#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); int (*cert_cb)(SSL *ssl, void *arg);
When cert_cb() is NULL, no callback function is used.
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 zero 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.
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 is a certificate is already set so the callback can modify or delete the existing certificate.
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.
Licensed under the OpenSSL license (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.