0 on failure.

SEE ALSO
ERR_get_error(3), CMS_sign(3), CMS_final(3),

COPYRIGHT
Copyright 2014-2026 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 <https://www.openssl.org/source/license.html>.