Message from Julius Caesar

Below is the complete code for the Message from Julius Caesar Try It Out! problem.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace CSharpBook.Examples.CaesarCipher
{
    class Program
    {
        static void Main(string[] args)
        {
            // PART 1: Basic encryption and decryption
            string message = "Hello World!";
            int key = 3;
            
            string encrypted = Encrypt(message, key);
            Console.WriteLine(encrypted);

            string decrypted = Decrypt(encrypted, key);
            Console.WriteLine(decrypted);

            Console.ReadKey();

            // PART 2: Cracking the Caesar cipher with an unknown key by
            // trying all possible keys. One of them will be readable, the
            // rest will be nonsense.
            string encryptedMessageToHack = "UX LNKX MH WKBGD RHNK HOTEMBGX";
            for (int index = 0; index <= 26; index++)
            {
                Console.WriteLine(Decrypt(encryptedMessageToHack, index));
            }

            Console.ReadKey();
        }

        /// <summary>
        /// Encrypts a complete message using a Caesar cipher with a given key.
        /// </summary>
        private static string Encrypt(string message, int key)
        {
            message = message.ToUpper();

            string encryptedMessage = "";

            for (int index = 0; index < message.Length; index++)
            {
                char encryptedLetter = Encrypt(message[index], key);
                encryptedMessage += encryptedLetter;
            }

            return encryptedMessage;
        }

        /// <summary>
        /// Encrypts a single letter with the given key using a Caesar cipher.
        /// </summary>
        private static char Encrypt(char letter, int key)
        {
            // Skip encryption of non-alphabetical letters.
            if (letter < 'A' || letter > 'Z') { return letter; }

            // Figure out what letter of the alphabet this one is.
            int letterAsNumber = (int)letter - 'A';

            // Do the actual encryption
            int encryptedLetter = (letterAsNumber + key) % 26;

            // Turn back into a letter
            return (char)(encryptedLetter + 'A');
        }

        /// <summary>
        /// Decrypts a complete message using a Caesar cipher with a given key.
        /// </summary>
        private static string Decrypt(string message, int key)
        {
            // We don't want to be worried about case issues, so we'll make it all 
            // upper case.
            message = message.ToUpper();

            // Start with an empty message.
            string encryptedMessage = "";

            // Convert one letter at a time and tack it on the end of our message.
            for (int index = 0; index < message.Length; index++)
            {
                char encryptedLetter = Decrypt(message[index], key);
                encryptedMessage += encryptedLetter;
            }

            return encryptedMessage;
        }

        /// <summary>
        /// Decrypts a single letter, using a Caesar cipher with the given
        /// key.
        /// </summary>
        private static char Decrypt(char letter, int key)
        {
            // Skip decryption of non-alphabetical letters.
            if (letter < 'A' || letter > 'Z') { return letter; }

            // Figure out what letter of the alphabet this one is.
            int letterAsNumber = (int)letter - 'A';

            // Do the actual decryption.
            // Depending on the letter and the key, this could go negative.
            // To compensate, we add the number of letters in the alphabet.
            int encryptedLetter = (letterAsNumber - key + 26) % 26;

            // Turn back into a letter
            return (char)(encryptedLetter + 'A');
        }
    }
}

The correct decoded message is:

BE SURE TO DRINK YOUR OVALTINE

Which comes from a well known movie: A Christmas Story