def sign_ssh_challenge(self, blob, identity):
        """Sign given blob using a private key on the device."""
        msg = _parse_ssh_blob(blob)
        log.debug('%s: user %r via %r (%r)',
                  msg['conn'], msg['user'], msg['auth'], msg['key_type'])
        log.debug('nonce: %r', msg['nonce'])
        fp = msg['public_key']['fingerprint']
        log.debug('fingerprint: %s', fp)
        log.debug('hidden challenge size: %d bytes', len(blob))

        log.info('please confirm user "%s" login to "%s" using %s...',
                 msg['user'].decode('ascii'), identity.to_string(),
                 self.device)

        with self.device:
            return self.device.sign(blob=blob, identity=identity)
Sign given blob using a private key on the device.