NIP-XX

Nostr-Specific Deterministic Private Key Generation from Ethereum Wallet Signature

draft optional author:0xc0de4c0ffee author:sshmatrix

Abstract

This specification provides an optional method for Nostr clients to generate deterministic private keys from Ethereum wallet signatures. This NIP proposes HMAC Key Derivation Function (HKDF) coupled with SHA-256 from ECDSA signatures (EIP-191) as an alternative to the Schnorr signatures (BIP-340), allowing Nostr to interact with Ethereum ecosystem.

Terminology

a) Username

Username can be either of the following:

petname or petname@domain.eth.limo or domain.eth.limo or sub.domain.eth.limo, where

petname is a NIP-02 compatible name,
petname@domain.eth.limo is a NIP-05 compatible name,
domain.eth.limo is NIP-05 equivalent of _@domain.eth.limo,
sub.domain.eth.limo is NIP-05 equivalent of _@sub.domain.eth.limo.

a) sub@domain.eth.limo and sub.domain.eth.limo are NOT equivalent as their signatures will be different

b) petname can be the same as domain

b) Password

Password is an optional string value used in HKDF salt,

let password = "horse staple battery"
let salt = await sha256(`eip155:${chainId}:${username}:${password?password:""}:${signature.slice(68)}`);

c) Message

Message is text on screen that should warn the users to not sign messages indiscriminately,

let message = `Login to Nostr as ${username}\n\nImportant: Please verify the integrity and authenticity of your Nostr client before signing this message.\n${info}`

d) Signature

Signature is the deterministic signature from connected Ethereum wallet. Ethereum signatures (v,r,s) are 65 bytes long, i.e. bytes1(v) + bytes32(r) + bytes32(s),

let signature = wallet.signMessage(message);

e) HKDF (HMAC Key Derivation Function)

HKDF-SHA-256 is used to derive the 42 bytes long hash key: hkdf(sha256, inputKey, salt, info, dkLen = 42)

Input key is SHA-256 hash of signature bytes.

let inputKey = await sha256(hexToBytes(signature.slice(2)));

Salt is SHA-256 hash of the following identifier string:
```
let salt = await sha256(`eip155:${chainId}:${username}:${password?password:""}:${signature.slice(68)}`);
```
where, signature.slice(68) is hex s value of Ethereum signature, i.e. the last 32 bytes.

Info is a string with the following format:

let info = `eip155:${chainId}:${username}:${address}`;

Derived Key Length dkLen is set to 42.
```
let dkLen = 42;
```
FIPS 186/4 B.4.1 requires hash length to be ≥ n+8, where n is the length of final private key, such that 42 ≥ 32 + 8.
hashToPrivateKey function is FIPS 186-4 B.4.1 implementation to convert hash keys derived using HKDF to valid secp256k1 private keys. This function is implemented in JavaScript library @noble/secp256k1 as hashToPrivateKey().
```
let hashKey = hkdf(sha256, inputKey, salt, info, dkLen=42);
let privKey = secp256k1.utils.hashToPrivateKey(hashKey);
let pubKey = secp256k1.schnorr.getPublicKey(privKey);
```

Implementation Requirements

Connected Ethereum wallet signer MUST be EIP191 and RFC6979 compatible.
The message MUST be string formatted as Login to Nostr as ${username}\n\nImportant: Please verify the integrity and authenticity of your Nostr client before signing this message.\n${info}.
HKDF input key MUST be generated as the SHA-256 hash of 65 bytes signature.
HKDF salt MUST be generated as SHA-256 hash of string eip155:${chainID}:${username}:${password?password:""}:${signature.slice(68)}.
HKDF derived key length MUST be 42.
HKDF info MUST be string formatted as eip155:${chainId}:${username}:${address}.

JS Example

const secp256k1 = require('@noble/secp256k1');
const {hexToBytes, bytesToHex} = require('@noble/hashes/utils');
const {hkdf} = require('@noble/hashes/hkdf');
const {sha256} = require('@noble/hashes/sha256');

// const wallet = connected ethereum wallet with ethers.js
let username = "me@domain.eth.limo"
let chainId = wallet.getChainId(); // get chainid from connected wallet
let address = wallet.getAddress(); // get address from wallet
let info = `eip155:${chainId}:${username}:${address}`;

let message = `Login to Nostr as ${username}\n\nImportant: Please verify the integrity and authenticity of your Nostr client before signing this message.\n${info}`
let signature = wallet.signMessage(message); // request signature from wallet

let password = "horse staple battery"
let inputKey = await sha256(hexToBytes(signature.slice(2))); //skip "0x"
let salt = await sha256(`eip155:${chainId}:${username}:${password?password:""}:${signature.slice(68)}`);
let dkLen = 42;

let hashKey = await hkdf(sha256, inputKey, salt, info, dkLen);
let privKey = secp256k1.utils.hashToPrivateKey(hashKey);
let pubKey = secp256k1.schnorr.getPublicKey(privKey);

Security Considerations

Users should always verify the integrity and authenticity of the Nostr client before signing the message.
Users should ensure that they only input their Nostr Username and Password in trusted and secure clients.

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