Unknown-Input Attacks in the Parallel Setting: Improving the Security of the CHES 2012 Leakage-Resilient PRF.

In this work we present a leakage-resilient PRF which makes use of parallel block cipher implementations with unknown-inputs. To the best of our knowledge this is the first work to study and exploit unknown-inputs as a form of key-dependent algorithmic noise. It turns out that such noise renders the problem of side-channel key recovery intractable under very little and easily satisfiable assumptions. That is, the construction stays secure even in a noise-free setting and independent of the number of traces and the used power model. The contributions of this paper are as follows. First, we present a PRF construction which offers attractive security properties, even when instantiated with the AES. Second, we study the effect of unknown-input attacks in parallel implementations. We put forward their intractability and explain it by studying the inevitable model errors obtained when building templates in such a scenario. Third, we compare the security of our construction to the CHES 2012 one and show that it is superior in many ways. That is, a standard block cipher can be used, the security holds for all intermediate variables and it can even partially tolerate local EM attacks and some typical implementation mistakes or hardware insufficiencies. Finally, we discuss the performance of a standard-cell implementation.