Paper 2026/924

RIC: Randomize Invalid Coefficients to Mitigate Side-Channel Assisted Chosen-Ciphertext Attacks on ML-KEM

Kentaro Imafuku, National Institute of Advanced Industrial Science and Technology

Abstract

Module lattice-based key encapsulation mechanisms (ML-KEM) are susceptible to side-channel-assisted chosen-ciphertext attacks (SCA-CCAs) that exploit leakage from the re-encryption process during decapsulation. These attacks enable adversaries to recover secret keys with hundreds to thousands of oracle accesses, thereby posing a critical threat to the practical deployment of post-quantum cryptography. This paper presents RandInvalidCoeff, a novel and lightweight countermeasure that introduces randomness into the decryption function to mitigate SCA-CCAs. By randomizing invalid coefficients in the decrypted message polynomial, RandInvalidCoeff injects probabilistic errors into side-channel observations, significantly reducing the attacker’s ability to perform reliable key recovery. This randomization slightly increases the decryption failure rate (DFR). Nevertheless, our analysis shows that strong resistance can be achieved with an acceptable DFR of approximately $2^{-80}$. We provide information-theoretic and statistical analyses of the countermeasure to quantify the reduction in information leakage for plaintext-checking (PC) and decryption-failure (DF) oracle attacks. Furthermore, we performed PC- and DF-oracle attacks on an implementation with RandInvalidCoeff to validate the theoretical analysis, confirming that the proposed method achieves the expected countermeasure effect. The results demonstrate that the number of observations required for successful key recovery increases by a factor of more than eight, with only a few percent performance overhead compared to unprotected implementations.