'Returns the key authorization for the given token
https://tools.ietf.org/html/draft-ietf-acme-acme-02#section-7.1'

def get_keyauthorization(self, token):

    accountkey_json = json.dumps(self.jws_header['jwk'], sort_keys=True, separators=(',', ':'))
    thumbprint = nopad_b64(hashlib.sha256(accountkey_json.encode('utf8')).digest())
    return '{0}.{1}'.format(token, thumbprint)
