'Generate validation.
:param JWK account_key:
:param OpenSSL.crypto.PKey cert_key: Optional private key used
in certificate generation. If not provided (``None``), then
fresh key will be generated.
:rtype: `tuple` of `OpenSSL.crypto.X509` and `OpenSSL.crypto.PKey`'

def validation(self, account_key, **kwargs):

    return self.response(account_key).gen_cert(key=kwargs.get('cert_key'))
