Researcher Finds Side-Channel Vulnerability in Post-Quantum Key Encapsulation Mechanism

Slashdot reader storagedude shared this report from The Cyber Express: A security researcher discovered an exploitable timing leak in the Kyber key encapsulation mechanism (KEM) that's in the process of being adopted by NIST as a post-quantum cryptographic standard. Antoon Purnal of PQShield detailed his findings in a blog post and on social media, and noted that the problem has been fixed with the help of the Kyber team. The issue was found in the reference implementation of the Module-Lattice-Based Key-Encapsulation Mechanism (ML-KEM) that's in the process of being adopted as a NIST post-quantum key encapsulation standard. "A key part of implementation security is resistance against side-channel attacks, which exploit the physical side-effects of cryptographic computations to infer sensitive information," Purnal wrote. To secure against side-channel attacks, cryptographic algorithms must be implemented in a way so that "no attacker-observable effect of their execution depends on the secrets they process," he wrote. In the ML-KEM reference implementation, "we're concerned with a particular side channel that's observable in almost all cryptographic deployment scenarios: time." The vulnerability can occur when a compiler optimizes the code, in the process silently undoing "measures taken by the skilled implementer." In Purnal's analysis, the Clang compiler was found to emit a vulnerable secret-dependent branch in the poly_frommsg function of the ML-KEM reference code needed in both key encapsulation and decapsulation, corresponding to the expand_secure implementation. While the reference implementation was patched, "It's important to note that this does not rule out the possibility that other libraries, which are based on the reference implementation but do not use the poly_frommsg function verbatim, may be vulnerable - either now or in the future," Purnal wrote. Purnal also published a proof-of-concept demo on GitHub. "On an Intel Core i7-13700H, it takes between 5-10 minutes to leak the entire ML-KEM 512 secret key using end-to-end decapsulation timing measurements."

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