GPGAuth stage0

def server_identity_is_verified(self):
        """ GPGAuth stage0 """
        # Encrypt a uuid token for the server
        server_verify_token = self.gpg.encrypt(self._nonce0,
                                               self.server_fingerprint, always_trust=True)
        if not server_verify_token.ok:
            raise GPGAuthStage0Exception(
                'Encryption of the nonce0 (%s) '
                'to the server fingerprint (%s) failed.' %
                (self._nonce0, self.server_fingerprint)
            )

        server_verify_response = post_server_verify_token(
            self,
            keyid=self.user_fingerprint,
            server_verify_token=str(server_verify_token)
        )

        if not check_server_verify_response(server_verify_response):
            raise GPGAuthStage0Exception("Verify endpoint wrongly formatted")

        if server_verify_response.headers.get('X-GPGAuth-Verify-Response') != self._nonce0:
            raise GPGAuthStage0Exception(
                'The server decrypted something different than what we sent '
                '(%s <> %s)' %
                (server_verify_response.headers.get('X-GPGAuth-Verify-Response'), self._nonce0))
        logger.info('server_identity_is_verified: OK')
        return True