Advancing the NFT standard: ERC721-Permit

And how to avoid the two step approve + transferFrom with ERC721-Permit (EIP-4494)

There's a new standard in the making. To understand how this really works, I recommend you take a look at my tutorials on:


But we'll try to cover the basics here also. You might be familiar already with ERC20-Permit (EIP-2612). It adds a new permit function. A user can sign an ERC20 approve transaction off-chain producing a signature that anyone could use and submit to the permit function. When permit is executed, it will execute the approve function. This allows for meta-transaction support of ERC20 transfers, but it also simply gets rid of the annoyance of needing two transactions: approve and transferFrom. Now you submit the signature to the smart contract which will call permit and then transferFrom in the same transaction.

Permit Police

ERC721-Permit: Preventing Misuse and Replays

The main issue we are facing is that a valid signature might be used several times or in other places where it's not intended to be used in. To prevent this we are adding several parameters. Under the hood we are using the already existing, widely used EIP-712 standard.

Okay I know this is getting confusing, EIP-712 and EIP-721 in one standard, but stay with me. We'll always call EIP-721 only ERC-721 from now on to make it easier.

EIP712 vs. EIP721 Meme

1. EIP-712 Domain Hash

With EIP-712, we define a domain separator for our ERC-721. 

bytes32 eip712DomainHash = keccak256(
    abi.encode(
        keccak256(
            "EIP712Domain(string name,string version,uint256 chainId,address verifyingContract)"
        ),
        keccak256(bytes(name())), // ERC-721 Name
        keccak256(bytes("1")),    // Version
        block.chainid,
        address(this)
    )
);

This ensures a signature is only used for our given token contract address on the correct chain id. The chain id was introduced to exactly identify a network after the Ethereum Classic fork which continued to use a network id of 1.  A list of existing chain ids can be seen here.

2. Permit Hash Struct

Now we can create a Permit specific signature:

bytes32 hashStruct = keccak256(
    abi.encode(
        keccak256("Permit(address spender,uint256 tokenId,uint256 nonce,uint256 deadline)"),
        spender,
        tokenId,
        nonces[tokenId],
        deadline
    )
);

This hashStruct will ensure that the signature can only used for

  • the permit function
  • to approve for spender
  • to approve the given tokenId
  • only valid before the given deadline
  • only valid for the given nonce

The nonce ensures someone can not replay a signature, i.e., use it multiple times on the same contract. As you can see these are the first real differences to ERC20-Permit. This now works on a tokenId basis rather than the ERC721 owner address. And also the nonce is only incremented upon transferring an ERC721, not upon calling permit. Why is that?

Well because with permits and NFT's you actually have a unique feature opportunity that's not possible with ERC20. You can allow a user to create multiple permit signatures for multiple spender addresses all for the same tokenId. All spenders can execute the permit function using the same nonce since we aren't incrementing the nonce inside permit. And only if the NFT is actually transferred, the nonce is incremented making the old signatures invalid. So make sure in your ERC721 contract to increase the nonce for every transfer:

function _transfer(address from, address to, uint256 tokenId) internal override {
    _nonces[tokenId]++;
    super._transfer(from, to, tokenId);
}

3. Final Hash

Now we can build the final signature starting with 0x1901 for an EIP-191-compliant 712 hash:
bytes32 hash = keccak256(
    abi.encodePacked("\x19\x01", eip712DomainHash, hashStruct)
);

4. Verifying the Signature

Using this hash we can use ecrecover to retrieve the signer of the function. But now we come to the next big difference to ERC20-Permit. The signature here is a hash and not the usually used v,r,s signature values. We'll come to why in a second, but first in the normal case you'll need to use assembly to recover the v,r,s values and then use them for the ecrecover:

bytes32 r;
bytes32 s;
uint8 v;
assembly {
    r := mload(add(signature, 0x20))
    s := mload(add(signature, 0x40))
    v := byte(0, mload(add(signature, 0x60)))
}

address signer = ecrecover(hash, v, r, s);
require(signer == owner, "ERC20Permit: invalid signature");
require(signer != address(0), "ECDSA: invalid signature");

Invalid signatures will produce an empty address, that's what the last check is for. So then why do we pass the signature as a string and not as v,r,s values? Seems a little more complicated passing it as string and needing assembly to verify the signature, doesn't it.

Well passing the signature as a string essentially allows for the most flexibility. If the signature is just a string, it's quite easy to extend the verification of it to more than just a regular ecrecover check. In fact already in the ERC721-Permit there are two additional signature verifications included:


EIP-2098 are a compact form standard for signatures. They make use of a mathematical trick in the representation of the signature on the elliptic curve. The details here don't matter, but essentially instead of requiring 65 bytes for a signature, you can represent it with only 64 bytes. But it requires a slightly different signature verification. But if you're using 64 or 65 bytes, obviously in both cases you can represent the signature as a string. Great.

And then EIP-1271 we looked at for meta-transactions already here, but it's essentially a way for smart contracts to create signatures. In our case imagine a smart contract is the owner of the NFT. Usually a smart contract cannot create signatures since there's not private key. When verifying a signature and it it's invalid, in a second step we can check if it's a valid smart contract signature:

We call the owner address of the NFT using staticcall, which ensures no further state modifications happen in the call. If the result succeeds and has a valid returnData length (this is very critical, see the previous 0x bug), we can check if the return value matches 0x1626ba7e, the magic value from EIP-1271 which implies 'it's a valid signature'. How the smart contract verifies the signature is up to the contract to decide.

Lastly if you read my post about ecrecover here, you will know there are some issues with it.

Complete Signature Verification

So you should handle those properly. Fortunately doing all of this is easier then it sounds when using the Openzeppelin contracts. A complete signature verification for ERC721-Permit might look like on the right.

First we make use of ECDSA.tryRecover. This will solve a couple things in one:

  • Support for EIP-2098 signatures.
  • Solving potential security problems with ecrecover.


Then if the signature verification recovers an address, we check if it's the owner of the NFT or if he is approved. You could of course only allow the owner, but since approved addresses in ERC721 should have full control, it's better to also give them control for permits.

And lastly if it's not a valid EOA so far, we can check if it's a valid contract signature. And since a contract can either be directly the owner or approved, we need to check it for both cases using the staticcall functionality.

Note that if a contract is approved via ERC721.setApprovalForAll, we won't be able to verify its signature and allow it to use permit. However, an approved for all contract could simply approve himself first using ERC721.approve.

(address signer, ) = ECDSA.tryRecover(hash, signature);
bool isValidEOASignature = signer != address(0) &&
    _isApprovedOrOwner(signer, tokenId);

require(
    isValidEOASignature ||
    _isValidContractERC1271Signature(
        ownerOf(tokenId),
        hash, signature
    ) || _isValidContractERC1271Signature(
        getApproved(tokenId),
        hash,
        signature
    ),
    "ERC721Permit: invalid signature"
);

function _isValidContractERC1271Signature(
    address signer,
    bytes32 hash,
    bytes memory signature
) private view returns (bool) {
    (bool success, bytes memory result) = signer.staticcall(
        abi.encodeWithSelector(
        IERC1271.isValidSignature.selector,
        hash,
        signature
        )
    );
    return (success &&
        result.length == 32 &&
        abi.decode(result,(bytes4))
            == IERC1271.isValidSignature.selector
    );
}

5. Approving the NFT

Now lastly we only have to increase the nonce for the owner and call the approve function:

_approve(owner, spender, amount);

You can see a full implementation example here.

Uniswap ERC721-Permit

The Uniswap implementation has some differences, see implementation here. In Uniswap V3 positions are wrapped in the ERC721 non-fungible token interface and come with permit functinoality.

However notable differences are:

  • signarure is passed via v,r,s values
  • nonce is incremented upon every permit execution, not allowing multiple permits to be executed
Uniswap Permit

ERC721-Permit Library

I have created an ERC-20 Permit library that you can import. You can find it at https://github.com/soliditylabs/ERC721-Permit.

Built using reference implementation as reference, the way it was meant to be used.

You can simply use it by installing via npm:

$ npm install @soliditylabs/erc721-permit --save-dev

Import it into your ERC-721 contract like this:

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

import {ERC721, ERC721Permit} from "@soliditylabs/erc20-permit/contracts/ERC20Permit.sol";

contract MyNFTContract is ERC721Permit("MyNFT", "MNFT") {
  uint256 private _lastTokenId;

  function mint() public {
    _mint(msg.sender, ++_lastTokenId);
  }

  function safeTransferFromWithPermit(
    address from,
    address to,
    uint256 tokenId,
    bytes memory _data,
    uint256 deadline,
    bytes memory signature
  ) external {
    _permit(msg.sender, tokenId, deadline, signature);
    safeTransferFrom(from, to, tokenId, _data);
  }
}

We also added a safeTransferFromWithPermit function here. This is not directly a standard function of the ERC721-Permit, but it can still be a pretty useful addition. With that you can approve and transfer in a single call, saving some extra gas and also not requiring any helper contracts.

Frontend Usage

Signing data via EIP-712 is directly supported in a lot of wallets these days. For example in MetaMask take a look here how to integrate it.

As always use with care

Be aware that the standard is not yet final. In fact it's at relatively early draft stage. I will keep the library updated in case the standard changes again. My library code was also not audited, use at your own risk. If you notice any issue or code being outdated, please contact me and I can update it.

You have reached the end of the article. I hereby permit you to comment and ask questions.


Markus Waas

Solidity Developer

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    What is Balancer? Balancer is very similar to Uniswap. If you're not familiar with Uniswap or Balancer yet, they are fully decentralized protocols for automated liquidity provision on Ethereum. An easier-to-understand description would be that they are decentralized exchanges (DEX) relying on...

  • mousetrap

    Navigating the pitfalls of securely interacting with ERC20 tokens

    Figuring out how to securely interact might be harder than you think

    You would think calling a few functions on an ERC-20 token is the simplest thing to do, right? Unfortunately I have some bad news, it's not. There are several things to consider and some errors are still pretty common. Let's start with the easy ones. Let's take a very common token: ... Now to...

  • Aave

    Why you should automatically generate interests from user funds

    How to integrate Aave and similar systems in your contracts

    If you're writing contracts that use, hold or manage user funds, you might want to consider using those funds for generating free extra income. What's the catch? That's right, it's basically free money and leaving funds unused in a contract is wasting a lot of potential. The way these...

  • Matic Logo

    How to use Polygon (Matic) in your Dapp

    Deploying and onboarding users to  Polygon  to avoid the high gas costs

    Gas costs are exploding again, ETH2.0 is still too far away and people are now looking at layer 2 solutions. Here's a good overview of existing layer 2 projects: https://github.com/Awesome-Layer-2/awesome-layer-2. Today we will take a closer look at Polygon (previously known as Matic) as a...

  • Migrating from Truffle to Buidler

    And why you should probably keep both.

    Why Buidler? Proper debugging is a pain with Truffle. Events are way too difficult to use as logging and they don't even work for reverted transactions (when you would need them most). Buidler gives you a console.log for your contracts which is a game changer. And you'll also get stack traces...

  • Factory

    Contract factories and clones

    How to deploy contracts within contracts as easily and gas-efficient as possible

    The factory design pattern is a pretty common pattern used in programming. The idea is simple, instead of creating objects directly, you have an object (the factory) that creates objects for you. In the case of Solidity, an object is a smart contract and so a factory will deploy new contracts for...

  • IPFS logo

    How to use IPFS in your Dapp?

    Using the interplanetary file system in your frontend and contracts

    You may have heard about IPFS before, the Interplanetary File System. The concept has existed for quite some time now, but with IPFS you'll get a more reliable data storage, thanks to their internal use of blockchain technology. Filecoin is a new system that is incentivizing storage for IPFS...

  • tiny-kitten

    Downsizing contracts to fight the contract size limit

    What can you do to prevent your contracts from getting too large?

    Why is there a limit? On November 22, 2016 the Spurious Dragon hard-fork introduced EIP-170 which added a smart contract size limit of 24.576 kb. For you as a Solidity developer this means when you add more and more functionality to your contract, at some point you will reach the limit and when...

  • EXTCODEHASH

    Using EXTCODEHASH to secure your systems

    How to safely integrate anyone's smart contract

    What is the EXTCODEHASH? The EVM opcode EXTCODEHASH was added on February 28, 2019 via EIP-1052. Not only does it help to reduce external function calls for compiled Solidity contracts, it also adds additional functionality. It gives you the hash of the code from an address. Since only contract...

  • Uniswap

    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...