Limited Entropy Dot Com Not so random thoughts on security featured by Eloi Sanfèlix


Crypto Series: Differential Fault Analysis by examples

Posted by Eloi Sanfèlix

So, after more than a year without writing anything here, I was bored today and thought it would be nice to share a new piece on attacking cryptographic implementations here 🙂

Differential Fault Analysis (DFA) attacks are part of what is known as fault injection attacks. This is, they are based on forcing a cryptographic implementation to compute incorrect results and attempt to take advantage from them. With fault injection attacks (also often called active side channel attacks) one can achieve things like unauthenticated access to sensitive functionality, bypassing secure boot implementations, and basically bypassing any security checks an implementation performs.

With DFA attacks, one is able to retrieve cryptographic keys by analyzing correct/faulty output pairs and comparing them. Of course, this assumes you are able to inject faults somehow... which is often true in hardware implementations: gaming consoles, STBs, smart cards, etc. At the software level, one can achieve similar things by debugging the implementation and changing data or by patching instructions... but this is something we have been doing for a long time, haven't we? 🙂 I often say that fault injection attacks are the analog version of 'nopping' instructions out in a program, although we often do not know exactly what kind of faults we are injecting (i.e. we often miss a fault model, but we still successfully attack implementations in this way).

There are ways to protect against this kind of attack as an application programmer, but this is not the objective of this post. In the remainder of this post, I will explain two powerful DFA attacks on two modern cryptographic algorithms: RSA and (T)DES. For some information on protecting from these attacks as a programmer, take a look at these slides. If there is some interest, I will outline the most common techniques to perform fault attacks in a future post.