using System;

​

public class EncryptionProgram

{

    static void Main()

    {

        Console.WriteLine("Welcome to the Encryption Program!");

        Console.WriteLine("---------------------------------");

​

        while (true)

        {

            Console.WriteLine("Choose an option:");

            Console.WriteLine("1. Encrypt a message");

            Console.WriteLine("2. Decrypt a message");

            Console.WriteLine("3. Exit");

            Console.Write("Enter your choice (1, 2, or 3): ");

​

            int choice;

            if (!int.TryParse(Console.ReadLine(), out choice))

            {

                Console.WriteLine("Invalid choice. Please try again.");

                continue;

            }

​

            if (choice == 1)

            {

                Console.Write("Enter the message to encrypt: ");

                string message = Console.ReadLine();

                string encryptedMessage = Encrypt(message);

                Console.WriteLine("Encrypted message: " + encryptedMessage);

            }

            else if (choice == 2)

            {

                Console.Write("Enter the message to decrypt: ");

                string message = Console.ReadLine();

                string decryptedMessage = Decrypt(message);

                Console.WriteLine("Decrypted message: " + decryptedMessage);

            }

            else if (choice == 3)

            {

                Console.WriteLine("Exiting the program. Goodbye!");

                break;

            }

            else

            {

                Console.WriteLine("Invalid choice. Please try again.");

            }

​

            Console.WriteLine();

        }

    }

​

    static private string Encrypt(string message)

    {

        return ShiftMessage(message, 9);

    }

​

    static private string Decrypt(string encryptedMessage)

    {

        return ShiftMessage(encryptedMessage, -9);

    }

​

    static private string ShiftMessage(string message, int shift)

    {

        string processedMessage = "";

        foreach (char c in message)

        {

            char shiftedChar = ShiftChar(c, shift);

            processedMessage += shiftedChar;

        }

        return processedMessage;

    }

​

    static private char ShiftChar(char c, int shift)

    {

        if (!char.IsLetter(c))

        {

            return c; // If the character is not a letter, return it as it is

        }

​

        char baseChar = char.IsUpper(c) ? 'A' : 'a'; // Determine the base character ('A' or 'a')

        int alphabetSize = 26; // Size of the alphabet

​

        // Apply the shift by finding the new character position and wrapping around the alphabet

        int newPosition = (c - baseChar + shift + alphabetSize) % alphabetSize;

        char shiftedChar = (char)(baseChar + newPosition);

​

        return shiftedChar;

    }

}