International Association
for Cryptologic Research

This paper presents KyberSlash1 and KyberSlash2 – two timing vulnerabilities in several implementations (including the official reference code) of the Kyber Post-Quantum Key Encapsulation Mechanism, recently standardized as ML-KEM. We demonstrate the exploitability of both KyberSlash1 and KyberSlash2 on two popular platforms: the Raspberry Pi 2 (Arm Cortex-A7) and the Arm Cortex-M4 microprocessor. Kyber secret keys are reliably recovered within minutes for KyberSlash2 and a few hours for KyberSlash1. We responsibly disclosed these vulnerabilities to maintainers of various libraries and they have swiftly been patched. We present two approaches for detecting and avoiding similar vulnerabilities. First, we patch the dynamic analysis tool Valgrind to allow detection of variable-time instructions operating on secret data, and apply it to more than 1000 implementations of cryptographic primitives in SUPERCOP. We report multiple findings. Second, we propose a more rigid approach to guarantee the absence of variable-time instructions in cryptographic software using formal methods.

Compared to elliptic curve cryptography, a primary drawback of latticebased schemes is the larger size of their public keys and ciphertexts. A common procedure for compressing these objects consists essentially of dropping some of their least significant bits. Albeit effective for compression, there is a limit to the number of bits to be dropped before we get a noticeable decryption failure rate (DFR), which is a security concern. To address this issue, this paper presents a family of error-correction codes that, by allowing an increased number of dropped bits while preserving a negligible DFR, can be used for both ciphertext and publickey compression in modern lattice-based schemes. To showcase the impact and practicality of our proposal, we use the highly optimized ML-KEM, a post-quantum lattice-based scheme recently standardized by NIST. We provide detailed procedures for tailoring our codes to ML-KEM’s specific noise distributions, and show how to analyze the DFR without independence assumptions on the noise coefficients. Among our results, we achieve between 4% and 8% ciphertext compression for MLKEM. Alternatively, we obtain 8% shorter public keys compared to the current standard. We also present isochronous implementations of the decoding procedure, achieving negligible performance impact in the full ML-KEM decapsulation even when considering optimized implementations for AVX2, Cortex-M4, and Cortex-A53.

BIKE is a code-based key encapsulation mechanism (KEM) that was recently selected as an alternate candidate by the NIST’s standardization process on post-quantum cryptography. This KEM is based on the Niederreiter scheme instantiated with QC-MDPC codes, and it uses the BGF decoder for key decapsulation. We discovered important limitations of BGF that we describe in detail, and then we propose a new decoding algorithm for QC-MDPC codes called PickyFix. Our decoder uses two auxiliary iterations that are significantly different from previous approaches and we show how they can be implemented efficiently. We analyze our decoder with respect to both its error correction capacity and its performance in practice. When compared to BGF, our constant-time implementation of PickyFix achieves speedups of 1.18, 1.29, and 1.47 for the security levels 128, 192 and 256, respectively.