code: def pack_key(self, owner_id='', kid=''):
          
          keys = pick_key(self.my_keys(owner_id, 'sig'), 'sig', alg=self.alg,
                          kid=kid)
  
          if not keys:
              raise NoSuitableSigningKeys('kid={}'.format(kid))
  
          return keys[0]
docs: Find a key to be used for signing the Json Web Token
  
          :param owner_id: Owner of the keys to chose from
          :param kid: Key ID
          :return: One key