In the last decade, an effort has been made by the research community to find efficient ways to thwart side channel analysis (SCA) against physical implementations of cryptographic algorithms. A common countermeasure for implementations of block ciphers is Boolean masking which randomizes the variables to be protected by the bitwise addition of one or several random value(s). However, advanced techniques called higher-order SCA attacks exist that overcome such a countermeasure. These attacks are greatly favored by the very nature of Boolean masking. In this paper, we revisit the affine masking initially introduced by Von Willich in 2001 as an alternative to Boolean masking. We show how to apply it to AES at the cost of a small timing overhead compared to Boolean masking. We then conduct an in-depth analysis pinpointing the leakage reduction implied by affine masking. Our results clearly show that the proposed scheme provides an excellent performance-security trade-off to protect AES against higher-order SCA.

Abstract. Cryptographic algorithms embedded on physical devices are particularly vulnerable to Side Channel Analysis (SCA). The most common countermeasure for block cipher implementations is masking, which randomizes the variables to be protected by combining them with one or several random values. In this paper, we propose an original masking scheme based on Shamir's Secret Sharing scheme [22] as an alternative to Boolean masking. We detail its implementation for the AES using the same tool than Rivain and Prouff in CHES 2010 [16]: multi-party computation. We then conduct a security analysis of our scheme in order to compare it to Boolean masking. Our results show that for a given amount of noise the proposed scheme -implemented to the first orderprovides the same security level as 3 rd up to 4 th order boolean masking, together with a better efficiency.

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FALCON is a very efficient and compact lattice-based signature finalist of the NIST’s Post-Quantum standardization campaign. This work assesses Falcon’s sidechannel resistance by analyzing two vulnerabilities, namely the pre-image computation and the trapdoor sampling. The first attack is an improvement of Karabulut and Aysu (DAC 2021). It overcomes several difficulties inherent to the structure of the stored key like the Fourier representation and directly recovers the key with a limited number of traces and a reduced complexity. The main part of this paper is dedicated to our second attack: we show that a simple power analysis during the signature execution could provide the exact value of the output of a subroutine called the base sampler. This intermediate value does not directly lead to the secret and we had toadapt the so-called hidden parallelepiped attack initially introduced by Nguyen and Regev in Eurocrypt 2006 and reused by Ducas and Nguyen in Asiacrypt 2012. We extensively quantify the resources for our attacks and experimentally demonstrate them with FALCON’s reference implementation on the ELMO simulator (McCann, Oswald and Whitnall USENIX 2017) and on a ChipWhisperer Lite with STM32F3 target (ARM Cortex M4).These new attacks highlight the need for side-channel protection for one of the three finalists of NIST’s standardization campaign by pointing out the vulnerable parts and quantifying the resources of the attacks.

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