Verified compilation from BitML to Bitcoin: an Agda odyssey

Blockchain technology has taken the financial world by storm in recent years, allowing for programmable contracts to be enacted amongst participants in a decentralised fashion. Bugs in those programs, however, can lead to huge monetary losses and cannot in principle be amended after detection, due to the blockchain being an immutable data structure. This incentivizes a high-assurance approach to developing smart contracts, which so far has mainly consisted of approximate methods of static analysis. Here, we strive for something more radical, namely the use of interactive proof assistants grounded in Type Theory to develop such contracts and formally verify their correctness by proving logical propositions within the same system. Specifically, we take existing work on the Bitcoin Modelling Language (BitML) — a high-level process calculus for expressing contracts that compile down to Bitcoin transactions — and encode its definitions, semantics, and translation procedure in the Agda proof assistant. BitML is one of the most mature works at the confluence of Blockchain and Programming Languages, which justifies the tremendous amount of effort required to mechanise the intricate results of the original paper, compared to various more lightweight alternatives such as model checking. We can then prove properties about BitML contracts as Agda programs, in particular the main meta-theoretical result of the BitML paper, compilation correctness, which states that it suffices to prove properties at the more abstract level of BitML contracts, and then provably transfer them to the low-level of Bitcoin transactions. By virtue of working in a type-theoretic proof assistant whose underlying logic is constructive, we can say that the central research goal of this thesis amounts to producing a verified compiler from BitML contracts to Bitcoin transactions. This whole dissertation is a type-checked Agda script, and the corresponding formalisations are publicly available in HTML format: - https://omelkonian.github.io/formal-bitcoin/ - https://omelkonian.github.io/formal-bitml/ - https://omelkonian.github.io/formal-bitml-to-bitcoin

Reconciling Event Structures with Modern Multiprocessors

Weakestmo is a recently proposed memory consistency model that uses event structures to resolve the infamous "out-of-thin-air" problem and to enable efficient compilation to hardware. Nevertheless, this latter property - compilation correctness - has not yet been formally established. This paper closes this gap by establishing correctness of the intended compilation schemes from Weakestmo to a wide range of formal hardware memory models (x86, POWER, ARMv7, ARMv8) in the Coq proof assistant. Our proof is the first that establishes correctness of compilation of an event-structure-based model that forbids "out-of-thin-air" behaviors, as well as the first mechanized compilation proof of a weak memory model supporting sequentially consistent accesses to such a range of hardware platforms. Our compilation proof goes via the recent Intermediate Memory Model (IMM), which we suitably extend with sequentially consistent accesses

Reconciling Event Structures with Modern Multiprocessors (Artifact)

The artifact is a virtual machine image containing two Coq packages which include mechanization of proofs stated in the paper. The first package imm contains a modified version of the Intermediate Memory Model, extended with the support of sequentially consistent atomics, and the compilation correctness proofs from it to hardware models. The second package weakestmoToImm contains a definition of the Weakestmo memory model as well as a compilation correctness proof from it to IMM

Having Fun in Learning Formal Specifications

There are many benefits in providing formal specifications for our software. However, teaching students to do this is not always easy as courses on formal methods are often experienced as dry by students. This paper presents a game called FormalZ that teachers can use to introduce some variation in their class. Students can have some fun in playing the game and, while doing so, also learn the basics of writing formal specifications in the form of pre- and post-conditions. Unlike existing software engineering themed education games such as Pex and Code Defenders, FormalZ takes the deep gamification approach where playing gets a more central role in order to generate more engagement. This short paper presents our work in progress: the first implementation of FormalZ along with the result of a preliminary users' evaluation. This implementation is functionally complete and tested, but the polishing of its user interface is still future work