using System;

class MainClass
{
    static void displaymenu()
    {
        Console.WriteLine("Menu");
        Console.WriteLine("1) Encrypt.");
        Console.WriteLine("2) Decrypt.");
        Console.WriteLine("3) Quit.");
    }

    static int GetInteger(int min, int max, string Prompt)
    {
        int value = 0;
        do
        {
            Console.Write("\n{0}", Prompt);
            try
            {
                value = Convert.ToInt32(Console.ReadLine());
                if (value < 1 || value > 25)
                {
                    Console.WriteLine("Please enter a value between {0} and {1}.", min, max);
                }
            }
            catch
            {
                Console.WriteLine("Please enter a whole number between {0} and {1},", min, max);
            }
        } while (value < 1 || value > 25);
        return value;
    }
    static string GetMessage(string Prompt)
    {
        string message = "";
        do
        {
            Console.Write("\n{0}", Prompt);
            message = Console.ReadLine();
            if (message == "")
            {
                Console.WriteLine("Please enter a message");
            }
        } while (message == "");
        return message;
    }
    static string Encode(string message, int key)
    {
        int ASCIICode, LetterNumber;
        string NewMessage = "";
        for (int i = 0; i < message.Length; i++)
        {
            if (message[i] >= 'a' && message[i] <= 'z')
            {
                LetterNumber = (int)message[i] - 97;
                LetterNumber = (LetterNumber + key) % 26;
                if (LetterNumber < 0) LetterNumber += 26;
                ASCIICode = LetterNumber + 97;
                NewMessage += Convert.ToChar(ASCIICode);
            }
            else if (message[i] >= 'A' && message[i] <= 'Z')
            {
                LetterNumber = (int)message[i] - 65;
                LetterNumber = (LetterNumber + key) % 26;
                if (LetterNumber < 0) LetterNumber += 26;
                ASCIICode = LetterNumber + 65;
                NewMessage += Convert.ToChar(ASCIICode);
            }
            else
                NewMessage += message[i];
        }
        return NewMessage;
    }
    static void Main(string[] args)
    {
        int MenuChoice = 0;
        string message, NewMessage;
        int key;
        do
        {
            displaymenu();
            MenuChoice = GetInteger(1, 3, "Enter menu option: ");
            switch (MenuChoice)
            {
                case 1:
                    message = GetMessage("Enter message to be Encrypted: ");
                    key = GetInteger(1, 25, "Enter key used: ");
                    NewMessage = Encode(message, key);
                    Console.WriteLine("\nEncrypted message: {0}", NewMessage);
                    break;
                case 2:
                    message = GetMessage("Enter message to be decrypted: ");
                    key = GetInteger(1, 25, "Enter key used: ");
                    NewMessage = Encode(message, -key);
                    Console.WriteLine("\nDecrypted message: {0}", NewMessage);
                    break;
            }
        } while (MenuChoice != 3);
    }
}