Lockable Extension for ERC-721
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正文
Abstract
An extension of ERC-721, this standard incorporates locking
features into NFTs, allowing for various uses while preventing sale or transfer. The token's owner
can lock
it, setting up locker address (either an EOA or a contract) that exclusively holds the power to unlock the token. Owner can also provide approval for tokenId
, enabling ability to lock asset while address holds the token approval. Token can also be locked by approved
, assigning locker to itself. Upon token transfer, these rights get purged.
Motivation
ERC-721 has sparked an unprecedented surge in demand for NFTs. However, despite this tremendous success, the NFT economy suffers from secondary liquidity where it remains illiquid in owner’s wallet. There are projects which aim to address the liquidity challenge, but they entail the below mentioned inconveniences and risks for owners as they necessitate transferring the participating NFTs to the projects' contracts.
- Loss of utility: The utility value of NFTs diminishes when they are transferred to an escrow account, no longer remaining under the direct custody of the owners.
- Lack of composability: The market could benefit from increased liquidity if NFT owners had access to multiple financial tools, such as leveraging loans and renting out their assets for maximum returns. Composability serves as the missing piece in creating a more efficient market.
- Smart contract vulnerabilities: NFTs are susceptible to loss or theft due to potential bugs or vulnerabilities present in the smart contracts they rely on.
The aforementioned issues contribute to a poor user experience (UX), and we propose enhancing the ERC-721 standard by implementing a native locking mechanism: Rather than being transferred to a smart contract, an NFT remains securely stored in self-custody but is locked. During the lock period, the NFT's transfer is restricted while its other properties remain unchanged. NFT Owner retains the ability to use or distribute it’s utility.
NFTs have numerous use cases where the NFT must remain within the owner's wallet, even when it serves as collateral for a loan. Whether it's authorizing access to a Discord server, or utilizing NFT within a play-to-earn (P2E) game, owner should have the freedom to do so throughout the lending period. Just as real estate owner can continue living in their mortgaged house, take personal loan or keep tenants to generate passive income, these functionalities should be available to NFT owners to bring more investors in NFT economy.
Lockable NFTs enable the following use cases :
- NFT-collateralized loans: Utilize NFT as collateral for a loan without locking it on the lending protocol contract. Instead, lock it within owner’s wallet while still enjoying all the utility of NFT.
- No collateral rentals of NFTs: Borrow an NFT for a fee without the need for significant collateral. Renter can use the NFT but not transfer it, ensuring the lender's safety. The borrowing service contract automatically returns the NFT to the lender once the borrowing period expires.
- Buy Now Pay Later (BNPL): The buyer receives the locked NFT and can immediately begin using it. However, they are unable to sell the NFT until all installments are paid. Failure to complete the full payment results in the NFT returning to the seller, along with a fee.
- Composability: Maximize liquidity by having access to multiple financial tools. Imagine taking a loan against NFT and putting it on rentals to generate passive income.
- Primary sales: Mint an NFT for a partial payment and settle the remaining amount once owner is satisfied with the collection's progress.
- Soulbound: Organization can mint and self-assign
locker
, send token to user and lock the asset. - Safety: Safely and conveniently use exclusive blue chip NFTs. Lockable extension allows owner to lock NFT and designate secure cold wallet as the unlocker. This way, owner can keep NFT on MetaMask and easily use it, even if a hacker gains access to MetaMask account. Without access to the cold wallet, the hacker cannot transfer NFT, ensuring its safety.
This proposal is different from other locking proposals in number of ways:
- This implementation provides a minimal implementation of
lock
andunlock
and believes other conditions like time-bound are great ideas but can be achieved without creating a specific implementation. Locking and Unlocking can be based on any conditions (e.g. repayment, expiry). Therefore time-bound unlocks a relatively specific use case that can be achieved via smart-contracts themselves without that being a part of the token contract. - This implementation proposes a separation of rights between locker and approver. Token can be locked with approval and approved can unlock and withdraw tokens (opening up opportunities like renting, lending, BNPL etc), and token can be locked lacking the rights to revoke token, yet can unlock if required (opening up opportunities like account-bound NFTs).
- Our proposal implement ability to
transferAndLock
which can be used to transfer, lock and optionally approve token. Enabling the possibility of revocation after transfer.
By extending the ERC-721 standard, the proposed standard enables secure and convenient management of underlying NFT assets. It natively supports prevalent NFTFi use cases such as staking, lending, and renting. We anticipate that this proposed standard will foster increased engagement of NFT owners in NFTFi projects, thereby enhancing the overall vitality of the NFT ecosystem.
Specification
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 and RFC 8174.
Overview
ERC-721 compliant contracts MAY implement this EIP to provide standard methods of locking and unlocking the token at its current owner address.
Token owner MAY lock
the token and assign locker
to some address
using lock(uint256 tokenId, address _locker)
function, this MUST set locker
to _locker
. Token owner or approved MAY lock
the token using lock(uint256 tokenId)
function, this MUST set locker
to msg.sender
. Token MAY be unlocked
by locker
using unlock
function. unlock
function MUST delete locker
mapping and default to address(0)
.
If the token is locked
, the lockerOf
function MUST return an address that is locker
and can unlock
the token. For tokens that are not locked
, the lockerOf
function MUST return address(0)
.
lock
function MUST revert if token is already locked
. unlock
function MUST revert if token is not locked
. ERC-721 approve
function MUST revert if token is locked
. ERC-721 functions that transfer ownership of a token MUST revert if token is locked
, unless msg.sender
is approved
and locker
both. After ERC-721 token transfer function call, values of locker
and approved
MUST be purged.
Token MAY be transferred and locked
, also assign approval
to locker
using transferAndLock
function. This is RECOMMENDED for use-cases where Token transfer and subsequent revocation is REQUIRED.
Interface
// SPDX-License-Identifier: CC0-1.0
pragma solidity >=0.7.0 <0.9.0;
/// @title Lockable Extension for ERC721
/// @dev Interface for the Lockable extension
/// @author StreamNFT
interface IERC7066{
/**
* @dev Emitted when tokenId is locked
*/
event Lock (uint256 indexed tokenId, address _locker);
/**
* @dev Emitted when tokenId is unlocked
*/
event Unlock (uint256 indexed tokenId);
/**
* @dev Lock the tokenId if msg.sender is owner or approved and set locker to msg.sender
*/
function lock(uint256 tokenId) external;
/**
* @dev Lock the tokenId if msg.sender is owner and set locker to _locker
*/
function lock(uint256 tokenId, address _locker) external;
/**
* @dev Unlocks the tokenId if msg.sender is locker
*/
function unlock(uint256 tokenId) external;
/**
* @dev Tranfer and lock the token if the msg.sender is owner or approved.
* Lock the token and set locker to caller
* Optionally approve caller if bool setApprove flag is true
*/
function transferAndLock(uint256 tokenId, address from, address to, bool setApprove) external;
/**
* @dev Returns the wallet, that is stated as unlocking wallet for the tokenId.
* If address(0) returned, that means token is not locked. Any other result means token is locked.
*/
function lockerOf(uint256 tokenId) external view returns (address);
}
Rationale
This proposal set locker[tokenId]
to address(0)
when token is unlocked
because we delete mapping on locker[tokenId]
freeing up space. Also, this assertion helps our contract to validate if token is locked
or unlocked
for internal function calls.
This proposal exposes transferAndLock(uint256 tokenId, address from, address to, bool setApprove)
which can be used to transfer token and lock at the receiver's address. This additionally accepts input bool setApprove
which on true
assign approval
to locker
, hence enabling locker
to revoke the token (revocation conditions can be defined in contracts and approval
provided to contract). This provides conditional ownership to receiver, without the privilege to transfer
token.
Backwards Compatibility
This standard is compatible with ERC-721 standards.
Existing Upgradedable ERC-721 can upgrade to this standard, enabling locking capability inherently and unlock underlying liquidity features.
Test Cases
Test cases can be found here.
Reference Implementation
Reference Interface can be found here.
Reference Implementation can be found here.
Security Considerations
There are no security considerations related directly to the implementation of this standard for the contract that manages ERC-721.
Considerations for the contracts that work with lockable tokens
- Once
locked
, token can not be furtherapproved
ortransfered
. - If token is
locked
and caller islocker
andapproved
both, caller can transfer the token. locked
token withlocker
as in-accesible account or un-verified contract address can lead to permanent lock of the token.- There are no MEV considerations regarding lockable tokens as only authorized parties are allowed to lock and unlock.
Copyright
Copyright and related rights waived via CC0.