Algebraic Side-Channel Attacks against ISAP's Re-Keying: one Ascon Round May not be Enough for Serial Implementations

We investigate the side-channel security of ISAP against Algebraic Side-Channel Attacks (ASCA) in a simulated setting where the Hamming weight leakages of its intermediate computations can be recovered. For this purpose, we first describe how these attacks, so far only used to target 8-bit implementations, can be applied to 16-bit or 32-bit implementations. We then use ASCA to discuss the side-channel security claims of ISAP's re-keying, where a single bit of nonce is absorbed per permutation call. Theoretically, this re-keying aims to ensure that attacking more than one permutation call jointly does not improve over attacking the same number of permutation calls independently. Yet, while this expectation is expected to be met for ISAP's conservative parameters (where permutation calls are made of 12 Ascon rounds), the extent to which it does (not) hold for ISAP's aggressive parameters (where permutation calls are made of a single Ascon round) remains an open question. We contribute to this question by showing that for 16-bit implementations, combining the leakages of multiple permutation calls can improve over attacking the same number of permutation calls independently, which contradicts ISAP's (theoretical) leakage-resistance claims. By contrast, for 32-bit leakages, we only show similar weaknesses by guessing a large part of the target state (i.e., more than 128 bits), which only impacts the initialization of ISAP's re-keying and does not contradict its security reduction. These results confirm that for hardware implementations with a sufficient level of parallelism, ISAP's aggressive parameters are probably sufficient, but that for more serial (e.g., software) implementations, slightly more conservative parameters, or the addition of implementation-level countermeasures, are needed.