Cryptanalysis of the CHES 2009/2010 Random Delay Countermeasure

Inserting random delays in cryptographic implementations is often used as a countermeasure against side-channel attacks. Most previ- ous works on the topic focus on improving the statistical distribution of these delays. For example, ecient random delay generation algorithms have been proposed at CHES 2009/2010. These solutions increase se- curity against attacks that solve the lack of synchronization between dierent leakage traces by integrating them. In this paper, we demon- strate that integration may not be the best tool to evaluate random delay insertions. For this purpose, we rst describe dierent attacks exploiting pattern recognition techniques and Hidden Markov Models. Using these tools, we succeed in cryptanalyzing a (straightforward) implementation of the CHES 2009/2010 proposal in an Atmel microcontroller, with the same data complexity as an unprotected implementation of the AES Ri- jndael. In other words, we completely cancel the countermeasure in this case. Next, we show that our cryptanalysis tools are remarkably robust to attack improved variants of the countermeasure, e.g. with additional noise or irregular dummy operations. We also exhibit that the attacks remain applicable in a non-proled adversarial scenario. Overall, these results suggest that the use of random delays may not be eective for pro- tecting small embedded devices against side-channel leakage. They also conrm the need of worst-case analysis in physical security evaluations.