Paper 2024/621 Original: eprint.iacr.org/2024/621 ↗

How to Lose Some Weight - A Practical Template Syndrome Decoding Attack

Abstract

We study the hardness of the Syndrome Decoding problem, the base of most code-based cryptographic schemes, such as Classic McEliece, in the presence of side-channel information. We use ChipWhisperer equipment to perform a template attack on Classic McEliece running on an ARM Cortex-M4, and accurately classify the Hamming weights of consecutive 32-bit blocks of the secret error vector. With these weights at hand, we optimize Information Set Decoding algorithms. Technically, we show how to speed up information set decoding via a dimension reduction, additional parity-check equations, and an improved information set search, all derived from the Hamming weight information. Consequently, using our template attack, we can practically recover an error vector in dimension n=2197 in a matter of seconds. Without side-channel information, such an instance has a complexity of around 88 bit. We also estimate how our template attack affects the security of the proposed McEliece parameter sets. Roughly speaking, even an error-prone leak of our Hamming weight information leads for n=3488 to a security drop of 89 bits.