Spotlight on an unprotected AES128 white-box implementation


I think it all began when I've worked on the NSC2013 crackme made by @elvanderb, long story short you had an AES128 heavily obfuscated white-box implementation to break. The thing was you could actually solve the challenge in different ways:

  1. the first one was the easiest one: you didn't need to know anything about white-box, crypto or even AES ; you could just see the function as a black-box & try to find "design flaws" in its inner-workings
  2. the elite way: this one involved to understand & recover the entire design of the white-box, then to identify design weaknesses that allows the challenger to directly attack & recover the encryption key. A really nice write-up has been recently written by @doegox, check it out, really :): Oppida/NoSuchCon challenge.

The annoying thing is that you don't have a lot of understandable available C code on the web that implement such things, nevertheless you do have quite some nice academic references ; they are a really good resource to build your own.

This post aims to present briefly, in a simple way what an AES white-box looks like, and to show how its design is important if you want to not have your encryption key extracted :). The implementation I'm going to talk about today is not my creation at all, I just followed the first part (might do another post talking about the second part? Who knows) of a really nice paper (even for non-mathematical / crypto guys like me!) written by James A. Muir.

The idea is simple: we will start from a clean AES128 encryption function in plain C, we will modify it & transform it into a white-box implementation in several steps. As usual, all the code are available on my github account; you are encourage to break & hack them!

Of course, we will use this post to briefly present what is the white-box cryptography, what are the goals & why it's kind of cool.

Before diving deep, here is the table of contents:

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