Securing Smart Contracts in Blockchain Blockchain is an emerging technology that underlies creation and exchange of the digital assets, including cryptocurrency such as Bitcoin and Ether, without the need for a central authority. It provides a public ledger for recording sequence of transactions in blocks that are linked as a chain. Smart contracts are computer programs governing participant agreements that are automatically enforced by consensus protocols in the blockchain. Together, blockchain and smart contracts revolutionize efficient transaction stores, services and workflows that work even among distrusting participants and without a trusted authority. Unfortunately, like most software, smart contracts are vulnerable as evidenced by a recent Decentralized Autonomous Organization (DAO) attack that lost cryptocurrency thenvalued about 60 million. Correctness of executions alone is not sufficient to guarantee security of smart contracts. This paper addresses how we can apply model checking, a wellestablished formal verification technique, to help alleviate security issues in smart contract development. Most existing studies have focused on verification of smart contracts on a specific language and specific platform. Smart contracts may have hidden operational side effects that impact software behaviors. Thus, applying model checking to smart contracts is not necessarily straightforward. This paper presents a general technique for building the core functional models applicable for model checking to identify all possible executions that lead to security breaches. It also shows how resulting executions can be systematically analyzed to help identify security issues. The models are language and system independent in that they can represent any smart contract in any language or any platform. We illustrate and evaluate the technique with a widely used example of a smart contract in a financial system along with experimental results using a wellknown model checker, NuSMV in various scenarios.

MPro: Combining Static and Symbolic Analysis for Scalable Testing of Smart Contract Smart contracts are executable programs that enable the building of a programmable trust mechanism between multiple entities without the need of a trusted thirdparty. At the time of this writing, there were over 10 million smart contracts deployed on the Ethereum networks and this number continues to grow at a rapid pace. Smart contracts are often written in a Turingcomplete programming language called Solidity, which is not easy to audit for subtle errors. Further, since smart contracts are immutable, errors have led to attacks resulting in losses of cryptocurrency worth 100s of millions of USD and reputational damage. Unfortunately, manual security analyses do not scale with size and number of smart contracts. Automated and scalable mechanisms are essential if smart contracts are to gain mainstream acceptance. Researchers have developed several security scanners in the past couple of years. However, many of these analyzer either do not scale well, or if they do, produce many false positives. This issue is exacerbated when bugs are triggered only after a series of interactions with the functions of the contractundertest. A depthn vulnerability, refers to a vulnerability that requires invoking a specific sequence of n functions to trigger. Depthn vulnerabilities are timeconsuming to detect by existing automated analyzers, because of the combinatorial explosion of sequences of functions that could be executed on smart contracts. In this paper, we present a technique to analyze depthn vulnerabilities in an efficient and scalable way by combining symbolic execution and data dependency analysis. A significant advantage of combining symbolic with static analysis is that it scales much better than symbolic alone and does not have the problem of false positive that static analysis tools typically have. We have implemented our technique in a tool called MPro, a scalable and automated smart contract analyzer based on the existing symbolic analysis tool MythrilClassic and the static analysis tool Slither. We analyzed 100 randomly chosen smart contracts on MPro and our evaluation shows that MPro is about ntimes faster than MythrilClassic for detecting depthn vulnerabilities, while preserving all the detection capabilities of MythrilClassic.