SSL_CTX_set_client_cert_cb(3OpenSSSSL_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 argu-
       ments 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 hand-
       shake 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 certifi-
       cate must only be sent, when the server did send the
       request.

       When a certificate was set using the SSL_CTX_use_cer-
       tificate(3) family of functions, it will be sent to the
       server. The TLS standard requires that only a certifi-
       cate 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 call-
       back 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 cer-
       tificate, the OpenSSL library will try to load the pri-
       vate 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 cer-
       tificate 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 cer-
       tificate.

BUGS
       The client_cert_cb() cannot return a complete certifi-
       cate chain, it can only return one client certificate.
       If the chain only has a length of 2, the root CA cer-
       tificate 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)



0.9.7c                     2002-06SSL_CTX_set_client_cert_cb(3)
