具有密钥和二进制文件，如何反向/解密流加密？

// I have all these fields (filled correctly)

    public byte[] Keychain;
    public uint Step, Mul, HeaderXor, Key;

public void Decrypt(byte[] packet) {
        fixed (byte* pp = packet, pk = Keychain) {
            uint size = (uint)GetPacketSize(packet);
            uint header = (first) ? /* Checks if it is a partial packet (It isnt!)*/
                0x000eb7e2 :
                *((uint*)&pp[0]) ^ HeaderXor; 
                // HeaderXor is an unsigned int
                // It also changes after each decryption and if the key changes

            if (first) 
                first = false;

            uint token = *((uint*)&pp[0]);
            *((uint*)&pp[0]) = header;
            token &= 0x3FFF;    // Get only last 14 bits
            token *= Mul * 4;   
            // Mul is an unsigned int and changes sometimes
            token = *((uint*)&pk[token]);

            uint i, r, t;
            size -= r = (size - 8) & 3; // Make size dividable by 4

            for (i = 8; i < size; i += 4) {
                t = *((uint*)&pp[i]);
                token ^= t;
                *((uint*)&pp[i]) = token;

                t &= 0x3FFF;
                token = *((uint*)&pk[t * Mul * 4]);
            }

            t = 0xFFFFFFFF >> 8 * (4 - (int)r);
            token &= t;
            *((uint*)&pp[i]) ^= token; // If something is left over ( if size - 8 == 5 then size & 3 has rest of 1)
            * ((uint*)&pp[4]) = 0;

            Step++;
            Step &= 0x3FFF;
            HeaderXor = *((uint*)&pk[Step * Mul * 4]);
        }
    }

// Encrypted data
// 5b 54 34 23
// cc c2 5a a3
// 81 7e d6 27
// 36 c4 8f 36
// b9 3b 6f ce
// f4 8e 72 5b
// 
// Decrypted data
// e2 b7 18 00
// 00 00 00 00
// be 00 56 00
// 2f 00 58 00
// 30 00 59 00
// 31 00 01 00