WhatsApp Desktop Client Code

using System;
using System.Collections.Generic;
using System.Linq;
using System.Security.Cryptography;
using System.Text;

static class Encryption
{
    public static RC4 encryptionOutgoing = null;
    public static RC4 encryptionIncoming = null;

    public static byte[] WhatsappEncrypt(byte[] key, byte[] data, bool appendHash)
    {
        if (encryptionOutgoing == null)
            encryptionOutgoing = new RC4(key, 256);
        HMACSHA1 h = new HMACSHA1(key);
        byte[] buff = new byte[data.Length];
        Buffer.BlockCopy(data, 0, buff, 0, data.Length);

        encryptionOutgoing.Cipher(buff);
        byte[] hashByte = h.ComputeHash(buff);
        byte[] response = new byte[4 + buff.Length];
        if (appendHash)
        {
            Buffer.BlockCopy(buff, 0, response, 0, buff.Length);
            Buffer.BlockCopy(hashByte, 0, response, buff.Length, 4);
        }
        else
        {
            Buffer.BlockCopy(hashByte, 0, response, 0, 4);
            Buffer.BlockCopy(buff, 0, response, 4, buff.Length);
        }

        return response;
    }

    public static byte[] WhatsappDecrypt(byte[] key, byte[] data)
    {
        if (encryptionIncoming == null)
            encryptionIncoming = new RC4(key, 256);
        byte[] buff = new byte[data.Length];
        Buffer.BlockCopy(data, 0, buff, 0, data.Length);
        encryptionIncoming.Cipher(buff);
        return buff;
    }
}