First Full-Fledged Side Channel Attack on HMAC-SHA-2

Side-channel attacks pose a threat to cryptographic algorithms. Hash functions, in particular those from the SHA-2 family, can also be an interesting target if some of their inputs are secret. HMAC is an important use case of a hash function, in which the input is partially secret and thus unknown to the attacker. Despite a few publications that discuss applications of power analysis techniques to attack HMAC-SHA2, no generic method that shows a full attack on its hardware implementation has been proposed so far. In this article, we present a novel practical template attack on HMAC-SHA-2 intended primarily against its implementations in hardware. To the best of our knowledge, it is the first practical attack on a true hardware implementation. We detail all the stages of the attack and validate it experimentally. Our experiments are based on an open-source hardware SHA-256 implementation that was implemented on two targets: (1) a pre-silicon side-channel leakage simulator and (2) an FPGA. In both cases, we show a full attack implementation up to the discovery of the key derivatives that allow for forging HMAC signatures. The setup used to attack the FPGA implementation cost less than $3K. The entire attack (the trace acquisition and the analysis) on the FPGA took about two hours including the profiling stage, and about half an hour excluding the profiling stage.