Biblio

After suffering decades of high-profile attacks, the need for formal verification of security-critical software has never been clearer. Verification-oriented programming languages like F* are now being used to build high-assurance cryptographic libraries and implementations of standard protocols like TLS. In this paper, we seek to apply these verification techniques to modern Web applications, like WhatsApp, that embed sophisticated custom cryptographic components. The problem is that these components are often implemented in JavaScript, a language that is both hostile to cryptographic code and hard to reason about. So we instead target WebAssembly, a new instruction set that is supported by all major JavaScript runtimes. We present a new toolchain that compiles Low*, a low-level subset of the F* programming language, into WebAssembly. Unlike other WebAssembly compilers like Emscripten, our compilation pipeline is focused on compactness and auditability: we formalize the full translation rules in the paper and implement it in a few thousand lines of OCaml. Using this toolchain, we present two case studies. First, we build WHACL*, a WebAssembly version of the existing, verified HACL* cryptographic library. Then, we present LibSignal*, a brand new, verified implementation of the Signal protocol in WebAssembly, that can be readily used by messaging applications like WhatsApp, Skype, and Signal.

We present a high-assurance software stack for secure function evaluation (SFE). Our stack consists of three components: i. a verified compiler (CircGen) that translates C programs into Boolean circuits; ii. a verified implementation of Yao's SFE protocol based on garbled circuits and oblivious transfer; and iii. transparent application integration and communications via FRESCO, an open-source framework for secure multiparty computation (MPC). CircGen is a general purpose tool that builds on CompCert, a verified optimizing compiler for C. It can be used in arbitrary Boolean circuit-based cryptography deployments. The security of our SFE protocol implementation is formally verified using EasyCrypt, a tool-assisted framework for building high-confidence cryptographic proofs, and it leverages a new formalization of garbled circuits based on the framework of Bellare, Hoang, and Rogaway (CCS 2012). We conduct a practical evaluation of our approach, and conclude that it is competitive with state-of-the-art (unverified) approaches. Our work provides concrete evidence of the feasibility of building efficient, verified, implementations of higher-level cryptographic systems. All our development is publicly available.