SSL_CTX_SET_CERT_CB(3ossl)

🌸 SSL_CTX_SET_CERT_CB(3ossl) — cert callback central 🌸

okay so bestie, welcome to the SSL_CTX_SET_CERT_CB(3ossl) manual page. This is the OpenSSL glow-up for SSL_CTX_set_cert_cb and SSL_set_cert_cb, aka the “handle certificate callback function” girlies. Let’s get into it. 💅

✨ NAME ✨

SSL_CTX_set_cert_cb, SSL_set_cert_cb — handle certificate callback function.

🎀 SYNOPSIS 🎀

#include <openssl/ssl.h>

void SSL_CTX_set_cert_cb(SSL_CTX *c, int (*cert_cb)(SSL *ssl, void *arg),
                         void *arg);
void SSL_set_cert_cb(SSL *s, int (*cert_cb)(SSL *ssl, void *arg), void *arg);

🦋 DESCRIPTION 🦋

So SSL_CTX_set_cert_cb() and SSL_set_cert_cb() set the cert_cb callback. The arg value is a pointer that gets passed to the application callback. 🫶

When cert_cb is NULL, no callback function is used. Simple as that.

cert_cb is the application-defined callback. It is called before a certificate will be used by a client or server. The callback can then inspect the passed ssl structure and set or clear any appropriate certificates. If the callback is successful it MUST return 1 — even if no certificates have been set. A 0 is returned on error, which will abort the handshake with a fatal internal error alert. A negative return value will suspend the handshake and the handshake function will return immediately. SSL_get_error(3) will return SSL_ERROR_WANT_X509_LOOKUP to indicate that the handshake was suspended. The next call to the handshake function will again lead to the call of cert_cb. It is the job of the cert_cb to store information about the state of the last call, if required to continue. 😭

🌈 NOTES 🌈

An application will typically call SSL_use_certificate() and SSL_use_PrivateKey() to set the end entity certificate and private key. It can add intermediate and optionally the root CA certificates using SSL_add1_chain_cert().

It might also call SSL_certs_clear() to delete any certificates associated with the SSL object.

The certificate callback functionality supersedes the (largely broken) functionality provided by the old client certificate callback interface. It is always called even if a certificate is already set, so the callback can modify or delete the existing certificate. No cap, it really does show up even when a cert is already there. 💀

A more advanced callback might examine the handshake parameters and set whatever chain is appropriate. For example, a legacy client supporting only TLSv1.0 might receive a certificate chain signed using SHA1, whereas a TLSv1.2 or later client which advertises support for SHA256 could receive a chain using SHA256.

Normal server sanity checks are performed on any certificates set by the callback. So if an EC chain is set for a curve the client does not support, it will not be used. 🧚

💖 RETURN VALUES 💖

SSL_CTX_set_cert_cb() and SSL_set_cert_cb() do not return values.

👑 SEE ALSO 👑

• ssl(7)
• SSL_use_certificate(3)
• SSL_add1_chain_cert(3)
• SSL_get_client_CA_list(3)
• SSL_clear(3)
• SSL_free(3)

🍓 COPYRIGHT 🍓

Copyright 2014-2020 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>.

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okay that’s the whole manual, bestie — OpenSSL 3.5.0, dated 2026-09-25. Stay iconic, and may your callbacks always return the right vibe. 💅✨