Crisis.Ionic.Zip.ZipEntry.ProcessExtraFieldWinZipAes C# (CSharp) Method

        private int ProcessExtraFieldWinZipAes(byte[] buffer, int j, Int16 dataSize, long posn)
        {
            if (this._CompressionMethod == 0x0063)
            {
                if ((this._BitField & 0x01) != 0x01)
                    throw new BadReadException(String.Format("  Inconsistent metadata at position 0x{0:X16}", posn));

                this._sourceIsEncrypted = true;

                //this._aesCrypto = new WinZipAesCrypto(this);
                // see spec at http://www.winzip.com/aes_info.htm
                if (dataSize != 7)
                    throw new BadReadException(String.Format("  Inconsistent size (0x{0:X4}) in WinZip AES field at position 0x{1:X16}", dataSize, posn));

                this._WinZipAesMethod = BitConverter.ToInt16(buffer, j);
                j += 2;
                if (this._WinZipAesMethod != 0x01 && this._WinZipAesMethod != 0x02)
                    throw new BadReadException(String.Format("  Unexpected vendor version number (0x{0:X4}) for WinZip AES metadata at position 0x{1:X16}",
                        this._WinZipAesMethod, posn));

                Int16 vendorId = BitConverter.ToInt16(buffer, j);
                j += 2;
                if (vendorId != 0x4541)
                    throw new BadReadException(String.Format("  Unexpected vendor ID (0x{0:X4}) for WinZip AES metadata at position 0x{1:X16}", vendorId, posn));

                int keystrength = (buffer[j] == 1) ? 128 : (buffer[j] == 3) ? 256 : -1;
                if (keystrength < 0)
                    throw new BadReadException(String.Format("Invalid key strength ({0})", keystrength));

                _Encryption_FromZipFile = this._Encryption = (keystrength == 128)
                    ? EncryptionAlgorithm.WinZipAes128
                    : EncryptionAlgorithm.WinZipAes256;

                j++;

                // set the actual compression method
                this._CompressionMethod_FromZipFile =
                this._CompressionMethod = BitConverter.ToInt16(buffer, j);
                j += 2; // for the next segment of the extra field
            }
            return j;
        }