- Antonio Antonino - KILT Protocol antonio@kilt.io
- v1.0 - July.22 2022: Initial definition and RFC for the new method
This document defines the Asset DID Method that conforms to the DID Core W3C Spec. By the term asset, the authors refer to both an entire asset (or token) class (e.g., the entire Bitcoin fungible asset class) as well as a specific instance within that class (e.g., a specific NFT - the instance - within an NFT collection - the asset class).
Hence, an Asset Decentralized Identifier (DID) is a string uniquely identifying assets on any existing and future blockchain.
The Asset DID method is identified by the asset scheme, as in the following example: did:asset:bip122:000000000019d6689c085ae165831e93.slip44:0.
Asset DIDs are composed of the following elements:
asset-did = "did:asset:" + <modified-caip-19-asset-id>
modified-caip-19-asset-id = <caip-2-chain-id> + "." + <asset-namespace> + ":" + <asset-reference> + [":" + <asset-id>]
caip-2-chain-id = <chain-namespace> + ":" + <chain-reference>
chain-namespace = [-a-z0-9]{3,8}
chain-reference = [-a-zA-Z0-9]{1,32}
asset-namespace = [-a-z0-9]{3,8}
asset-reference = [-a-zA-Z0-9]{1,64}
asset-id = [-a-zA-Z0-9]{1,78}
The caip-2-chain-id is a CAIP (Chain Agnostic Improvement Proposal) chain identifier as described in the CAIP-2 Blockchain ID Specification.
The modified-caip-19-asset-id is derived from a CAIP asset identifier as described in the CAIP-19 Asset Type and Asset ID Specification, with the / symbol separating the chain identifier from the asset identifier being replaced with the . symbol, and the optional / symbol preceding the asset-id component being replaced by the : symbol.
This is required to render the resulting string a valid URI according to the DID Core Specification.
Chain and asset namespaces are defined with Improvement Proposals and are stored under the CAIP repository. For instance, the namespaces for EIP155 (Ethereum-based) and BIP122 (Bitcoin-based) chains as well as for SLIP44 (native fungible tokens), ERC20 (smart contract based tokens), and ERC721 (NFTs) assets have already been defined.
An Asset DID as such is not directly created, but it is generated at any time from a specific asset on as specific network. Because of the chain agnostic nature of Asset DIDs, this process can take place on any blockchain that is identifiable by a CAIP-2 Identifier.
Furthermore, as far as Asset DIDs are concerned, the assets identified do not need to already exist. For instance, a "right to create" a new NFT collection on a given blockchain for a user could be encoded in a Verifiable Credential where the identifier of the asset is an Asset DID.
The Asset DID method does not support direct updates, being a purely generative method, but its resolution reflects any changes to the underlying asset or to its credentials.
The Asset DID method does not support deactivations, being a purely generative method.
The information required to resolve an Asset DID is the following:
- The chain on which the asset lives, e.g., the Ethereum mainnet
- The namespace defining the asset, e.g., the ERC721 namespace
- The reference to the asset class within the defined namespace, e.g., the smart contract address for an NFT collection
- [OPTIONAL] The unique identifier of the asset instance within the asset class, e.g., the NFT ID within a given collection
The resolution process then follows a hierarchical structure.
The chain identifier is decomposed into its namespace and reference. Then, the resolver for the specified namespace is used to resolve the specified reference.
The same process is followed for the asset identifier. The identifier is decomposed into its namespace, reference, and optional ID. Then, the resolver for the specified asset namespace is used to resolve the asset reference and ID.
As an example, the returned DID Document for one of the CryptoKitties NFTs, identified with the Asset DID did:asset:eip155:1.erc721:0x06012c8cf97bead5deae237070f9587f8e7a266d:634446, is the following:
{
"@context": [
"https://www.w3.org/ns/did/v1",
"https://kilt.io/asset-did-context.json"
],
"id": "did:asset:eip155:1.erc721:0x06012c8cf97bead5deae237070f9587f8e7a266d:634446",
"chain": {
"namespace": "eip155",
"reference": "0"
},
"asset": {
"namespace": "erc721",
"reference": "0x06012c8cf97bead5deae237070f9587f8e7a266d",
"identifier": "634446"
}
}At the present time, the resolution result does not include any document metadata.
Beyond simple identifiers, the DID standard defines URLs to enhance the semantics of the resolution process, allowing resolvers to include additional information to the resolution result or to transform such information according to the client's needs.
Following the DID URL specification, an Asset DID URL has the following syntax:
asset-did-url = "did:asset:" + <path-abempty> + ["?" + <query>] + ["#" + <fragment>]
where path-abempty, query and fragment are defined in RFC3986.
The list of supported paths, queries, and fragments is shown below and follows the rules presented in the DID Resolution specification.
At the present time, not path is allowed in an Asset DID URL. The specification of paths in an Asset DID URL will result in a parsing error.
At the present time, not query parameter is allowed in an Asset DID URL. The presence of any query parameters in an Asset DID URL will be disregarded.
At the present time, not fragments is allowed in an Asset DID URL. The specification of fragments in an Asset DID URL will result in a parsing error.
Being purely generative, Asset DIDs do not have support for verification and encryption keys.
Furthermore, since Asset DIDs are meant to identify public, passive assets, correlation is a needed feature. It is possible for two different Asset DIDs to logically refer to the same asset (for instance NFTs that have been bridged across chains), and research is undergoing to provide a link between all the different instances of it.