Tool Demonstration: FSolidM for Designing Secure Ethereum Smart Contracts
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Blockchain-based distributed computing platforms enable the trusted execution of computation—defined in the form of smart contracts—without trusted agents. Smart contracts are envisioned to have a variety of applications, ranging from financial to IoT asset tracking. Unfortunately, the development of smart contracts has proven to be extremely error prone. In practice, contracts are riddled with security vulnerabilities comprising a critical issue since bugs are by design non-fixable and contracts may handle financial assets of significant value. To facilitate the development of secure smart contracts, we have created the FSolidM framework, which allows developers to define contracts as finite state machines (FSMs) with rigorous and clear semantics. FSolidM provides an easy-to-use graphical editor for specifying FSMs, a code generator for creating Ethereum smart contracts, and a set of plugins that developers may add to their FSMs to enhance security and functionality.
KeywordsSmart contract Security Finite state machine Ethereum Solidity Automatic code generation Design patterns
In recent years, blockchains have seen wide adoption. For instance, the market capitalization of Bitcoin, the leading blockchain-based cryptocurrency, has grown from $15 billion to more than $100 billion in 2017. The goal of the first generation of blockchains was only to provide cryptocurrencies and payment systems. In contrast, more recent blockchains, such as Ethereum, strive to provide distributed computing platforms [1, 2]. Blockchain-based distributed computing platforms enable the trusted execution of general purpose computation, implemented in the form of smart contracts, without any trusted parties. Blockchains and smart contracts are envisioned to have a variety of applications, ranging from finance to IoT asset tracking . As a result, they are embraced by an increasing number of organizations and companies, including major IT and financial firms, such as Cisco, IBM, Wells Fargo, and J.P. Morgan .
However, the development of smart contracts has proven to be extremely error prone in practice. Recently, an automated analysis of a large sample of smart contracts from the Ethereum blockchain found that more than 43% of contracts have security issues . These issues often result in security vulnerabilities, which may be exploited by cyber-criminals to steal cryptocurrencies and other digital assets. For instance, in 2016, $50 million worth of cryptocurrencies were stolen in the infamous “The DAO” attack, which exploited a combination of smart-contract vulnerabilities . In addition to theft, malicious attackers may also be able to cause damage by leading a smart contract into a deadlock, which prevents account holders from spending or withdrawing their own assets.
The prevalence of smart-contract vulnerabilities poses a severe problem in practice due to multiple reasons. First, smart contracts handle assets of significant financial value: at the time of writing, contracts deployed on the Ethereum blockchain together hold more than $6 billion worth of cryptocurrency. Second, it is by design impossible to fix bugs in a contract (or change its functionality in any way) once the contract has been deployed. Third, due to the “code is law” principle , it is also by design impossible to remove a faulty or malicious transaction from the blockchain, which means that it is often impossible to recover from a security incident.1
Previous work focused on alleviating security issues in existing smart contracts by providing tools for verifying correctness  and for identifying common vulnerabilities . In contrast, we take a different approach by developing a framework, called FSolidM , which helps developers to create smart contracts that are secure by design. The main features of our framework are as follows.
Formal Model: One of the key factors contributing to the prevalence of security issues is the semantic gap between the developers’ assumptions about the underlying execution semantics and the actual semantics of smart contracts . To close this semantic gap, FSolidM is based on a simple, formal, finite-state machine (FSM) based model for smart contracts, which we introduced in . The model was designed to support Ethereum smart contracts, but it could easily be extended to other platforms.
Graphical Editor: To further decrease the semantic gap and facilitate development, FSolidM provides an easy-to-use graphical editor that enables developers to design smart contracts as FSMs.
Code Generator: FSolidM provides a tool for translating FSMs into Solidity, the most widely used high-level language for developing Ethereum contracts. Solidity code can be translated into Ethereum Virtual Machine bytecode, which can be deployed and executed on the platform.
Open Source: FSolidM is open-source and available online (see Sect. 3).
The advantages of our framework, which helps developers to create secure contracts instead of trying to fix existing ones, are threefold. First, we decrease the semantic gap and eliminate the issues arising from it by providing a formal model and an easy-to-use graphical editor. Second, since the process is rooted in rigorous semantics, our framework may be connected to formal analysis tools [12, 13]. Third, the code generator and plugins enable developers to implement smart contracts with minimal amount of error-prone manual coding.
The rest of this paper is organized as follows. In Sect. 2, we present blind auction as a motivating example, which we implement as an FSM-based smart contract. In Sect. 3, we describe our FSolidM tool and its built-in plugins. Finally, in Sect. 4, we offer concluding remarks and outline future work.
2 Defining Smart Contracts as FSMs
AcceptingBlindedBids: blinded bids and deposits may be submitted;
RevealingBids: bidders may reveal their bids (i.e., they can send their actual bids and the contract checks if the hash value is the same as the one submitted in the previous state and if they made sufficient deposit);
Finished: the highest bid wins the auction; bidders can withdraw their deposits except for the winner, who can withdraw only the difference between her deposit and bid;
Canceled: bidders can retract bids and withdraw their deposits.
Figure 1 presents the blind auction example in the form of an FSM. For simplicity, we have abbreviated AcceptingBlindedBids, RevealingBids, Finished, and Canceled to ABB, RB, F, and C, respectively. ABB is the initial state of the FSM. Each transition (e.g., bid, reveal, cancel) is associated to a set of actions that a user can perform during the blind auction. For instance, a bidder can execute the bid transition at the ABB state to send a blind bid and a deposit value. Similarly, a user can execute the close transition, which signals the end of the bidding period, if the associated guard now>= creationTime + 5 days evaluates to true. To differentiate transition names from guards, we use square brackets for the latter. A bidder can reveal her bids by executing the reveal transition. The finish transition signals the completion of the auction, while the cancelABB and cancelRB transitions signal the cancellation of the auction. Finally, the unbid and withdraw transitions can be executed by the bidders to withdraw their deposits. For ease of presentation, we omit from Fig. 1 the actions that correspond to each transition. For instance, during the execution of the withdraw transition, the following action is performed amount = pendingReturns[msg.sender].
3 The FSolidM Tool
FSolidM is an open-source2, web-based tool that is built on top of WebGME . FSolidM enables collaboration between multiple users during the development of smart contracts. Changes in FSolidM are committed and versioned, which enables branching, merging, and viewing the history of a contract. We present the FSolidM tool in more detail in .
Notice that in Fig. 2, parts of the code shown in the code editor are darker (lines 1–10) than other parts (lines 12–15). The darker lines of code include code that was generated from the FSM model defined in the graphical editor and are locked—cannot be altered in the code editor. The non-dark parts indicate code that was directly specified in the code editor.
Locking: When an Ethereum contract calls a function of another contract, the caller has to wait for the call to finish. This allows the callee—who may be malicious—to exploit the intermediate state of the caller, e.g., by invoking a function of the caller. This re-entrancy issue is one of the most well-known vulnerabilities, which was also exploited in the infamous “The DAO” attack.
To prevent re-entrancy, we provide a security plugin for locking the smart contract. Locking eliminates re-entrancy vulnerabilities in a “foolproof” manner: functions within the contract cannot be nested within each other in any way.
Transition Counter: If multiple functions calls are invoked around the same time, then the order in which these calls are executed on the Ethereum blockchain may be unpredictable. Hence, when a user invokes a function, she may be unable to predict what the state and the values stored within a contract will be when the function is actually executed. This issue has been referred to as “transaction-ordering dependence”  and “unpredictable state” , and it can lead to various security vulnerabilities.
We provide a plugin that prevents unpredictable-state vulnerabilities by enforcing a strict ordering on function call executions. The plugin expects a transition number in every function as a parameter and ensures that the number is incremented by one for each function execution. As a result, when a user invokes a function with the next transition number in sequence, she can be sure that the function is executed before any other state changes can take place.
Automatic Timed Transitions: We provide a plugin for implementing time-constraint patterns. We extend our language with timed transitions, which are similar to non-timed transitions, but (1) their guards and assignments do not use input or output data and (2) they include a number specifying transition time.
We implement timed transitions as a modifier that is applied to every function, and which ensures that timed transitions are executed automatically if their time and data guards are satisfied. Writing such modifiers manually could lead to vulnerabilities. For example, a developer might forget to add a modifier to a function, which enables malicious users to invoke functions without the contract progressing to the correct state (e.g., place bids in an auction even though the auction should have already been closed due to a time limit).
Access Control: In many contracts, access to certain transitions (i.e., functions) needs to be controlled and restricted. For example, any user can participate in a typical blind auction by submitting a bid, but only the creator should be able to cancel the auction. To facilitate the enforcement of such constraints, we provide a plugin that (1) manages a list of administrators at runtime (identified by their addresses) and (2) enables developers to forbid non-administrators from accessing certain functions.
4 Conclusion and Future Work
Blockchain-based decentralized computing platforms with smart-contract functionality are envisioned to have a significant technological and economic impact in the future. However, if we are to avoid an equally significant risk of security incidents, we must ensure that smart contracts are secure. To facilitate the development of smart contracts that are secure by design, we created the FSolidM framework, which enables designing contracts as FSMs. Our framework is rooted in rigorous yet clear semantics, and it provides an easy-to-use graphical editor and code generator. We also implemented a set of plugins that developers can use to enhance the security or functionality of their contracts. In the future, we plan to integrate model checkers and compositional verification tools into our framework [12, 13] to enable the verification of security and safety properties.
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