X509_CHECK_PURPOSE(3ossl)

📦 version 15.1, in freebsd 15.1

💅 X509_CHECK_PURPOSE — the OpenSSL 3ossl era, bestie

okay so buckle up 🎀 this is the whole x509v3.h purpose-checking family: X509_check_purpose, X509_PURPOSE_get_count, X509_PURPOSE_get_unused_id, X509_PURPOSE_get_by_sname, X509_PURPOSE_get_by_id, X509_PURPOSE_add, X509_PURPOSE_cleanup, X509_PURPOSE_get0, X509_PURPOSE_get_id, X509_PURPOSE_get0_name, X509_PURPOSE_get0_sname, X509_PURPOSE_get_trust, and X509_PURPOSE_set — literally all the functions related to checking the purpose of a certificate. iconic honestly 👑

✨ SYNOPSIS ✨

#include <openssl/x509v3.h>

int X509_check_purpose(X509 *x, int id, int ca);

int X509_PURPOSE_get_count(void);
int X509_PURPOSE_get_unused_id(OSSL_LIB_CTX *libctx);
int X509_PURPOSE_get_by_sname(const char *sname);
int X509_PURPOSE_get_by_id(int id);
int X509_PURPOSE_add(int id, int trust, int flags,
                     int (*ck) (const X509_PURPOSE *, const X509 *, int),
                     const char *name, const char *sname, void *arg);
void X509_PURPOSE_cleanup(void);

X509_PURPOSE *X509_PURPOSE_get0(int idx);
int X509_PURPOSE_get_id(const X509_PURPOSE *);
char *X509_PURPOSE_get0_name(const X509_PURPOSE *xp);
char *X509_PURPOSE_get0_sname(const X509_PURPOSE *xp);
int X509_PURPOSE_get_trust(const X509_PURPOSE *xp);
int X509_PURPOSE_set(int *p, int purpose);

🌸 DESCRIPTION 🌸

the main character: X509_check_purpose

X509_check_purpose() asks the question "was certificate x literally made for the purpose represented by id?" 🦋 If ca is nonzero, then certificate x gets checked to see whether it's a possible CA, and you may get back various levels of certainty. heads up tho: certificate x must be a complete certificate, otherwise the function just returns an error. no vibes, no partial certs, periodt 💀

Below are the potential ID's that can be checked — copy these exactly, iykyk:

# define X509_PURPOSE_SSL_CLIENT        1
# define X509_PURPOSE_SSL_SERVER        2
# define X509_PURPOSE_NS_SSL_SERVER     3
# define X509_PURPOSE_SMIME_SIGN        4
# define X509_PURPOSE_SMIME_ENCRYPT     5
# define X509_PURPOSE_CRL_SIGN          6
# define X509_PURPOSE_ANY               7
# define X509_PURPOSE_OCSP_HELPER       8
# define X509_PURPOSE_TIMESTAMP_SIGN    9
# define X509_PURPOSE_CODE_SIGN        10

The checks performed take into account the X.509 extensions keyUsage, extendedKeyUsage, and basicConstraints. so like, it's actually reading the receipts 📜

the rest of the squad ✨

  • X509_PURPOSE_get_count() — returns the number of currently defined purposes. that's the headcount, bestie 🫶
  • X509_PURPOSE_get_unused_id() — returns the smallest purpose id not yet used, which is guaranteed to be unique and larger than X509_PURPOSE_MAX. the libctx parameter should be used to provide the library context — but it is currently ignored, since the purpose mapping table is global. yeah. it's giving placeholder 🙈
  • X509_PURPOSE_get_by_sname() — returns the index of the purpose with the given short name, or -1 if not found.
  • X509_PURPOSE_get_by_id() — returns the index of the purpose with the given id, or -1 if not found.
  • X509_PURPOSE_add() — adds or modifies a purpose entry identified by sname. Unless the id stays the same for an existing entry, id must be fresh, which can be achieved by using the result of X509_PURPOSE_get_unused_id(). This function also sets in the entry the trust id trust, the given flags, the purpose (long) name name, the short name sname, the purpose checking function ck of type int (*) (const X509_PURPOSE *, const X509 *, int), and its user data arg which may be retrieved via the X509_PURPOSE pointer. so much going on in one call, she's multitasking 💅
  • X509_PURPOSE_cleanup() — removes all purposes that are not pre-defined. spring cleaning energy 🧹
  • X509_PURPOSE_get0() — returns an X509_PURPOSE pointer or NULL on error.
  • X509_PURPOSE_get_id() — returns the id of the given X509_PURPOSE structure.
  • X509_PURPOSE_get0_name() — returns the (long) name of the given X509_PURPOSE.
  • X509_PURPOSE_get0_sname() — returns the short name of the given X509_PURPOSE.
  • X509_PURPOSE_get_trust() — returns the trust id of the given X509_PURPOSE.
  • X509_PURPOSE_set() — assigns the given purpose id to the location pointed at by p. oh and it resets to the any purpose if purpose is X509_PURPOSE_DEFAULT_ANY. plot twist 🌀

💖 RETURN VALUES 💖

non-CA checks 🍓

X509_check_purpose() returns the following values. For non-CA checks:

  • -1 — an error condition has occurred
  • 1 — the certificate was created to perform the purpose represented by id
  • 0 — the certificate was not created to perform the purpose represented by id

CA checks 👑

For CA checks the below integers could be returned with the following meanings:

  • -1 — an error condition has occurred
  • 0 — not a CA or does not have the purpose represented by id
  • 1 — is a CA.
  • 2 — Only possible in old versions of openSSL when basicConstraints are absent. New versions will not return this value. May be a CA
  • 3 — basicConstraints absent but self signed V1.
  • 4 — basicConstraints absent but keyUsage present and keyCertSign asserted.
  • 5 — legacy Netscape specific CA Flags present

everyone else 🌈

  • X509_PURPOSE_get_count() — returns the number of currently defined purposes.
  • X509_PURPOSE_get_unused_id() — returns the smallest purpose id not yet used.
  • X509_PURPOSE_get_by_sname() — returns the index of the purpose with the given short name or -1 if not found.
  • X509_PURPOSE_get_by_id() — returns the index of the purpose with the given id or -1 if not found.
  • int X509_PURPOSE_add() — returns 1 on success, 0 on error.
  • X509_PURPOSE_cleanup() — does not return anything. she simply leaves 💨
  • X509_PURPOSE_get0() — returns an X509_PURPOSE pointer or NULL on error.
  • X509_PURPOSE_get_id() — returns the id of the given X509_PURPOSE structure.
  • X509_PURPOSE_get0_name() — returns the (long) name of the given X509_PURPOSE.
  • X509_PURPOSE_get0_sname() — returns the short name of the given X509_PURPOSE.
  • X509_PURPOSE_get_trust() — returns the trust id of the given X509_PURPOSE.
  • X509_PURPOSE_set() — returns 1 on success, 0 on error.

💀 BUGS 💀

not thread-safe, bestie. the X509_PURPOSE implementation so far is not thread-safe. There may be race conditions retrieving purpose information while X509_PURPOSE_add() or X509_PURPOSE_cleanup(void) is being called. so like… don't do that. fr fr 😭

🦋 HISTORY 🦋

X509_PURPOSE_get_unused_id() was added in OpenSSL 3.5. glow-up era ✨

Copyright 2019-2025 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>.

and that's the whole page, bestie 🫶 this was the OpenSSL 3ossl manual, version 3.5.6, dated 2026-04-07 — go forth and check those purposes 💜🌈

📦 where this page comes from