//Programmer Joe Zeng

//DATE 5/20/2025 - 5/21/2025

//Program name: U9 Simple Cryptography

//Description: Using ASCII value in coding

using System;

​

public class Program

{

    public static string Decrypt(string input2)//Method to decrypt a method.

    {

        string decrypted = "";//put it into string and change it after progress.

        foreach (char c in input2)//check every letter/number/char in the second input

        {

            if ((c >= 'A' && c <= 'Z'))//if the letter range is in A to Z.

            {

                char nC = (char)(c - 3);//move the letter left 3, and make this value as nC

                decrypted += nC;//make decrypted be same as nC every loop, exept non-letters

            }

            else if ((c >= 'a' && c <= 'z')) //lowercase letters, do  same thing as uppercase one.

            {

                char nC = (char)(c - 3);

                decrypted += nC;

            }

            else

            {

                decrypted += c; 

            }

        }

        return decrypted;

    }

    public static string Encrypt(string m){//method to encrypt

        string encrypted = "";

     foreach (char c in m)//same as decrypt part, but will move letter right 3 instead of left.

        {

            if ((c >= 'A' && c <= 'Z')) 

            {

                char nC = (char)(c + 3);

                if (nC > 'Z')

                {

                    nC = (char)(nC - 26); //+ 26 will become to -26

                }

                encrypted +=nC;

            }

            else if ((c >= 'a' && c <= 'z')) // lowercase letters

            {

                char nC = (char)(c + 3);

                if (nC > 'z')

                {

                    nC = (char)(nC - 26); // wrap around

                }

                 encrypted +=nC;

            }

            else

            {

                 encrypted += c;

            }

        }

     return encrypted;

     }

​

    public static void Main()

    {

    start://why go back to here after program ends.

        Console.WriteLine("Welcome to U9A3 - Encryption APP! Enter in a secret message:");

        string m = Console.ReadLine();//m is encrypt input

        string encrypted = Encrypt(m);

        Console.WriteLine("The message after Encrypted is:");

        Console.WriteLine(encrypted);

​

        Console.WriteLine(); //new line

​

        //Decrypt part

        Console.WriteLine("Now input a message to decrypt:");

        string input2 = Console.ReadLine();

        string decrypted = Decrypt(input2);

        ;

        Console.WriteLine("The message after decryption is:");

        Console.WriteLine(decrypted);

        goto start;

    }

}

​