Lightning login (LNURL-auth): a site's login lnurl in Send logs you in instead of trying to pay it. A community member pasted Stacker News's lightning-login lnurl into Send and got "stacker.news: no sig query variable provided" (2026-10-16): every lnurl1 was taken for LNURL-pay, their login endpoint was fetched as if for pay parameters, and its refusal came back as the error. Now an lnurl whose URL carries tag=login and a 32-byte k1 (parseLnurlAuth; lightning: prefix and upper case fine; action register|login|link|auth) is a login: the wallet asks first — the site's name, what it is asking (log in / create an account / link / confirm it's you), that the challenge is signed with a key kept for this site alone and that nothing is paid or shared — then signs k1 (ECDSA, DER) with the LUD-05 linking key (m/138'/0 as hashing key, HMAC-SHA256 of the domain read as four 32-bit child indexes under m/138'; the same wallet is the same user on a site every time and a different one elsewhere; from the mnemonic, or an imported xprv) and calls back with sig and key (lnurlAuthKey, lnurlAuthLogin). The site's OK shows "Logged in to <site>", its reason otherwise, with Try again and Back. It takes no balance (checked before canPay in zaps' matchSendText), so it works on an empty wallet and from an Android lightning: intent. tools/lnurl-auth-unit-test.js (login vs pay vs bad k1; one key per site per wallet; the DER signature verifies under the key sent; the server's reason is the error) and tools/lnurl-auth-test.js (a stand-in site that verifies the signature: the ask, nothing sent before Log in, OK, the same key next time, a register link, a refusal's words); send-dest-test passes.

Co-Authored-By: Claude Fable 5.1 <[email protected]>
Claude-Session: https://claude.ai/code/session_01J1Kon8bxJoHRpJdSzshCkp

1 files changed, 42 insertions, 0 deletions

diff --git a/tools/lnurl-auth-unit-test.js b/tools/lnurl-auth-unit-test.js
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+// LNURL-auth (LUD-04/05) without a browser: a login lnurl is told from a
+// pay one; the linking key is the same for a domain every time and different
+// per domain; the signature over k1 verifies under the key sent.
+// Run: bun tools/lnurl-auth-unit-test.js
+import assert from 'node:assert/strict';
+import { HDKey } from '@scure/bip32';
+import { mnemonicToSeedSync } from '@scure/bip39';
+import { secp256k1 } from '@noble/curves/secp256k1';
+import { bytesToHex, hexToBytes } from '@noble/hashes/utils';
+import { encodeLnurl, parseLnurlAuth, lnurlAuthKey, lnurlAuthLogin } from '../src/lnurl.js';
+const k1 = 'a'.repeat(64);
+const loginUrl = 'https://stacker.news/api/lnauth?tag=login&k1=' + k1 + '&action=login';
+const a = parseLnurlAuth(encodeLnurl(loginUrl));
+assert.deepEqual(a, { url: loginUrl, domain: 'stacker.news', k1, action: 'login' });
+assert.equal(parseLnurlAuth('lightning:' + encodeLnurl(loginUrl).toUpperCase()).domain, 'stacker.news', 'a lightning: prefix and upper case are fine');
+assert.equal(parseLnurlAuth(encodeLnurl('https://stacker.news/api/lnauth?tag=login&k1=zz')), null, 'a k1 that is not 32 bytes is not a login');
+assert.equal(parseLnurlAuth(encodeLnurl('https://coinos.io/.well-known/lnurlp/adam')), null, 'a pay lnurl is not a login');
+assert.equal(parseLnurlAuth('[email protected]'), null);
+assert.equal(parseLnurlAuth(encodeLnurl(loginUrl.replace('action=login', 'action=register'))).action, 'register');
+assert.equal(parseLnurlAuth(encodeLnurl(loginUrl.replace('action=login', 'action=what'))).action, 'login', 'an unknown action is a login');
+const master = HDKey.fromMasterSeed(mnemonicToSeedSync('abandon abandon abandon abandon abandon abandon abandon abandon abandon abandon abandon about'));
+const k = lnurlAuthKey(master, 'stacker.news');
+assert.equal(bytesToHex(k.publicKey), bytesToHex(lnurlAuthKey(master, 'stacker.news').publicKey), 'the same key for a domain every time');
+assert.notEqual(bytesToHex(k.publicKey), bytesToHex(lnurlAuthKey(master, 'coinos.io').publicKey), 'another domain, another key');
+assert.equal(k.publicKey.length, 33, 'a compressed public key');
+assert.notEqual(bytesToHex(lnurlAuthKey(HDKey.fromMasterSeed(mnemonicToSeedSync('zoo zoo zoo zoo zoo zoo zoo zoo zoo zoo zoo wrong')), 'stacker.news').publicKey), bytesToHex(k.publicKey), 'another wallet, another user');
+// the callback, with a server that checks the signature the way theirs does
+let got = null;
+globalThis.fetch = async (url) => {
+  const u = new URL(url);
+  got = { sig: u.searchParams.get('sig'), key: u.searchParams.get('key'), k1: u.searchParams.get('k1'), tag: u.searchParams.get('tag') };
+  const okSig = secp256k1.verify(secp256k1.Signature.fromDER(got.sig), hexToBytes(got.k1), hexToBytes(got.key));
+  return new Response(JSON.stringify(okSig ? { status: 'OK' } : { status: 'ERROR', reason: 'bad sig' }), { headers: { 'content-type': 'application/json' } });
+};
+const r = await lnurlAuthLogin(a, k);
+assert.equal(got.tag, 'login'); assert.equal(got.k1, k1);
+assert.equal(got.key, bytesToHex(k.publicKey), 'the key sent is the linking key');
+assert.equal(r.key, got.key);
+assert.match(got.sig, /^30[0-9a-f]+$/, 'a DER signature');
+globalThis.fetch = async () => new Response(JSON.stringify({ status: 'ERROR', reason: 'no such user' }), { headers: { 'content-type': 'application/json' } });
+await assert.rejects(lnurlAuthLogin(a, k), /no such user/, "the server's reason is the error");
+console.log('✓ lnurl-auth: a login is told from a payment, one key per site per wallet, the signature verifies, the server has the last word');