code: def sign_ssh_challenge(self, blob, identity):
          
          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)
docs: Sign given blob using a private key on the device.