A Formal Model of Bitcoin Transactions

113

The adoption of blockchain based distributed ledgers is growing fast due to their ability to provide reliability, integrity, and auditability without trusted entities. One of the key capabilities of these emerging platforms is the ability to create self-enforcing smart contracts. However, the development of smart contracts has proven to be error-prone in practice, and as a result, contracts deployed on public platforms are often riddled with security vulnerabilities. This issue is exacerbated by the design of these platforms, which forbids updating contract code and rolling back malicious transactions. In light of this, it is crucial to ensure that a smart contract is secure before deploying it and trusting it with significant amounts of cryptocurrency. To this end, we introduce the VeriSolid framework for the formal verification of contracts that are specified using a transition-system based model with rigorous operational semantics. Our model-based approach allows developers to reason about and verify contract behavior at a high level of abstraction. VeriSolid allows the generation of Solidity code from the verified models, which enables the correct-by-design development of smart contracts.

Section: G2 the King Of The Ether Throne Modelsmentioning

confidence: 99%

Section: G3 Resource Allocation Contractmentioning

confidence: 99%

VeriSolid: Correct-by-Design Smart Contracts for Ethereum

Mavridou

Lászka

Stachtiari

et al. 2019

113

“…Smart contracts in Bitcoin have a completely different flavour compared to Ethereum, since they are usually expressed as cryptographic protocols, rather than as programs. Despite the limited expressiveness of the scripts in Bitcoin transactions [10], several kinds of contracts for Bitcoin have been proposed [9]: they range from lotteries [6,7,13,29], to general multiparty computations [4,17,26], to contingent payments [11,28], etc. All these works focus on proving the security of a fixed contract, unlike the above-mentioned works on Ethereum, where the goal is to verify arbitrary contracts.…”

Section: Contributionsmentioning

confidence: 99%

Verifying Liquidity of Bitcoin Contracts

Bartoletti

Zunino

2019

Self Cite

A landmark security property of smart contracts is liquidity: in a non-liquid contract, it may happen that some funds remain frozen. The relevance of this issue is witnessed by a recent liquidity attack to the Ethereum Parity Wallet, which has frozen ∼160M USD within the contract, making this sum unredeemable by any user. We address the problem of verifying liquidity of Bitcoin contracts. Focussing on BitML, a contracts DSL with a computationally sound compiler to Bitcoin, we study various notions of liquidity. Our main result is that liquidity of BitML contracts is decidable, in all the proposed variants. To prove this, we first transform the infinite-state semantics of BitML into a finitestate one, which focusses on the behaviour of any given set of contracts, abstracting the context moves. With respect to the chosen contracts, this abstraction is sound and complete. Our decision procedure for liquidity is then based on model-checking the finite space of states of the abstraction.1. We formalize a notion of liquidity (Definition 2), and we illustrate several meaningful variants. Our notion of liquidity takes into account both the contract and the strategy that a participant follows to perform contract actions. Roughly, a strategy is liquid when following it ensures that funds do not remain frozen within the contract, even in the presence of adversaries. 2. We introduce an abstraction of the semantics of BitML which is finite-state (Theorem 1), and sound and complete w.r.t. the concrete (infinite-state) semantics, given a set of contracts under observation (Theorems 2 and 3). 3. We devise a verification technique for liquidity in BitML. Our technique can establish whether a strategy is liquid for a given contract, and also to synthesise a liquid strategy, when it exists (Theorem 4).Our finite-state abstraction is general-purpose: verifying liquidity is only one of its possible applications (some other applications are discussed in Sect. 6). Related Works.Several recent works study security issues related to Ethereum smart contracts. A few papers address EVM, the bytecode language which is the target of compilation of Solidity. Among them, [27] introduces an operational semantics of a simplified version of EVM, and develops Oyente, a tool to detect some vulnerability patterns of EVM contracts through symbolic execution. Securify [35] checks vulnerability patterns by analysing dependency graphs extracted

“…In this section we give a minimalistic introduction to Bitcoin [21,38], focussing on the crucial notion of transaction. To this purpose, we rely on the model of Bitcoin transactions in [11]. Here, instead of repeating the formal machinery of [11], we introduce the needed concepts through a series of examples.…”

Section: Background On Bitcoin Transactionsmentioning

confidence: 99%

“…To this purpose, we rely on the model of Bitcoin transactions in [11]. Here, instead of repeating the formal machinery of [11], we introduce the needed concepts through a series of examples. We will however follow the same notation of [11], and point to the formal definitions therein, to allow the reader to make precise the intuitions provided in this paper.…”

Section: Background On Bitcoin Transactionsmentioning

confidence: 99%

SoK: Unraveling Bitcoin Smart Contracts

Atzei

Bartoletti

Cimoli

et al. 2018

Self Cite

Albeit the primary usage of Bitcoin is to exchange currency, its blockchain and consensus mechanism can also be exploited to securely execute some forms of smart contracts. These are agreements among mutually distrusting parties, which can be automatically enforced without resorting to a trusted intermediary. Over the last few years a variety of smart contracts for Bitcoin have been proposed, both by the academic community and by that of developers. However, the heterogeneity in their treatment, the informal (often incomplete or imprecise) descriptions, and the use of poorly documented Bitcoin features, pose obstacles to the research. In this paper we present a comprehensive survey of smart contracts on Bitcoin, in a uniform framework. Our treatment is based on a new formal specification language for smart contracts, which also helps us to highlight some subtleties in existing informal descriptions, making a step towards automatic verification. We discuss some obstacles to the diffusion of smart contracts on Bitcoin, and we identify the most promising open research challenges.