Roberto Zunino’s research while affiliated with University of Trento and other places

Current UTXO-based smart contracts face an efficiency bottleneck, requiring any transaction sent to a contract to specify the entire updated contract state. This requirement becomes particularly burdensome when the contract state contains dynamic data structures, such as maps, which are needed in many use cases for tracking users interactions with the contract. The problem is twofold: on the one hand, a large state in transactions implies a large transaction fee; on the other hand, a large centralized state is detrimental to the parallelization of transactions, which should be one of the main selling points of UTXO-based blockchains compared to account-based ones. We propose a technique to efficiently execute smart contracts on an extended UTXO blockchain, which allows the contract state to be distributed across multiple UTXOs. In this way, transactions only need to specify the part of the state they need to access, reducing their size (and fees). We also show how to exploit our model to parallelize the validation of transactions on multi-core CPUs. We implement our technique and provide an empirical validation of its effectiveness.

Decentralized blockchain platforms support the secure exchange of assets among users without relying on trusted third parties. These exchanges are programmed with smart contracts, computer programs directly executed by blockchain nodes. Multiple smart contract languages are available nowadays to developers, each with its own distinctive features, strengths, and weaknesses. In this paper, we examine the smart contract languages used in six major blockchain platforms: Ethereum, Solana, Cardano, Algorand, Aptos, and Tezos. Starting with a high-level overview of their design choices, we provide a comprehensive assessment that focuses on programming style, security, code readability, and usability, drawing on an original benchmark that encompasses a common set of use cases across all the smart contract languages under examination.

Most blockchain platforms from Ethereum onwards render smart contracts as stateful reactive objects that update their state and transfer crypto-assets in response to transactions. In this way, they support the development of contracts in the imperative procedural paradigm, familiar to most programmers. A drawback of this design choice is that when a user submits a transaction, they cannot predict in which state it will be executed, exposing them to transaction-ordering attacks. The UTXO model is an alternative blockchain design that thwarts these attacks by requiring new transactions to spend past ones: since transactions have unique identifiers, reordering attacks are ineffective. Currently, the blockchains following the UTXO model either provide contracts with limited expressiveness (Bitcoin), or require complex run-time environments and unfamiliar programming abstractions (Cardano). We present a framework for smart contracts in the UTXO model, that allows expressive contracts to be securely executed by bare-bone UTXO blockchains with loop-free scripts enriched with covenants, and supports the familiar procedural programming style.

We tackle the problem of establishing the soundness of approximate bisimilarity with respect to PCTL and its relaxed semantics. To this purpose, we consider a notion of bisimilarity inspired by the one introduced by Desharnais, Laviolette, and Tracol, and parametric with respect to an approximation error $\delta$, and to the depth n of the observation along traces. Essentially, our soundness theorem establishes that, when a state q satisfies a given formula up-to error $\delta$ and steps n, and q is bisimilar to $q'$ up-to error $\delta'$ and enough steps, we prove that $q'$ also satisfies the formula up-to a suitable error $\delta"$ and steps n. The new error $\delta"$ is computed from $\delta$, $\delta'$ and the formula, and only depends linearly on n. We provide a detailed overview of our soundness proof. We extend our bisimilarity notion to families of states, thus obtaining an asymptotic equivalence on such families. We then consider an asymptotic satisfaction relation for PCTL formulae, and prove that asymptotically equivalent families of states asymptotically satisfy the same formulae.

Extractable Value refers to a wide class of economic attacks to public blockchains, where adversaries with the power to reorder, drop or insert transactions in a block can "extract" value from user transactions. Empirical research has shown that mainstream protocols, like e.g. decentralized exchanges, are massively targeted by these attacks, with detrimental effects on their users and on the blockchain network. Despite the growing impact of these attacks in the real world, theoretical foundations are still missing. In this paper we propose a formal theory of Extractable Value, based on a general, abstract model of blockchains and smart contracts. Our theory is the basis for formal proofs of security against Extractable Value attacks.

We tackle the problem of establishing the soundness of approximate bisimilarity with respect to PCTL and its relaxed semantics. To this purpose, we consider a notion of bisimilarity similar to the one introduced by Desharnais, Laviolette, and Tracol, which is parametric with respect to an approximation error δ, and to the depth n of the observation along traces. Essentially, our soundness theorem establishes that, when a state q satisfies a given formula up-to error δ and steps n, and q is bisimilar to q′ up-to error δ′ and enough steps, we prove that q′ also satisfies the formula up-to a suitable error δ′′ and steps n. The new error δ′′ is computed from δ,δ′ and the formula, and only depends linearly on n. We provide a detailed overview of our soundness proof.