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LUD-13: signMessage-based seed generation for auth protocol.

author: akumaigorodski author: fiatjaf author: hampus_s discussion: https://t.me/lnurl/5155

Seed generation method for the auth protocol

This is based on the signmessage API provided by some Lightning node implementations. It signs an HMAC of a message in a standard way using ECDSA with deterministic nonces, so it's always the same signature given the same key and message.

Here we define a canonical phrase to be signed, and from that we will derive the LNURL-auth seed.

linkingKey derivation for wallets which don't have an access to master privKey:

In this case neither hashingKey nor domain-specific linkingKeys can be derived by the path. To overcome this limitation a different scheme is used for this class of wallets:

  1. The following canonical phrase is defined: DO NOT EVER SIGN THIS TEXT WITH YOUR PRIVATE KEYS! IT IS ONLY USED FOR DERIVATION OF LNURL-AUTH HASHING-KEY, DISCLOSING ITS SIGNATURE WILL COMPROMISE YOUR LNURL-AUTH IDENTITY AND MAY LEAD TO LOSS OF FUNDS!.
  2. LN WALLET obtains an RFC6979 deterministic signature of sha256(utf8ToBytes(canonical phrase)) using secp256k1 with node private key.
  3. LN WALLET defines hashingKey as PrivateKey(sha256(obtained signature)).
  4. SERVICE domain name is extracted from auth LNURL and then service-specific linkingPrivKey is defined as PrivateKey(hmacSha256(hashingKey, service domain name)).

LN WALLET must make sure it is not possible to accidentally or automatically sign and hand out a signature of canonical phrase.