public function signMessage(Swift_Message $message)
    {
        if (null === $this->signCertificate && null === $this->encryptCert) {
            return $this;
        }

        if ($this->signThenEncrypt) {
            $this->smimeSignMessage($message);
            $this->smimeEncryptMessage($message);
        } else {
            $this->smimeEncryptMessage($message);
            $this->smimeSignMessage($message);
        }
    }