<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def pack_key(self, owner_id='', kid=''):
 """ 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 """

        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]