C#-Code

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

namespace trainYourProgrammer
{
	class MainClass
	{
		public static void Main(string[] args) {
			string path = "/home/eulerschezahl/Dokumente/Programmieren/challenges/TrainYourProgrammer/";
			//einpacken
			HideFile ((Bitmap)Bitmap.FromFile (path + "Glueckwunsch_org.png"), 
			          new FileInfo (path + "TrainYourProgrammer.zip"))
				.Save(path + "Glueckwunsch.png");
			HideFile ((Bitmap)Bitmap.FromFile (path + "logo.png"), 
			          new FileInfo (path + "Glueckwunsch.png"))
				.Save(path + "Steganographie.png");

			//auspacken zur Probe
			ShowFile ((Bitmap)Bitmap.FromFile (path + "Steganographie.png"), path + "found/");
			ShowFile ((Bitmap)Bitmap.FromFile (path + "Glueckwunsch.png"), path + "found/");
		}

		public static Bitmap HideFile(Bitmap original, FileInfo file) {
			int size = (int)file.Length;
			if (original.Width * original.Height * 6 < (4 + file.Name.Length + 1 + file.Length) * 8)
				throw new NotSupportedException ("file too large for image");
			byte[] byte8 = new byte[4 + file.Name.Length + 1 + file.Length];
			byte8 [0] = (byte)(size >> 24);
			byte8 [1] = (byte)(size >> 16);
			byte8 [2] = (byte)(size >> 8);
			byte8 [3] = (byte)(size);
			for (int i = 0; i < file.Name.Length; i++)
				byte8[i + 4] = (byte)file.Name [i];
			byte[] data = File.ReadAllBytes (file.FullName);
			Array.Copy (data, 0, byte8, 5 + file.Name.Length, data.Length);
			byte[] byte6 = Byte8ToByte6 (byte8);
			Bitmap target = (Bitmap)original.Clone ();
			for (int i = 0; i < byte6.Length; i++) {
				int x = i % target.Width; 
				int y = i / target.Width;
				Color cl = target.GetPixel (x, y);
				cl = Color.FromArgb ((cl.R & 0xFC) | byte6[i] >> 4,
				                     (cl.G & 0xFC) | (byte6[i] >> 2) & 0x3,
				                     (cl.B & 0xFC) | byte6[i] & 0x3);
				target.SetPixel (x, y, cl);
			}
			return target;
		}

		public static void ShowFile(Bitmap bmp, string target) {
			byte[] byte6 = new byte[bmp.Width * bmp.Height];
			for (int i = 0; i < byte6.Length; i++) {
				int x = i % bmp.Width; 
				int y = i / bmp.Width;
				Color cl = bmp.GetPixel (x, y);
				byte6[i] = (byte)((cl.R & 0x3) << 4 | (cl.G & 0x3) << 2 | (cl.B & 0x3));
			}
			byte[] byte8 = Byte6ToByte8 (byte6);

			int size = byte8 [0] << 24 | byte8 [1] << 16 | byte8 [2] << 8 | byte8 [3];
			string filename = "";
			int index = 4;
			do {
				filename += (char)byte8 [index];
			} while (byte8[++index] != 0);
			byte[] file = new byte[size];
			Array.Copy (byte8, index + 1, file, 0, file.Length);
			File.WriteAllBytes (target + filename, file);
		}

		private static byte[] Byte8ToByte6(byte[] byte8) {
			Array.Resize (ref byte8, (byte8.Length / 3 + 2) * 3);
			byte[] byte6 = new byte[(byte8.Length * 4 + 2) / 3];
			for (int i = 0; i < byte8.Length / 3; i++) {
				byte6 [4 * i] = (byte)(byte8 [3 * i] >> 2);
				byte6 [4 * i + 1] = (byte)((byte8 [3 * i] << 4 | byte8 [3 * i + 1] >> 4) & 0x3F);
				byte6 [4 * i + 2] = (byte)((byte8 [3 * i + 1] << 2 | byte8[3*i + 2] >> 6) & 0x3F);
				byte6 [4 * i + 3] =(byte)( byte8 [3 * i + 2] & 0x3F);
			}
			return byte6;
		}

		private static byte[] Byte6ToByte8(byte[] byte6) {
			Array.Resize (ref byte6, (byte6.Length / 4 + 3) * 4);
			byte[] byte8 = new byte[(byte6.Length * 3 + 3) / 4];
			for (int i = 0; i < byte6.Length / 4; i++) {
				byte8 [3 * i] =(byte)( byte6 [4 * i] << 2 | byte6 [4 * i + 1] >> 4);
				byte8 [3 * i + 1] = (byte)(byte6 [4 * i + 1] << 4 | byte6 [4 * i + 2] >> 2);
				byte8 [3 * i + 2] = (byte)(byte6 [4 * i + 2] << 6 | byte6 [4 * i + 3]);
			}
			return byte8;
		}
	}
}