What is verified software?¶
Formal verification involves using software tools to analyze all possible behaviors of a program and prove mathematically that they comply with the code’s specification (i.e., a machine-readable description of the developer’s intentions). Unlike software testing, formal verification provides strong guarantees that a program behaves as expected and is free from entire classes of errors.
Our specifications cover a range of properties, including:
- Memory safety: our code never violates memory abstractions, and, as a consequence, is free from common bugs and vulnerabilities like buffer overflows, null-pointer dereferences, use-after-frees, double-frees, etc.
- Type safety: our code respects the interfaces amongs its components including any abstraction boundaries; e.g., one component never passes the wrong kind of parameters to another, or accesses its private state.
- Functional correctness: our code’s input/output behavior is fully characterized by simple mathematical functions derived directly from the official cryptographic standards (e.g., from NIST or the IETF).
- Secret Independence: Observations about our code’s low-level behavior (specifically, the time it takes to execute or the memory addresses that it accesses) are independent of the secrets manipulated by the library. Hence, an adversary monitoring these “side-channels” learns nothing about the secrets.
All of these guarantees do not prevent our code from achieving good performance! HACL* meets or beats the performance of most existing cryptographic implementations in C, and on certain platforms, Vale code meets or beats the performance of hand-tuned assembly code in state-of-the-art libraries. By bringing them together, the EverCrypt provider provides best-in-class performance on most platforms.
History and publications¶
Our cryptographic code is the culmination of several years of research carried through Project Everest.
Early attempts at verifying cryptographic code in F* were presented at CSF 2016: A Verified Extensible Library of Elliptic Curves (Jean Karim Zinzindohoué, Evmorfia-Iro Bartzia, Karthikeyan Bhargavan). This work established an initial body of verified libraries, but extracted to OCaml and had substandard performance. More on this early work be found in J-K Zinzindohoué’s Ph.D. thesis.
To deliver better performance, we established a C-like subset of F* that would compile to C, called Low*. Its foundations were presented at ICFP 2017: Verified Low-Level Programming Embedded in F* (Jonathan Protzenko, Jean-Karim Zinzindohoué, Aseem Rastogi, Tahina Ramananandro, Peng Wang, Santiago Zanella-Béguelin, Antoine Delignat-Lavaud, Cătălin Hriţcu, Karthikeyan Bhargavan, Cédric Fournet, and Nikhil Swamy).
Using Low*, and inspired by discussions at the HACS series of workshops, we presented the first version of the HACL* library at CCS 2017: HACL*: A Verified Modern Cryptographic Library (Jean-Karim Zinzindohoué, Karthikeyan Bhargavan, Jonathan Protzenko, Benjamin Beurdouche).
In parallel to work on HACL*, the Vale team set out to design a DSL for assembly programming, for those algorithms which could not achieve maximal performance unless written in hand-tuned assembly. The first version of Vale, which used Dafny as its verified backend, was presented at Usenix 2017 (distinguished paper award): Vale: Verifying High-Performance Cryptographic Assembly Code (Barry Bond, Chris Hawblitzel, Manos Kapritsos, K. Rustan M. Leino, Jacob R. Lorch, Bryan Parno, Ashay Rane, Srinath Setty, Laure Thompson).
In order to unify C-like and ASM-like programming, Vale was overhauled to use F* as its verification infrastructure; this second version of Vale was presented at POPL 2019: A Verified, Efficient Embedding of a Verifiable Assembly Language (Aymeric Fromherz, Nick Giannarakis, Chris Hawblitzel, Bryan Parno, Aseem Rastogi, Nikhil Swamy).
Having both C-like and ASM-like implementations all expressed in F* allowed us to connect the two semantics and establish deep integrations between parts of the codebase written in C and those written in assembly. Connecting C and ASM also enabled implementation multiplexing and algorithmic agility, while establishing strong abstraction boundaries to serve as a foundation for other verified clients. We call the result EverCrypt: EverCrypt: A Fast, Verified, Cross-Platform Cryptographic Provider (Jonathan Protzenko, Bryan Parno, Aymeric Fromherz, Chris Hawblitzel, Marina Polubelova, Karthikeyan Bhargavan, Benjamin Beurdouche, Joonwon Choi, Antoine Delignat-Lavaud, Cedric Fournet, Tahina Ramananandro, Aseem Rastogi, Nikhil Swamy, Christoph Wintersteiger and Santiago Zanella-Beguelin).
Our Verification Tools¶
The Vale cryptographic libraries (used in EverCrypt) rely on the Vale tool, which compiles the Vale DSL to F*, and is responsible for compiling the Vale code to assembly.