PKCS7_SIGN(3)

📦 version 4.4.0, in alpine 3.24, arch rolling

💅 NAME

PKCS7_sign — create a PKCS#7 signedData structure.

Okay so this is PKCS7_sign(3), and she lives in the Library Functions Manual for the libcrypto library. She’s the one you call when you need a PKCS#7 signedData structure, and honestly? iconic. 💖

✨ SYNOPSIS ✨

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

PKCS7 *
PKCS7_sign(X509 *signcert, EVP_PKEY *pkey, STACK_OF(X509) *certs,
    BIO *data, int flags);

🎀 DESCRIPTION 🎀

Bestie, PKCS7_sign() creates and returns a PKCS#7 signedData structure. Here’s the cast of characters:

  • signcert — the certificate to sign with.
  • pkey — the corresponding private key.
  • certs — an optional additional set of certificates to include in the PKCS#7 structure (for example any intermediate CAs in the chain).

The data to be signed is read from BIO data.

flags is an optional set of flags. Any of the following flags (OR’ed together) can be passed in the flags parameter:

  • PKCS7_TEXT — Many S/MIME clients expect the signed content to include valid MIME headers. If this flag is set, MIME headers for type text/plain are prepended to the data.
  • 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 signer’s certificate can be obtained by other means: for example a previously signed message.
  • PKCS7_DETACHED — The data being signed is included in the PKCS7 structure, unless this is set, in which case it is omitted. This is used for PKCS7 detached signatures which are used in S/MIME plaintext signed messages for example.
  • PKCS7_BINARY — Normally the supplied content is translated into MIME canonical format (as required by the S/MIME specifications). If this is set, no translation occurs. This option should be used if the supplied data is in binary format; otherwise, the translation will corrupt it. Not a drill, bestie — binary data needs this flag or it gets wrecked. 😭
  • PKCS7_NOATTR — 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 this flag is 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_STREAM — If set, then the returned PKCS7 structure is just initialized ready to perform the signing operation. The signing is however not performed and the data to be signed is not read from the data parameter. Signing is deferred until after the data has been written. In this way data can be signed in a single pass.
  • PKCS7_PARTIAL — If set, a partial PKCS7 structure is output to which additional signers and capabilities can be added before finalization.
⚠️ If the flag PKCS7_STREAM is set, the returned PKCS7 structure is not complete and outputting its contents via a function that does not properly finalize the PKCS7 structure will give unpredictable results. Several functions including PKCS7_final(3), SMIME_write_PKCS7(3), PEM_write_bio_PKCS7_stream(3), and i2d_PKCS7_bio_stream(3) finalize the structure. Alternatively finalization can be performed by obtaining the streaming ASN.1 BIO directly using BIO_new_PKCS7().

If a signer is specified, it will use the default digest for the signing algorithm. This is SHA1 for both RSA and DSA keys.

In OpenSSL 1.0.0, the certs, signcert, and pkey parameters can all be NULL if the PKCS7_PARTIAL flag is set. One or more signers can be added using the function PKCS7_sign_add_signer(3) and attributes can be added using the functions described in PKCS7_add_attribute(3). PKCS7_final(3) must also be called to finalize the structure if streaming is not enabled. Alternative signing digests can also be specified using this method.

In OpenSSL 1.0.0, if signcert and pkey are NULL, then a certificate-only PKCS#7 structure is output.

In versions of OpenSSL before 1.0.0 the signcert and pkey parameters must NOT be NULL. Periodt. 💅

🫶 RETURN VALUES 🫶

PKCS7_sign() returns either a valid PKCS7 structure or NULL if an error occurred. The error can be obtained from ERR_get_error(3). That’s the whole vibe check. ✨

🌈 SEE ALSO 🌈

🦋 HISTORY 🦋

PKCS7_sign() first appeared in OpenSSL 0.9.5 and have been available since OpenBSD 2.7.

The PKCS7_PARTIAL and PKCS7_STREAM flags were added in OpenSSL 1.0.0. We love a glow-up. 💖

🙈 BUGS 🙈

Some advanced attributes such as counter signatures are not supported. That’s the tea. ☕

Footer cuteness, as promised: the source/version line says Debian, and the date is June 8, 2025 — mwah, bestie 💋
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