SSL_CTX_set_client_cert_cb(3)

     SSL_CTX_set_client_cert<I>00.9.6h (2002-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 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
          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.

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     SSL_CTX_set_client_cert<I>00.9.6h (2002-SSL_CTX_set_client_cert_cb(3)

          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)

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