A smart contract is a computer program or a transaction protocol that is intended to automatically execute, control or document legally-relevant events and actions according to the terms of a contract or an agreement. The objectives of smart contracts are the reduction of need for trusted intermediators, arbitration costs, and fraud losses, as well as the reduction of malicious and accidental exceptions. Smart contracts are commonly associated with cryptocurrencies, and the smart contracts introduced by Ethereum are generally considered a fundamental building block for decentralized finance (DeFi) and NFT applications.

Vending machines are mentioned as the oldest piece of technology equivalent to smart contract implementation. The original Ethereum white paper by Vitalik Buterin in 2014 describes the Bitcoin protocol as a weak version of the smart contract concept as originally defined by Nick Szabo, and proposes a stronger version based on the Solidity language, which is Turing complete. Since Bitcoin, various cryptocurrencies support scripting languages which allow for more advanced smart contracts between untrusted parties.

A smart contract should not be confused with a smart legal contract, which refers to a traditional, natural-language, legally-binding agreement that has selected terms expressed and implemented in machine-readable code.

Workings

Similar to a transfer of value on a blockchain, deployment of a smart contract on a blockchain occurs by sending a transaction from a wallet for the blockchain. The transaction includes the compiled code for the smart contract as well as a special receiver address. That transaction must then be included in a block that is added to the blockchain, at which point the smart contract’s code will execute to establish the initial state of the smart contract. Byzantine fault-tolerant algorithms secure the smart contract in a decentralized way from attempts to tamper with it. Once a smart contract is deployed, it cannot be updated. Smart contracts on a blockchain can store arbitrary state and execute arbitrary computations. End clients interact with a smart contract through transactions. Such transactions with a smart contract can invoke other smart contracts. These transactions might result in changing the state and sending coins from one smart contract to another or from one account to another.