Creates EIP-5114: Soulbound Token

EIPS/eip-5114.md

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+---
+eip: 5114
+title: Soulbound Token
+description: A token that is attached to a "soul" at mint time and cannot be transferred after that.
+author: Micah Zoltu (@MicahZoltu)
+discussions-to: https://ethereum-magicians.org/t/eip-5114-soulbound-token/9417
+status: Draft
+type: Standards Track
+category: ERC
+created: 2022-05-30
+requires: 721
+---
+
+## Abstract
+A soulbound token is a token that is bound to another Non-Fungible Token (NFT; e.g., a EIP-721 token) when it is minted, and cannot be transferred/moved after that.
+
+## Specification
+```solidity
+interface IERC5114 {
+	// fired anytime a new instance of this token is minted
+	// this event **MUST NOT** be fired twice for the same `tokenId`
+	event Mint(uint256 indexed tokenId, address indexed nftAddress, uint256 indexed nftTokenId);
+
+	// returns the NFT token that owns this token.
+	// this function **MUST** throw if the token hasn't been minted yet
+	// this function **MUST** always return the same result every time it is called after it has been minted
+	// this function **MUST** return the same value as found in the original `Mint` event for the token
+	function ownerOf(uint256 index) external view returns (address nftAddress, uint256 nftTokenId);
+
+	// returns a censorship resistant URI with details about this token collection
+	// the metadata returned by this is merged with the metadata return by `tokenUri(uint256)`
+	// the collectionUri **MUST** be immutable and content addressable (e.g., ipfs://)
+	// the collectionUri **MUST NOT** point at mutable/censorable content (e.g., https://)
+	// data from `tokenUri` takes precedence over data returned by this method
+	// any external links referenced by the content at `collectionUri` also **MUST** follow all of the above rules
+	function collectionUri() external view returns (string collectionUri);
+
+	// returns a censorship resistant URI with details about this token instance
+	// the tokenUri **MUST** be immutable and content addressable (e.g., ipfs://)
+	// the tokenUri **MUST NOT** point at mutable/censorable content (e.g., https://)
+	// data from this takes precedence over data returned by `collectionUri`
+	// any external links referenced by the content at `tokenUri` also **MUST** follow all of the above rules
+	function tokenUri(uint256 tokenId) external view returns (string tokenUri);
+}
+```
+
+## Rationale
+### Immutability
+By requiring that tokens can never move, we both guarantee non-separability and non-mergeability among collections of soulbound tokens that are bound to a single NFT while simultaneously allowing users to aggressively cache results.
+### Content Addressable URIs Required
+Soulbound tokens are meant to be permanent badges/indicators attached to a persona.
+This means that not only can the user not transfer ownership, but the minter also cannot withdraw/transfer/change ownership as well.
+This includes mutating or removing any remote content as a means of censoring or manipulating specific users.
+### No Specification for tokenUri Data Format
+The format of the data pointed to by `collectionUri()` and `tokenUri(uint256)` is intentionally left out of this standard in favor of separate standards that can be iterated on in the future.
+The immutability constraints are the only thing defined by this to ensure that the spirit of this token is maintained, regardless of the specifics of the data format.
+
+## Backwards Compatibility
+This is a new token type and is not meant to be backward compatible with any existing tokens other than existing viable souls (like EIP-721 tokens).
+
+## Security Considerations
+Users of tokens that claim to implement this EIP must be diligent in verifying they actually do.
+A token author can create a token that, upon initial probing of the API surface, may appear to follow the rules when in reality it doesn't.
+For example, the contract could allow transfers via some mechanism and simply not utilize them initially.
+
+It should also be made clear that soulbound tokens are not bound to a human, they are bound to a persona.
+A persona is any actor (which could be a group of humans) that collects multiple soulbound tokens over time to build up a collection of badges.
+This persona may transfer to another human, or to another group of humans, and anyone interacting with a persona should not assume that there is a single permanent immutable human behind that persona.
+
+It is possible for a soulbound token to be bound to another soulbound token.
+In theory, if all tokens in the chain are created at the same time they could form a loop.
+Software that tries to walk such a chain should take care to have an exit strategy if a loop is detected.
+
+## Copyright
+Copyright and related rights waived via [CC0](../LICENSE.md).