Timing side-channels are arguably one of the main sources of vulnerabilities in cryptographic implementations. One effective mitigation against timing side-channels is to write programs that do not perform secret-dependent branches and memory accesses. This mitigation, known as łcryptographic constant-timež, is adopted by several popular cryptographic libraries. This paper focuses on compilation of cryptographic constant-time programs, and more specifically on the following question: is the code generated by a realistic compiler for a constant-time source program itself provably constant-time? Surprisingly, we answer the question positively for a mildly modified version of the CompCert compiler, a formally verified and moderately optimizing compiler for C. Concretely, we modify the CompCert compiler to eliminate sources of potential leakage. Then, we instrument the operational semantics of CompCert intermediate languages so as to be able to capture cryptographic constant-time. Finally, we prove that the modified CompCert compiler preserves constant-time. Our mechanization maximizes reuse of the CompCert correctness proof, through the use of new proof techniques for proving preservation of constant-time. These techniques achieve complementary trade-offs between generality and tractability of proof effort, and are of independent interest. CCS Concepts: • Security and privacy → Logic and verification.

Constant-time programming is an established discipline to secure programs against timing attackers. Several real-world secure C libraries such as NaCl, mbedTLS, or Open Quantum Safe, follow this discipline. We propose an advanced static analysis, based on state-of-theart techniques from abstract interpretation, to report time leakage during programming. To that purpose, we analyze source C programs and use full context-sensitive and arithmetic-aware alias analyses to track the tainted flows. We give semantic evidences of the correctness of our approach on a core language. We also present a prototype implementation for C programs that is based on the CompCert compiler toolchain and its companion Verasco static analyzer. We present verification results on various realworld constant-time programs and report on a successful verification of a challenging SHA-256 implementation that was out of scope of previous tool-assisted approaches.

Code obfuscation is emerging as a key asset in security by obscurity. It aims at hiding sensitive information in programs so that they become more difficult to understand and reverse engineer. Since the results on the impossibility of perfect and universal obfuscation, many obfuscation techniques have been proposed in the literature, ranging from simple variable encoding to hiding the control flow of a program. In this paper, we formally verify in Coq an advanced code obfuscation called control-flow graph flattening, that is used in stateof-the-art program obfuscators. Our control-flow graph flattening is a program transformation operating over C programs, that is integrated into the CompCert formally verified compiler. The semantics preservation proof of our program obfuscator relies on a simulation proof performed on a realistic language, the Clight language of CompCert. The automatic extraction of our program obfuscator into OCaml yields a program with competitive results.