The Game Among Bribers in a Smart Contract System
Blockchain has been used to build various applications, and the introduction of smart contracts further extends its impacts. Most of existing works consider the positive usage of smart contracts but ignore the other side of it: smart contracts can be used
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Abstract. Blockchain has been used to build various applications, and the introduction of smart contracts further extends its impacts. Most of existing works consider the positive usage of smart contracts but ignore the other side of it: smart contracts can be used in a destructive way, particularly, they can be utilized to carry out bribery. The hardness of tracing a briber in a blockchain system may even motivate bribers. Furthermore, an adversary can utilize bribery smart contracts to influence the execution results of other smart contracts in the same system. To better understand this threat, we propose a formal framework to analyze bribery in the smart contract system using game theory. We give a full characterization on how the bribery budget of a briber may influence the execution of a smart contract if the briber tries to manipulate its execution result by bribing users in the system.
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Introduction
Various applications are developed on top of blockchain technology [31–33]. However, most of these works assume that the blockchain is a perfect system, e.g., all records stored in the system are correct, and ignore the complexity of the way that the decentralized system achieves consensus. For purely cryptocurrency systems, both static model [24] and game theory model [21,27] have been used to analyze their security features. The introduction of the smart contract makes the situation trickier while extending the applicability of blockchain technology. A smart contract can involve multiple users/participants and have a high value stake. Thus, it has the potential to be more critical than mining in pure cryptocurrency systems (e.g., Bitcoin), in which only a fixed reward is paid to successful miners. The amount of cryptocurrency involved in a contract may be many times and significantly higher than the cost of running the contract itself. Therefore, users involved in a smart contract have the incentive to push through a certain outcome. In particular, they may achieve such a goal through bribery, i.e., offering cryptocurrencies to other users in the system. Interestingly, bribery itself can also be carried out using smart contracts. A recent work discussed this c International Financial Cryptography Association 2019 A. Zohar et al. (Eds.): FC 2018 Workshops, LNCS 10958, pp. 294–307, 2019. https://doi.org/10.1007/978-3-662-58820-8_20
The Game Among Bribers in a Smart Contract System
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concept and proposed a straightforward framework to implement bribery on blockchain [20] where the briber offers incentive to the bribee through a smart contract. Bribery is a serious problem as it may help to compromise the fundamental assumption of smart contract execution model based on consensus or majority accepted outcome. Note that a user is honest in mining does not necessarily means that he/she will remain honest when offered with monetary reward in making decisions. Their honesty is even more questionable when taking into consideration the unlinkability of users’ identities to real persons, and the fact that there is
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