Solvent: liquidity verification of smart contracts

Smart contracts are programs executed by blockchains networks to regulate the exchange of crypto-assets between untrusted users. Due to their immutability, public accessibility and high value at stake, smart contracts are an attractive target for attackers, as evidenced by a long history of security incidents. This has been a driving factor for the application of formal methods to Ethereum, the leading smart contract platform, and Solidity, its main smart contract language, which have become the target of dozens of verification tools with varying objectives. A current limitation of these tools is that they are not really effective in expressing and verifying liquidity properties regarding the exchange of crypto-assets: for example, is it true that in every reachable state a user can fire a sequence of transactions to withdraw a given amount of crypto-assets? We propose Solvent, a tool aimed at verifying these kinds of properties, which are beyond the reach of existing verification tools for Solidity. We evaluate the effectiveness and performance of Solvent through a common benchmark of smart contracts.