RC4

using System;
using System.Collections.Generic;
using System.Linq;

namespace Шифр_Вернана
{
    class Program
    {
        static void Main(string[] args)
        {
            var msg = "hello, world";
            var key = "key";

            var rc4 = new RC4(Encoding.ASCII.GetBytes(key));

            var cryptedMsg = rc4.Crypt(Encoding.ASCII.GetBytes(msg));

            Console.WriteLine(Encoding.ASCII.GetString(cryptedMsg));

            var deCryptedMsg = rc4.DeCrypt(cryptedMsg);

            Console.WriteLine(Encoding.ASCII.GetString(deCryptedMsg));

        }


    }

    class RC4
    {
        private byte[] S = new byte[256];

        private byte[] Key;

        public RC4(byte[] key)
        {
            Key = key.Clone() as byte[];
            InitS();
        }

        public RC4()
        {
            for (byte i = 0; i < S.Length; i++) S[i] = i;
        }

        public byte GetNewKey()
        {
           byte i = 0;
           byte j = 0;
           i = (byte)((i + 1) % 256);
           j = (byte)((j + S[i]) % 256);
           Swap(ref S[i], ref S[j]);
           var t = (S[i] + S[j]) % 256;
           return   S[t];

        }
        public void InitS()
        {
            for (int i = 0; i < 256; i++) S[i] = (byte)i;

            int j = 0;

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

        }

        public byte[] Crypt(byte[] arr)=>
          (from item in arr select (byte)(item ^ GetNewKey())).ToArray();

        public byte[] DeCrypt(byte [] msg)
        {
            InitS();
            return Crypt(msg);
        }

        public static void Swap(ref byte left, ref byte right)
        {
            left ^= right;
            right ^= left;
            left ^= right;
        }

    }
}