This standard addresses an extension to the ERC-721 specification by allowing signatures on NFTs representing works of art. This provides improved provenance by creating functionality for an artist to designate an original and signed limited-edition prints of their work.
ERC-3440 is an ERC-721 extension specifically designed to make NFTs more robust for works of art. This extends the original ERC-721 spec by providing the ability to designate the original and limited-edition prints with a specialized enumeration extension similar to the original 721 extension built-in. The key improvement of this extension is allowing artists to designate the limited nature of their prints and provide a signed piece of data that represents their unique signature to a given token Id, much like an artist would sign a print of their work.
Currently the link between a NFT and the digital work of art is only enforced in the token metadata stored in the shared tokenURI state of a NFT. While the blockchain provides an immutable record of history back to the origin of an NFT, often the origin is not a key that an artist maintains as closely as they would a hand written signature.
An edition is a printed replica of an original piece of art. ERC-721 is not specifically designed to be used for works of art, such as digital art and music. ERC-721 (NFT) was originally created to handle deeds and other contracts. Eventually ERC-721 evolved into gaming tokens, where metadata hosted by servers may be sufficient. This proposal takes the position that we can create a more tangible link between the NFT, digital art, owner, and artist. By making a concise standard for art, it will be easier for an artist to maintain a connection with the Ethereum blockchain as well as their fans that purchase their tokens.
The use cases for NFTs have evolved into works of digital art, and there is a need to designate an original NFT and printed editions with signatures in a trustless manner. ERC-721 contracts may or may not be deployed by artists, and currently, the only way to understand that something is uniquely touched by an artist is to display it on 3rd party applications that assume a connection via metadata that exists on servers, external to the blockchain. This proposal helps remove that distance with readily available functionality for artists to sign their work and provides a standard for 3rd party applications to display the uniqueness of a NFT for those that purchase them. The designation of limited-editions combined with immutable signatures, creates a trustlessly enforced link. This signature is accompanied by view functions that allow applications to easily display these signatures and limited-edition prints as evidence of uniqueness by showing that artists specifically used their key to designate the total supply and sign each NFT.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119.
ERC-721 compliant contracts MAY implement this ERC for editions to provide a standard method for designating the original and limited-edition prints with signatures from the artist.
Implementations of ERC-3440 MUST designate which token Id is the original NFT (defaulted to Id 0), and which token Id is a unique replica. The original print SHOULD be token Id number 0 but MAY be assigned to a different Id. The original print MUST only be designated once. The implementation MUST designate a maximum number of minted editions, after which new Ids MUST NOT be printed / minted.
Artists MAY use the signing feature to sign the original or limited edition prints but this is OPTIONAL. A standard message to sign is RECOMMENDED to be simply a hash of the integer of the token Id.
Signature messages MUST use the EIP-712 standard.
A contract that is compliant with ERC-3440 shall implement the following abstract contract (referred to as ERC3440.sol):
A major role of NFTs is to display uniqueness in digital art. Provenance is a desired feature of works of art, and this standard will help improve a NFT by providing a better way to verify uniqueness. Taking this extra step by an artist to explicitly sign tokens provides a better connection between the artists and their work on the blockchain. Artists can now retain their private key and sign messages in the future showing that the same signature is present on a unique NFT.
This proposal combines already available 721 extensions and is backwards compatible with the ERC-721 standard.
An example implementation including tests can be found here.
This extension gives an artist the ability to designate an original edition, set the maximum supply of editions as well as print the editions and uses the tokenURI extension to supply a link to the art work. To minimize the risk of an artist changing this value after selling an original piece this function can only happen once. Ensuring that these functions can only happen once provides consistency with uniqueness and verifiability. Due to this, the reference implementation handles these features in the constructor function. An edition may only be signed once, and care should be taken that the edition is signed correctly before release of the token/s.
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