zrc-6.md

ZRC	Title	Status	Type	Author	Created (yyyy-mm-dd)	Updated (yyyy-mm-dd)
6	Non-Fungible Token Standard	Implemented	Standard	Neuti Yoo [email protected] Jun Hao Tan [email protected]	2021-10-01	2022-03-09

Table of Contents

• I. What are NFTs and NFT royalties?
• II. Abstract
• III. Motivation
• IV. Specification
  • A. Immutable Parameters
  • B. Mutable Fields
  • C. Roles
  • D. Error Codes
  • E. Transitions
• V. Implementations
• VI. References
• VII. Copyright

I. What are NFTs and NFT royalties?

An NFT, or Non-Fungible Token is a digital asset. Unlike fungible tokens, each token is unique and non-interchangeable.

NFT royalties give an NFT creator or rights holder a percentage of the sale price each time the NFT is sold or re-sold.

II. Abstract

ZRC-6 defines a new minimum interface of an NFT smart contract while improving upon ZRC-1.

The main advantages of this standard are:

1. ZRC-6 standardizes royalty information retrieval with a percentage-based royalty fee model and unit-less royalty payments to a single address. Funds will be paid for secondary sales only if a marketplace chooses to implement royalty payments. Marketplace contracts should transfer the actual funds.

2. ZRC-6 includes base URI to support token URI with the concatenation of base URI and token ID. A token URI is an IPFS, HTTP, or data URL. This URL must return a JSON blob of data with the metadata for the NFT when queried. ZRC-7 covers metadata and token URI in detail.

3. ZRC-6 is designed for remote state read (x <- & c.f) such that logic to get data from a ZRC-6 contract is straightforward. ZRC-6 exposes immutable parameters via mutable fields and includes only transitions that mutate the state of the contract.

4. ZRC-6 is designed for failure. Therefore minting, burning, and token transfers are pausable.

5. ZRC-6 features batch operations for minting, burning and token transfers such that only a single transaction is required.

6. ZRC-6 features a single transition for token transfer with destination validation. The transition can be called by a token owner, a spender, or an operator. ZRC-6 prevents transferring tokens to the zero address or the address of a ZRC-6 contract.

7. ZRC-6 is compatible with ZRC-X since every callback name is prefixed with ZRC6_.

III. Motivation

1. Many of the largest NFT marketplaces have implemented incompatible royalty payment solutions.

2. The concatenated token URI can reduce gas cost and is more optimal for contract state.

3. Using callbacks to get data can complicate the logic easily. Unlike immutable parameters, mutable fields are available for remote state read.

4. Without an emergency stop mechanism, it's hard to respond to bugs and vulnerabilities gracefully.

5. Without batch operations, it can be very inefficient to transfer or mint multiple tokens with multiple transactions.

6. ZRC-1 includes Transfer and TransferFrom for the token transfer. The two transitions have the same type signature and the only difference is the access control. This has added unnecessary complexity. ZRC-1 does not validate the destination for token transfer and it is not safe.

7. The ZRC-1 and ZRC-2 contracts can share the same callback names. Contracts must have unique names for callback transitions.

IV. Specification

A. Immutable Parameters

Name	Type	Description
initial_contract_owner	ByStr20	Address of contract owner. It must not be the zero address i.e., 0x0000000000000000000000000000000000000000.
initial_base_uri	String	Base URI. e.g. https://creatures-api.zilliqa.com/api/creature/.
name	String	NFT name. It must not be an empty string.
symbol	String	NFT symbol. It must not be an empty string.

B. Mutable Fields

Name	Type	Description	Required
is_paused	Bool	True if the contract is paused. Otherwise, False. is_paused defaults to False.
contract_owner	ByStr20	Address of the contract owner. contract_owner defaults to initial_contract_owner.	✓
royalty_recipient	ByStr20	Address to send royalties to. royalty_recipient defaults to initial_contract_owner.
royalty_fee_bps	Uint128	Royalty fee BPS. For example, 1 = 0.01%, 10000 = 100%. royalty_fee_bps ranges from 1 to 10000 and defaults to 1000. When calculating the royalty amount, you must only use division to avoid integer overflow. royalty amount = sale price ÷ ( 10000 ÷ royalty fee bps ) For example, if royalty_fee_bps is 1000 (10%) and sale price is 999, royalty amount is 99.
base_uri	String	Base URI. For example, if base_uri is https://creatures-api.zilliqa.com/api/creature/ and token_id is 1, token URI is https://creatures-api.zilliqa.com/api/creature/1. If there is no specific token URI for a token in token_uris, the concatenated token URI is the token URI. If every token has its own token URI, base_uri can be an empty string. base_uri defaults to initial_base_uri. This field must not be mutated unless there is a strong reason.	✓
token_uris	Map Uint256 String	Mapping from token ID to its specific token URI. When it is required to set a specific token URI per token, use this field. If there is a specific token URI for a token in token_uris, the URI is the token URI. If every token does not have its own token URI, token_uris can be an empty map.	✓
minters	Map ByStr20 Bool	Set of minters.	✓
token_owners	Map Uint256 ByStr20	Mapping from token ID to its owner.	✓
spenders	Map Uint256 ByStr20	Mapping from token ID to a spender.	✓
operators	Map ByStr20 (Map ByStr20 Bool)	Mapping from token owner to set of operators.	✓
token_id_count	Uint256	The total number of tokens minted. Defaults to 0.	✓
balances	Map ByStr20 Uint256	Mapping from token owner to the number of existing tokens.	✓
total_supply	Uint256	The total number of existing tokens. Defaults to 0.	✓
token_name	String	Token name. Defaults to name. This field is for remote state read. This field must not be mutated.	✓
token_symbol	String	Token symbol. Defaults to symbol. This field is for remote state read. This field must not be mutated.	✓
contract_ownership_recipient	ByStr20	Address of the contract ownership recipient. Defaults to zero address.

C. Roles

Name	Description	Required
contract_owner	The contract owner can: pause/unpause the contract set contract ownership recipient set royalty recipient set royalty fee BPS set base URI add/remove a minter	✓
royalty_recipient	The royalty recipient gets a royalty amount each time the NFT is sold or re-sold. Initially, the royalty recipient is the contract owner.
minter	A minter can mint tokens. Initially, the contract owner is a minter.	✓
token_owner	Each token has an token owner. A token owner can: transfer a token burn a token add/remove a spender of a token add/remove an operator	✓
spender	On behalf of the token owner, a spender can transfer a token. There can only be one spender per token at any given time.	✓
operator	On behalf of the token owner, an operator can: transfer a token burn a token (optional) add/remove a spender of a token (optional) An operator is not bound to a single token, unlike a spender.	✓
contract_ownership_recipient	If the contract owner wants to transfer contract ownership to someone, the contract owner can set the person as the contract ownership recipient. The contract ownership recipient can accept the contract ownership and become the new contract owner.

Transition	contract_owner	minter	token_owner	spender	operator	contract_ownership_recipient
Pause	✓
Unpause	✓
SetRoyaltyRecipient	✓
SetRoyaltyFeeBPS	✓
SetBaseURI	✓
Mint		✓
BatchMint		✓
Burn			✓		✓
BatchBurn			✓		✓
AddMinter	✓
RemoveMinter	✓
SetSpender			✓		✓
AddOperator			✓
RemoveOperator			✓
TransferFrom			✓	✓	✓
BatchTransferFrom			✓	✓	✓
SetContractOwnershipRecipient	✓
AcceptContractOwnership						✓

D. Error Codes

The NFT contract must define the following constants for use as error codes for the Error exception.

Name	Type	Code	Description	Required
NotPausedError	Int32	-1	Emit when the contract is not paused.
PausedError	Int32	-2	Emit when the contract is paused.
SelfError	Int32	-3	Emit when the address is self.	✓
NotContractOwnerError	Int32	-4	Emit when the address is not a contract owner.	✓
NotTokenOwnerError	Int32	-5	Emit when the address is not a token owner.	✓
NotMinterError	Int32	-6	Emit when the address is not a minter.	✓
NotOwnerOrOperatorError	Int32	-7	Emit when the address is neither a token owner nor a token operator.	✓
MinterNotFoundError	Int32	-8	Emit when the minter is not found.	✓
MinterFoundError	Int32	-9	Emit when the minter is found.	✓
SpenderFoundError	Int32	-10	Emit when the spender is found.	✓
OperatorNotFoundError	Int32	-11	Emit when the operator is not found.	✓
OperatorFoundError	Int32	-12	Emit when the operator is found.	✓
NotAllowedToTransferError	Int32	-13	Emit when _sender is not allowed to transfer the token.	✓
TokenNotFoundError	Int32	-14	Emit when the token is not found.	✓
InvalidFeeBPSError	Int32	-15	Emit when the fee bps does not range from 1 to 10000.
ZeroAddressDestinationError	Int32	-16	Emit when the destination is the zero address.	✓
ThisAddressDestinationError	Int32	-17	Emit when the destination is _this_address.	✓
NotContractOwnershipRecipientError	Int32	-18	Emit when the address is not a contract ownership recipient.

E. Transitions

	Transition	Required
1	Pause()
2	Unpause()
3	SetRoyaltyRecipient(to: ByStr20)
4	SetRoyaltyFeeBPS(fee_bps: Uint128)
5	SetBaseURI(uri: String)
6	Mint(to: ByStr20, token_uri: String)	✓
7	BatchMint(to_token_uri_pair_list: List (Pair ByStr20 String)
8	Burn(token_id: Uint256)
9	BatchBurn(token_id_list: List Uint256)
10	AddMinter(to: ByStr20)	✓
11	RemoveMinter(to: ByStr20)	✓
12	SetSpender(to: ByStr20, token_id: Uint256)	✓
13	AddOperator(to: ByStr20)	✓
14	RemoveOperator(to: ByStr20)	✓
15	TransferFrom(to: ByStr20, token_id: Uint256)	✓
16	BatchTransferFrom(to_token_id_pair_list: List (Pair ByStr20 Uint256)
17	SetContractOwnershipRecipient(to: ByStr20)
18	AcceptContractOwnership()

1. Pause (Optional)

Pauses the contract. Use this only if things are going wrong ('circuit breaker').

Requirements:

• The contract must not be paused. Otherwise, it must throw PausedError.
• _sender must be the contract owner. Otherwise, it must throw NotContractOwnerError.

Messages:

	Name	Description	Callback Parameters
_tag	ZRC6_PauseCallback	Provide the sender a boolean for whether the contract is paused or not.	is_paused : Bool True if paused, otherwise False

Events:

	Name	Description	Event Parameters
_eventname	Pause	The contract has been paused.	is_paused : Bool True if paused, otherwise False

2. Unpause (Optional)

Unpauses the contract.

Requirements:

• The contract must be paused. Otherwise, it must throw NotPausedError.
• _sender must be the contract owner. Otherwise, it must throw NotContractOwnerError.

Messages:

	Name	Description	Callback Parameters
_tag	ZRC6_UnpauseCallback	Provide the sender a boolean for whether the contract is paused or not.	is_paused : Bool True if paused, otherwise False

Events:

	Name	Description	Event Parameters
_eventname	Unpause	The contract has been unpaused.	is_paused : Bool True if paused, otherwise False

3. SetRoyaltyRecipient (Optional)

Sets to as the royalty recipient.

Arguments:

Name	Type	Description
to	ByStr20	Address that royalties are sent to.

Requirements:

• _sender must be the contract owner. Otherwise, it must throw NotContractOwnerError.
• to must not be the zero address. Otherwise, it must throw ZeroAddressDestinationError.
• to must not be _this_address. Otherwise, it must throw ThisAddressDestinationError.

Messages:

	Name	Description	Callback Parameters
_tag	ZRC6_SetRoyaltyRecipientCallback	Provide the sender the address of royalty recipient.	to : ByStr20 Address of the royalty recipient

Events:

	Name	Description	Event Parameters
_eventname	SetRoyaltyRecipient	Royalty recipient has been updated.	to : ByStr20 Address of the royalty recipient

4. SetRoyaltyFeeBPS (Optional)

Sets fee_bps as royalty fee bps.

Arguments:

Name	Type	Description
fee_bps	Uint128	Royality fee BPS. e.g. 1 = 0.01%, 10000 = 100%.

Requirements:

• _sender must be the contract owner. Otherwise, it must throw NotContractOwnerError.
• fee_bps must range from 1 to 10000. Otherwise, it must throw InvalidFeeBPSError.

Messages:

	Name	Description	Callback Parameters
_tag	ZRC6_SetRoyaltyFeeBPSCallback	Provide the sender the royalty fee BPS.	royalty_fee_bps : Uint128 Royalty Fee BPS

Events:

	Name	Description	Event Parameters
_eventname	SetRoyaltyFeeBPS	Royalty fee BPS has been updated.	royalty_fee_bps : Uint128 Royalty Fee BPS

5. SetBaseURI (Optional)

Sets uri as the base URI. Use this only if there is a strong reason to change the base_uri.

Arguments:

Name	Type	Description
uri	String	Base URI.

Requirements:

• _sender must be the contract owner. Otherwise, it must throw NotContractOwnerError.

Messages:

	Name	Description	Callback Parameters
_tag	ZRC6_SetBaseURICallback	Provide the sender the base URI.	base_uri : String Base URI

Events:

	Name	Description	Event Parameters
_eventname	SetBaseURI	Base URI has been updated.	base_uri : String Base URI

6. Mint

Mints a token with a specific token_uri and transfers it to to. Pass empty string to token_uri to use the concatenated token URI. i.e. <base_uri><token_id>.

Arguments:

Name	Type	Description
to	ByStr20	Address of the recipient of the token to be minted.
token_uri	String	URI of a token.

Requirements:

• The contract must not be paused. Otherwise, it must throw PausedError.
• to must not be the zero address. Otherwise, it must throw ZeroAddressDestinationError
• to must not be _this_address. Otherwise, it must throw ThisAddressDestinationError
• _sender must be a minter. Otherwise, it must throw NotMinterError.

Messages:

	Name	Description	Callback Parameters
_tag	ZRC6_RecipientAcceptMint	Dummy callback to prevent invalid recipient contract.
_tag	ZRC6_MintCallback	Provide the sender the address of token recipient and token ID.	to : ByStr20 Address of a recipient token_id : Uint256 Unique ID of a token token_uri : String URI of a token

Events:

	Name	Description	Event Parameters
_eventname	Mint	Token has been minted with a specific URI.	to : ByStr20 Address of a recipient token_id : Uint256 Unique ID of a token token_uri : String URI of a token

7. BatchMint (Optional)

Mints multiple tokens with token_uris and transfers them to multiple tos. Pass empty string to token_uri to use the concatenated token URI. i.e. <base_uri><token_id>.

Arguments:

Name	Type	Description
to_token_uri_pair_list	List (Pair ByStr20 String)	List of Pair (to, token_uri) to : ByStr20 Address of a recipient token_uri : String URI of a token

Requirements:

• The contract must not be paused. Otherwise, it must throw PausedError.
• to_list must not include the zero address. Otherwise, it must throw ZeroAddressDestinationError
• to_list must not include _this_address. Otherwise, it must throw ThisAddressDestinationError
• _sender must be a minter. Otherwise, it must throw NotMinterError.

Messages:

	Name	Description	Callback Parameters
_tag	ZRC6_BatchMintCallback	An empty callback.

Events:

	Name	Description	Event Parameters
_eventname	BatchMint	Multiple tokens have been minted with token URIs.	to_token_uri_pair_list : List (Pair ByStr20 String) List of Pair (to, token_uri) to : ByStr20 Address of a recipient token_uri : String URI of a token start_id : Uint256 The token ID minted firstly end_id : Uint256 The token ID minted lastly

8. Burn (Optional)

Destroys token_id.

Arguments:

Name	Type	Description
token_id	Uint256	Unique ID of an existing token to be destroyed.

Requirements:

• The contract must not be paused. Otherwise, it must throw PausedError.
• token_id must exist. Otherwise, it must throw TokenNotFoundError.
• _sender must be a token owner or an operator. Otherwise, it must throw NotOwnerOrOperatorError.

Messages:

	Name	Description	Callback Parameters
_tag	ZRC6_BurnCallback	Provide the sender the burn address and token ID.	token_owner : ByStr20 Address of the token owner token_id : Uint256 Unique ID of a token

Events:

	Name	Description	Event Parameters
_eventname	Burn	Token has been burned.	token_owner : ByStr20 Address of the token owner token_id : Uint256 Unique ID of a token

9. BatchBurn (Optional)

Destroys token_id_list.

Arguments:

Name	Type	Description
token_id_list	List Uint256	List of unique IDs of the NFT to be destroyed.

Requirements:

• The contract must not be paused. Otherwise, it must throw PausedError.
• token_id must exist. Otherwise, it must throw TokenNotFoundError.
• _sender must be a token owner or an operator. Otherwise, it must throw NotOwnerOrOperatorError.

Messages:

	Name	Description	Callback Parameters
_tag	ZRC6_BatchBurnCallback	An empty callback.

Events:

Equivalent to multiple Burn events.

10. AddMinter

Adds minter.

Arguments:

Name	Type	Description
minter	ByStr20	Address to be added as minter.

Requirements:

• _sender must be the contract owner. Otherwise, it must throw NotContractOwnerError.
• minter must not be already a minter. Otherwise, it must throw MinterFoundError.

Messages:

	Name	Description	Callback Parameters
_tag	ZRC6_AddMinterCallback	Provide the sender the address of the minter.	minter : ByStr20 Address that has been added

Events:

	Name	Description	Event Parameters
_eventname	AddMinter	Minter has been added.	minter : ByStr20 Address that has been added

11. RemoveMinter

Removes minter.

Arguments:

Name	Type	Description
minter	ByStr20	Address to be removed from minter.

Requirements:

• _sender must be the contract owner. Otherwise, it must throw NotContractOwnerError.
• minter must be already a minter. Otherwise, it must throw MinterNotFoundError.

Messages:

	Name	Description	Callback Parameters
_tag	ZRC6_RemoveMinterCallback	Provide the sender the address that has been removed.	minter : ByStr20 Address that has been removed

Events:

	Name	Description	Event Parameters
_eventname	RemoveMinter	Minter has been removed.	minter : ByStr20 Address that has been removed

12. SetSpender

Sets spender for token_id. To remove spender for a token, use zero_address i.e., 0x0000000000000000000000000000000000000000

Arguments:

Name	Type	Description
spender	ByStr20	Address to be set as a spender for a given token.
token_id	Uint256	Unique ID of an existing token.

Requirements:

• token_id must exist. Otherwise, it must throw TokenNotFoundError.
• _sender must be a token owner or an operator. Otherwise, it must throw NotOwnerOrOperatorError.
• _sender must not be spender. Otherwise, it must throw SelfError.
• spender must not be already a spender. Otherwise, it must throw SpenderFoundError.

Messages:

	Name	Description	Callback Parameters
_tag	ZRC6_SetSpenderCallback	Provide the sender the address of the spender and token ID.	spender : ByStr20 Address that has been updated token_id : Uint256 Unique ID of a token

Events:

	Name	Description	Event Parameters
_eventname	SetSpender	Spender has been updated.	token_owner : ByStr20 Address of the token owner spender : ByStr20 Address that has been updated token_id : Uint256 Unique ID of a token

13. AddOperator

Adds operator for _sender.

Arguments:

Name	Type	Description
operator	ByStr20	Address to be added as operator.

Requirements:

• _sender must be the token owner. Otherwise, it must throw NotTokenOwnerError.
• _sender must not be operator. Otherwise, it must throw SelfError.
• operator must not be already an operator. Otherwise, it must throw OperatorFoundError.

Messages:

	Name	Description	Callback Parameters
_tag	ZRC6_AddOperatorCallback	Provide the sender the address of the operator.	operator : ByStr20 Address that has been added

Events:

	Name	Description	Event Parameters
_eventname	AddOperator	Operator has been added.	token_owner : ByStr20 Address of the token owner operator : ByStr20 Address that has been added

14. RemoveOperator

Removes operator for _sender.

Arguments:

Name	Type	Description
operator	ByStr20	Address to be removed from operator.

Requirements:

• operator must be already an operator. Otherwise, it must throw OperatorNotFoundError.

Messages:

	Name	Description	Callback Parameters
_tag	ZRC6_RemoveOperatorCallback	Provide the sender the address that has been removed.	operator : ByStr20 Address that has been removed

Events:

	Name	Description	Event Parameters
_eventname	RemoveOperator	Operator has been removed.	token_owner : ByStr20 Address of the token owner operator : ByStr20 Address that has been removed

15. TransferFrom

Transfers token_id from the token owner to to.

Arguments:

Name	Type	Description
to	ByStr20	Recipient address of the token.
token_id	Uint256	Unique ID of the token to be transferred.

Requirements:

• The contract must not be paused. Otherwise, it must throw PausedError.
• to must not be the zero address. Otherwise, it must throw ZeroAddressDestinationError.
• to must not be _this_address. Otherwise, it must throw ThisAddressDestinationError.
• token_id must exist. Otherwise, it must throw TokenNotFoundError.
• _sender must be a token owner, spender, or operator. Otherwise, it must throw NotAllowedToTransferError.
• _sender must not be to. Otherwise, it must throw SelfError.

Messages:

	Name	Description	Callback Parameters
_tag	ZRC6_RecipientAcceptTransferFrom	Dummy callback to prevent invalid recipient contract.	from : ByStr20 Address of the token owner to : ByStr20 Address of a recipient token_id : Uint256 Unique ID of a token
_tag	ZRC6_TransferFromCallback	Provide the sender the result of the token transfer.	from : ByStr20 Address of the token owner to : ByStr20 Address of a recipient token_id : Uint256 Unique ID of a token

Events:

	Name	Description	Event Parameters
_eventname	TransferFrom	Token has been transferred.	from : ByStr20 Address of the token owner to : ByStr20 Address of a recipient token_id : Uint256 Unique ID of a token

16. BatchTransferFrom (Optional)

Transfers multiple token_id to multiple to.

Arguments:

Name	Type	Description
to_token_id_pair_list	List (Pair ByStr20 Uint256)	List of Pair (to, token_id) to : ByStr20 Address of a recipient token_id : Uint256 Unique ID of a token

Requirements:

• The contract must not be paused. Otherwise, it must throw PausedError.
• to must not be the zero address. Otherwise, it must throw ZeroAddressDestinationError.
• to must not be _this_address. Otherwise, it must throw ThisAddressDestinationError.
• token_id must exist. Otherwise, it must throw TokenNotFoundError.
• _sender must be a token owner, spender, or operator. Otherwise, it must throw NotAllowedToTransferError.
• _sender must not be to. Otherwise, it must throw SelfError.

Messages:

	Name	Description	Callback Parameters
_tag	ZRC6_BatchTransferFromCallback	An empty callback.

Events:

Equivalent to multiple TransferFrom events.

17. SetContractOwnershipRecipient (Optional)

Sets to as the contract ownership recipient. To reset contract_ownership_recipient, use zero_address i.e., 0x0000000000000000000000000000000000000000.

Arguments:

Name	Type	Description
to	ByStr20	Address to be set as the contract ownership recipient.

Requirements:

• _sender must be the contract owner. Otherwise, it must throw NotContractOwnerError.
• _sender must not be to. Otherwise, it must throw SelfError.

Messages:

	Name	Description	Callback Parameters
_tag	ZRC6_SetContractOwnershipRecipientCallback	Provide the sender the address of the contract ownership recipient.	to : ByStr20 Address of the contract ownership recipient

Events:

	Name	Description	Event Parameters
_eventname	SetContractOwnershipRecipient	The contract ownership recipient has been updated.	to : ByStr20 Address of the contract ownership recipient

18. AcceptContractOwnership (Optional)

Sets contract_ownership_recipient as the contract owner.

Requirements:

• _sender must be the contract ownership recipient. Otherwise, it must throw NotContractOwnershipRecipientError.

Messages:

	Name	Description	Callback Parameters
_tag	ZRC6_AcceptContractOwnershipCallback	Provide the sender the result of the contract ownership transfer.	contract_owner : ByStr20 Address of the contract owner

Events:

	Name	Description	Event Parameters
_eventname	AcceptContractOwnership	Contract ownership has been transferred.	contract_owner : ByStr20 Address of the contract owner

V. Implementations

zrc6.scilla - a reference implementation

• includes test cases written using Zilliqa Isolated Server and Jest
• MIT licensed

VI. References

• Ethereum Smart Contract Best Practices
• EIP-721: Non-Fungible Token Standard
• EIP-2981: NFT Royalty Standard
• EIP-1155: Multi Token Standard
• OpenZeppelin ERC721.sol Implementation
• OpenZeppelin IERC2981.sol Implementation
• OpenZeppelin ERC1155.sol Implementation
• OpenZeppelin Pausable.sol Implementation
• OpenSea - Metadata Standards
• ZRC-1: Standard for Non Fungible Tokens
• ZRC issue #88 - ZRC contracts must have unique names for callback transitions
• SWC Registry - Smart Contract Weakness Classification and Test Cases
• RFC 1738 - Uniform Resource Locators (URL)
• RFC 2397 - The "data" URL scheme
• ZRC-7: NFT Metadata Standard

VII. Copyright

Copyright and related rights waived via CC0.