Communications of the ACM
Secure Multiparty Computations on Bitcoin
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Is it possible to design an online protocol for playing a lottery, in a completely decentralized way, that is, without relying on a trusted third party? Or can one construct a fully decentralized protocol for selling secret information, so that neither the seller nor the buyer can cheat in it? Until recently, it seemed that every online protocol that has financial consequences for the participants needs to rely on some sort of a trusted server that ensures that the money is transferred between them. In this work, we propose to use Bitcoin (a digital currency, introduced in 2008) to design such fully decentralized protocols that are secure even if no trusted third party is available. As an instantiation of this idea, we construct protocols for secure multiparty lotteries using the Bitcoin currency, without relying on a trusted authority. Our protocols guarantee fairness for the honest parties no matter how the loser behaves. For example, if one party interrupts the protocol, then her money is transferred to the honest participants. Our protocols are practical (to demonstrate it, we performed their transactions in the actual Bitcoin system) and in principle could be used in real life as a replacement for the online gambling sites.
1. Introduction
One of the most attractive features of the Internet is its decentralization: the TCP/IP protocol itself, and several other protocols running on top of it do not rely on a single server, and often can be executed between parties that do not need to trust each other, or even do not need to know each other's true identity. Examples of such protocols include: the SMTP and the HTTP protocols, the peer-to-peer content distributions platforms, messaging systems, and many others. A natural question to ask is how far can the "decentralization" of the digital world go? In other words, what are the real-life applications which one can implement on the Internet without the need of a trusted third party? Until recently, one notable example of a task that seemed to always require some sort of a "trusted server" was the online financial transactions (that had to rely on a bank or a credit card company). This situation changed radically in 2009 when the first fully decentralized digital currency, called Bitcoin, was deployed by Nakamoto.17, a The huge success of Bitcoin (its current market capitalization is around $5 billion) is due precisely to its distributed nature and the lack of a central authority that controls Bitcoin transactions. We describe Bitcoin in more detail in Section 2.
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