Cardano Improvement Proposals


CIP 68 - Datum Metadata Standard

Contents

Abstract

This proposal defines a metadata standard for native assets making use of output datums not only for NFTs but any asset class.

Motivation

This proposal addresses a few shortcomings of CIP-0025:

Besides these shortcomings CIP-0025 has some flaws in its design. For people unaware of CIP-0025 or want to use a different way of minting or want to use a different metadata format/mechanism you open up a protocol to metadata spoofing, because this standard is so established and metadata in minting transactions are interpreted by most platforms by default. Since this standard is not enforced at the protocol level there is no guarantee everyone will be aware of it or follow the rules. At the same time you limit and constraint the capabilities of the ledger if everyone was forced to follow the rules of CIP-0025.

This standard tackles all these problems and offers many more advantages, not only for NFTs, but also for any asset class that may follow. Additionally, this CIP will introduce a way to classify tokens so that third parties like wallets can easily know what the kind of token it is.

Specification

Considerations

The basic idea is to have two assets issued, where one references the other. We call these two a reference NFT and an user token, where the user token can be an NFT, FT or any other asset class that is transferable and represents any value. So, the user token is the actual asset that lives in a user's wallet.

To find the metadata for the user token you need to look for the output, where the reference NFT is locked in. How this is done concretely will become clear below. Moreover, this output contains a datum, which holds the metadata. The advantage of this approach is that the issuer of the assets can decide how the transaction output with the reference NFT is locked and further handled. If the issuer wants complete immutable metadata, the reference NFT can be locked at the address of an unspendable script. Similarly, if the issuer wants the NFTs/FTs to evolve or wants a mechanism to update the metadata, the reference NFT can be locked at the address of a script with arbitrary logic that the issuer decides.

Lastly and most importantly, with this construction, the metadata can be used by a PlutusV2 script with the use of reference inputs CIP-0031. This will drive further innovation in the token space.

Reference NFT label

This is the registered asset_name_label value

assetnamelabel class description
100 NFT Reference NFT locked at a script containing the datum

Constraints and conditions

For a correct relationship between the user token and the reference NFT a few conditions must be met.

Some remarks about the above,

  1. The user token and reference NFT do not need to be minted in the same transaction. The order of minting is also not important.
  2. It may be the case that there can be multiple user tokens (multiple asset names or quantity greater than 1) referencing the same reference NFT.

The datum in the output with the reference NFT contains the metadata at the first field of the constructor 0. The version number is at the second field of this constructor:

big_int = int / big_uint / big_nint
    big_uint = #6.2(bounded_bytes)
    big_nint = #6.3(bounded_bytes)
    
    metadata =
      { * metadata => metadata }
      / [ * metadata ]
      / big_int
      / bounded_bytes
    
    version = int
    
    datum = #6.121([metadata, version])
    

222 NFT Standard

Besides the necessary standard for the reference NFT we're introducing two specific token standards in this CIP. Note that the possibilities are endless here and more standards can be built on top of this CIP for FTs, other NFTs, semi fungible tokens, etc. The first is the 222 NFT standard with the registered asset_name_label prefix value

assetnamelabel class description
222 NFT NFT hold by the user's wallet making use of CIP-0025 inner structure

Class

The user token represents an NFT (non-fungible token).

Pattern

The user token and reference NFT must have an identical name, preceded by the asset_name_label prefix.

Example:\ user token: (222)Test123\ reference NFT: (100)Test123

Metadata

This is a low-level representation of the metadata, following closely the structure of CIP-0025. All UTF-8 encoded keys and values need to be converted into their respective byte's representation when creating the datum on-chain.

files_details = 
      {
        ? name : bounded_bytes, ; UTF-8
        mediaType : bounded_bytes, ; UTF-8
        src : bounded_bytes ; UTF-8
      }
    
    metadata = 
      {
        name : bounded_bytes, ; UTF-8
        image : bounded_bytes, ; UTF-8
        ? mediaType : bounded_bytes, ; UTF-8
        ? description : bounded_bytes, ; UTF-8
        ? files : [* files_details]
      }
    
    datum = #6.121([metadata, 1]) ; version 1
    

Example datum as JSON:

{"constructor" : 0, "fields": [{"map": [{"k": "6E616D65", "v": "5370616365427564"}, {"k": "696D616765", "v": "697066733A2F2F74657374"}]}, {"int": 1}]}
    

Retrieve metadata as 3rd party

A third party has the following NFT d5e6bf0500378d4f0da4e8dde6becec7621cd8cbf5cbb9b87013d4cc.(222)TestToken they want to lookup. The steps are

  1. Construct reference NFT from user token: d5e6bf0500378d4f0da4e8dde6becec7621cd8cbf5cbb9b87013d4cc.(100)TestToken
  2. Look up reference NFT and find the output it's locked in.
  3. Get the datum from the output and lookup metadata by going into the first field of constructor 0.
  4. Convert to JSON and encode all string entries to UTF-8 if possible, otherwise leave them in hex.

Retrieve metadata from a Plutus validator

We want to bring the metadata of the NFT d5e6bf0500378d4f0da4e8dde6becec7621cd8cbf5cbb9b87013d4cc.(222)TestToken in the Plutus validator context. To do this we

  1. Construct reference NFT from user token: d5e6bf0500378d4f0da4e8dde6becec7621cd8cbf5cbb9b87013d4cc.(100)TestToken (off-chain)
  2. Look up reference NFT and find the output it's locked in. (off-chain)
  3. Reference the output in the transaction. (off-chain)
  4. Verify validity of datum of the referenced output by checking if policy ID of reference NFT and user token and their asset names without the asset_name_label prefix match. (on-chain)

333 FT Standard

The second introduced standard is the 333 FT standard with the registered asset_name_label prefix value

assetnamelabel class description
333 FT FT hold by the user's wallet making use of Cardano foundation off-chain registry inner structure

Class

The user token is an FT (fungible token).

Pattern

The user token and reference NFT must have an identical name, preceded by the asset_name_label prefix.

Example:\ user token: (333)Test123\ reference NFT: (100)Test123

Metadata

This is a low-level representation of the metadata, following closely the structure of the Cardano foundation off-chain metadata registry. All UTF-8 encoded keys and values need to be converted into their respective byte's representation when creating the datum on-chain.

; Explanation here: https://developers.cardano.org/docs/native-tokens/token-registry/cardano-token-registry/
    
    metadata = 
      {
        name : bounded_bytes, ; UTF-8
        description : bounded_bytes, ; UTF-8
        ? ticker: bounded_bytes, ; UTF-8
        ? url: bounded_bytes, ; UTF-8
        ? logo: bounded_bytes, ; UTF-8
        ? decimals: big_int
      }
    
    datum = #6.121([metadata, 1]) ; version 1
    

Example datum as JSON:

{"constructor" : 0, "fields": [{"map": [{"k": "6E616D65", "v": "5370616365427564"}, {"k": "6465736372697074696F6E", "v": "54686973206973206D79207465737420746F6B656E"}]}, {"int": 1}]}
    

Retrieve metadata as 3rd party

A third party has the following FT d5e6bf0500378d4f0da4e8dde6becec7621cd8cbf5cbb9b87013d4cc.(333)TestToken they want to lookup. The steps are

  1. Construct reference NFT from user token: d5e6bf0500378d4f0da4e8dde6becec7621cd8cbf5cbb9b87013d4cc.(100)TestToken
  2. Look up reference NFT and find the output it's locked in.
  3. Get the datum from the output and lookup metadata by going into the first field of constructor 0.
  4. Convert to JSON and encode all string entries to UTF-8 if possible, otherwise leave them in hex.

Retrieve metadata from a Plutus validator

We want to bring the metadata of the FT d5e6bf0500378d4f0da4e8dde6becec7621cd8cbf5cbb9b87013d4cc.(333)TestToken in the Plutus validator context. To do this we

  1. Construct reference NFT from user token: d5e6bf0500378d4f0da4e8dde6becec7621cd8cbf5cbb9b87013d4cc.(100)TestToken (off-chain)
  2. Look up reference NFT and find the output it's locked in. (off-chain)
  3. Reference the output in the transaction. (off-chain)
  4. Verify validity of datum of the referenced output by checking if policy ID of reference NFT and user token and their asset names without the asset_name_label prefix match. (on-chain)

Rationale

Without seperation of reference NFT and user token you lose all flexibility and moving the user token would be quite cumbersome as you would need to add the metadata everytime to the new output where the user token is sent to. Hence you separate metadata and user token and lock the metadata inside another UTxO, so you can freely move the user token around.

In order to reference the correct UTxO containing the metadata, it needs to be authenticated, otherwise metadata spoofing attacks become possible. One way to achieve that is by adding an NFT (reference NFT) to the UTxO. This NFT needs to under the same Policy ID as the user token, followed by an asset name pattern defined in the standard. This way you create a secure link between reference NFT and user token without the need for any extra data and you can make use of this off-chain and on-chain.

The security for the link is derived from the minting policy itself, so it's important to write the validator with the right constraints and rules since this CIP solely defines the interface to keep flexibility as high as possible.

Backward Compatibility

To keep metadata compatibility with changes coming in the future, we introduce a version field in the datum.

Path to Active

References

This CIP is licensed under CC-BY-4.0.