ARC-72: Algorand Smart Contract NFT Specification

Base specification for non-fungible tokens implemented as smart contracts.

Algorand Smart Contract NFT Specification

Abstract

This specifies an interface for non-fungible tokens (NFTs) to be implemented on Algorand as smart contracts. This interface defines a minimal interface for NFTs to be owned and traded, to be augmented by other standard interfaces and custom methods.

Motivation

Currently most NFTs in the Algorand ecosystem are implemented as ASAs. However, to provide rich extra functionality, it can be desirable to implement NFTs as a smart contract instead. To foster an interoperable NFT ecosystem, it is necessary that the core interfaces for NFTs be standardized.

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.

Core NFT specification

A smart contract NFT that is compliant with this standard must implement the interface detection standard defined in ARC-73.

Additionally, the smart contract MUST implement the following interface:

{
  "name": "ARC-72",
  "desc": "Smart Contract NFT Base Interface",
  "methods": [
    {
      "name": "arc72_ownerOf",
      "desc": "Returns the address of the current owner of the NFT with the given tokenId",
      "readonly": true,
      "args": [
        { "type": "uint256", "name": "tokenId", "desc": "The ID of the NFT" },
      ],
      "returns": { "type": "address", "desc": "The current owner of the NFT." }
    },
    {
      "name": "arc72_transferFrom",
      "desc": "Transfers ownership of an NFT",
      "readonly": false,
      "args": [
        { "type": "address", "name": "from" },
        { "type": "address", "name": "to" },
        { "type": "uint256", "name": "tokenId" }
      ],
      "returns": { "type": "void" }
    },
  ],
  "events": [
    {
      "name": "arc72_Transfer",
      "desc": "Transfer ownership of an NFT",
      "args": [
        {
          "type": "address",
          "name": "from",
          "desc": "The current owner of the NFT"
        },
        {
          "type": "address",
          "name": "to",
          "desc": "The new owner of the NFT"
        },
        {
          "type": "uint256",
          "name": "tokenId",
          "desc": "The ID of the transferred NFT"
        }
      ]
    }
  ]
}

Ownership of a token ID by the zero address indicates that ID is invalid. The arc72_ownerOf method MUST return the zero address for invalid token IDs. The arc72_transferFrom method MUST error when from is not the owner of tokenId. The arc72_transferFrom method MUST error unless called by the owner of tokenId or an approved operator as defined by an extension such as the transfer management extension defined in this ARC. The arc72_transferFrom method MUST emit a arc72_Transfer event a transfer is successful. A arc72_Transfer event SHOULD be emitted, with from being the zero address, when a token is first minted. A arc72_Transfer event SHOULD be emitted, with to being the zero address, when a token is destroyed.

All methods in this and other interfaces defined throughout this standard that are marked as readonly MUST be read-only as defined by ARC-22.

The ARC-73 interface selector for this core interface is 0x53f02a40.

Metadata Extension

A smart contract NFT that is compliant with this metadata extension MUST implement the interfaces required to comply with the Core NFT Specification, as well as the following interface:

{
  "name": "ARC-72 Metadata Extension",
  "desc": "Smart Contract NFT Metadata Interface",
  "methods": [
    {
      "name": "arc72_tokenURI",
      "desc": "Returns a URI pointing to the NFT metadata",
      "readonly": true,
      "args": [
        { "type": "uint256", "name": "tokenId", "desc": "The ID of the NFT" },
      ],
      "returns": { "type": "byte[256]", "desc": "URI to token metadata." }
    }
  ],
}

URIs shorter than the return length MUST be padded with zero bytes at the end of the URI. The token URI returned SHOULD be an ipfs://... URI so the metadata can’t expire or be changed by a lapse or takeover of a DNS registration. The token URI SHOULD NOT be an http:// URI due to security concerns. The URI SHOULD resolve to a JSON file following :

• the JSON Metadata File Schema defined in ARC-3.
• the standard for declaring traits defined in ARC-16.

Future standards could define new recommended URI or file formats for metadata.

The ARC-73 interface selector for this metadata extension interface is 0xc3c1fc00.

Transfer Management Extension

A smart contract NFT that is compliant with this transfer management extension MUST implement the interfaces required to comply with the Core NFT Specification, as well as the following interface:

{
  "name": "ARC-72 Transfer Management Extension",
  "desc": "Smart Contract NFT Transfer Management Interface",
  "methods": [
    {
      "name": "arc72_approve",
      "desc": "Approve a controller for a single NFT",
      "readonly": false,
      "args": [
        { "type": "address", "name": "approved", "desc": "Approved controller address" },
        { "type": "uint256", "name": "tokenId", "desc": "The ID of the NFT" },
      ],
      "returns": { "type": "void" }
    },
    {
      "name": "arc72_setApprovalForAll",
      "desc": "Approve an operator for all NFTs for a user",
      "readonly": false,
      "args": [
        { "type": "address", "name": "operator", "desc": "Approved operator address" },
        { "type": "bool", "name": "approved", "desc": "true to give approval, false to revoke" },
      ],
      "returns": { "type": "void" }
    },
    {
      "name": "arc72_getApproved",
      "desc": "Get the current approved address for a single NFT",
      "readonly": true,
      "args": [
        { "type": "uint256", "name": "tokenId", "desc": "The ID of the NFT" },
      ],
      "returns": { "type": "address", "desc": "address of approved user or zero" }
    },
    {
      "name": "arc72_isApprovedForAll",
      "desc": "Query if an address is an authorized operator for another address",
      "readonly": true,
      "args": [
        { "type": "address", "name": "owner" },
        { "type": "address", "name": "operator" },
      ],
      "returns": { "type": "bool", "desc": "whether operator is authorized for all NFTs of owner" }
    },
  ],
  "events": [
    {
      "name": "arc72_Approval",
      "desc": "An address has been approved to transfer ownership of the NFT",
      "args": [
        {
          "type": "address",
          "name": "owner",
          "desc": "The current owner of the NFT"
        },
        {
          "type": "address",
          "name": "approved",
          "desc": "The approved user for the NFT"
        },
        {
          "type": "uint256",
          "name": "tokenId",
          "desc": "The ID of the NFT"
        }
      ]
    },
    {
      "name": "arc72_ApprovalForAll",
      "desc": "Operator set or unset for all NFTs defined by this contract for an owner",
      "args": [
        {
          "type": "address",
          "name": "owner",
          "desc": "The current owner of the NFT"
        },
        {
          "type": "address",
          "name": "operator",
          "desc": "The approved user for the NFT"
        },
        {
          "type": "bool",
          "name": "approved",
          "desc": "Whether operator is authorized for all NFTs of owner "
        }
      ]
    },
  ]
}

The arc72_Approval event MUST be emitted when the arc72_approve method is called successfully. The zero address for the arc72_approve method and the arc72_Approval event indicate no approval, including revocation of previous single NFT controller. When a arc72_Transfer event emits, this also indicates that the approved address for that NFT (if any) is reset to none. The arc72_ApprovalForAll event MUST be emitted when the arc72_setApprovalForAll method is called successfully. The contract MUST allow multiple operators per owner. The arc72_transferFrom method, when its nftId argument is owned by its from argument, MUST succeed for when called by an address that is approved for the given NFT or approved as operator for the owner.

The ARC-73 interface selector for this transfer management extension interface is 0xb9c6f696.

Enumeration Extension

A smart contract NFT that is compliant with this enumeration extension MUST implement the interfaces required to comply with the Core NFT Specification, as well as the following interface:

{
  "name": "ARC-72 Enumeration Extension",
  "desc": "Smart Contract NFT Enumeration Interface",
  "methods": [
    {
      "name": "arc72_balanceOf",
      "desc": "Returns the number of NFTs owned by an address",
      "readonly": true,
      "args": [
        { "type": "address", "name": "owner" },
      ],
      "returns": { "type": "uint256" }
    },
    {
      "name": "arc72_totalSupply",
      "desc": "Returns the number of NFTs currently defined by this contract",
      "readonly": true,
      "args": [],
      "returns": { "type": "uint256" }
    },
    {
      "name": "arc72_tokenByIndex",
      "desc": "Returns the token ID of the token with the given index among all NFTs defined by the contract",
      "readonly": true,
      "args": [
        { "type": "uint256", "name": "index" },
      ],
      "returns": { "type": "uint256" }
    },
  ],
}

The sort order for NFT indices is not specified. The arc72_tokenByIndex method MUST error when index is greater than arc72_totalSupply.

The ARC-73 interface selector for this enumeration extension interface is 0xa57d4679.

Rationale

This specification is based on ERC-721, with some differences.

Core Specification

The core specification differs from ERC-721 by:

• removing safeTransferFrom, since there is not a test for whether an address on Algorand corresponds to a smart contract
• moving management functionality out of the base specification into an extension
• moving balance query functionality out of the base specification into the enumeration extension

Moving functionality out of the core specification into extensions allows the base specification to be much simpler, and allows extensions for extra capabilities to evolve separately from the core idea of owning and transferring ownership of non-fungible tokens. It is recommended that NFT contract authors make use of extensions to enrich the capabilities of their NFTs.

Metadata Extension

The metadata extension differns from the ERC-721 metadata extension by using a fixed-length URI return and removing the symbol and name operations. Metadata such as symbol or name can be included in the metadata pointed to by the URI.

Transfer Management Extension

The transfer management extension is taken from the set of methods and events from the base ERC-721 specification that deal with approving other addresses to transfer ownership of an NFT. This functionality is important for trusted NFT galleries like OpenSea to list and sell NFTs on behalf of users while allowing the owner to maintain on-chain ownership. However, this set of functionality is the bulk of the complexity of the ERC-721 standard, and moving it into an extension vastly simplifies the core NFT specification. Additionally, other interfaces have been proposed to allow for the sale of NFTs in decentralized manners without needing to give transfer control to a trusted third party.

Enumeration Extension

The enumeration extension is taken from the ERC-721 enumeration extension. However, it also includes the arc72_balanceOf function that is included in the base ERC-721 specification. This change simplifies the core standard and groups the arc72_balanceOf function with related functionality for contracts where supply details are desired.

Backwards Compatibility

This standard introduces a new kind of NFT that is incompatible with NFTs defined as ASAs. Applications that want to index, manage, or view NFTs on Algorand will need to handle these new smart NFTs as well as the already popular ASA implementation of NFTs will need to add code to handle both, and existing smart contracts that handle ASA-based NFTs will not work with these new smart contract NFTs.

While this is a severe backwards incompatibility, smart contract NFTs are necessary to provide richer and more diverse functionality for NFTs.

Security Considerations

The fact that anybody can create a new implementation of a smart contract NFT standard opens the door for many of those implementations to contain security bugs. Additionally, malicious NFT implementations could contain hidden anti-features unexpected by users. As with other smart contract domains, it is difficult for users to verify or understand security properties of smart contract NFTs. This is a tradeoff compared with ASA NFTs, which share a smaller set of security properties that are easier to validate, to gain the possibility of adding novel features.

Copyright

Copyright and related rights waived via CCO.

Citation

Please cite this document as:

William G Hatch, "ARC-72: Algorand Smart Contract NFT Specification," Algorand Requests for Comments, no. 72, January 2023. [Online serial]. Available: https://github.com/algorandfoundation/ARCs/blob/main/ARCs/arc-0072.md.