XORBruteForce.cs

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

namespace XORBruteForce
{
    class Program
    {
        //https://stackoverflow.com/questions/1344221/how-can-i-generate-random-alphanumeric-strings
        private static Random random = new Random();
        public static string RandomKey(int length)
        {
            const string chars = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
            return new string(Enumerable.Repeat(chars, length)
              .Select(s => s[random.Next(s.Length)]).ToArray());
        }

        private static byte[] xorEncDec(byte[] input, string theKeystring)
        {
            byte[] theKey = Encoding.UTF8.GetBytes(theKeystring);
            byte[] mixed = new byte[input.Length];
            for (int i = 0; i < input.Length; i++)
            {
                mixed[i] = (byte)(input[i] ^ theKey[i % theKey.Length]);
            }
            return mixed;
        }

        //https://stackoverflow.com/questions/11454004/calculate-a-md5-hash-from-a-string
        public static string CreateMD5(string input)
        {
            // Use input string to calculate MD5 hash
            using (System.Security.Cryptography.MD5 md5 = System.Security.Cryptography.MD5.Create())
            {
                byte[] inputBytes = System.Text.Encoding.ASCII.GetBytes(input);
                byte[] hashBytes = md5.ComputeHash(inputBytes);

                // Convert the byte array to hexadecimal string
                StringBuilder sb = new StringBuilder();
                for (int i = 0; i < hashBytes.Length; i++)
                {
                    sb.Append(hashBytes[i].ToString("X2"));
                }
                return sb.ToString();
            }
        }

        static void Main(string[] args)
        {
            //How long do we want to key to be? This will ofc effect decrypt time / CPU usage over time
            int keyLength = 4;

            //Declare some data, this could be shellcode
            var orgData = DateTime.UtcNow.ToString();
            //Create a hash of the org data
            var orgHash = CreateMD5(orgData);
            //Generate a key
            var orgKey = RandomKey(keyLength);

            //Encrypt the data
            var encryptedData = xorEncDec(Encoding.UTF8.GetBytes(orgData), orgKey);

            Console.WriteLine($"[+] Encrypted data with key {orgKey}, Hash -> {orgHash}");

            // Create new stopwatch.
            Stopwatch stopwatch = new Stopwatch();

            // Begin timing.
            stopwatch.Start();


            int crackAttempts = 0;
            bool cracked = false;
            while (!cracked)
            {
                //Generate a random key
                var runKey = RandomKey(keyLength);
                //Attempt Decrypt
                var outPut = xorEncDec(encryptedData, runKey);

                //Compare the hashes, does it match?
                if (CreateMD5(Encoding.UTF8.GetString(outPut)).Equals(orgHash))
                {
                    //Of so, let's stop
                    Console.WriteLine($"[+] Found it at attempt {crackAttempts} , key -> {runKey}, result-> {Encoding.UTF8.GetString(outPut)}");

                    cracked = true;
                }
                crackAttempts++;
            }
            // Stop timing.
            stopwatch.Stop();

            // Write result.
            Console.WriteLine("[+] Time elapsed: {0} seconds", stopwatch.Elapsed.TotalSeconds);

            Console.ReadLine();
        }
    }
}