EIP-2304: Multichain address resolution for ENS

Abstract

This EIP introduces new overloads for the the addr field for ENS resolvers, which permit resolution of addresses for other blockchains via ENS.

Motivation

With the increasing uptake of ENS by multi-coin wallets, wallet authors have requested the ability to resolve addresses for non-Ethereum chains inside ENS. This specification standardises a way to enter and retrieve these addresses in a cross-client fashion.

Specification

A new accessor function for resolvers is specified:

function addr(bytes32 node, uint coinType) external view returns(bytes memory);

The EIP165 interface ID for this function is 0xf1cb7e06.

When called on a resolver, this function must return the cryptocurrency address for the specified namehash and coin type. A zero-length string must be returned if the specified coin ID does not exist on the specified node.

coinType is the cryptocurrency coin type index from SLIP44.

The return value is the cryptocurency address in its native binary format. Detailed descriptions of the binary encodings for several popular chains are provided in the Address Encoding section below.

A new event for resolvers is defined:

event AddressChanged(bytes32 indexed node, uint coinType, bytes newAddress);

Resolvers MUST emit this event on each change to the address for a name and coin type.

Recommended accessor functions

The following function provides the recommended interface for changing the addresses stored for a node. Resolvers SHOULD implement this interface for setting addresses unless their needs dictate a different interface.

function setAddr(bytes32 node, uint coinType, bytes calldata addr);

setAddr adds or replaces the address for the given node and coin type. The parameters for this function are as per those described in addr() above.

This function emits an AddressChanged event with the new address; see also the backwards compatibility section below for resolvers that also support addr(bytes32).

Address Encoding

In general, the native binary representation of the address should be used, without any checksum commonly used in the text representation.

A table of encodings for common blockchains is provided, followed by a more detailed description of each format. In the table, ‘encodings’ lists the address encodings supported by that chain, along with any relevant parameters. Details of those address encodings are described in the following sections.

P2PKH(version)

Pay to Public Key Hash addresses are base58check encoded. After decoding, the first byte is a version byte. For example, the Bitcoin address 1A1zP1eP5QGefi2DMPTfTL5SLmv7DivfNa base58check decodes to the 21 bytes 0062e907b15cbf27d5425399ebf6f0fb50ebb88f18.

P2PKH addresses have a version byte, followed by a 20 byte pubkey hash. Their canonical encoding is their scriptPubkey encoding (specified here) is OP_DUP OP_HASH160 <pubKeyHash> OP_EQUALVERIFY OP_CHECKSIG.

The above example address is thus encoded as the 25 bytes 76a91462e907b15cbf27d5425399ebf6f0fb50ebb88f1888ac.

P2SH(version)

P2SH addresses are base58check encoded in the same manner as P2PKH addresses. P2SH addresses have a version, followed by a 20 byte script hash. Their scriptPubkey encoding (specified here) is OP_HASH160 <scriptHash> OP_EQUAL. A Bitcoin address of 3Ai1JZ8pdJb2ksieUV8FsxSNVJCpoPi8W6 decodes to the 21 bytes 0562e907b15cbf27d5425399ebf6f0fb50ebb88f18 and is encoded as the 23 bytes a91462e907b15cbf27d5425399ebf6f0fb50ebb88f1887.

SegWit(hrp)

SegWit addresses are encoded with bech32. Bech32 addresses consist of a human-readable part - ‘bc’ for Bitcoin mainnet - and a machine readable part. For SegWit addresses, this decodes to a ‘witness version’, between 0 and 15, and a ‘witness program’, as defined in BIP141.

The scriptPubkey encoding for a bech32 address, as defined in BIP141, is OP_n, where n is the witness version, followed by a push of the witness program. Note this warning from BIP173:

Implementations should take special care when converting the address to a scriptPubkey, where witness version n is stored as OP_n. OP_0 is encoded as 0x00, but OP_1 through OP_16 are encoded as 0x51 though 0x60 (81 to 96 in decimal). If a bech32 address is converted to an incorrect scriptPubKey the result will likely be either unspendable or insecure.

For example, the Bitcoin SegWit address BC1QW508D6QEJXTDG4Y5R3ZARVARY0C5XW7KV8F3T4 decodes to a version of 0 and a witness script of 751e76e8199196d454941c45d1b3a323f1433bd6, and then encodes to a scriptPubkey of 0014751e76e8199196d454941c45d1b3a323f1433bd6.

ChecksummedHex(chainId?)

To translate a text format checksummed hex address into binary format, simply remove the ‘0x’ prefix and hex decode it. 0x314159265dD8dbb310642f98f50C066173C1259b is hex-decoded and stored as the 20 bytes 314159265dd8dbb310642f98f50c066173c1259b.

A checksum format is specified by EIP-55, and extended by RSKIP60, which specifies a means of including the chain ID in the checksum. The checksum on a text format address must be checked. Addresses with invalid checksums that are not all uppercase or all lowercase MUST be rejected with an error. Implementations may choose whether to accept non-checksummed addresses, but the authors recommend at least providing a warning to users in this situation.

When encoding an address from binary to text, an EIP55/RSKIP60 checksum MUST be used - so the correct encoding of the above address for Ethereum is 0x314159265dD8dbb310642f98f50C066173C1259b.

Ripple

Ripple addresses are encoded using a version of base58check with an alternative alphabet, described here. Two types of ripple addresses are supported, ‘r-addresses’, and ‘X-addresss’. r-addresses consist of a version byte followed by a 20 byte hash, while X-addresses consist of a version byte, a 20 byte hash, and a tag, specified here.

Both address types should be stored in ENS by performing ripple’s version of base58check decoding and storing them directly (including version byte). For example, the ripple address rf1BiGeXwwQoi8Z2ueFYTEXSwuJYfV2Jpn decodes to and is stored as 004b4e9c06f24296074f7bc48f92a97916c6dc5ea9, while the address X7qvLs7gSnNoKvZzNWUT2e8st17QPY64PPe7zriLNuJszeg decodes to and is stored as 05444b4e9c06f24296074f7bc48f92a97916c6dc5ea9000000000000000000.

CashAddr

Bitcoin Cash defines a new address format called ‘CashAddr’, specified here. This uses a variant of bech32 encoding to encode and decode (non-segwit) Bitcoin Cash addresses, using a prefix of ‘bitcoincash:’. A CashAddr should be decoded using this bech32 variant, then converted and stored based on its type (P2PKH or P2SH) as described in the relevant sections above.

Bech32

Bech32 addresses consist of a human-readable part - for example, ‘bnb’ for Binance - and a machine readable part. The encoded data is simply the address, which can be converted to binary and stored directly.

For example, the BNB address bnb1grpf0955h0ykzq3ar5nmum7y6gdfl6lxfn46h2 decodes to the binary representation 40c2979694bbc961023d1d27be6fc4d21a9febe6, which is stored directly in ENS.

Example

An example implementation of a resolver that supports this EIP is provided here:

pragma solidity ^0.5.8;

contract AddrResolver is ResolverBase {
    bytes4 constant private ADDR_INTERFACE_ID = 0x3b3b57de;
    bytes4 constant private ADDRESS_INTERFACE_ID = 0xf1cb7e06;
    uint constant private COIN_TYPE_ETH = 60;

    event AddrChanged(bytes32 indexed node, address a);
    event AddressChanged(bytes32 indexed node, uint coinType, bytes newAddress);

    mapping(bytes32=>mapping(uint=>bytes)) _addresses;

    /**
     * Sets the address associated with an ENS node.
     * May only be called by the owner of that node in the ENS registry.
     * @param node The node to update.
     * @param a The address to set.
     */
    function setAddr(bytes32 node, address a) external authorised(node) {
        setAddr(node, COIN_TYPE_ETH, addressToBytes(a));
    }

    /**
     * Returns the address associated with an ENS node.
     * @param node The ENS node to query.
     * @return The associated address.
     */
    function addr(bytes32 node) public view returns (address) {
        bytes memory a = addr(node, COIN_TYPE_ETH);
        if(a.length == 0) {
            return address(0);
        }
        return bytesToAddress(a);
    }

    function setAddr(bytes32 node, uint coinType, bytes memory a) public authorised(node) {
        emit AddressChanged(node, coinType, a);
        if(coinType == COIN_TYPE_ETH) {
            emit AddrChanged(node, bytesToAddress(a));
        }
        _addresses[node][coinType] = a;
    }

    function addr(bytes32 node, uint coinType) public view returns(bytes memory) {
        return _addresses[node][coinType];
    }

    function supportsInterface(bytes4 interfaceID) public pure returns(bool) {
        return interfaceID == ADDR_INTERFACE_ID || interfaceID == ADDRESS_INTERFACE_ID || super.supportsInterface(interfaceID);
    }
}

Implementation

An implementation of this interface is provided in the ensdomains/resolvers repository.

Backwards Compatibility

If the resolver supports the addr(bytes32) interface defined in EIP137, the resolver MUST treat this as a special case of this new specification in the following ways:

1. The value returned by addr(node) from EIP137 should always match the value returned by addr(node, 60) (60 is the coin type ID for Ethereum).
2. Anything that causes the AddrChanged event from EIP137 to be emitted must also emit an AddressChanged event from this EIP, with the coinType specified as 60, and vice-versa.

Tests

The table below specifies test vectors for valid address encodings for each cryptocurrency described above.

Copyright

Copyright and related rights waived via CC0.

Citation

Please cite this document as:

Nick Johnson, "EIP-2304: Multichain address resolution for ENS," Ethereum Improvement Proposals, no. 2304, September 2019. [Online serial]. Available: https://eips.ethereum.org/EIPS/eip-2304.