International Association for Cryptologic Research

International Association
for Cryptologic Research


Boris Köpf


A Provably Secure And Efficient Countermeasure Against Timing Attacks
Boris Köpf Markus Dürmuth
We show that the expected number of key bits that an unknown-message attacker can extract from a deterministic side-channel is bounded from above by |O| log_2 (n+1), where n is the number of side-channel measurements and O is the set of possible observations. We use this bound to derive a novel countermeasure against timing attacks, where the strength of the security guarantee can be freely traded for the resulting performance penalty. We give algorithms that efficiently and optimally adjust this trade-off for given constraints on the side-channel leakage or on the efficiency of the cryptosystem. Finally, we perform a case-study that shows that applying our countermeasure leads to implementations with minor performance overhead and strong security guarantees.
Formally Bounding the Side-Channel Leakage in Unknown-Message Attacks
Michael Backes Boris Köpf
We propose a novel approach for quantifying a system's resistance to unknown-message side-channel attacks. The approach is based on a measure of the secret information that an attacker can extract from a system from a given number of side-channel measurements. We provide an algorithm to compute this measure, and we use it to analyze the resistance of hardware implementations of cryptographic algorithms with respect to power and timing attacks. In particular, we show that message-blinding -- the common countermeasure against timing attacks -- reduces the rate at which information about the secret is leaked, but that the complete information is still eventually revealed. Finally, we compare information measures corresponding to unknown-message, known-message, and chosen-message attackers and show that they form a strict hierarchy.


Michael Backes (1)
Markus Dürmuth (1)