This standard is an extension of EIP-721. It proposes a minimal interface to make tokens soulbound using the feature detection functionality of EIP-165. A soulbound token is a non-fungible token bound to a single account.
The Ethereum community has expressed a need for non-transferrable, non-fungible, and socially-priced tokens similar to World of Warcraft’s soulbound items. But the lack of a token standard leads many developers to simply throw errors upon a user's invocation of transfer functionalities. Over the long term, this will lead to fragmentation and less composability.
In this document, we outline a minimal addition to EIP-721 that allows wallet implementers to check for a token contract's permanent (non-)transferability using EIP-165.
The keywords "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.
A token with a uint256 tokenId may be bound to a receiving account with function locked(...) returning true. In this case, all EIP-721 functions of the contract that transfer the token from one account to another must throw.
To aid recognition that an EIP-721 token implements "soulbinding" via this EIP upon calling EIP-721's function supportsInterface(bytes4 interfaceID) external view returns (bool) with interfaceID=0xb45a3c0e, a contract implementing this EIP must return true.
The above model is the simplest possible path towards a canonical interface for Soulbound tokens. It reflects upon the numerous Soulbound token implementations that simply revert upon transfers.
This proposal is fully backward compatible with EIP-721.
There are no security considerations related directly to the implementation of this standard.
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