This is a parallel implementation of the symmetric substitution cypher ATBASH in C#.

ATBASH.cs

#region
 
using System;
using System.Linq;
using OpenMetaverse;
 
#endregion
 
namespace AtBash
{
    internal class Program
    {
        private static void Main(string[] args)
        {
            // Prints out "Hello there! Good day!" on the console.
            Console.WriteLine(wasATBASH("Svool gsviv! Tllw wzb!"));
            Console.ReadKey();
        }
 
        ///////////////////////////////////////////////////////////////////////////
        //  Copyright (C) Wizardry and Steamworks 2015 - License: GNU GPLv3      //
        ///////////////////////////////////////////////////////////////////////////
        /// <summary>
        ///     An implementation of the ATBASH cypher for latin alphabets.
        /// </summary>
        /// <param name="data">the data to encrypt or decrypt</param>
        /// <returns>the encrypted or decrypted data</returns>
        private static string wasATBASH(string data)
        {
            char[] a =
            {
                'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm',
                'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z'
            };
 
            char[] input = data.ToArray();
 
            Parallel.ForEach(Enumerable.Range(0, data.Length), i =>
            {
                char e = input[i];
                if (!char.IsLetter(e)) return;
                int x = 25 - Array.BinarySearch(a, char.ToLowerInvariant(e));
                if (!char.IsUpper(e))
                {
                    input[i] = a[x];
                    return;
                }
                input[i] = char.ToUpperInvariant(a[x]);
            });
 
            return new string(input);
        }
    }
}