NAME    [Toc]    [Back]

       SSL_CTX_set_client_cert_cb,  SSL_CTX_get_client_cert_cb  -
       Handle client certificate callback function

SYNOPSIS    [Toc]    [Back]

       #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    [Toc]    [Back]

       The   SSL_CTX_set_client_cert_cb()   function   sets   the
       client_cert_cb() callback that is  called  when  a  client
       certificate  is  requested  by a server and no certificate
       was  set for the  SSL  object.  When  client_cert_cb()  is
       NULL,     no    callback    function    is    used.    The
       SSL_CTX_get_client_cert_cb()function returns a pointer  to
       the  currently set callback function. The 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 WARNINGS sections.) If no  certificate is  set,  ``0''
       must  be returned and no certificate will be sent. A negative
 return value  will  suspend  the  handshake  and  the
       handshake    function   will   return   immediately.   The
       SSL_get_error()        function        will         return
       SSL_ERROR_WANT_X509_LOOKUP to  indicate that the handshake
       was suspended. The next call to  the   handshake  function
       will  lead to the call of client_cert_cb(). It  is the job
       of the  client_cert_cb()  function  to  store  information
       about the state of the last call, if required to continue.

NOTES    [Toc]    [Back]

       During a handshake (or renegotiation) a server may request
       a  certificate  from the client. A client certificate must
       only be sent when the server sent a request. When  a  certificate
  was set using the SSL_CTX_use_certificate() family
 of functions, it will be  sent to the server. The  TLS
       standard  requires  that  a certificate is sent only 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  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(). If the  callback
       returns  no certificate, the OpenSSL library will not send
       a certificate.

WARNINGS    [Toc]    [Back]

       The client_cert_cb() function  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  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()function.                The
       SSL_CTX_add_extra_chain_cert()function  is  only available
       for the SSL_CTX object as a whole and  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() is called.  It is  mandatory
       to  destroy  the  SSL object using SSL_free() and create a
       new one to return to the previous state.

SEE ALSO    [Toc]    [Back]

       Files:         ssl(3),         SSL_CTX_use_certificate(3),
       SSL_CTX_add_extra_chain_cert(3),                      (3),
       SSL_get_client_CA_list(3), SSL_clear(3), SSL_free(3)



                                    SSL_CTX_set_client_cert_cb(3)