bmphide solver - Flare-On 2019 CTF

Program.cs

using System;
using System.Drawing;
using System.IO;

namespace BMPExtractor {
    
    internal class Program {
	    
        private static int len_key = 6;

        private static byte[] ExtractBytes(Bitmap bm)
        {
            byte[] data = new byte[len_key];
            int num = 0;
            for (int i = 0; i < bm.Width; i++)
            {
                for (int j = 0; j < bm.Height; j++)
                {
                    if (num >= len_key)
                        break;

                    Color pixel = bm.GetPixel(i, j);
                    data[num] = (byte)((pixel.R & 7) | ((pixel.G & 7) << 3) | ((pixel.B & 3) << 6));
                    num++;
                }

            }
            
            return data;
            
        }
        
        private static byte Reverse_b(byte target, int num)
        {

	        for (int i = 0; i < num; i++)
	        {
		        byte aux = (byte)(target << 7);
		        target = (byte)(target >> 1 | aux);
	        }
	        
	        return target;

        }
        
        private static byte Reverse_d(byte target, int num)
        {

	        for (int i = 0; i < num; i++)
	        {
		        byte aux = (byte)(target >> 7);
		        target = (byte)(target << 1 | aux);
	        }
            
	        return target;

        }

        private static byte Reverse_e(byte target, byte k)
        {
	        
	        return (byte)(target ^ k);

        }
        
        private static byte g(int idx)
        {
	        byte b = (byte)((long)(idx + 1) * (long)((ulong)309030853));
	        byte k = (byte)((idx + 2) * 209897853);
	        return Reverse_e(b, k);
        }
        
        private static byte[] DecryptBytes(byte[] target)
        {

	        int num = 0;
            for (int i = 0; i < target.Length; i++)
            {
	            num += 2;
                target[i] = Reverse_d(target[i], 3);
                target[i] = Reverse_e(target[i], g(num - 1));
                target[i] = Reverse_b(target[i], 7);
                target[i] = Reverse_e(target[i], g(num - 2));
            }
            
            return target;

        }

        private static int getSizeHeader(byte[] bytes)
        {

	        int result = 0;
	        for (int i = 0; i < 4; i++)
		        result += bytes[i + 2] << 8 * i;

	        return result;

        }
        
        public static void Main(string[] args) {
            
            string path = Path.GetFullPath(args[0]);
            string out_path = Path.GetFullPath(args[1]);
            Bitmap bitmap = new Bitmap(path);
            Console.WriteLine("[*] Trying to extract header...");
            byte[] data = ExtractBytes(bitmap);
            Console.Write("[+] Header bytes extracted successfully: ");
            Console.WriteLine(BitConverter.ToString(data).Replace("-", " "));
            Console.WriteLine("[*] Decrypting header...");
            byte[] decrypted = DecryptBytes(data);
            Console.Write("[+] Header bytes decrypted successfully: ");
            Console.WriteLine(BitConverter.ToString(decrypted).Replace("-", " "));
            Program.len_key = getSizeHeader(decrypted);
            Console.WriteLine("[+] File length: " + Program.len_key);
            Console.WriteLine("[*] Trying to extract file...");
            data = ExtractBytes(bitmap);
            Console.WriteLine("[+] File bytes extracted successfully.");
            Console.WriteLine("[*] Decrypting file...");
            decrypted = DecryptBytes(data);
            Console.WriteLine("[*] Writing file...");
            File.WriteAllBytes(out_path, decrypted);
            Console.WriteLine("[+] File wrote successfully.");

        }

    }
    
}