OurContract：比特幣智能合約的設計

Abstract

以太坊實作了第一個基於區塊鏈的智能合約平臺，並成功促使眾多去中心化應用程式建置於其上。然而，對於比特幣及其衍生區塊鏈，目前尚未有成熟的智能合約功能實作。並且，為了實現智能合約所需的可追蹤帳戶，以太坊區塊鏈採用了帳戶模型而非類似比特幣的未花費交易輸出（unspent transaction output, UTXO）模型，但多少犧牲了用戶隱私。在這篇論文中，我們呈現了一種建構在比特幣上的智能合約平臺，稱為OurContract。我們在UTXO之上添加了帳戶層，以實現可追蹤的帳戶，同時保有UTXO模型的用戶隱私特性。為了使OurContract更實際，我們設計了合約帳戶與使用者之間的幣轉移，以及根據礦工執行合約的消耗來決定交易手續費等機制。為了讓使用者能開發OurContract應用程式，我們提供了一個功能受限的C作為程式語言，稱為C-，及其附帶函式庫用以與區塊鏈進行交互，稱為libourcontract。這項研發不僅能套用於比特幣，也能套用在其他任何基於UTXO模型的區塊鏈。

Ethereum implements the first blockchain-based smart contract platform and successfully brings lots of decentralized applications. For Bitcoin and its derivatives, however, there are currently no mature implementations with smart contracts. Moreover, the Ethereum blockchain adopts the account model instead of the UTXO model as in Bitcoin to achieve account tracking, which is necessary for smart contracts but compromises some user privacy. In this thesis, we present OurContract, an alternative solution to smart contract platform built on the Bitcoin blockchain. We construct an additional account layer on top of the UTXO model to achieve account tracking while preserving the privacy property of the UTXO model. To make OurContract more practical, we design mechanisms to enable coin transfer among contract accounts and users, and we allow transaction fee to be determined according to the cost for miners to execute contracts. For users to develop OurContract applications, we provide a limited version of C, called C-, as the programming language, and an associated library, called libourcontract, for C- programmers to interact with the blockchain. Our work can be applied to not only Bitcoin, but any blockchains based on the UTXO model.