PKCS7_SIGN_ADD_SIGNER(3)

📦 version 4.4.0, in alpine 3.24, arch rolling

okay bestie, this is the PKCS7_SIGN_ADD_SIGNER(3) page from the Library Functions Manual — yes, the one all about PKCS7_sign_add_signer — and it's giving “add a signer to a SignedData structure” energy 💅✨

🎀 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 💅

Okay so PKCS7_sign_add_signer() adds a signer to a PKCS7 signed data structure p7, using certificate signcert, private key pkey, and message digest md. Main character behavior, honestly 🫶

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, as the page says — a valid PKCS7 signed data structure.

If the md parameter is NULL, then the default digest for the public key algorithm will be used.

🚨 real talk: 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. It's giving “I need options” and honestly same 💀

Any of the following flags (OR'ed together) can be passed in the flags parameter.

  • PKCS7_REUSE_DIGEST — if 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.
  • PKCS7_PARTIAL — if 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. (Yes bestie, that's PKCS7_SIGNER_INO — we keep the page's exact spelling 🙈)
  • PKCS7_NOCERTS — if 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.

  • PKCS7_NOATTR — if set, then no authenticatedAttributes will be used.
  • PKCS7_NOSMIMECAP — if 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. Iconic little handle, no cap ✨

🫶 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). Check those errors, bestie! 🔥

🔍 SEE ALSO 🔍

🌸 HISTORY 🌸

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

xoxo, your Unix bestie 💖 — this page is from Debian, dated June 8, 2025, and it's the PKCS7_SIGN_ADD_SIGNER(3) manual. Bye bestie, go add those signers responsibly! 🫶✨

📦 where this page comes from