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);

        int (*cert_cb)(SSL *ssl, void *arg);

DESCRIPTION

       SSL_CTX_set_cert_cb() and SSL_set_cert_cb() sets the cert_cb() callback, arg value is
       pointer which is passed to the application callback.

       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.

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 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.

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 2014-2018 The OpenSSL Project Authors. All Rights Reserved.

       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>.