Aptos Token Standard (Legacy)

Overview of NFT

An NFT is a non-fungible token or data stored on a blockchain that uniquely defines ownership of an asset. NFTs were first defined in EIP-721 and later expanded upon in EIP-1155. NFTs are typically defined using the following properties:

• name: The name of the asset. It must be unique within a collection.
• description: The description of the asset.
• uri: A URL pointer to off-chain for more information about the asset. The asset could be media such as an image or video or more metadata.
• supply: The total number of units of this NFT. Many NFTs have only a single supply, while those that have more than one are referred to as editions.

Additionally, most NFTs are part of a collection or a set of NFTs with a common attribute, for example, a theme, creator, or minimally contract. Each collection has a similar set of attributes:

• name: The name of the collection. The name must be unique within the creator’s account.
• description: The description of the collection.
• uri: A URL pointer to off-chain for more information about the asset. The asset could be media such as an image or video or more metadata.
• supply: The total number of NFTs in this collection.
• maximum: The maximum number of NFTs that this collection can have. If maximum is set to 0, then supply is not tracked.

Design principles

The Aptos token standard is developed with the following principles:

• Interoperability: Provide a standard implementation to improve interoperability across the ecosystem projects. Moreover, Move being a static language without dynamic dispatch makes this principle even more imperative.
• Liquidity: Achieve maximal liquidity by defining the NFT, fungible (non-decimal) and semi-fungible tokens in one contract. These different types of tokens can be easily stored, transferred and transacted in the same way. As a consequence, it becomes easier to achieve maximal interoperability across the marketplaces, exchanges, and other methods of exchange.
• Rich on-chain token properties: Enable the customization of on-chain token properties. Users can define their own properties and store them on-chain. This can potentially eliminate the need for the off-chain metadata.
• Reduced overhead: Reduce the cost of creating large amounts of NFTs from fungible tokens. This can lead to, for example, reduced overhead for similar tokens by the reuse of on-chain metadata for certain fungible tokens.

Storing customized token properties on-chain

The Aptos token standard uses the PropertyMap module to store on-chain properties of tokens. PropertyMap maps a string key to a property value on-chain, which stores the value in Binary Canonical Serialization (BCS) format and its type. Currently, only primitive types (bool, u8, u64, u128, address and String) are supported in PropertyMap. Applications, such as Aptos Names, define application specific properties that are read and written by the applications smart contract.

Default properties

You can add your properties to default_properties in the TokenData. The properties defined here are shared by all tokens by default.

The default_properties field is a key-value store with type information. It leverages the PropertyMap module which contain functions for serializing and deserializing different primitive types to the property value.

Token properties

You can also use the token_properties defined in the token itself for customization on-chain. You can create customized values for a property of this specific token, thereby allowing a token to have a different property value from its default.

Note that limits exist to storing customized token properties on-chain, namely 1000 properties per token with field names limited to 128 characters.

Evolving from fungible token to NFT

Fungible tokens share the same default property values. However, these property values can evolve over time and become different from each other. To support such evolution of token properties, the Aptos token standard provides the property_version field. Here is how it works:

• During the token creation (minting), all tokens initially have property_version set to 0 and these tokens can be stacked together as fungible token.
• When the creators mutate the default properties of a token, the mutated token will be assigned a unique property_version to create a new token_id to differentiate it from other fungible tokens. This unique token_id allows the token to have its own property values, and all further mutation of this token does not change the property_version again. This token essentially becomes an NFT now.

Configuring mutability

To make mutability explicit for both the creator and owner, the Aptos token standard provides mutability_config at both the collection level and the token level to control which fields are mutable. Configurable here means the creator can configure this field to be mutable or immutable during creation.

Storing metadata off-chain

Follow the standard below to ensure your NFT can be correctly displayed by various wallets.

You should store the metadata in a JSON file located in an off-chain storage solution such as arweave and provide the URL to the JSON file in the uri field of the token or the collection. We recommend the developers follow the ERC-1155 off-chain data schema to format their JSON files.

{
  "image": "https://www.arweave.net/abcd5678?ext=png",
  "animation_url": "https://www.arweave.net/efgh1234?ext=mp4",
  "external_url": "https://petra.app/",
  "attributes": [
    {
      "trait_type": "web",
      "value": "yes"
    },
    {
      "trait_type": "mobile",
      "value": "yes"
    },
    {
      "trait_type": "extension",
      "value": "yes"
    }
  ],
  "properties": {
    "files": [
      {
        "uri": "https://www.arweave.net/abcd5678?ext=png",
        "type": "image/png"
      },
      {
        "uri": "https://watch.videodelivery.net/9876jkl",
        "type": "unknown",
        "cdn": true
      },
      {
        "uri": "https://www.arweave.net/efgh1234?ext=mp4",
        "type": "video/mp4"
      }
    ],
    "category": "video"
  }
}

• image: URL to the image asset. You may use the ?ext={file_extension} query to provide information on the file type.
• animation_url: URL to the multimedia attachment of the asset. You may use the same file_extension query to provide the file type.
• external_url: URL to an external website where the user can also view the image.
• attributes - Object array, where an object should contain trait_type and value fields. value can be a string or a number.
• properties.files: Object array, where an object should contain the URI and type of the file that is part of the asset. The type should match the file extension. The array should also include files specified in image and animation_url fields, as well as any other files associated with the asset. You may use the ?ext={file_extension} query to provide information on the file type.
• properties.category: Has supported categories:
• image - PNG, GIF, JPG
• video - MP4, MOV
• audio - MP3, FLAC, WAV
• vr - 3D models; GLB, GLTF
• html - HTML pages; scripts and relative paths within the HTML page are also supported

You can also host your files on CDN to provide faster loading time by using the cdn flag in the file object. When the file exists, this should be the primary location to read the media file (video, audio, vr) by wallet. If the file is no longer available, the wallet can fall back to use the animation_url to load the file.

{
  "properties": {
    "files": [
      {
        "uri": "https://watch.videodelivery.net/52a52c4a261c88f19d267931426c9be6",
        "type": "unknown",
        "cdn": true
      }
    ]
  }
}

Token data model

The following diagram depicts the flow of token data through Aptos.

Token resources

As shown in the diagram above, token-related data are stored at both the creator’s account and the owner’s account.

Struct-level resources

The following tables describe fields at the struct level. For the definitive list, see the Aptos Token Framework.

Resource stored at the creator’s address

Resource stored at the owner’s address

For more detailed descriptions, see Aptos Token Framework.

Token lifecycle

Token creation

Every Aptos token belongs to a collection. The developer first needs to create a collection through create_collection_script and then create the token belonging to the collection create_token_script. To achieve parallel TokenData and Token creation, a developer can create unlimited collection and TokenData where the maximum of the collection and TokenData are set as 0. With this setting, the token contract won’t track the supply of types of token (TokenData count) and supply of token within each token type. As the result, the TokenData and token can be created in parallel.

Aptos also enforces simple validation of the input size and prevents duplication:

• Token name - unique within each collection
• Collection name - unique within each account
• Token and collection name length - smaller than 128 characters
• URI length - smaller than 512 characters
• Property map - can hold at most 1000 properties, and each key should be smaller than 128 characters

Token mutation

Our standard supports mutation with a principle that the mutable fields are specified during the token creation. This allows the token owner to be informed which fields are mutable when they get the token from the creator. Our contract uses CollectionMutabilityConfig to check if a field is mutable. Our contract uses TokenMutabilityConfig to check if a TokenData field is mutable.

For mutation of properties, we have both

• default_properties stored in TokenData shared by all tokens belonging to the TokenData
• token_properties stored in the token itself

To mutate default_properties, developers can use mutate_tokendata_property to mutate the properties when TokenMutabilityConfig is set to true.

CAUTION: Set the TokenMutabilityConfig field to false unless it is absolutely necessary. Allowing default_properties to be mutable provides creators too much power; creators can change the burnable config to provide themselves the authority to burn tokens after token creation.

To mutate token_properties stored in the token, our standard uses the TOKEN_PROPERTY_MUTABLE property stored in default_properties. When the creator creates the TokenData with the TOKEN_PROPERTY_MUTABLE property set to true, the creator can mutate token_properties. Note that if the mutate_tokendata_property is set to true, creators can mutate the token_properties anyway since they can overwrite the setting in default_properties.

Token burn

We provide burn and burn_by_creator functions for token owners and token creators to burn (or destroy) tokens. However, these two functions are also guarded by configs that are specified during the token creation so that both creator and owner are clear on who can burn the token. Burn is allowed only when the BURNABLE_BY_OWNER property is set to true in default_properties. Burn by creator is allowed when BURNABLE_BY_CREATOR is true in default_properties. Once all the tokens belonging to a TokenData are burned, the TokenData will be removed from the creator’s account. Similarly, if all TokenData belonging to a collection are removed, the CollectionData will be removed from the creator’s account.

Token transfer

We provide three different modes for transferring tokens between the sender and receiver.

Two-step transfer

To protect users from receiving undesired NFTs, they must be first offered NFTs, and then accept the offered NFTs. Then only the offered NFTs will be deposited in the users’ token stores. This is the default token transfer behavior. For example:

1. If Alice wants to send Bob an NFT, she must first offer Bob this NFT. This NFT is still stored under Alice’s account.
2. Only when Bob claims the NFT, will the NFT be removed from Alice’s account and stored in Bob’s token store.

Transfer with opt-in

If a user wants to receive direct transfer of the NFT, skipping the initial steps of offer and claim, then the user can call opt_in_direct_transfer to allow other people to directly transfer the NFTs into the user’s token store. After opting into direct transfer, the user can call transfer to transfer tokens directly. For example, Alice and anyone can directly send a token to Bob’s token store once Bob opts in.

Multi-agent transfer

The sender and receiver can both sign a transfer transaction to directly transfer a token from the sender to receiver direct_transfer_script. For example, once Alice and Bob both sign the transfer transaction, the token will be directly transferred from Alice’s account to Bob.