byte[] sk = this.signKey;
        if ( sk == null ) {
            throw new CIFSException("Signing is not initialized");
        }

        int seqNum = this.signSequence.getAndIncrement();
        byte[] seqBytes = new byte[4];
        SMBUtil.writeInt4(seqNum, seqBytes, 0);

        MessageDigest mac = Crypto.getHMACT64(sk);
        mac.update(seqBytes); // sequence