using System;

public class Program

{

    public static void Main()

    {

    string key = "helloworld";

    string input = "thisisasecretmessage";

    string encrypted = VigenereCipher.Encipher(input, key);

    Console.WriteLine(encrypted); // lweoh``uaqjtpid`rccs

    string decrypted = VigenereCipher.Decipher(encrypted, key);

    Console.WriteLine(decrypted); // thisisasecretmessage

    }

}

​

public class VigenereCipher

{

    public static string Encipher(string input, string key)

    {

        string output = string.Empty;

        for (int i = 0, j = 0; i < input.Length; i++)

        {

            char c = input[i];

            if (c < 'A' || c > 'Z' && c < 'a' || c > 'z')

                continue;

​

            output += (char)((c & 96) + (c - 'A' + key[j] - 'A') % 26);

            j = ++j % key.Length;

        }

        return output;

    }

​

    public static string Decipher(string input, string key)

    {

        string output = string.Empty;

        for (int i = 0, j = 0; i < input.Length; i++)

        {

            char c = input[i];

            if (c < 'A' || c > 'Z' && c < 'a' || c > 'z')

                continue;

​

            output += (char)((c & 96) + (c - 'A' - (key[j] - 'A') + 26) % 26);

            j = ++j % key.Length;

        }

        return output;

    }

}