using System;

public class Program

{

    static public string Encrypt(string a)

        {   char[] temp = a.ToCharArray();

        for (int i = 0; i < a.Length; i++)

        {

            if (temp[i] == 'A' || temp[i] == 'a')

                temp[i] = 'P';

            else if 

                (temp[i] == 'B'|| temp[i] == 'b')

                temp[i] = 'R';

            else if 

                (temp[i] == 'C'|| temp[i] == 'c')

                temp[i] = 'I';

            else if 

                (temp[i] == 'D'|| temp[i] == 'd')

                temp[i] = 'z';

            else if 

                (temp[i] == 'E'|| temp[i] == 'e')

                temp[i] = 'E';

            else if 

                (temp[i] == 'F'|| temp[i] == 'f')

                temp[i] = 'A';

            else if 

                (temp[i] == 'G'|| temp[i] == 'g')

                temp[i] = 'B';

            else if 

                (temp[i] == 'H'|| temp[i] == 'h')

                temp[i] = 'C';

            else if 

                (temp[i] == 'I'|| temp[i] == 'i')

                temp[i] = 'D';

            else if 

                (temp[i] == 'J'|| temp[i] == 'j')

                temp[i] = 'F';

            else if 

                (temp[i] == 'K'|| temp[i] == 'k')

                temp[i] = 'G';

            else if 

                (temp[i] == 'L'|| temp[i] == 'l')

                temp[i] = 'H';

            else if 

                (temp[i] == 'M'|| temp[i] == 'm')

                temp[i] = 'J';

            else if 

                (temp[i] == 'N'|| temp[i] == 'n')

                temp[i] = 'K';

            else if 

                (temp[i] == 'O'|| temp[i] == 'o')

                temp[i] = 'L';

            else if 

                (temp[i] == 'P'|| temp[i] == 'p')

                temp[i] = 'M';

            else if 

                (temp[i] == 'Q'|| temp[i] == 'q')

                temp[i] = 'N';

            else if 

                (temp[i] == 'R'|| temp[i] == 'r')

                temp[i] = 'O';

            else if 

                (temp[i] == 'S'|| temp[i] == 's')

                temp[i] = 'Q';

            else if 

                (temp[i] == 'T'|| temp[i] == 't')

                temp[i] = 'S';

            else if 

                (temp[i] == 'U'|| temp[i] == 'u')

                temp[i] = 'T';

            else if 

                (temp[i] == 'V'|| temp[i] == 'v')

                temp[i] = 'U';

            else if 

                (temp[i] == 'W'|| temp[i] == 'w')

                temp[i] = 'V';

            else if 

                (temp[i] == 'X'|| temp[i] == 'x')

                temp[i] = 'W';

            else if 

                (temp[i] == 'Y'|| temp[i] == 'y')

                temp[i] = 'X';

            else if 

                (temp[i] == 'Z'|| temp[i] == 'z')

                temp[i] = 'Y';

        }

        string b = new string(temp);

          return b;

        }

    static public string Decrypt(string c)

        {   char[] temp2 = c.ToCharArray();

        for (int i = 0; i < c.Length; i++)

        {

            if (temp2[i] == 'P'|| temp2[i] == 'p')

                temp2[i] = 'A';

            else if 

                (temp2[i] == 'R'|| temp2[i] == 'r')

                temp2[i] = 'B';

            else if 

                (temp2[i] == 'I'|| temp2[i] == 'i')

                temp2[i] = 'C';

            else if 

                (temp2[i] == 'Z'|| temp2[i] == 'z')

                temp2[i] = 'D';

            else if 

                (temp2[i] == 'E'|| temp2[i] == 'e')

                temp2[i] = 'E';

            else if 

                (temp2[i] == 'A'|| temp2[i] == 'a')

                temp2[i] = 'F';

            else if 

                (temp2[i] == 'B'|| temp2[i] == 'b')

                temp2[i] = 'G';

            else if 

                (temp2[i] == 'C'|| temp2[i] == 'c')

                temp2[i] = 'H';

            else if 

                (temp2[i] == 'D'|| temp2[i] == 'd')

                temp2[i] = 'I';

            else if 

                (temp2[i] == 'F'|| temp2[i] == 'f')

                temp2[i] = 'J';

            else if 

                (temp2[i] == 'G'|| temp2[i] == 'g')

                temp2[i] = 'K';

            else if 

                (temp2[i] == 'H'|| temp2[i] == 'h')

                temp2[i] = 'L';

            else if 

                (temp2[i] == 'J'|| temp2[i] == 'j')

                temp2[i] = 'M';

            else if 

                (temp2[i] == 'K'|| temp2[i] == 'k')

                temp2[i] = 'N';

            else if 

                (temp2[i] == 'L'|| temp2[i] == 'l')

                temp2[i] = 'O';

            else if 

                (temp2[i] == 'M'|| temp2[i] == 'm')

                temp2[i] = 'P';

            else if 

                (temp2[i] == 'N'|| temp2[i] == 'n')

                temp2[i] = 'Q';

            else if 

                (temp2[i] == 'O'|| temp2[i] == 'o')

                temp2[i] = 'R';

            else if 

                (temp2[i] == 'Q'|| temp2[i] == 'q')

                temp2[i] = 'S';

            else if 

                (temp2[i] == 'S'|| temp2[i] == 's')

                temp2[i] = 'T';

            else if 

                (temp2[i] == 'T'|| temp2[i] == 't')

                temp2[i] = 'U';

            else if 

                (temp2[i] == 'U'|| temp2[i] == 'u')

                temp2[i] = 'V';

            else if 

                (temp2[i] == 'V'|| temp2[i] == 'v')

                temp2[i] = 'W';

            else if 

                (temp2[i] == 'W'|| temp2[i] == 'w')

                temp2[i] = 'X';

            else if 

                (temp2[i] == 'X'|| temp2[i] == 'x')

                temp2[i] = 'Y';

            else if 

                (temp2[i] == 'Y'|| temp2[i] == 'y')

                temp2[i] = 'Z';

        }

        string d = new string(temp2);

          return d;

        }

    static public void Main()

    {

        string choice = "This value does not matter";

         while(choice != "q" && choice != "Q")

         {Console.WriteLine("Would you like to encrypt or decrypt?");

        string temp = Console.ReadLine();

        if(temp == "Encrypt" || temp == "encrypt")

        {Console.WriteLine("What would you like to encrypt?");

        string encrypt = Console.ReadLine();

         Console.WriteLine("Your enrypted message is: " + Encrypt(encrypt));

        }

        else if(temp == "Decrypt" || temp == "decrypt")

        {Console.WriteLine("What would you like to decrypt?");

        string decrypt = Console.ReadLine();

         Console.WriteLine("Your decrypted message is: " + Decrypt(decrypt));

        }

          {Console.WriteLine("Type anything to restart the program, or type q to quit the program");

     choice = Console.ReadLine();

         Console.Clear();}

        }

    }

}

​