CEP-78 Modalities

The enhanced NFT implementation supports various 'modalities' that dictate the behavior of a specific contract instance. Modalities represent the common expectations around contract usage and behavior.

Ownership

This modality specifies the behavior regarding ownership of NFTs and whether the owner of the NFT can change over the contract's lifetime. There are three modes:

1. Minter: Minter mode is where the ownership of the newly minted NFT is attributed to the minter of the NFT and cannot be specified by the minter. In the Minter mode the owner of the NFT will not change and thus cannot be transferred to another entity.
2. Assigned: Assigned mode is where the owner of the newly minted NFT must be specified by the minter of the NFT. In this mode, the assigned entity can be either minter themselves or a separate entity. However, similar to the Minter mode, the ownership in this mode cannot be changed, and NFTs minted in this mode cannot be transferred from one entity to another.
3. Transferable: In the Transferable mode the owner of the newly minted NFT must be specified by the minter. However, in the Transferable mode, NFTs can be transferred from the owner to another entity.

In all the three mentioned modes, the owner entity is currently restricted to Accounts on the Casper network.

Note: In the Transferable mode, it is possible to transfer the NFT to an Account that does not exist.

This Ownership mode is a required installation parameter and cannot be changed once the contract has been installed. The mode is passed in as u8 value to the "ownership_mode" runtime argument.

Ownership	u8
Minter	0
Assigned	1
Transferable	2

The ownership mode of a contract can be determined by querying the ownership_mode entry within the contract's NamedKeys.

NFTKind

The NFTKind modality specifies the commodity that NFTs minted by a particular contract will represent. Currently, the NFTKind modality does not alter or govern the behavior of the contract itself and only exists to specify the correlation between on-chain data and off-chain items. There are three different variations of the NFTKind mode.

1. Physical: The NFT represents a real-world physical item e.g., a house.
2. Digital: The NFT represents a digital item, e.g., a unique JPEG or digital art.
3. Virtual: The NFT is the virtual representation of a physical notion, e.g., a patent or copyright.

The NFTKind mode is a required installation parameter and cannot be changed once the contract has been installed. The mode is passed in as a u8 value to nft_kind runtime argument.

NFTKind	u8
Physical	0
Digital	1
Virtual	2

NFTHolderMode

The NFTHolderMode dictates which entities on a Casper network can own and mint NFTs. There are three different options currently available:

1. Accounts: In this mode, only Accounts can own and mint NFTs.
2. Contracts: In this mode, only Contracts can own and mint NFTs.
3. Mixed: In this mode both Accounts and Contracts can own and mint NFTs.

If the NFTHolderMode is set to Contracts a ContractHash whitelist must be provided. This whitelist dictates which Contracts are allowed to mint NFTs in the restricted Installer minting mode.

NFTHolderMode	u8
Accounts	0
Contracts	1
Mixed	2

This modality is an optional installation parameter and will default to the Mixed mode if not provided. However, this mode cannot be changed once the contract has been installed. The mode is passed in as a u8 value to nft_holder_mode runtime argument.

WhitelistMode

The WhitelistMode dictates if the ACL whitelist restricting access to the mint entry point can be updated. There are currently two options:

1. Unlocked: The ACL whitelist is unlocked and can be updated via the set variables endpoint.
2. Locked: The ACL whitelist is locked and cannot be updated further.

If the WhitelistMode is set to Locked an ACL whitelist of entity keys must be provided on installation. This whitelist dictates which entities can mint NFTs in the restricted ACL minting mode. These entities include Accounts and/or Contracts.

This WhitelistMode is an optional installation parameter and will be set to unlocked if not passed. However, the whitelist mode itself cannot be changed once the contract has been installed. The mode is passed in as a u8 value to whitelist_mode runtime argument.

WhitelistMode	u8
Unlocked	0
Locked	1

Minting

The minting mode governs the behavior of contract when minting new tokens. The minting modality provides two options:

1. Installer: This mode restricts the ability to mint new NFT tokens only to the installing account of the NFT contract.
2. Public: This mode allows any account to mint NFT tokens.
3. ACL: This mode allows whitelisted accounts or contracts to mint NFT tokens.

This modality is an optional installation parameter and will default to the Installer mode if not provided. However, this mode cannot be changed once the contract has been installed. The mode is set by passing a u8 value to the minting_mode runtime argument.

MintingMode	u8
Installer	0
Public	1
ACL	2

NFTMetadataKind

This modality dictates the schema for the metadata for NFTs minted by a given instance of an NFT contract. There are four supported modalities:

1. CEP78: This mode specifies that NFTs minted must have valid metadata conforming to the CEP-78 schema.
2. NFT721: This mode specifies that NFTs minted must have valid metadata conforming to the NFT-721 metadata schema.
3. Raw: This mode specifies that metadata validation will not occur and raw strings can be passed to token_metadata runtime argument as part of the call to mint entrypoint.
4. CustomValidated: This mode specifies that a custom schema provided at the time of install will be used when validating the metadata as part of the call to mint entrypoint.

During installation, one NFTMetadataKind must be chosen as the base metadata kind for the contract instance. Additional kinds may be included using either the additional_required_metadata or optional_metadata arguments.

CEP-78 metadata example

{
  "name": "John Doe",
  "token_uri": "https://www.barfoo.com",
  "checksum": "940bffb3f2bba35f84313aa26da09ece3ad47045c6a1292c2bbd2df4ab1a55fb"
}

NFT-721 metadata example

{
  "name": "John Doe",
  "symbol": "abc",
  "token_uri": "https://www.barfoo.com"
}

Custom Validated

The CEP-78 implementation allows installers of the contract to provide their custom schema at the time of installation. The schema is passed as a String value to json_schema runtime argument at the time of installation. Once provided, the schema for a given instance of the contract cannot be changed.

The custom JSON schema must contain a top-level properties field. An example of a valid JSON schema is provided. In this example, each property has a name, the description of the property itself, and whether the property is required to be present in the metadata. If the metadata kind is not set to custom validated, then the value passed to the json_schema runtime argument will be ignored.

Example Custom Validated schema

{
  "properties": {
    "deity_name": {
      "name": "deity_name",
      "description": "The name of deity from a particular pantheon.",
      "required": true
    },
    "mythology": {
      "name": "mythology",
      "description": "The mythology the deity belongs to.",
      "required": true
    }
  }
}

Example Custom Metadata

{
  "deity_name": "Baldur",
  "mythology": "Nordic"
}

NFTMetadataKind	u8
CEP78	0
NFT721	1
Raw	2
CustomValidated	3

NFTIdentifierMode

The identifier mode governs the primary identifier for NFTs minted for a given instance on an installed contract. This modality provides two options:

1. Ordinal: NFTs minted in this modality are identified by a u64 value. This value is determined by the number of NFTs minted by the contract at the time the NFT is minted.
2. Hash: NFTs minted in this modality are identified by a base16 encoded representation of the blake2b hash of the metadata provided at the time of mint.

Since the primary identifier in the Hash mode is derived by hashing over the metadata, making it a content-addressed identifier, the metadata for the minted NFT cannot be updated after the mint.

Attempting to install the contract with the MetadataMutability modality set to Mutable in the Hash identifier mode will raise an error.

This modality is a required installation parameter and cannot be changed once the contract has been installed.

It is passed in as a u8 value to the identifier_mode runtime argument.

NFTIdentifierMode	u8
Ordinal	0
Hash	1

Metadata Mutability

The metadata mutability mode governs the behavior around updates to a given NFTs metadata. This modality provides two options:

1. Immutable: Metadata for NFTs minted in this mode cannot be updated once the NFT has been minted.
2. Mutable: Metadata for NFTs minted in this mode can update the metadata via the set_token_metadata entrypoint.

The Mutable option cannot be used in conjunction with the Hash modality for the NFT identifier; attempting to install the contract with this configuration raises InvalidMetadataMutability error. This modality is a required installation parameter and cannot be changed once the contract has been installed. It is passed in as a u8 value to the metadata_mutability runtime argument.

MetadataMutability	u8
Immutable	0
Mutable	1

BurnMode

The BurnMode modality dictates whether tokens minted by a given instance of an NFT contract can be burnt. This modality provides two options:

1. Burnable: Minted tokens can be burnt.
2. NonBurnable: Minted tokens cannot be burnt.

BurnMode	u8
Burnable	0
NonBurnable	1

This modality is an optional installation parameter and will default to the Burnable mode if not provided. However, this mode cannot be changed once the contract has been installed. The mode is set by passing a u8 value to the burn_mode runtime argument.

OwnerReverseLookupMode

The OwnerReverseLookupMode modality is set at install and determines if a given contract instance writes necessary data to allow reverse lookup by owner in addition to by ID.

This modality provides the following options:

1. NoLookup: The reporting and receipt functionality is not supported. In this option, the contract instance does not maintain a reverse lookup database of ownership and therefore has more predictable gas costs and greater scaling.
2. Complete: The reporting and receipt functionality is supported. Token ownership will be tracked by the contract instance using the system described here.
3. TransfersOnly: The reporting and receipt functionality is supported like Complete. However, it does not begin tracking until the first transfer. This modality is for use cases where the majority of NFTs are owned by a private minter and only NFT's that have been transferred benefit from reverse lookup tracking. Token ownership will also be tracked by the contract instance using the system described here.

Additionally, when set to Complete, causes a receipt to be returned by the mint or transfer entrypoints, which the caller can store in their account or contract context for later reference.

Further, two special entrypoints are enabled in Complete mode. First, register_owner which when called will allocate the necessary tracking record for the imputed entity. This allows isolation of the one time gas cost to do this per owner, which is convenient for accounting purposes. Second, updated_receipts, which allows an owner of one or more NFTs held by the contract instance to attain up to date receipt information for the NFTs they currently own.

OwnerReverseLookupMode	u8
NoLookup	0
Complete	1
TransfersOnly	2

This modality is an optional installation parameter and will default to the NoLookup mode if not provided. The mode is set by passing a u8 value to the owner_reverse_lookup_mode runtime argument. This mode cannot be changed once the contract has been installed.

Note : if ownership_mode is set to Minter and the minting_mode is set to Installer only, OwnerReverseLookupMode will be set to NoLookup. This is because the minter, by definition, owns all of the tokens forever. Therefore, there is no reason to do a reverse lookup for that owner. This rule applies only to newly installed contract instances.

Note : if OwnerReverseLookupMode is set to TransfersOnly then ownership_mode has to be set to Transferable only. This is because other ownership modes do not allow transfer.

If you are upgrading a contract from CEP-78 version 1.0 to 1.1, OwnerReverseLookupMode will be set to Complete, as this was the standard behavior of CEP-78 1.0. In addition to being set to Complete, existing records will be migrated into the CEP-78 1.1 format, which will impose a one-time gas cost to cover the migration.

If you have an existing CEP-78 version 1.0 contract instance, and would prefer the newer functionality with no lookup, the only option is to install a separate, new contract instance and mint all of the NFTs anew in that instance and then burn the corresponding NFTs from the old instance. If you do not own all the NFTs held by the old contract instance, you do not have this option.

NamedKeyConventionMode

The NamedKeyConvention modality dictates whether the Wasm passed will attempt to install a version 1.1.1 instance of CEP-78 or attempt to migrate a version 1.0 CEP-78 instance to version 1.1.1.

This modality provides three options:

1. DerivedFromCollectionName: This modality will signal the contract to attempt to install a new version 1.1.1 instance of the CEP-78 contract. The contract package hash and the access URef will be saved in the installing account's NamedKeys as cep78_contract_package_<collection_name> and cep78_contract_package_access_<collection_name>.
2. V_1_0_standard: This modality will signal the contract to attempt to upgrade from version 1.0 to version 1.1.1. In this scenario, the contract will retrieve the package hash and the access URef from the NamedKey entries originally created during the 1.0 installation.
3. V_1_0_custom: This modality will signal the contract to attempt to upgrade from version 1.0 to version 1.1.1. In this scenario, the calling account must provide the NamedKey entries under which the package hash and the access URef are saved. Additionally, this requires the passing of the runtime arguments access_key_name and hash_key_name for the access URef and package hash, respectively. In this modality, these arguments are required and must be passed in.

NamedKeyConvention	u8
DerivedFromCollectionName	0
V_1_0_standard	1
V_1_0_custom	2

EventsMode

The EventsMode modality determines how the installed instance of CEP-78 will handle the recording of events that occur from interacting with the contract.

The modality provides three options:

1. NoEvents: This modality will signal the contract to not record events at all. This is the default mode.
2. CEP47: This modality will signal the contract to record events using the CEP47 event schema. Further information can be found below.
3. CES: This modality will signal the contract to record events using the Casper Event Standard.

EventsMode	u8
NoEvents	0
CEP47	1
CES	2

Transfer Filter Hook

The transfer filter modality, if enabled, specifies a contract package hash pointing to a contract that will be called when the transfer method is invoked on the contract. CEP-78 will call the can_transfer method on the specified callback contract, which is expected to return a value of TransferFilterContractResult, represented as a u8.

• TransferFilterContractResult::DenyTransfer will block the transfer regardless of the outcome of other checks
• TransferFilterContractResult::ProceedTransfer will allow the transfer to proceed if other checks also pass

The transfer filter can be enabled by passing a ARG_TRANSFER_FILTER_CONTRACT argument to the install method, with a value of type Option<Key>

CEP47 Mode

The CEP47 EventsMode modality mimics the event schema previously used in the CEP47 NFT standard. Events are stored as a BTreeMap within a dictionary (EVENTS) in the contract's context. Entries consist of the PREFIX_HASH_KEY_NAME, followed by the EVENT_TYPE and then variable data as listed in the table below. The events can be retrieved directly via their dictionary entry using the JSON-RPC, with more information on this process available here.

Event name	Included values and type
Mint	recipient (Key), token_id (String)
Transfer	owner (Key), operator (Option<Key>), recipient (Key), token_id (String)
Burn	owner (Key), token_id (String)
ApprovalGranted	owner (Key), spender (Key), token_id (String)
ApprovalRevoked	owner (Key), token_id (String)
ApprovalForAll	owner (Key), operator (Key)
RevokedForAll	owner (Key), operator (Key)
MetadataUpdate	token_id (String)
Migration	-
VariablesSet	-

Casper Event Standard

CES is an option within the EventsMode modality that determines how changes to tokens issued by the contract instance will be recorded. Any changes are recorded in the __events dictionary and can be observed via a node's Server Side Events stream. They may also be viewed by querying the dictionary at any time using the JSON-RPC interface.

The emitted events are encoded according to the Casper Event Standard, and the schema is visible to an observer reading the __events_schema contract named key.

For this CEP-78 reference implementation, the events schema is as follows:

Event name	Included values and type
Mint	recipient (Key), token_id (String), data (String)
Transfer	owner (Key), operator (Option<Key>), recipient (Key), token_id (String)
Burn	owner (Key), token_id (String)
Approval	owner (Key), spender (Key), token_id (String)
ApprovalRevoked	owner (Key), token_id (String)
ApprovalForAll	owner (Key), operator (Key)
RevokedForAll	owner (Key), operator (Key)
MetadataUpdated	token_id (String), data (String)
Migration	-
VariablesSet	-

Modality Conflicts

The MetadataMutability option set to Mutable cannot be used in conjunction with the NFTIdentifierMode modality set to Hash.