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Most blockchain platforms from Ethereum onwards render smart contracts asstateful reactive objects that update their state and transfer crypto-assets inresponse to transactions. A drawback of this design is that when users submit atransaction, they cannot predict in which state it will be executed. Thisexposes them to transaction-ordering attacks, a widespread class of attackswhere adversaries with the power to construct blocks of transactions canextract value from smart contracts (the so-called MEV attacks). The UTXO modelis an alternative blockchain design that thwarts these attacks by requiring newtransactions to spend past ones: since transactions have unique identifiers,reordering attacks are ineffective. Currently, the blockchains following theUTXO model either provide contracts with limited expressiveness (Bitcoin), orrequire complex run-time environments (Cardano). We present ILLUM, anIntermediate-Level Language for the UTXO Model. ILLUM can express real-worldsmart contracts, e.g. those found in Decentralized Finance. We define acompiler from ILLUM to a bare-bone UTXO blockchain with loop-free scripts. Ourcompilation target only requires minimal extensions to Bitcoin Script: inparticular, we exploit covenants, a mechanism for preserving scripts alongchains of transactions. We prove the security of our compiler: namely, anyattack targeting the compiled contract is also observable at the ILLUM level.Hence, the compiler does not introduce new vulnerabilities that were notalready present in the source ILLUM contract. Finally, we discuss thesuitability of ILLUM as a compilation target for higher-level contractlanguages.

Secure compilation of rich smart contracts on poor UTXO blockchains