* The signature field of the SMB is overwritted with the sequence number;
     * The MD5 digest of the MAC signing key + the entire SMB is taken;
     * The first 8 bytes of this are placed in the signature field.
     *
     * @param data The data.
     * @param offset The starting offset at which the SMB header begins.
     * @param length The length of the SMB data starting at offset. 
     */