Verifying liquidity of recursive Bitcoin contracts

Smart contracts - computer protocols that regulate the exchange of crypto-assets in trustless environments have become popular with spread blockchain technologies. A landmark security property smart is liquidity: a non-liquid contract, it may happen some assets remain frozen, i.e. not redeemable by anyone. The relevance this issue witnessed recent liquidity attacks to Ethereum, which frozen hundreds USD millions. We address problem verifying on BitML, DSL for secure compiler Bitcoin, featuring primitives currency transfers, contract renegotiation and consensual recursion. Our main result verification technique liquidity. first transform infinite-state semantics BitML into finite-state one, focusses behaviour chosen set contracts, abstracting from moves context. With respect abstraction sound, if abstracted liquid, then also concrete one such. verify model-checking abstraction. implement toolchain automatically verifies compiles them we assess through benchmark representative contracts.