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A library for secure smart contract development. Build on a solid foundation of community-vetted code.

• Implementations of standards like ERC20 and ERC721.
• Flexible role-based permissioning scheme.
• Reusable Solidity components to build custom contracts and complex decentralized systems.
• First-class integration with the Gas Station Network for systems with no gas fees!
• Audited by leading security firms (last full audit on v2.0.0).

Overview

Installation

$ npm install @openzeppelin/contracts

OpenZeppelin Contracts features a stable API, which means your contracts won't break unexpectedly when upgrading to a newer minor version.

Usage

Once installed, you can use the contracts in the library by importing them:

pragma solidity ^0.8.0;

import "@openzeppelin/contracts/token/ERC721/ERC721.sol";

contract MyCollectible is ERC721 {
    constructor() ERC721("MyCollectible", "MCO") public {
    }
}

If you're new to smart contract development, head to Developing Smart Contracts to learn about creating a new project and compiling your contracts.

To keep your system secure, you should always use the installed code as-is, and neither copy-paste it from online sources, nor modify it yourself. The library is designed so that only the contracts and functions you use are deployed, so you don't need to worry about it needlessly increasing gas costs.

Learn More

The guides in the docs site will teach about different concepts, and how to use the related contracts that OpenZeppelin Contracts provides:

• Access Control: decide who can perform each of the actions on your system.
• Tokens: create tradeable assets or collectives, and distribute them via Crowdsales.
• Gas Station Network: let your users interact with your contracts without having to pay for gas themselves.
• Utilities: generic useful tools, including non-overflowing math, signature verification, and trustless paying systems.

The full API is also thoroughly documented, and serves as a great reference when developing your smart contract application. You can also ask for help or follow Contracts's development in the community forum.

Finally, you may want to take a look at the guides on our blog, which cover several common use cases and good practices.. The following articles provide great background reading, though please note, some of the referenced tools have changed as the tooling in the ecosystem continues to rapidly evolve.

• The Hitchhiker’s Guide to Smart Contracts in Ethereum will help you get an overview of the various tools available for smart contract development, and help you set up your environment.
• A Gentle Introduction to Ethereum Programming, Part 1 provides very useful information on an introductory level, including many basic concepts from the Ethereum platform.
• For a more in-depth dive, you may read the guide Designing the Architecture for Your Ethereum Application, which discusses how to better structure your application and its relationship to the real world.

Security

This project is maintained by OpenZeppelin, and developed following our high standards for code quality and security. OpenZeppelin is meant to provide tested and community-audited code, but please use common sense when doing anything that deals with real money! We take no responsibility for your implementation decisions and any security problems you might experience.

The core development principles and strategies that OpenZeppelin is based on include: security in depth, simple and modular code, clarity-driven naming conventions, comprehensive unit testing, pre-and-post-condition sanity checks, code consistency, and regular audits.

The latest audit was done on October 2018 on version 2.0.0.

Please report any security issues you find to [e-mail].

Contribute

OpenZeppelin exists thanks to its contributors. There are many ways you can participate and help build high quality software. Check out the contribution guide!

License

OpenZeppelin is released under the MIT License.

Changelog

3.4.0 (2021-02-02)

• BeaconProxy: added new kind of proxy that allows simultaneous atomic upgrades. (#2411)
• EIP712: added helpers to verify EIP712 typed data signatures on chain. (#2418)
• ERC20Permit: added an implementation of the ERC20 permit extension for gasless token approvals. (#2237)
• Presets: added token presets with preminted fixed supply ERC20PresetFixedSupply and ERC777PresetFixedSupply. (#2399)
• Address: added functionDelegateCall, similar to the existing functionCall. (#2333)
• Clones: added a library for deploying EIP 1167 minimal proxies. (#2449)
• Context: moved from contracts/GSN to contracts/utils. (#2453)
• PaymentSplitter: replace usage of .transfer() with Address.sendValue for improved compatibility with smart wallets. (#2455)
• UpgradeableProxy: bubble revert reasons from initialization calls. (#2454)
• SafeMath: fix a memory allocation issue by adding new SafeMath.tryOp(uint,uint)→(bool,uint) functions. SafeMath.op(uint,uint,string)→uint are now deprecated. (#2462)
• EnumerableMap: fix a memory allocation issue by adding new EnumerableMap.tryGet(uint)→(bool,address) functions. EnumerableMap.get(uint)→string is now deprecated. (#2462)
• ERC165Checker: added batch getSupportedInterfaces. (#2469)
• RefundEscrow: beneficiaryWithdraw will forward all available gas to the beneficiary. (#2480)
• Many view and pure functions have been made virtual to customize them via overrides. In many cases this will not imply that other functions in the contract will automatically adapt to the overridden definitions. People who wish to override should consult the source code to understand the impact and if they need to override any additional functions to achieve the desired behavior.

Security Fixes

• ERC777: fix potential reentrancy issues for custom extensions to ERC777. (#2483)

If you're using our implementation of ERC777 from version 3.3.0 or earlier, and you define a custom _beforeTokenTransfer function that writes to a storage variable, you may be vulnerable to a reentrancy attack. If you're affected and would like assistance please write to [e-mail]. Read more in the pull request.