Abstract:

A developing field of interest for the distributed systems and applied cryptography community is that of smart contracts: self-executing financial instruments that synchronize their state, often through a blockchain. One such smart contract system that has seen widespread practical adoption is Ethereum, which has grown to secure approximately 30 billion USD of currency value and in excess of 300,000 daily transactions. Unfortunately, the rise of these technologies has been marred by a repeated series of security vulnerabilities and high pro file contract failures. To address these failures, the Ethereum community has turned to formal verification and program analysis which show great promise due to the computational simplicity and bounded-time execution inherent to smart contracts. Despite this, no fully formal, rigorous, comprehensive, and executable semantics of the EVM (Ethereum Virtual Machine) currently exists, leaving a lack of rigor on which to base such tools. In this work, we present KEVM, the first fully executable formal semantics of the EVM, the bytecode language in which smart contracts are executed. We create this semantics in a framework for executable semantics, the K framework. We show that our semantics not only passes the official 40,683-test stress test suite for EVM implementations, but also reveals ambiguities and potential sources of error in the existing on-paper formalization of EVM semantics on which our work is based. These properties make KEVM an ideal formal reference implementation against which other implementations can be evaluated. We proceed to argue for a semantics-first formal verification approach for EVM contracts, and demonstrate its practicality by using KEVM to verify practically important properties over the arithmetic operation of an example smart contract and the correct operation of a token transfer function in a second contract. We show that our approach is feasible and not computationally restrictive. We hope that our work serves as the base for the development of a wide range of useful formally derived tools for Ethereum, including model checkers, certified compilers, and program equivalence checkers.