using System;

using System.Text;

​

public class Program

{

​

    public static string Decrypt(string key, string message)

    {

        byte[] phone = Encoding.UTF8.GetBytes(key);

        byte[] messageChars = Convert.FromBase64String(message);

        byte[] output = new byte[messageChars.Length];

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

        {

            output[i] = Convert.ToByte((messageChars[i] ^ phone[i % phone.Length]) & byte.MaxValue);

        }

        return Encoding.UTF8.GetString(output);

    }

    public static string Encrypt(string key, string message)

        {

            byte[] phone = Encoding.UTF8.GetBytes(key);

            byte[] messageChars = Encoding.UTF8.GetBytes(message);

            byte[] output = new byte[messageChars.Length];

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

            {

                output[i] = Convert.ToByte((messageChars[i] ^ phone[i % phone.Length]) & byte.MaxValue);

            }

            return Convert.ToBase64String(output);

        }

    public static void Main()

    {

        string TelephoneNumber = "+445558473622";

        string Flag = "JyApHhkfQDQMCmo5JRkmGydyYklRMx03BnUCdCcPNDMOLn0IYh5JDT4OWwYKdQJyLg==";

        string password = Decrypt(TelephoneNumber, "alhYbFpASnZWQFNzQE52UVlaW19gWGZFH2BOR11GQVBWV1J6RXRHX11YUA==");

        Console.WriteLine("Password is: " + password);

        string key = Encrypt(password, TelephoneNumber);

        Console.WriteLine("Key is: " + key);

        string flag = Decrypt(key, Flag);

        Console.WriteLine("Flag is: " + flag);

    }

}