International Association for Cryptologic Research

International Association
for Cryptologic Research


Kostas Papagiannopoulos


Bitslice Masking and Improved Shuffling:: How and When to Mix Them in Software?
We revisit the popular adage that side-channel countermeasures must be combined to be efficient, and study its application to bitslice masking and shuffling. Our main contributions are twofold. First, we improve this combination: by shuffling the shares of a masked implementation rather than its tuples, we can amplify the impact of the shuffling exponentially in the number of shares, while this impact was independent of the masking security order in previous works. Second, we evaluate the masking and shuffling combination’s performance vs. security tradeoff under sufficient noise conditions: we show that the best approach is to mask first (i.e., fill the registers with as many shares as possible) and shuffle the independent operations that remain. We conclude that with moderate but sufficient noise, the “bitslice masking + shuffling” combination of countermeasures is practically relevant, and its interest increases when randomness is expensive and many independent operations are available for shuffling. When these conditions are not met, masking only is the best option. As additional side results, we improve the best known attack against the shuffling countermeasure from ASIACRYPT 2012. We also recall that algorithmic countermeasures like masking and shuffling, and therefore their combination, cannot be implemented securely without a minimum level of physical noise.
Friet: an Authenticated Encryption Scheme with Built-in Fault Detection 📺
In this work we present a duplex-based authenticated encryption scheme Friet based on a new permutation called Friet-P. We designed Friet-P with a novel approach for cryptographic permutations and block ciphers that takes fault-attack resistance into account and that we introduce in this paper. In this method, we build a permutation f_C to be embedded in a larger one f. First, we define f as a sequence of steps that all abide a chosen error-correcting code C, i.e., that map C-codewords to C-codewords. Then, we embed f_C in f by first encoding its input to an element of C, applying f and then decoding back from C. This last step detects a fault when the output of f is not in C. We motivate the design of the permutation we use in Friet and report on performance in soft- and hardware. We evaluate the fault-detection capabilities of the software and simulated hardware implementations with attacks. Finally, we perform a leakage evaluation. Our code is available at
Practical Evaluation of Protected Residue Number System Scalar Multiplication 📺
The Residue Number System (RNS) arithmetic is gaining grounds in public key cryptography, because it offers fast, efficient and secure implementations over large prime fields or rings of integers. In this paper, we propose a generic, thorough and analytic evaluation approach for protected scalar multiplication implementations with RNS and traditional Side Channel Attack (SCA) countermeasures in an effort to assess the SCA resistance of RNS. This paper constitutes the first robust evaluation of RNS software for Elliptic Curve Cryptography against electromagnetic (EM) side-channel attacks. Four different countermeasures, namely scalar and point randomization, random base permutations and random moduli operation sequence, are implemented and evaluated using the Test Vector Leakage Assessment (TVLA) and template attacks. More specifically, variations of RNS-based Montgomery Powering Ladder scalar multiplication algorithms are evaluated on an ARM Cortex A8 processor using an EM probe for acquisition of the traces. We show experimentally and theoretically that new bounds should be put forward when TVLA evaluations on public key algorithms are performed. On the security of RNS, our data and location dependent template attacks show that even protected implementations are vulnerable to these attacks. A combination of RNS-based countermeasures is the best way to protect against side-channel leakage.
Location, Location, Location: Revisiting Modeling and Exploitation for Location-Based Side Channel Leakages
Near-field microprobes have the capability to isolate small regions of a chip surface and enable precise measurements with high spatial resolution. Being able to distinguish the activity of small regions has given rise to the location-based side-channel attacks, which exploit the spatial dependencies of cryptographic algorithms in order to recover the secret key. Given the fairly uncharted nature of such leakages, this work revisits the location side-channel to broaden our modeling and exploitation capabilities. Our contribution is threefold. First, we provide a simple spatial model that partially captures the effect of location-based leakages. We use the newly established model to simulate the leakage of different scenarios/countermeasures and follow an information-theoretic approach to evaluate the security level achieved in every case. Second, we perform the first successful location-based attack on the SRAM of a modern ARM Cortex-M4 chip, using standard techniques such as difference of means and multivariate template attacks. Third, we put forward neural networks as classifiers that exploit the location side-channel and showcase their effectiveness on ARM Cortex-M4, especially in the context of single-shot attacks and small memory regions. Template attacks and neural network classifiers are able to reach high spacial accuracy, distinguishing between 2 SRAM regions of 128 bytes each with 100% success rate and distinguishing even between 256 SRAM byte-regions with 32% success rate. Such improved exploitation capabilities revitalize the interest for location vulnerabilities on various implementations, ranging from RSA/ECC with large memory footprint, to lookup-table-based AES with smaller memory usage.
Low Randomness Masking and Shuffling: An Evaluation Using Mutual Information
Kostas Papagiannopoulos
Side-channel countermeasure designers often face severe performance overheads when trying to protect a device. Widely applied countermeasures such as masking and shuffling entail generating a large amount of random numbers, which can result in a computational bottleneck. To mitigate the randomness cost, this work evaluates low-randomness versions of both masking and shuffling, namely Recycled Randomness Masking (RRM) and Reduced Randomness Shuffling (RRS). These countermeasures employ memory units to store generated random numbers and reuse them in subsequent computations,making them primarily suitable for implementation on devices with sufficient memory. Both RRM and RRS are evaluated using the MI-based framework in the context of horizontal attacks. The evaluation exhibits the tradeoff between the randomness cost and the noisy leakage security level offered by the countermeasures, enabling the designer to fine-tune a masking or shuffling scheme and maximize the security level achieved for a certain cost.