Kalpa Publications in Computing

This paper presents a method for constructing an operation sequence of sliding window exponentiation from the noisy cache information of RSA, which can be used for a cache attack using sliding windows leak (SWL). SWL, which was reported in CHES 2017, is a kind of cache side-channel leak of a sequence of operations (i.e., multiplication and squaring) from software RSA decryption using the sliding window method for modular exponentiation. It was shown that an SWL attack can retrieve the secret keys of 1,024-bit and 2,048-bit RSA with non-negligible probability if the SWL is correctly captured. However, in practice, it is not always possible for an attacker to acquire a complete and correct operation sequence from cache information observation. In addition, no concrete method for deriving a fully correct operation sequence from a partially acquired operation sequence has been reported in literature. In this paper, we first show that the capture errors in an operation sequence can be evaluated based on the Levenshtein distance between correct and estimated sequences. The dynamic time warping (DTW) algorithm is employed for quantitative evaluation. Then, we present a method of accurately estimating a complete and correct operation sequence from noisy sequences obtained through multiple observations. The basic idea of the proposed method and DTW-based evaluation is to divide the acquired operation sequence into short subsequences referred to as “operation patterns.” Furthermore, we show the effectiveness of the proposed method through a set of experiments performed using RSA software in Libgcrypt, which is one of the most common open source software in cryptography.