InterPlanetary NFT (IPNFT)

Simple Summary

A standard encoding and addressing for non-fungible tokens (NFTs) for upholding chain immutablity gurantees off chain.

Abstract

Following standard defines a standard encoding for ERC-721 Non-Fungible Token assets & metadata (addressed by tokenURI) which is compatible with the existing specification, while it attempts to address some of the issues found in the wild.

Motivation

ERC-721: Non-Fungible Token Standard defines tokenURI as a distinct Uniform Resource Identifier (URI) for a given asset where it may point to a JSON file that conforms to the "ERC721 Metadata JSON Schema".

Metadata schema in turn defines name, description and image string fields, where image is a URI pointing to a resource with mime type image/* representing the asset to which this NFT represents.

Loose requirements lead to number of problems in the wild:

• Autheticity - Many NFTs use URIs to web 2.0 host that
  1. Can go down and make assets inaccessible (404)
  2. Serve unauthentic resource for maliciously or unintentionally.
• Ambiguity - While specification only defines image field for the asset, in practice many new fields with URI are used. This introduces ambiguity in which URIs are part of NFT asset and which are not.

IPNFT specification defines IPLD format for encoding NFT metadata and all the assets into a hash linked DAG which can be addressed via ipfs:// URLs and used in tokenURI in ERC-721 compatible way while addressing outlined problems as follows:

• Autheticity - Content addressing of IPFS guarantees that content can be retreived from arbitrary nodes in the network and that content is verifiably be authentic.
• Ambiguity - All sssets included in the DAG are part of NFT any URL inside the metadata is not.

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.

Asset Link

MUST be an IPLD link to DAG-PB directory, which SHOULD contain exactly one named file.

IPNFT

Every IPNFT MUST be DAG-CBOR encoded data structure which MUST have:

1. type property with "nft" value.
2. name property with a string vaule.
3. description property with a string value.
4. Have image asset link.

IPFNT data structure MAY contain arbitrary other properties however it is RECOMMENDED to put those under properties as per "ERC721 Metadata JSON Schema".

All NFT assets MUST be included in IPNFT data structure as asset links which MAY be any top level property except (type, name, description, image, properties, metadata.json) or any nested property.

metadata.json content

It is RECOMMENDED for IPNFT to have a metadata.json IPLD link to a JSON source in DAG-PB or RAW encoding mirroring IPNFT data structure with following caveats:

1. type property is OPTIONAL and can be omitted. If present it MUST have value "nft".
2. Asset links MUST be substituted for corresponding ipfs://{asset_cid}/file.png URLs in string format (substitute file.png with actual file name).
3. SHOULD NOT have a property named metadata.json so it does not diverge from IPNFT data structure.

Caveats

1. IPNFT root block should not exceed 1MiB in size.

   Larger blocks are not well supported by IPFS. If CBOR encoded metadata still exceeds this block limit author shoud consider refatoring metadata into multiple block e.g. taking JSON substructure, encoding it in DAG-CBOR and linking to it from metadata.