International Association
for Cryptologic Research

How will future microarchitectures impact the security of existing cryptographic implementations? As we cannot keep reducing the size of transistors, chip vendors have started developing new microarchitectural optimizations to speed up computation. A recent study (Sanchez Vicarte et al., ISCA 2021) suggests that these optimizations might open the Pandora’s box of microarchitectural attacks. However, there is little guidance on how to evaluate the security impact of future optimization proposals. To help chip vendors explore the impact of microarchitectural optimizations on cryptographic implementations, we develop (i) an expressive domain-specific language, called LmSpec, that allows them to specify the leakage model for the given optimization and (ii) a testing framework, called LmTest, to automatically detect leaks under the specified leakage model within the given implementation. Using this framework, we conduct an empirical study of 18 proposed microarchitectural optimizations on 25 implementations of eight cryptographic primitives in five popular libraries. We find that every implementation would contain secret-dependent leaks, sometimes sufficient to recover a victim’s secret key, if these optimizations were realized. Ironically, some leaks are possible only because of coding idioms used to prevent leaks under the standard constant-time model.

At RWC 2020, Carruth gave an overview of what Spectre attacks mean for the development for cryptographic software. One central message of his talk was that while certain Spectre-related attacks are considered CPU bugs that should (and are being) fixed in hardware, “Spectre v1 is here for decades. . . ” Among other coding guidelines, he recommends protecting against such Spectre v1 attacks by: * moving operations involving long-term keys to a separate agent process; and * hardening this agent process with speculative load hardening (SHL), if it is affordable. In this presentation we will show that SLH is insufficient as a protection against Spectre v1, in particular when applied to cryptographic software. While this observation may seem like it contradicts earlier analyses, it is a result of taking declassification of data into account, which is a very common, albeit often implicit, construct in cryptographic software. On the positive side we show that two small modifications to SLH yield a countermeasure that provably protects against Spectre v1 attacks. What is even more positive is that this countermeasure is—in particular for cryptographic software—expected to be much cheaper than SLH. In order to widely deploy this countermeasure it is necessary to augment type systems of mainstream programming languages and compilers to distinguish between secret and public data. Such modifications to type systems are already being discussed to systematically protect against traditional timing attacks.