RC4(компактно и понятно)

using System.Linq;

namespace RC4_compact_
{
    class RC4
    {
        static public byte[] EnCrypt(byte [] msg , byte[] key)
        {
            int j = 0;
            int i = 0;

            var S = (from n in Enumerable.Range(0, 256) select (byte)n).ToArray();

            var encrypted = new byte[msg.Length];

            for (i = 0; i < 256; i++)
            {
                  j = (j + S[i] + key[i % key.Length]) % 256;
                  Swap(ref S[i] ,ref S[j]);
            }

            i = j = 0;

            for (int r = 0; r < msg.Length; r++)
            {
                i = (i + 1) % 256;
                j = (j + S[i]) % 256;
                Swap(ref S[i], ref S[j]);
                encrypted[r] = (byte) (msg[r] ^ S[(S[i] + S[j]) % 256]);
            }

            return encrypted;
        }

        static public byte[] DeCrypt(byte[] msg, byte[] key) =>
            EnCrypt(msg, key);
        static void Swap(ref byte a, ref byte b)
        {
            a ^= b;b ^= a;a ^= b;
        }
    }
}