Constant-Time Foundations for the New Spectre Era

TL;DR

This paper extends the concept of constant-time programming to counter micro-architectural attacks like Spectre, providing formal semantics and a static analysis tool to verify cryptographic code security.

Contribution

It introduces a formal semantics for constant-time in the presence of speculative execution and develops Pitchfork, a static analysis tool for real-world cryptographic libraries.

Findings

Our semantics accurately models Spectre-like attacks.

Pitchfork detects constant-time violations in cryptographic libraries.

Extended constant-time notion remains tractable and rigorous.

Abstract

The constant-time discipline is a software-based countermeasure used for protecting high assurance cryptographic implementations against timing side-channel attacks. Constant-time is effective (it protects against many known attacks), rigorous (it can be formalized using program semantics), and amenable to automated verification. Yet, the advent of micro-architectural attacks makes constant-time as it exists today far less useful. This paper lays foundations for constant-time programming in the presence of speculative and out-of-order execution. We present an operational semantics and a formal definition of constant-time programs in this extended setting. Our semantics eschews formalization of microarchitectural features (that are instead assumed under adversary control), and yields a notion of constant-time that retains the elegance and tractability of the usual notion. We demonstrate the relevance of our semantics in two ways: First, by contrasting existing Spectre-like attacks with our definition of constant-time. Second, by implementing a static analysis tool, Pitchfork, which detects violations of our extended constant-time property in real world cryptographic libraries.