Bounded, yet Sufficient? How to Determine Whether Limited Side Channel Information Enables Key Recovery

This work presents a novel algorithm to quantify the relation between three factors that characterize a side channel adversary: the amount of observed side channel leakage, the workload of full key recovery, and its achievable success rate. The proposed algorithm can be used by security evaluators to derive a realistic bound on the capabilities of a side channel adversary. Furthermore, it provides an optimal strategy for combining subkey guesses to achieve any predefined success rate. Hence, it can be used by a side channel adversary to determine whether observed leakage suffices for key recovery before expending computation time. The algorithm is applied to a series of side channel measurements of a microcontroller AES implementation and simulations. A comparison to related work shows that the new algorithm improves on existing algorithms in several respects.