NAME

       SSL_CTX_set_client_cert_cb, SSL_CTX_get_client_cert_cb - handle client certificate
       callback function

SYNOPSIS

        #include <openssl/ssl.h>

        void SSL_CTX_set_client_cert_cb(SSL_CTX *ctx,
                                        int (*client_cert_cb)(SSL *ssl, X509 **x509,
                                                              EVP_PKEY **pkey));
        int (*SSL_CTX_get_client_cert_cb(SSL_CTX *ctx))(SSL *ssl, X509 **x509,
                                                        EVP_PKEY **pkey);
        int (*client_cert_cb)(SSL *ssl, X509 **x509, EVP_PKEY **pkey);

DESCRIPTION

       SSL_CTX_set_client_cert_cb() sets the client_cert_cb() callback, that is called when a
       client certificate is requested by a server and no certificate was yet set for the SSL
       object.

       When client_cert_cb() is NULL, no callback function is used.

       SSL_CTX_get_client_cert_cb() returns a pointer to the currently set callback function.

       client_cert_cb() is the application defined callback. If it wants to set a certificate, a
       certificate/private key combination must be set using the x509 and pkey arguments and "1"
       must be returned. The certificate will be installed into ssl, see the NOTES and BUGS
       sections.  If no certificate should be set, "0" has to be returned and no certificate will
       be sent. 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 client_cert_cb(). It is the job of the client_cert_cb() to store
       information about the state of the last call, if required to continue.

NOTES

       During a handshake (or renegotiation) a server may request a certificate from the client.
       A client certificate must only be sent, when the server did send the request.

       When a certificate was set using the SSL_CTX_use_certificate(3) family of functions, it
       will be sent to the server. The TLS standard requires that only a certificate is sent, if
       it matches the list of acceptable CAs sent by the server. This constraint is violated by
       the default behavior of the OpenSSL library. Using the callback function it is possible to
       implement a proper selection routine or to allow a user interaction to choose the
       certificate to be sent.

       If a callback function is defined and no certificate was yet defined for the SSL object,
       the callback function will be called.  If the callback function returns a certificate, the
       OpenSSL library will try to load the private key and certificate data into the SSL object
       using the SSL_use_certificate() and SSL_use_private_key() functions.  Thus it will
       permanently install the certificate and key for this SSL object. It will not be reset by
       calling SSL_clear(3).  If the callback returns no certificate, the OpenSSL library will
       not send a certificate.

RETURN VALUES

       SSL_CTX_get_client_cert_cb() returns function pointer of client_cert_cb() or NULL if the
       callback is not set.

BUGS

       The client_cert_cb() cannot return a complete certificate chain, it can only return one
       client certificate. If the chain only has a length of 2, the root CA certificate may be
       omitted according to the TLS standard and thus a standard conforming answer can be sent to
       the server. For a longer chain, the client must send the complete chain (with the option
       to leave out the root CA certificate). This can only be accomplished by either adding the
       intermediate CA certificates into the trusted certificate store for the SSL_CTX object
       (resulting in having to add CA certificates that otherwise maybe would not be trusted), or
       by adding the chain certificates using the SSL_CTX_add_extra_chain_cert(3) function, which
       is only available for the SSL_CTX object as a whole and that therefore probably can only
       apply for one client certificate, making the concept of the callback function (to allow
       the choice from several certificates) questionable.

       Once the SSL object has been used in conjunction with the callback function, the
       certificate will be set for the SSL object and will not be cleared even when SSL_clear(3)
       is being called. It is therefore mandatory to destroy the SSL object using SSL_free(3) and
       create a new one to return to the previous state.

SEE ALSO

       ssl(7), SSL_CTX_use_certificate(3), SSL_CTX_add_extra_chain_cert(3),
       SSL_get_client_CA_list(3), SSL_clear(3), SSL_free(3)

COPYRIGHT

       Copyright 2002-2018 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>.