private function signingKey()
    {
        if (isset($this->signingKeyId)) {
            return $this->keyStore()->fetch($this->signingKeyId);
        } else {
            throw new Exception('no implicit or specified signing key');
        }
    }
private function signingKey()
    {
        if (isset($this->signingKeyId)) {
            return $this->keyStore()->fetch($this->signingKeyId);
        } else {
            throw new Exception('no implicit or specified signing key');
        }
    }
@return Key

@throws Exception
@throws KeyStoreException