ethereum · TimDaub · Apr 1, 2022
@@ -0,0 +1,152 @@
+---
+eip: 1238
+title: Non-Transferrable Non-Fungible Tokens (Soulbound Tokens)
+author: Nicola Greco, Tim Daubenschütz <tim@daubenschuetz.de>
+discussions-to: https://github.com/ethereum/eips/issues/1238
+type: Standards Track
+category: ERC
+status: Draft
+created: 2022-04-01
+requires: 165, 721
+---
+
+## Simple Summary
+
+A standard interface for non-transferrable non-fungible tokens, also known as
+"soulbound tokens" (short "SBT").
+
+## Abstract
+
+The following standard allows for the implementation of a standard API for SBTs
+within smart contracts. This standard provides basic functionality to gift,
+mint and track SBTs.
+
+## Motivation
+
+`ERC1238` tokens inhert their metadata model from `ERC721`'s metadata. Wallets
+can differentiate between `ERC1238` and transferrable `ERC721` tokens through
+`ERC165`'s `supportsInterface(bytes4 interfaceID);` method.
+
+When a wallet detects support for `IERC721Metadata` (`interfaceID`:
+`0x5b5e139f`), it can additonal check if transfer functionality via
+
+- `IERC721` (`interfaceID`: `0x80ac58cd`); or
+- soulbound properties via `IERC1238` (`interfaceID`: `0xad5ec850`)
+
+are supported.
+
+Since `ERC1238` supports `ERC721Metadata`, if implemented correctly, `ERC721` wallets
+should be able to display `ERC1238` tokens without the need for any modifications
+from developers. `ERC721`'s transfer functionality could be hidden from the UI if
+`interfaceID` `0x80ac58cd` isn't supported or `ERC1238`'s `interfaceID` is
+detected.
+
+## Specification
+
+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.
+
+**Every `ERC1238` compliant contract must implement the `ERC1238`,
+`ERC721Metadata` and `ERC165` interfaces** (subject to "caveats" below):
+
+```solidity
+pragma solidity ^0.8.6;
+
+/// @title ERC-1238 Non-Transferrable Non-Fungible Token Standard
+/// @dev See https://eips.ethereum.org/EIPS/eip-1238
+///  Note: the ERC-165 identifier for this interface is 0xad5ec850.
+interface IERC1238 /* is ERC165, ERC721Metadata */ {
+  /// @dev This emits when bond of any SBT is established by any mechanism.
+  ///  This event emits when SBTs are created (`from` == 0) and destroyed
+  ///  (`to` == 0). Exception: during contract creation, any number of SBTs
+  ///  may be created and assigned without emitting Bond.
+  event Bond(address indexed _from, address indexed _to, uint256 indexed _tokenId);
+  /// @notice Find the address bound to an ERC1238 soulbound token (short "SBT")
+  /// @dev SBTs assigned to zero address are considered invalid, and queries
+  ///  about them do throw.
+  /// @param _tokenId The identifier for an SBT
+  /// @return The address of the owner bound to the SBT
+  function boundTo(uint256 _tokenId) external view returns (address);
+}
+
+interface ERC165 {
+  /// @notice Query if a contract implements an interface
+  /// @param interfaceID The interface identifier, as specified in ERC-165
+  /// @dev Interface identification is specified in ERC-165. This function
+  ///  uses less than 30,000 gas.
+  /// @return `true` if the contract implements `interfaceID` and
+  ///  `interfaceID` is not 0xffffffff, `false` otherwise
+  function supportsInterface(bytes4 interfaceID) external view returns (bool);
+}
+
+/// @title ERC-721 Non-Fungible Token Standard, optional metadata extension
+/// @dev See https://eips.ethereum.org/EIPS/eip-721
+///  Note: the ERC-165 identifier for this interface is 0x5b5e139f.
+interface ERC721Metadata /* is ERC721 */ {
+  /// @notice A descriptive name for a collection of NFTs in this contract
+  function name() external view returns (string _name);
+
+  /// @notice An abbreviated name for NFTs in this contract
+  function symbol() external view returns (string _symbol);
+
+  /// @notice A distinct Uniform Resource Identifier (URI) for a given asset.
+  /// @dev Throws if `_tokenId` is not a valid NFT. URIs are defined in RFC
+  ///  3986. The URI may point to a JSON file that conforms to the "ERC721
+  ///  Metadata JSON Schema".
+  function tokenURI(uint256 _tokenId) external view returns (string);
+}
+```
+
+For "ERC721 Metadata JSON Schema", see `ERC721` document.
+
+### Caveats
+
+No caveats known for now.
+
+## Rationale
+
+Since 2018, the Ethereum community has been seeking consensus for a standard
+for non-transferrable non-fungible token.
+
+For `ERC1238`, we propose:
+
+- `event Bond(address indexed _from, address indexed _to, uint256 indexed _tokenId)`, essentially `ERC721`'s Transfer event but renamed to "Bond".
+- `function boundTo(uint256 _tokenId) external view returns (address)`,
+  essentially `ERC721`'s `ownerOf(uint256 _tokenId)` but renamed to `boundTo`.
+
+The benefit of using the `ERC721Metadata` interface is that implementers (e.g.
+wallets) of `ERC1238` can identify transfer and tracking functionality using the
+`ERC165` `supportsInterface(bytes4 interfaceID)` function.
+
+## Backwards Compatibility
+
+We have adopted the `ERC165` and `ERC721Metadata` functions. `ERC1238`'s
+`boundTo` function is the pendant to `ERC721`'s `ownerOf` function.
+
+## Test Cases
+
+See "Implementations" section.
+
+## Implementations
+
+- [rugpullindex/ERC1238](https://github.com/rugpullindex/ERC1238) is a minimal
+  implementation using `burn` and `mint` and test cases released under GNU
+  General Public License v3.0.
+
+## References
+
+**Standards**
+
+1. [ERC-165](./eip-165.md) Standard Interface Detection.
+1. [ERC-721](https://github.com/ethereum/EIPs/issues/721) Token Standard.
+1. Soulbound. https://vitalik.ca/general/2022/01/26/soulbound.html
+1. RFC 2119 Key words for use in RFCs to Indicate Requirement Levels. https://www.ietf.org/rfc/rfc2119.txt
+
+**Issues**
+
+1. The Original ERC-1238 Issue. https://github.com/ethereum/eips/issues/1238
+
+## Copyright
+
+Copyright and related rights waived via [CC0](https://creativecommons.org/publicdomain/zero/1.0/).