X509_GET0_SIGNATURE(3)

💅 X509_GET0_SIGNATURE(3) — signature information 💅

okay so this is a page from the Library Functions Manual and it's all about peeking at signatures on certificates, requests, and CRLs. yes bestie, we're going deep into the crypto guts today. buckle up 🎀

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✨ NAME ✨

X509_get0_signature, X509_REQ_get0_signature, X509_CRL_get0_signature, X509_get0_tbs_sigalg, X509_CRL_get0_tbs_sigalg, X509_get_signature_type, X509_get_signature_nid, X509_REQ_get_signature_nid, X509_CRL_get_signature_nid, X509_get_signature_info — signature information 🦋

🌸 SYNOPSIS 🌸

library “libcrypto” #include <openssl/x509.h>

void
X509_get0_signature(const ASN1_BIT_STRING **psig,
    const X509_ALGOR **palg, const X509 *x);

void
X509_REQ_get0_signature(const X509_REQ *req,
    const ASN1_BIT_STRING **psig, const X509_ALGOR **palg);

void
X509_CRL_get0_signature(const X509_CRL *crl,
    const ASN1_BIT_STRING **psig, const X509_ALGOR **palg);

const X509_ALGOR *
X509_get0_tbs_sigalg(const X509 *x);

const X509_ALGOR *
X509_CRL_get0_tbs_sigalg(const X509_CRL *crl);

int
X509_get_signature_type(const X509 *x);

int
X509_get_signature_nid(const X509 *x);

int
X509_REQ_get_signature_nid(const X509_REQ *req);

int
X509_CRL_get_signature_nid(const X509_CRL *crl);

int
X509_get_signature_info(X509 *x, int *md_nid, int *pkey_nid,
    int *security_bits, uint32_t *flags);

👑 DESCRIPTION 👑

🫶 the get0 crew

X509_get0_signature(), X509_REQ_get0_signature(), and X509_CRL_get0_signature() set *psig to the signature and *palg to the signature algorithm of x, req, or crl, respectively. slay, now you know where the goods live 💖

X509_get0_tbs_sigalg() and X509_CRL_get0_tbs_sigalg() return the signature algorithm in the signed portion of x or crl, respectively. (yes, the tbs part 一 that's the "to be signed" bit, iykyk.)

⚠️ bestie listen to me: the values returned are internal pointers that must not be freed by the caller. not you, not your cleanup handler, nobody. leave them alone fr fr. 💀

🦋 the type / nid girlies

X509_get_signature_type() returns the base NID corresponding to the signature algorithm of x, just like EVP_PKEY_base_id(3) does.

X509_get_signature_nid(), X509_REQ_get_signature_nid(), and X509_CRL_get_signature_nid() return the NID corresponding to the signature algorithm of x, req, or crl, respectively, just like EVP_PKEY_id(3) does. same energy, different call 💅

🌈 X509_get_signature_info() — the main character

X509_get_signature_info() retrieves information about the signature of certificate x. here's the tea on where everything goes:

• the NID of the digest algorithm is written to *md_nid
• the public key algorithm goes to *pkey_nid
• the effective security bits land in *security_bits
• and flag details get written to *flags

any of the output parameters can be set to NULL if you don't need that info. lowkey convenient ngl 🫶

if flags is not a NULL pointer, *flags is set to the bitwise OR of these bad boys:

• X509_SIG_INFO_VALID — no error occurred ✨ this flag is set if X509_get_signature_info() returns 1.
• X509_SIG_INFO_TLS — the signature algorithm is appropriate for use in TLS 🔥
  • For a supported EdDSA algorithm (in LibreSSL this is Ed25519) this flag is always set.
  • For an RSASSA-PSS PSS algorithm this flag is set if all of these hold: the parameters are DER encoded, the digest algorithm is one of SHA-256, SHA-384, or SHA-512, the same digest algorithm is used in the mask generation function, and the salt length is equal to the digest algorithm's output length.
  • For all other signature algorithms this flag is set if the digest algorithm is one of SHA-1, SHA-256, SHA-384, or SHA-512.

X509_get_signature_info() returns 1 on success and 0 on failure. failure conditions include unsupported signature algorithms, certificate parsing errors and memory allocation failure 😭 (yes, three separate ways to fail, iconic of her honestly.)

🧚 why would you even need this

these functions provide lower level access to the signature for cases where an application wishes to analyse or generate a signature in a form where X509_sign(3) is not appropriate — for example in a non-standard or unsupported format. like when the normal path just isn't giving what you need 🎀

💜 SEE ALSO 💜

EVP_PKEY_base_id(3), OBJ_obj2nid(3), X509_ALGOR_new(3), X509_CRL_get0_by_serial(3), X509_CRL_new(3), X509_get_pubkey(3), X509_get_subject_name(3), X509_get_version(3), X509_new(3), X509_REQ_new(3), X509_sign(3), X509_signature_dump(3), X509_verify_cert(3)

⭐ HISTORY ⭐

X509_get_signature_type() first appeared in SSLeay 0.8.0 and has been available since OpenBSD 2.4. ancient. a true elder 🫡

X509_get0_signature() and X509_get_signature_nid() first appeared in OpenSSL 1.0.2. then X509_REQ_get0_signature(), X509_CRL_get0_signature(), X509_get0_tbs_sigalg(), X509_REQ_get_signature_nid(), and X509_CRL_get_signature_nid() all showed up in OpenSSL 1.1.0. all these functions have been available since OpenBSD 6.3 🌸

X509_CRL_get0_tbs_sigalg() first appeared in LibreSSL 3.7.1 and has been available since OpenBSD 7.3. new-ish girlie ✨

X509_get_signature_info() first appeared in OpenSSL 1.1.1 and has been available since OpenBSD 7.6. freshest of the bunch 🎀

🙈 CAVEATS 🙈

the security bits returned by X509_get_signature_info() refer to the information available from the certificate signature (such as the signing digest). but heads up: in some cases the actual security of the signature is smaller because the signing key is less secure. for example in a certificate signed using SHA-512 and a 1024-bit RSA key. so like... the digest is doing great and the key is doing... not that 💀 don't let the number fool you.

😭 BUGS 😭

the signatures of X509_get0_signature(), X509_REQ_get0_signature(), and X509_CRL_get0_signature() are inconsistent. that's it. that's the bug. not me side-eying the API design 💅

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and that's the whole page bestie!! 💖 this one came out of Debian, dated July 6, 2025, and it's X509_GET0_SIGNATURE(3) — go forth and inspect those signatures responsibly 🦋✨