int index = offset + SIGNATURE_OFFSET;
        for ( int i = 0; i < SIGNATURE_LENGTH; i++ )
            data[ index + i ] = 0;

        // set signed flag
        int oldFlags = SMBUtil.readInt4(data, offset + 16);
        int flags = oldFlags | ServerMessageBlock2.SMB2_FLAGS_SIGNED;
        SMBUtil.writeInt4(flags, data, offset + 16);

        this.digest.update(data, offset, length);