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SSL_CTX_set_client_cert_cb(3)                      OpenSSL                     SSL_CTX_set_client_cert_cb(3)



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 immediatly. 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.

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(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)



50                                               2013-03-05                    SSL_CTX_set_client_cert_cb(3)