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Programming securely with cryptography

In recent years, an increasing amount of sensitive data is being generated, manipulated, and accessed online, from bank accounts to health records. Both national security and individual privacy have come to rely on the security of web-based software applications. But even a single design flaw or implementation bug in an application may be exploited by a malicious criminal to steal, modify, or forge the private records of innocent users. Such attacks are becoming increasingly common and now affect millions of users every year.

The risks of deploying insecure software are too great to tolerate anything less than mathematical proof, but applications have become too large for security experts to examine by hand, and automated verification tools do not scale. Today, there is not a single widely-used web application for which we can give a proof of security, even against a small class of attacks. In fact, design and implementation flaws are still found in widely-distributed and thoroughly-vetted security libraries designed and implemented by experts.

Software security is in crisis. A focused research effort is needed if security programming and analysis techniques are to keep up with the rapid development and deployment of security-critical distributed applications based on new cryptographic protocols and secure hardware devices. The goal of our team Prosecco is to draw upon our expertise in cryptographic protocols and program verification to make decisive contributions in this direction.

Our vision is that, over its lifetime, Prosecco will contribute to making the use of formal techniques when programming with cryptography as natural as the use of a software debugger. To this end, our long-term goals are to design and implement programming language abstractions, cryptographic models, verification tools, and verified security libraries that developers can use to deploy provably secure distributed applications. Our target applications include cryptographic protocol implementations, hardware-based security APIs, smartphone- and browser-based web applications, and cloud-based web services. In particular, we aim to verify the full application: both the cryptographic core and the high-level application code. We aim to verify implementations, not just models. We aim to account for computational cryptography, not just its symbolic abstraction.

We identify five key focus areas for our research in the short- to medium term.

New programming languages for verified software

Building realistic verified applications requires new programming languages that enable the systematic development of efficient software hand-in-hand with their proofs of correctness. Our current focus is on designing and implementing the programming language F*, in collaboration with Microsoft Research. F* (pronounced F star) is an ML-like functional programming language aimed at program verification. Its type system includes polymorphism, dependent types, monadic effects, refinement types, and a weakest precondition calculus. Together, these features allow expressing precise and compact specifications for programs, including functional correctness and security properties. The F* type-checker aims to prove that programs meet their specifications using a combination of SMT solving and manual proofs. Programs written in F* can be translated to efficient OCaml, F#, or C for execution. The main ongoing use case of F* is building a verified, drop-in replacement for the whole HTTPS stack in Project Everest (a larger collaboration with Microsoft Research). This includes verified implementations of TLS 1.2 and 1.3 and of the underlying cryptographic primitives.

Symbolic verification of cryptographic applications

We aim to develop our own security verification tools for models and implementations of cryptographic protocols and security APIs using symbolic cryptography. Our starting point is the tools we have previously developed: the specialized cryptographic prover ProVerif, the reverse engineering and formal test tool Tookan, and the F* verification system. These tools are already used to verify industrial-strength cryptographic protocol implementations and commercial cryptographic hardware. We plan to extend and combine these approaches to capture more sophisticated attacks on applications consisiting of protocols, software, and hardware, as well as to prove symbolic security properties for such composite systems.

Computational verification of cryptographic applications

We aim to develop our own cryptographic application verification tools that use the computational model of cryptography. The tools include the computational prover CryptoVerif, and the F* verification system. Working together, we plan to extend these tools to analyze, for the first time, cryptographic protocols, security APIs, and their implementations under fully precise cryptographic assumptions. We also plan to pursue links between symbolic and computational verification, such as computational soundness results that enable computational proofs by symbolic techniques.

Efficient formally secure compilers for tagged architectures

We aim to leverage emerging hardware capabilities for fine-grained protection to build the first, efficient secure compilation chains for realistic low-level programming languages (the C language, and Low* a safe subset of C embedded in F* for verification). These compilation chains will provide a secure semantics for all programs and will ensure that high-level abstractions cannot be violated even when interacting with untrusted low-level code. To achieve this level of security without sacrificing efficiency, our secure compilation chains target a tagged architecture, which associates a metadata tag to each word and efficiently propagates and checks tags according to software-defined rules.

Building provably secure web applications

We aim to develop analysis tools and verified libraries to help programmers build provably secure web applications. The tools will include static and dynamic verification tools for client- and server-side JavaScript web applications, their verified deployment within HTML5 websites and browser extensions, as well as type-preserving compilers from high-level applications written in F* to JavaScript. In addition, we plan to model new security APIs in browsers and smartphones and develop the first formal semantics for various HTML5 web standards. We plan to combine these tools and models to analyze the security of multi-party web applications, consisting of clients on browsers and smartphones, and servers in the cloud.

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