PKCS7_SIGN_ADD_SIGNER(3) | Library Functions Manual | PKCS7_SIGN_ADD_SIGNER(3)

# NAME

`PKCS7_sign_add_signer` — add a signer to a SignedData structure

# SYNOPSIS

library “libcrypto”
`#include <openssl/pkcs7.h>`

PKCS7_SIGNER_INFO *
`PKCS7_sign_add_signer`(PKCS7 *p7, X509 *signcert, EVP_PKEY *pkey, const EVP_MD *md, int flags);

# DESCRIPTION

`PKCS7_sign_add_signer`() adds a signer with certificate signcert and private key pkey using message digest md to a PKCS7 signed data structure p7.

The PKCS7 structure should be obtained from an initial call to PKCS7_sign(3) 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 by a call to PKCS7_final(3).

The main purpose of this function is to provide finer control over a PKCS#7 signed data structure where the simpler PKCS7_sign(3) function defaults are not appropriate, for example if multiple signers or non default digest algorithms are needed.

Any of the following flags (OR'ed 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 structure: 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, though the signer's certificate must still be supplied in the signcert parameter. 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 authenticatedAttributes 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_signer`() returns an internal pointer to the PKCS7_SIGNER_INFO structure just added, which can be used to set additional attributes with the functions described in PKCS7_add_attribute(3) before it is finalized.

# RETURN VALUES

`PKCS7_sign_add_signer`() returns an internal pointer to the PKCS7_SIGNER_INFO structure just added or `NULL` if an error occurs. In some cases of failure, the reason can be determined with ERR_get_error(3).

# SEE ALSO

EVP_DigestInit(3), PKCS7_add_attribute(3), PKCS7_final(3), PKCS7_get_signer_info(3), PKCS7_new(3), PKCS7_sign(3)

# HISTORY

`PKCS7_sign_add_signer`() first appeared in OpenSSL 1.0.0 and has been available since OpenBSD 4.9.

June 8, 2025 | Debian