How to build and use ERC-777 tokens

An intro to the new upgraded standard for ERC-20 tokens

The new upgraded standard for ERC-20 tokens is becoming more and more popular. It's fully backwards compatible, you can easily create one using the Openzeppelin contracts and there are many interesting new features not available in ERC-20.

Should you upgrade from ERC-20? Well let's look into what ERC-777 is.

xkcd upgrades

The Features of ERC-777

Let's explore all features with direct code examples that you can simply follow via Remix. Let's create an ERC-777 contract:

// SPDX-License-Identifier: MIT
pragma solidity 0.7.4;

import "http://github.com/OpenZeppelin/openzeppelin-contracts/blob/v3.2.1-solc-0.7/contracts/token/ERC777/ERC777.sol";
import "http://github.com/OpenZeppelin/openzeppelin-contracts/blob/v3.2.1-solc-0.7/contracts/token/ERC777/IERC777Sender.sol";
import "http://github.com/OpenZeppelin/openzeppelin-contracts/blob/v3.2.1-solc-0.7/contracts/token/ERC777/IERC777Recipient.sol";
import "http://github.com/OpenZeppelin/openzeppelin-contracts/blob/v3.2.1-solc-0.7/contracts/introspection/ERC1820Implementer.sol";
import "http://github.com/OpenZeppelin/openzeppelin-contracts/blob/v3.2.1-solc-0.7/contracts/introspection/IERC1820Registry.sol";

contract TestERC777 is ERC777 {
    constructor(
        uint256 initialSupply,
        address[] memory defaultOperators
    ) ERC777("Gold", "GLD", defaultOperators) {
        _mint(msg.sender, initialSupply, "", "");
    }
}

Don't worry about the imports and defaultOperators yet, we will need them later on.

1. Similar to sending ETH + Data

Now that we have a deployed token, we can use it similarly to ERC-20. One difference though is a new transfer function. While in ERC-20 we used to do token.transfer(receiver, amount), now with ERC-777 we do token.send(receiver, amount, "").

Why this change?

First of all it's now more similar to the way of sending ETH via the send function. And then we also have a new bytes data field which enables one to send arbitrary data along with the transfer. This can be used freely and thus adds extra functionality in the token transfer call. In the regular transfer case you would just leave it empty.

2. Registry (EIP-1820)

This is technically its own standard as the 777 standard was getting too large. So a new standard EIP-1820 was created. One reason the standard was needed was to enable the hooks functionality (see below). But let's look at the 1820 standard now.

We have previously looked at the EIP-165 standard here. As a quick recap EIP-165 allows smart contracts to register as implementing a specific interface. This can ensure no invalid smart contract addresses are used, for example preventing to send tokens to a contract that doesn't have functions to retrieve those tokens. In EIP-165 a contract must return true for the supportsInterface(interfaceId) function if it implements the given interface.

So what's different in EIP-1820?

Single Registry Contract

In contrast to EIP-165 we don't have contracts themselves implementing a supportsInterface function themselves. Instead there is a single registry contract. This registry is always deployed on the same address for every Ethereum network: 0x1820a4b7618bde71dce8cdc73aab6c95905fad24.

How is it ensured that the registry is always available at the same address for each network?

The solution for this is quite interesting. The so-called 'Nick method' was first described here. The cryptographic signature for signed transactions in Ethereum consists of three values, v, r and s. Usually these have to be generated by a private key and the ecrecover function will retrieve the signer public key, i.e., the Ethereum address. But as it turns out you can choose these values at random and in 50% of the times will still get a valid signature.

This is being used by the EIP-1820 standard to define fixed values for v, r and s which produce a valid signature:

{
    v: 27,
    r: '0x1820182018201820182018201820182018201820182018201820182018201820'
    s: '0x1820182018201820182018201820182018201820182018201820182018201820'
}

Now looking at these values, you can see that those are not random. So you also now know that nobody actually owns the private key that could have created this transaction signature. Since nobody owns the key and this signature is only valid to deploy the regular registry contract at the 1820... address, we have a guarantee that in every Ethereum network under the 0x1820a4b7618bde71dce8cdc73aab6c95905fad24 address:

  • we can find the correct registry contract
  • or no contract at all (not yet deployed)

Take a look at  https://etherscan.io/address/0x1820a4b7618bde71dce8cdc73aab6c95905fad24#code to see the registry in action for the mainnet.

Registering an Interface

Given the single registry contract, anyone can call

function setInterfaceImplementer(address _addr, bytes32 _interfaceHash, address _implementer)

This can be another smart contract, but even EOA (human controlled addresses) can register an interface for their own address. This was not possible with EIP-165.

Fully EIP-165 backwards compatible + Caching

And EIP-1820 comes fully backwards compatible. When you call the function:

function getInterfaceImplementer(address _addr, bytes32 _interfaceHash) returns (address)

with an EIP-165 interface hash (ending with 28 zeroes), the call is just forwarded => _addr.supportsInterface(_interfaceHash). This also allows for some extra caching to save gas and store the EIP-165 call result for future usage.

3. Key feature: Hooks

Now that we understand the registry, we can look at the key feature of ERC-777 which are the new hooks. They allow someone to register a smart contract function to be executed every time tokens are sent from this address and/or received to this address.

Let's look at how one would implement both of these cases:

Send Hooks

First a send hook. This will be our hook that executes every time tokens are about to be sent from the given address. If someone wants to use this hook for their own address, they would call

erc1820.setInterfaceImplementer(
    myAddress,
    TOKENS_SENDER_INTERFACE_HASH,
    usingERC777SenderHook
)
usingERC777SenderHook.registerHookForAccount(
    myAddress
)

Now every time tokens are sent from myAddress, the tokensToSend function will be executed. If you're curious how this is implemented in the ERC-777 token contract, take a look here:

address implementer = registry.getInterfaceImplementer(
    from,
    TOKENS_SENDER_INTERFACE_HASH
);
if (implementer != address(0)) {
    IERC777Sender(implementer).tokensToSend(
        operator,
        from,
        to,
        amount,
        userData,
        operatorData
    );
}
contract UsingERC777SenderHook is IERC777Sender, ERC1820Implementer {
    // keccak256("ERC777TokensSender")
    bytes32 constant private TOKENS_SENDER_INTERFACE_HASH =
        0x29ddb589b1fb5fc7cf394961c1adf5f8c6454761adf795e67fe149f658abe895;

    function registerHookForAccount(address account) public {
        _registerInterfaceForAddress(
            TOKENS_SENDER_INTERFACE_HASH,
            account
        );
    }

    function tokensToSend(
        address operator,
        address from,
        address to,
        uint256 amount,
        bytes calldata userData,
        bytes calldata operatorData
    ) external override {
        // this will be run for every registered
        // 'from' token transfers
    }
}

Receive Hook

Likewise we can create a hook that is run after the registered address has received tokens. In our example on the right we use this to register the contract itself as receiver and implementer.

Now every time tokens are received to our contract, the tokensReceived function will be executed. If you're curious how this is implemented in the ERC-777 token contract, take a look here:

address implementer = registry.getInterfaceImplementer(
    to,
    TOKENS_RECIPIENT_INTERFACE_HASH
);

if (implementer != address(0)) {
    IERC777Recipient(implementer).tokensReceived(
        operator,
        from,
        to,
        amount,
        userData,
        operatorData
    );
} else {
    require(!to.isContract());
}

As you can see, for contracts we actually revert in case no implementer is registered. This is a good thing! Now only contracts that are registered to receive tokens as our example on the right actually can receive tokens.

contract UsingERC777ReceiverHook is IERC777Recipient {
    ERC777 public token;
    IERC1820Registry public registry
        = IERC1820Registry(0x1820a4B7618BdE71Dce8cdc73aAB6C95905faD24);
    
    // keccak256('ERC777TokensRecipient')
    bytes32 constant private TOKENS_RECIPIENT_INTERFACE_HASH
        = 0xb281fc8c12954d22544db45de3159a39272895b169a852b314f9cc762e44c53b;
        
    mapping(address => uint256) private _balances;

    constructor() {
        token = new TestERC777(100 ether, new address[](0));
        token.transfer(msg.sender, 100 ether);
        
        registry.setInterfaceImplementer(
            address(this),
            TOKENS_RECIPIENT_INTERFACE_HASH,
            address(this)
        );
    }

    function tokensReceived(
        address /*operator*/,
        address from,
        address /*to*/,
        uint256 amount,
        bytes calldata /*userData*/,
        bytes calldata /*operatorData*/
    ) external override {
        require(msg.sender == address(token), "Invalid token");

        // like approve + transferFrom, but only one tx
        _balances[from] += amount;
    }
}

Approve + TransferFrom in one Transaction

Just like the ERC20-Permit standard, this also allows us to do approve + transferFrom in one transaction. What previously was

  1. Approve 10 tokens to contract (token.approve(contractAddress, 10e18))
  2. Call function contractAddress.execute (internally using token.transferFrom(msg.sender, address(this), 10e18)).

Now becomes:

  1. Send 10 tokens to contract (token.send(contractAddress, 10e18, "")). That's it, our registered tokensReceived function can internally call execute.

4. Operators Functionality

With ERC-777 you also get functionality to set operators. This can be a set of default operators defined in the constructor which will be able to transfer tokens on behalf of any address. Or they can be defined by the token holders themselves as being allowed to transfer on their behalfs.

Obviously you have to fully trust the operators. That's why for the most part you won't set a regular address as operator. Rather those operators are intended to be verified smart contracts such as an exchange, a cheque processor or an automatic charging system. Ensuring they cannot steal any money and behave in an expected manner.

5. Backwards Compatible

One of the good things about 777 is that it's fully backwards compatible with ERC-20. This means all the same functions must exist including the identical events. Meaning you can actually just treat it as an ERC-20. But be aware of hooks.

Yo Dawg Hooks

If you treat it as ERC-20 or not, any registered send or receive hooks will still be triggered regardless. People can abuse this for reentrancy attacks. This has happened earlier this year for 300k USD lost on Uniswap v1. Simple solution: use reentrancy guards.

So should I use ERC-777 instead of ERC-20?

I wouldn't say this is a clear decision yet. Out in the wild the amount of popular ERC-777 tokens is still pretty small. There are additional risks involved with 777 as mentioned above with reentrancy. Also added complexity and people not being very familiar with it yet are reasons to not use it.

Ask yourself if any of the ERC-777 features would be of particular value to you. If not, stick to ERC-20. Otherwise you might want to give ERC-777 a try.


Markus Waas

Solidity Developer

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    Using the new Uniswap v2 in your contracts

    What's new in Uniswap v2 and how to integrate Uniswap v2

    Note : For Uniswap 3 check out the tutorial here. What is UniSwap? If you're not familiar with Uniswap yet, it's a fully decentralized protocol for automated liquidity provision on Ethereum. An easier-to-understand description would be that it's a decentralized exchange (DEX) relying on external...

  • Continuous Integration

    Solidity and Truffle Continuous Integration Setup

    How to setup Travis or Circle CI for Truffle testing along with useful plugins.

    Continuous integration (CI) with Truffle is great for developing once you have a basic set of tests implemented. It allows you to run very long tests, ensure all tests pass before merging a pull request and to keep track of various statistics using additional tools. We will use the Truffle...

  • Devcon 6

    Upcoming Devcon 2021 and other events

    The Ethereum Foundation just announced the next Devcon in 2021 in Colombia

    Biggest virtual hackathon almost finished First of all, the current HackMoney event has come to an end and it has been a massive success. One can only imagine what kind of cool projects people have built in a 30 days hackathon. All final projects can be seen at:...

  • ERC-2020

    The Year of the 20: Creating an ERC20 in 2020

    How to use the latest and best tools to create an ERC-20 token contract

    You know what an ERC-20 is, you probably have created your own versions of it several times (if not, have a look at: ERC-20). But how would you start in 2020 using the latest tools? Let's create a new ERC-2020 token contract with some basic functionality which focuses on simplicity and latest...

  • hiring

    How to get a Solidity developer job?

    There are many ways to get a Solidity job and it might be easier than you think!

    You have mastered the basics of Solidity, created your first few useful projects and now want to get your hands on some real-world projects. Getting a Solidity developer job might be easier than you think. There are generally plenty of options to choose from and often times not a lot of...

  • People making fun

    Design Pattern Solidity: Mock contracts for testing

    Why you should make fun of your contracts

    Mock objects are a common design pattern in object-oriented programming. Coming from the old French word 'mocquer' with the meaning of 'making fun of', it evolved to 'imitating something real' which is actually what we are doing in programming. Please only make fun of your smart contracts if you...

  • React and Ethereum

    Kickstart your Dapp frontend development with create-eth-app

    An overview on how to use the app and its features

    Last time we looked at the big picture of Solidity and already mentioned the create-eth-app. Now you will find out how to use it, what features are integrated and additional ideas on how to expand on it. Started by Paul Razvan Berg, the founder of sablier, this app will kickstart your frontend...

  • Solidity Overview

    The big picture of Solidity and Blockchain development in 2020

    Overview of the most important technologies, services and tools that you need to know

    Now, I do not know about you, but I remember when I first started with Solidity development being very confused by all the tools and services and how they work in connection with one another. If you are like me, this overview will help you understand the big picture of Solidity development. As I...

  • Design Pattern Solidity: Free up unused storage

    Why you should clean up after yourself

    You may or may not be used to a garbage collectors in your previous programming language. There is no such thing in Solidity and even if there was a similar concept, you would still be better off managing state data yourself. Only you as a programmer can know exactly which data will not be used...

  • How to setup Solidity Developer Environment on Windows

    What you need to know about developing on Windows

    Using Windows for development, especially for Solidity development, can be a pain sometimes, but it does not have to be. Once you have configured your environment properly, it can actually be extremely efficient and Windows is a very, very stable OS, so your overall experience can be amazing. The...

  • Avoiding out of gas for Truffle tests

    How you do not have to worry about gas in tests anymore

    You have probably seen this error message a lot of times: Error: VM Exception while processing transaction: out of gas Disclaimer : Unfortunately, this does not always actually mean what it is saying when using Truffle , especially for older versions. It can occur for various reasons and might be...

  • Design Pattern Solidity: Stages

    How you can design stages in your contract

    Closely related to the concept of finite-state machines, this pattern will help you restrict functions in your contract. You will find a lot of situations where it might be useful. Any time a contract should allow function calls only in certain stages. Let's look at an example: contract Pool {...

  • Web3 1.2.5: Revert reason strings

    How to use the new feature

    A new Web3 version was just released and it comes with a new feature that should make your life easier. With the latest version 1.2.5, you can now see the the revert reason if you use the new handleRevert option. You can activate it easily by using web3.eth.handleRevert = true . Now when you use...

  • Gaining back control of the internet

    How Ocelot is decentralizing cloud computing

    I recently came across an ambitious company that will completely redefine the way we are using the internet. Or rather, the way we are using its underlying infrastructure which ultimately is the internet. While looking at their offering, I also learned how to get anonymous cloud machines, you...

  • Devcon 5 - Review

    Impressions from the conference

    I had a lot to catch up on after Devcon. Also things didn't go quite as planned, so please excuse my delayed review! This year's Devcon was certainly stormy with a big typhoon warning already on day 1. Luckily (for us, not the people in Tokyo), it went right past Osaka. Nevertheless, a lot of...

  • Devcon 5 - Information, Events, Links, Telegram

    What you need to know

    Devcon 5 is coming up soon and there are already lots of events available, information about Osaka and more. Here is a short overview: Events Events Calendar Events Google Docs Events Kickback Most events are in all three, but if you really want to see all, you will have to look at all three...

  • Design Pattern Solidity: Off-chain beats on-chain

    Why you should do as much as possible off-chain

    As you might have realized, Ethereum transactions are anything but cheap. In particular, if you are computing complex things or storing a lot of data. That means sometimes we cannot put all logic inside Solidity. Instead, we can utilize off-chain computations to help us. A very simple example...

  • Design Pattern Solidity: Initialize Contract after Deployment

    How to use the Initializable pattern

    There are a few reasons why you might want to initialize a contract after deployment and not directly by passing constructor arguments. But first let's look at an example: contract MyCrowdsale { uint256 rate; function initialize(uint256 _rate) public { rate = _rate; } } What's the advantage over...

  • Consensys Blockchain Jobs Report

    What the current blockchain job market looks like

    Consensys published their blockchain jobs report which you can checkout in their Blockchain Developer Job Kit. The most interesting aspects are Blockchain developer jobs have been growing at a rate of 33x of the previous year according to LinkedIns jobs report Typical salary is about...

  • Provable — Randomness Oracle

    How the Oraclize random number generator works

    One particularly interesting approach by Provable is the usage of a hardware security device, namely the Ledger Nano S. It uses a trusted execution environment to generate random numbers and provides a Provable Connector Contract as interface. How to use the Provable Randomness Oracle? Use the...

  • Solidity Design Patterns: Multiply before Dividing

    Why the correct order matters!

    There has been a lot of progress since the beginning of Ethereum about best practices in Solidity. Unfortunately, I have the feeling that most of the knowledge is within the circle of experienced people and there aren’t that many online resources about it. That is why I would like to start this...

  • Devcon 5 Applications closing in one week

    Devcon 5 Applications closing

    Watch out for the Devcon 5 applications. You only have one week left to apply either as Buidler Student Scholarship Press Devcon is by far the biggest and most impressive Ethereum conference in the world. And it's full of developers! I am especially excited about the cool location this year in...

  • Randomness and the Blockchain

    How to achieve secure randomness for Solidity smart contracts?

    When we talk about randomness and blockchain, these are really two problems: 1. How to generate randomness in smart contracts? 2. How to produce randomness for proof-of-stake (POS) systems? Or more generally, how to produce trusted randomness in public distributed systems? There is some overlap...