SleepingGuard.cs

using System;
using System.IO;
using System.Text;

namespace CTFSleepingGuard
{
    internal static class Program
    {
        private static void Test()
        {
            var plain = Encoding.UTF8.GetBytes("PlainText");
            var encKey = Encoding.UTF8.GetBytes("S0m3Text");
            var encrypted = new byte[plain.Length];
            var obtainedKey = new byte[encKey.Length];

            for (var i = 0; i < plain.Length; i++)
                encrypted[i] = (byte) (plain[i] ^ encKey[i%encKey.Length]);

            for (var i = 0; i < encKey.Length; i++)
                obtainedKey[i] = (byte) (encrypted[i] ^ plain[i]);

            if (Encoding.UTF8.GetString(obtainedKey) == Encoding.UTF8.GetString(encKey))
                Console.WriteLine("y");
        }

        private static void Main(string[] args)
        {
            var b64Decoded = Convert.FromBase64String(File.ReadAllText("F:\\Progs\\NetCat\\b64"));
            var normalPng = File.ReadAllBytes("F:\\Progs\\NetCat\\g.png");

            var key = new byte[12];

            Test();

            var decoded = new byte[b64Decoded.Length];

            for (var i = 0; i < 12; i++)
                key[i] = (byte) (b64Decoded[i] ^ normalPng[i]);

            for (var i = 0; i < b64Decoded.Length; i++)
                decoded[i] = (byte) (b64Decoded[i] ^ key[i%key.Length]);

            File.WriteAllBytes("F:\\Progs\\NetCat\\d.png", decoded);

            Console.ReadLine();
        }
    }
}