NFT Dynamic Traits | Ethereum Improvement Proposals

Abstract

This specification introduces a new interface that extends ERC-721 and ERC-1155 that defines methods for setting and getting dynamic onchain traits associated with non-fungible tokens. These dynamic traits can be used to represent properties, characteristics, redeemable entitlements, or other attributes that can change over time. By defining these traits onchain, they can be used and modified by other onchain contracts.

Motivation

Trait values for non-fungible tokens are often stored offchain. This makes it difficult to query and mutate these values in contract code. Specifying the ability to set and get traits onchain allows for new use cases like redeeming onchain entitlements and transacting based on a token’s traits.

Onchain traits can be used by contracts in a variety of different scenarios. For example, a contract that wants to entitle a token to a consumable benefit (e.g. a redeemable) can robustly reflect that onchain. Marketplaces can allow bidding on these tokens based on the trait value without having to rely on offchain state and exposing users to frontrunning attacks. The motivating use case behind this proposal is to protect users from frontrunning attacks on marketplaces where users can list NFTs with certain traits where they are expected to be upheld during fulfillment.

Specification

The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “NOT RECOMMENDED”, “MAY”, and “OPTIONAL” in this document are to be interpreted as described in RFC 2119 and RFC 8174.

Contracts implementing this EIP MUST include the events, getters, and setters as defined below, and MUST return true for ERC-165 supportsInterface for 0xaf332f3e, the 4 byte interfaceId for this ERC.

interface IERC7496 {
    /* Events */
    event TraitUpdated(bytes32 indexed traitKey, uint256 tokenId, bytes32 traitValue);
    event TraitUpdatedRange(bytes32 indexed traitKey, uint256 fromTokenId, uint256 toTokenId);
    event TraitUpdatedRangeUniformValue(bytes32 indexed traitKey, uint256 fromTokenId, uint256 toTokenId, bytes32 traitValue);
    event TraitUpdatedList(bytes32 indexed traitKey, uint256[] tokenIds);
    event TraitUpdatedListUniformValue(bytes32 indexed traitKey, uint256[] tokenIds, bytes32 traitValue);
    event TraitMetadataURIUpdated();

    /* Getters */
    function getTraitValue(uint256 tokenId, bytes32 traitKey) external view returns (bytes32 traitValue);
    function getTraitValues(uint256 tokenId, bytes32[] calldata traitKeys) external view returns (bytes32[] traitValues);
    function getTraitMetadataURI() external view returns (string memory uri);

    /* Setters */
    function setTrait(uint256 tokenId, bytes32 traitKey, bytes32 newValue) external;
}

Keys & Names

The traitKey is used to identify a trait. The traitKey MUST be a unique bytes32 value identifying a single trait.

The traitKey SHOULD be a keccak256 hash of a human readable trait name.

Metadata

Trait metadata is necessary to provide information about which traits are present in a contract, how to display trait names and values, and other optional features.

The trait metadata must be compliant with the specified schema.

The trait metadata URI MAY be a data URI or point to an offchain resource.

The keys in the traits object MUST be unique trait names. If the trait name is 32 byte hex string starting with 0x then it is interpreted as a literal traitKey. Otherwise, the traitKey is defined as the keccak256 hash of the trait name. A literal traitKey MUST NOT collide with the keccak256 hash of any other traits defined in the metadata.

The displayName values MUST be unique and MUST NOT collide with the displayName of any other traits defined in the metadata.

The validateOnSale value provides a signal to marketplaces on how to validate the trait value when a token is being sold. If the validation criteria is not met, the sale MUST not be permitted by the marketplace contract. If specified, the value of validateOnSale MUST be one of the following (or it is assumed to be none):

none: No validation is necessary.
requireEq: The bytes32 traitValue MUST be equal to the value at the time the offer to purchase was made.
requireUintGte: The bytes32 traitValue MUST be greater than or equal to the value at the time the offer to purchase was made. This comparison is made using the uint256 representation of the bytes32 value.
requireUintLte: The bytes32 traitValue MUST be less than or equal to the value at the time the offer to purchase was made. This comparison is made using the uint256 representation of the bytes32 value.

Note that even though this specification requires marketplaces to validate the required trait values, buyers and sellers cannot fully rely on marketplaces to do this and must also take their own precautions to research the current trait values prior to initiating the transaction.

Here is an example of the specified schema:

{
  "traits": {
    "color": {
      "displayName": "Color",
      "dataType": {
        "type": "string",
      }
    },
    "points": {
      "displayName": "Total Score",
      "dataType": {
        "type": "decimal",
        "signed": false,
        "decimals": 0
      },
      "validateOnSale": "requireUintGte"
    },
    "name": {
      "displayName": "Name",
      "dataType": {
        "type": "string",
        "minLength": 1,
        "maxLength": 32,
        "valueMappings": {
          "0x0000000000000000000000000000000000000000000000000000000000000000": "Unnamed",
          "0x92e75d5e42b80de937d204558acf69c8ea586a244fe88bc0181323fe3b9e3ebf": "🙂"
        }
      },
      "tokenOwnerCanUpdateValue": true
    },
    "birthday": {
      "displayName": "Birthday",
      "dataType": {
        "type": "epochSeconds",
        "valueMappings": {
          "0x0000000000000000000000000000000000000000000000000000000000000000": null
        }
      }
    },
    "0x77c2fd45bd8bdef5b5bc773f46759bb8d169f3468caab64d7d5f2db16bb867a8": {
      "displayName": "🚢 📅",
      "dataType": {
        "type": "epochSeconds",
        "valueMappings": {
          "0x0000000000000000000000000000000000000000000000000000000000000000": 1696702201
        }
      }
    }
  }
}

`string` Metadata Type

The string metadata type allows for a string value to be set for a trait.

The dataType object MAY have a minLength and maxLength value defined. If minLength is not specified, it is assumed to be 0. If maxLength is not specified, it is assumed to be a reasonable length.

The dataType object MAY have a valueMappings object defined. If the valueMappings object is defined, the valueMappings object MUST be a mapping of bytes32 values to string or unset null values. The bytes32 values SHOULD be the keccak256 hash of the string value. The string values MUST be unique. If the trait for a token is updated to null, it is expected offchain indexers to delete the trait for the token.

`decimal` Metadata Type

The decimal metadata type allows for a numeric value to be set for a trait in decimal form.

The dataType object MAY have a signed value defined. If signed is not specified, it is assumed to be false. This determines whether the traitValue returned is interpreted as a signed or unsigned integer.

The dataType object MAY have minValue and maxValue values defined. These should be formatted with the decimals specified. If minValue is not specified, it is assumed to be the minimum value of signed and decimals. If maxValue is not specified, it is assumed to be the maximum value of the signed and decimals.

The dataType object MAY have a decimals value defined. The decimals value MUST be a non-negative integer. The decimals value determines the number of decimal places included in the traitValue returned onchain. If decimals is not specified, it is assumed to be 0.

The dataType object MAY have a valueMappings object defined. If the valueMappings object is defined, the valueMappings object MUST be a mapping of bytes32 values to numeric or unset null values.

`boolean` Metadata Type

The boolean metadata type allows for a boolean value to be set for a trait.

If valueMappings is not used, the default trait values for boolean should be bytes32(0) for false and bytes32(uint256(1)) (0x0000000000000000000000000000000000000000000000000000000000000001) for true.

`epochSeconds` Metadata Type

The epochSeconds metadata type allows for a numeric value to be set for a trait in seconds since the Unix epoch.

The dataType object MAY have a valueMappings object defined. If the valueMappings object is defined, the valueMappings object MUST be a mapping of bytes32 values to integer or unset null values.

Events

Updating traits MUST emit one of:

TraitUpdated
TraitUpdatedRange
TraitUpdatedRangeUniformValue
TraitUpdatedList
TraitUpdatedListUniformValue

For the Range events, the fromTokenId and toTokenId MUST be a consecutive range of tokens IDs and MUST be treated as an inclusive range.

For the List events, the tokenIds MAY be in any order.

It is RECOMMENDED to use the UniformValue events when the trait value is uniform across all token ids, so offchain indexers can more quickly process bulk updates rather than fetching each trait value individually.

Updating the trait metadata MUST emit the event TraitMetadataURIUpdated so offchain indexers can be notified to query the contract for the latest changes via getTraitMetadataURI().

`setTrait`

If a trait defines tokenOwnerCanUpdateValue as true, then the trait value MUST be updatable by the token owner by calling setTrait.

If the value the token owner is attempting to set is not valid, the transaction MUST revert. If the value is valid, the trait value MUST be updated and one of the TraitUpdated events MUST be emitted.

If the trait has a valueMappings entry defined for the desired value being set, setTrait MUST be called with the corresponding traitValue.

Rationale

The design of this specification is primarily a key-value mapping for maximum flexibility. This interface for traits was chosen instead of relying on using regular getFoo() and setFoo() style functions to allow for brevity in defining, setting, and getting traits. Otherwise, contracts would need to know both the getter and setter function selectors including the parameters that go along with it. In defining general but explicit get and set functions, the function signatures are known and only the trait key and values are needed to query and set the values. Contracts can also add new traits in the future without needing to modify contract code.

The traits metadata allows for customizability of both display and behavior. The valueMappings property can define human-readable values to enhance the traits, for example, the default label of the 0 value (e.g. if the key was “redeemed”, “0” could be mapped to “No”, and “1” to “Yes”). The validateOnSale property lets the token creator define which traits should be protected on order creation and fulfillment, to protect end users against frontrunning.

Backwards Compatibility

As a new EIP, no backwards compatibility issues are present, except for the point in the specification above that it is explicitly required that the onchain traits MUST override any conflicting values specified by the ERC-721 or ERC-1155 metadata URIs.

Test Cases

Authors have included Foundry tests covering functionality of the specification in the assets folder.

Reference Implementation

Authors have included reference implementations of the specification in the assets folder.

Security Considerations

The set* methods exposed externally MUST be permissioned so they are not callable by everyone but only by select roles or addresses.

Marketplaces SHOULD NOT trust offchain state of traits as they can be frontrunned. Marketplaces SHOULD check the current state of onchain traits at the time of transfer. Marketplaces MAY check certain traits that change the value of the NFT (e.g. redemption status, defined by metadata values with validateOnSale property) or they MAY hash all the trait values to guarantee the same state at the time of order creation.