ERC-721: Non-Fungible Token Standard

Metadata
Status: FinalStandards Track: ERCCreated: 2018-01-24
Authors
William Entriken (@fulldecent), Dieter Shirley (dete@axiomzen.co), Jacob Evans (jacob@dekz.net), Nastassia Sachs (nastassia.sachs@protonmail.com)
Requires
EIP-165
Links
GitHub pageDiscussion on Ethereum Magicians

Simple Summary

A standard interface for non-fungible tokens, also known as deeds.

Abstract

The following standard allows for the implementation of a standard API for NFTs within smart contracts. This standard provides basic functionality to track and transfer NFTs.

We considered use cases of NFTs being owned and transacted by individuals as well as consignment to third party brokers/wallets/auctioneers ("operators"). NFTs can represent ownership over digital or physical assets. We considered a diverse universe of assets, and we know you will dream up many more:

  • Physical property — houses, unique artwork
  • Virtual collectibles — unique pictures of kittens, collectible cards
  • "Negative value" assets — loans, burdens and other responsibilities

In general, all houses are distinct and no two kittens are alike. NFTs are distinguishable and you must track the ownership of each one separately.

Motivation

A standard interface allows wallet/broker/auction applications to work with any NFT on Ethereum. We provide for simple ERC-721 smart contracts as well as contracts that track an arbitrarily large number of NFTs. Additional applications are discussed below.

This standard is inspired by the ERC-20 token standard and builds on two years of experience since EIP-20 was created. EIP-20 is insufficient for tracking NFTs because each asset is distinct (non-fungible) whereas each of a quantity of tokens is identical (fungible).

Differences between this standard and EIP-20 are examined below.

Specification

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119.

Every ERC-721 compliant contract must implement the ERC721 and ERC165 interfaces (subject to "caveats" below):



A wallet/broker/auction application MUST implement the wallet interface if it will accept safe transfers.




The metadata extension is OPTIONAL for ERC-721 smart contracts (see "caveats", below). This allows your smart contract to be interrogated for its name and for details about the assets which your NFTs represent.




This is the "ERC721 Metadata JSON Schema" referenced above.




The enumeration extension is OPTIONAL for ERC-721 smart contracts (see "caveats", below). This allows your contract to publish its full list of NFTs and make them discoverable.




Caveats


The 0.4.20 Solidity interface grammar is not expressive enough to document the ERC-721 standard. A contract which complies with ERC-721 MUST also abide by the following:


    

  • Solidity issue #3412: The above interfaces include explicit mutability guarantees for each function. Mutability guarantees are, in order weak to strong: payable, implicit nonpayable, view, and pure. Your implementation MUST meet the mutability guarantee in this interface and you MAY meet a stronger guarantee. For example, a payable function in this interface may be implemented as nonpayable (no state mutability specified) in your contract. We expect a later Solidity release will allow your stricter contract to inherit from this interface, but a workaround for version 0.4.20 is that you can edit this interface to add stricter mutability before inheriting from your contract.
    • 

  • Solidity issue #3419: A contract that implements ERC721Metadata or ERC721Enumerable SHALL also implement ERC721. ERC-721 implements the requirements of interface ERC-165.
    • 

  • Solidity issue #2330: If a function is shown in this specification as external then a contract will be compliant if it uses public visibility. As a workaround for version 0.4.20, you can edit this interface to switch to public before inheriting from your contract.
    • 

  • Solidity issues #3494, #3544: Use of this.*.selector is marked as a warning by Solidity, a future version of Solidity will not mark this as an error.
    • 



If a newer version of Solidity allows the caveats to be expressed in code, then this EIP MAY be updated and the caveats removed, such will be equivalent to the original specification.


Rationale


There are many proposed uses of Ethereum smart contracts that depend on tracking distinguishable assets. Examples of existing or planned NFTs are LAND in Decentraland, the eponymous punks in CryptoPunks, and in-game items using systems like DMarket or EnjinCoin. Future uses include tracking real-world assets, like real-estate (as envisioned by companies like Ubitquity or Propy). It is critical in each of these cases that these items are not "lumped together" as numbers in a ledger, but instead each asset must have its ownership individually and atomically tracked. Regardless of the nature of these assets, the ecosystem will be stronger if we have a standardized interface that allows for cross-functional asset management and sales platforms.


"NFT" Word Choice


"NFT" was satisfactory to nearly everyone surveyed and is widely applicable to a broad universe of distinguishable digital assets. We recognize that "deed" is very descriptive for certain applications of this standard (notably, physical property).


Alternatives considered: distinguishable asset, title, token, asset, equity, ticket


NFT Identifiers


Every NFT is identified by a unique uint256 ID inside the ERC-721 smart contract. This identifying number SHALL NOT change for the life of the contract. The pair (contract address, uint256 tokenId) will then be a globally unique and fully-qualified identifier for a specific asset on an Ethereum chain. While some ERC-721 smart contracts may find it convenient to start with ID 0 and simply increment by one for each new NFT, callers SHALL NOT assume that ID numbers have any specific pattern to them, and MUST treat the ID as a "black box". Also note that NFTs MAY become invalid (be destroyed). Please see the enumeration functions for a supported enumeration interface.


The choice of uint256 allows a wide variety of applications because UUIDs and sha3 hashes are directly convertible to uint256.


Transfer Mechanism


ERC-721 standardizes a safe transfer function safeTransferFrom (overloaded with and without a bytes parameter) and an unsafe function transferFrom. Transfers may be initiated by:


    

  • The owner of an NFT
    • 

  • The approved address of an NFT
    • 

  • An authorized operator of the current owner of an NFT
    • 



Additionally, an authorized operator may set the approved address for an NFT. This provides a powerful set of tools for wallet, broker and auction applications to quickly use a large number of NFTs.


The transfer and accept functions' documentation only specify conditions when the transaction MUST throw. Your implementation MAY also throw in other situations. This allows implementations to achieve interesting results:


    

  • Disallow transfers if the contract is paused — prior art, CryptoKitties deployed contract, line 611
    • 

  • Blocklist certain address from receiving NFTs — prior art, CryptoKitties deployed contract, lines 565, 566
    • 

  • Disallow unsafe transfers — transferFrom throws unless _to equals msg.sender or countOf(_to) is non-zero or was non-zero previously (because such cases are safe)
    • 

  • Charge a fee to both parties of a transaction — require payment when calling approve with a non-zero _approved if it was previously the zero address, refund payment if calling approve with the zero address if it was previously a non-zero address, require payment when calling any transfer function, require transfer parameter _to to equal msg.sender, require transfer parameter _to to be the approved address for the NFT
    • 

  • Read only NFT registry — always throw from safeTransferFrom, transferFrom, approve and setApprovalForAll
    • 



Failed transactions will throw, a best practice identified in ERC-223, ERC-677, ERC-827 and OpenZeppelin's implementation of SafeERC20.sol. ERC-20 defined an allowance feature, this caused a problem when called and then later modified to a different amount, as on OpenZeppelin issue #438. In ERC-721, there is no allowance because every NFT is unique, the quantity is none or one. Therefore we receive the benefits of ERC-20's original design without problems that have been later discovered.


Creation of NFTs ("minting") and destruction of NFTs ("burning") is not included in the specification. Your contract may implement these by other means. Please see the event documentation for your responsibilities when creating or destroying NFTs.


We questioned if the operator parameter on onERC721Received was necessary. In all cases we could imagine, if the operator was important then that operator could transfer the token to themself and then send it -- then they would be the from address. This seems contrived because we consider the operator to be a temporary owner of the token (and transferring to themself is redundant). When the operator sends the token, it is the operator acting on their own accord, NOT the operator acting on behalf of the token holder. This is why the operator and the previous token owner are both significant to the token recipient.


Alternatives considered: only allow two-step ERC-20 style transaction, require that transfer functions never throw, require all functions to return a boolean indicating the success of the operation.


ERC-165 Interface


We chose Standard Interface Detection (ERC-165) to expose the interfaces that a ERC-721 smart contract supports.


A future EIP may create a global registry of interfaces for contracts. We strongly support such an EIP and it would allow your ERC-721 implementation to implement ERC721Enumerable, ERC721Metadata, or other interfaces by delegating to a separate contract.


Gas and Complexity (regarding the enumeration extension)


This specification contemplates implementations that manage a few and arbitrarily large numbers of NFTs. If your application is able to grow then avoid using for/while loops in your code (see CryptoKitties bounty issue #4). These indicate your contract may be unable to scale and gas costs will rise over time without bound.


We have deployed a contract, XXXXERC721, to Testnet which instantiates and tracks 340282366920938463463374607431768211456 different deeds (2^128). That's enough to assign every IPV6 address to an Ethereum account owner, or to track ownership of nanobots a few micron in size and in aggregate totalling half the size of Earth. You can query it from the blockchain. And every function takes less gas than querying the ENS.


This illustration makes clear: the ERC-721 standard scales.


Alternatives considered: remove the asset enumeration function if it requires a for-loop, return a Solidity array type from enumeration functions.


Privacy


Wallets/brokers/auctioneers identified in the motivation section have a strong need to identify which NFTs an owner owns.


It may be interesting to consider a use case where NFTs are not enumerable, such as a private registry of property ownership, or a partially-private registry. However, privacy cannot be attained because an attacker can simply (!) call ownerOf for every possible tokenId.


Metadata Choices (metadata extension)


We have required name and symbol functions in the metadata extension. Every token EIP and draft we reviewed (ERC-20, ERC-223, ERC-677, ERC-777, ERC-827) included these functions.


We remind implementation authors that the empty string is a valid response to name and symbol if you protest to the usage of this mechanism. We also remind everyone that any smart contract can use the same name and symbol as your contract. How a client may determine which ERC-721 smart contracts are well-known (canonical) is outside the scope of this standard.


A mechanism is provided to associate NFTs with URIs. We expect that many implementations will take advantage of this to provide metadata for each NFT. The image size recommendation is taken from Instagram, they probably know much about image usability. The URI MAY be mutable (i.e. it changes from time to time). We considered an NFT representing ownership of a house, in this case metadata about the house (image, occupants, etc.) can naturally change.


Metadata is returned as a string value. Currently this is only usable as calling from web3, not from other contracts. This is acceptable because we have not considered a use case where an on-blockchain application would query such information.


Alternatives considered: put all metadata for each asset on the blockchain (too expensive), use URL templates to query metadata parts (URL templates do not work with all URL schemes, especially P2P URLs), multiaddr network address (not mature enough)


Community Consensus


A significant amount of discussion occurred on the original ERC-721 issue, additionally we held a first live meeting on Gitter that had good representation and well advertised (on Reddit, in the Gitter #ERC channel, and the original ERC-721 issue). Thank you to the participants:


    

  • @ImAllInNow Rob from DEC Gaming / Presenting Michigan Ethereum Meetup Feb 7
    • 

  • @Arachnid Nick Johnson
    • 

  • @jadhavajay Ajay Jadhav from AyanWorks
    • 

  • @superphly Cody Marx Bailey - XRAM Capital / Sharing at hackathon Jan 20 / UN Future of Finance Hackathon.
    • 

  • @fulldecent William Entriken
    • 



A second event was held at ETHDenver 2018 to discuss distinguishable asset standards (notes to be published).


We have been very inclusive in this process and invite anyone with questions or contributions into our discussion. However, this standard is written only to support the identified use cases which are listed herein.


Backwards Compatibility


We have adopted balanceOf, totalSupply, name and symbol semantics from the ERC-20 specification. An implementation may also include a function decimals that returns uint8(0) if its goal is to be more compatible with ERC-20 while supporting this standard. However, we find it contrived to require all ERC-721 implementations to support the decimals function.


Example NFT implementations as of February 2018:


    

  • CryptoKitties -- Compatible with an earlier version of this standard.
    • 

  • CryptoPunks -- Partially ERC-20 compatible, but not easily generalizable because it includes auction functionality directly in the contract and uses function names that explicitly refer to the assets as "punks".
    • 

  • Auctionhouse Asset Interface -- The author needed a generic interface for the Auctionhouse ÐApp (currently ice-boxed). His "Asset" contract is very simple, but is missing ERC-20 compatibility, approve() functionality, and metadata. This effort is referenced in the discussion for EIP-173.
    • 



Note: "Limited edition, collectible tokens" like Curio Cards and Rare Pepe are not distinguishable assets. They're actually a collection of individual fungible tokens, each of which is tracked by its own smart contract with its own total supply (which may be 1 in extreme cases).


The onERC721Received function specifically works around old deployed contracts which may inadvertently return 1 (true) in certain circumstances even if they don't implement a function (see Solidity DelegateCallReturnValue bug). By returning and checking for a magic value, we are able to distinguish actual affirmative responses versus these vacuous trues.


Test Cases


0xcert ERC-721 Token includes test cases written using Truffle.


Implementations


0xcert ERC721 -- a reference implementation


    

  • MIT licensed, so you can freely use it for your projects
    • 

  • Includes test cases
    • 

  • Active bug bounty, you will be paid if you find errors
    • 



Su Squares -- an advertising platform where you can rent space and place images


    

  • Complete the Su Squares Bug Bounty Program to seek problems with this standard or its implementation
    • 

  • Implements the complete standard and all optional interfaces
    • 



ERC721ExampleDeed -- an example implementation


    

  • Implements using the OpenZeppelin project format
    • 



XXXXERC721, by William Entriken -- a scalable example implementation


    

  • Deployed on testnet with 1 billion assets and supporting all lookups with the metadata extension. This demonstrates that scaling is NOT a problem.
    • 



References


Standards


    

  1. ERC-20 Token Standard.
    2. 

  2. ERC-165 Standard Interface Detection.
    3. 

  3. ERC-173 Owned Standard.
    4. 

  4. ERC-223 Token Standard.
    5. 

  5. ERC-677 transferAndCall Token Standard.
    6. 

  6. ERC-827 Token Standard.
    7. 

  7. Ethereum Name Service (ENS). https://ens.domains
    8. 

  8. Instagram -- What's the Image Resolution? https://help.instagram.com/1631821640426723
    9. 

  9. JSON Schema. https://json-schema.org/
    10. 

  10. Multiaddr. https://github.com/multiformats/multiaddr
    11. 

  11. RFC 2119 Key words for use in RFCs to Indicate Requirement Levels. https://www.ietf.org/rfc/rfc2119.txt
    12. 



Issues


    

  1. The Original ERC-721 Issue. https://github.com/ethereum/eips/issues/721
    2. 

  2. Solidity Issue #2330 -- Interface Functions are External. https://github.com/ethereum/solidity/issues/2330
    3. 

  3. Solidity Issue #3412 -- Implement Interface: Allow Stricter Mutability. https://github.com/ethereum/solidity/issues/3412
    4. 

  4. Solidity Issue #3419 -- Interfaces Can't Inherit. https://github.com/ethereum/solidity/issues/3419
    5. 

  5. Solidity Issue #3494 -- Compiler Incorrectly Reasons About the selector Function. https://github.com/ethereum/solidity/issues/3494
    6. 

  6. Solidity Issue #3544 -- Cannot Calculate Selector of Function Named transfer. https://github.com/ethereum/solidity/issues/3544
    7. 

  7. CryptoKitties Bounty Issue #4 -- Listing all Kitties Owned by a User is O(n^2). https://github.com/axiomzen/cryptokitties-bounty/issues/4
    8. 

  8. OpenZeppelin Issue #438 -- Implementation of approve method violates ERC20 standard. https://github.com/OpenZeppelin/zeppelin-solidity/issues/438
    9. 

  9. Solidity DelegateCallReturnValue Bug. https://solidity.readthedocs.io/en/develop/bugs.html#DelegateCallReturnValue
    10. 



Discussions


    

  1. Reddit (announcement of first live discussion). https://www.reddit.com/r/ethereum/comments/7r2ena/friday_119_live_discussion_on_erc_nonfungible/
    2. 

  2. Gitter #EIPs (announcement of first live discussion). https://gitter.im/ethereum/EIPs?at=5a5f823fb48e8c3566f0a5e7
    3. 

  3. ERC-721 (announcement of first live discussion). https://github.com/ethereum/eips/issues/721#issuecomment-358369377
    4. 

  4. ETHDenver 2018. https://ethdenver.com
    5. 



NFT Implementations and Other Projects


    

  1. CryptoKitties. https://www.cryptokitties.co
    2. 

  2. 0xcert ERC-721 Token. https://github.com/0xcert/ethereum-erc721
    3. 

  3. Su Squares. https://tenthousandsu.com
    4. 

  4. Decentraland. https://decentraland.org
    5. 

  5. CryptoPunks. https://www.larvalabs.com/cryptopunks
    6. 

  6. DMarket. https://www.dmarket.io
    7. 

  7. Enjin Coin. https://enjincoin.io
    8. 

  8. Ubitquity. https://www.ubitquity.io
    9. 

  9. Propy. https://tokensale.propy.com
    10. 

  10. CryptoKitties Deployed Contract. https://etherscan.io/address/0x06012c8cf97bead5deae237070f9587f8e7a266d#code
    11. 

  11. Su Squares Bug Bounty Program. https://github.com/fulldecent/su-squares-bounty
    12. 

  12. XXXXERC721. https://github.com/fulldecent/erc721-example
    13. 

  13. ERC721ExampleDeed. https://github.com/nastassiasachs/ERC721ExampleDeed
    14. 

  14. Curio Cards. https://mycuriocards.com
    15. 

  15. Rare Pepe. https://rarepepewallet.com
    16. 

  16. Auctionhouse Asset Interface. https://github.com/dob/auctionhouse/blob/master/contracts/Asset.sol
    17. 

  17. OpenZeppelin SafeERC20.sol Implementation. https://github.com/OpenZeppelin/zeppelin-solidity/blob/master/contracts/token/ERC20/SafeERC20.sol
    18. 



Copyright


Copyright and related rights waived via CC0.