'Generate validation.
:param .JWK account_key: Private account key.
:param .JWA alg:
:returns: This challenge wrapped in `.JWS`
:rtype: .JWS'

def gen_validation(self, account_key, alg=jose.RS256, **kwargs):

    return jose.JWS.sign(payload=self.json_dumps(sort_keys=True).encode('utf-8'), key=account_key, alg=alg, **kwargs)
