Side-Channel Analysis of Keymill

One prominent countermeasure against side-channel attacks, especially differential power analysis (DPA), is fresh re-keying. In such schemes, the so-called re-keying function takes the burden of protecting a cryptographic primitive against DPA. To ensure the security of the scheme against side-channel analysis, the re-keying function has to with-stand both simple power analysis (SPA) and differential power analysis (DPA). Recently, at SAC 2016, Taha et al. proposed Keymill, a side-channel resilient key generator (or re-keying function), which is claimed to be inherently secure against side-channel attacks. In this work, however, we present a DPA attack on Keymill, which is based on the dynamic power consumption of a digital circuit that is tied to the 0 -> 1 and 1 -> 0 switches of its logical gates. Hence, the power consumption of the shift-registers used in Keymill depends on the 0 -> 1 and 1 -> 0 switches of its internal state. This information is sufficient to obtain the internal differential pattern (up to a small number of bits, which have to be brute-forced) of the 4 shift-registers of Keymill after the nonce has been absorbed. This leads to a practical key-recovery attack on Keymill.