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

     When cclliieenntt_cceerrtt_ccbb(()) 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.

1.0.2t               Last change: 2019-09-10                    1


     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.


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

1.0.2t               Last change: 2019-09-10                    2

See also SSL_CTX_get_client_cert_cb(3)

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