SSL_CTX_set_client_cert_cb, SSL_CTX_get_client_cert_cb - handle client
       certificate callback function

        #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);

       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.

       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.

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

       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)

       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.

       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 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

1.1.1l                             2021-08-24      SSL_CTX_SET_CLIENT_CERT_CB(3)