Template Attack Assisted Linear Cryptanalysis on Outer Rounds Protected DES Implementations.

This article focuses on a novel combined attack method, termed template attack (TA) assisted linear cryptanalysis (LC), designed to recover the secret key from an outer rounds protected Data Encryption Standard (DES) implementation. The approach first uses TA, a profiling side channel attack exploiting power consumption leakages, to recover the inner round outputs of the cipher, which are then used in LC to derive key hypotheses. To enhance efficiency, the optimal key enumeration algorithm (OKEA) is applied to prioritize key candidates. Simulation results demonstrate that with 800 correct samples and a key enumeration threshold of 2^10, the attack achieves an 83% success rate, outperforming previous differential cryptanalysis-based combined attacks in data complexity and applicability to known plaintext scenarios. The study highlights that protecting only the outer rounds of DES is insufficient against such combined attacks and suggests that TA assisted LC can serve as a powerful tool for evaluating the security of block cipher implementations, including extensions to Triple DES.