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

2 files changed, 144 insertions, 0 deletions

diff --git a/tools/lnurl-auth-test.js b/tools/lnurl-auth-test.js
new file mode 100644
index 0000000..767e85c
--- /dev/null
+++ b/tools/lnurl-auth-test.js
@@ -0,0 +1,102 @@
+// A site's login lnurl (LNURL-auth) pasted into Send is a login, not a
+// payment: the wallet asks first, naming the site, then signs the challenge
+// and tells the site; the site's refusal is shown as its own words. Against
+// a stand-in login server that checks the signature the way a real one does.
+// Run: bun tools/lnurl-auth-test.js
+import puppeteer from './puppeteer.js';
+import { generateMnemonic } from '@scure/bip39';
+import { wordlist } from '@scure/bip39/wordlists/english';
+import { secp256k1 } from '@noble/curves/secp256k1';
+import { hexToBytes } from '@noble/hashes/utils';
+import { encodeLnurl } from '../src/lnurl.js';
+import { buildHtml } from '../build.js';
+const sleep = (ms) => new Promise((r) => setTimeout(r, ms));
+let ok = true;
+const check = (n, c, d = '') => { console.log(` ${c ? '✓' : '✗'} ${n}${d ? ' — ' + d : ''}`); if (!c) ok = false; };
+const html = await buildHtml({ minify: true, pwa: false });
+const logins = []; // what the stand-in site was told
+const server = Bun.serve({ port: 0, fetch: (req) => {
+  const u = new URL(req.url), p = u.pathname;
+  if (p === '/lnauth') {
+    const sig = u.searchParams.get('sig'), key = u.searchParams.get('key'), k1 = u.searchParams.get('k1');
+    if (!sig) return Response.json({ status: 'ERROR', reason: 'no sig query variable provided' });
+    let good = false;
+    try { good = secp256k1.verify(secp256k1.Signature.fromDER(sig), hexToBytes(k1), hexToBytes(key)); } catch {}
+    logins.push({ k1, key, good });
+    return Response.json(good ? { status: 'OK' } : { status: 'ERROR', reason: 'bad signature' });
+  }
+  if (p === '/lnauth-closed') return Response.json({ status: 'ERROR', reason: 'registrations are closed' });
+  if (p.startsWith('/punks') || p === '/verify-worker.js' || p.startsWith('/locales') || /\.js$/.test(p)) { const f = Bun.file('dist' + p); return f.size ? new Response(f) : new Response('', { status: 404 }); }
+  return new Response(html, { headers: { 'content-type': 'text/html' } });
+} });
+const k1 = 'c0ffee'.repeat(10) + 'abcd';
+const loginLnurl = encodeLnurl(server.url.origin + '/lnauth?tag=login&k1=' + k1 + '&action=login');
+const closedLnurl = encodeLnurl(server.url.origin + '/lnauth-closed?tag=login&k1=' + k1 + '&action=register');
+const browser = await puppeteer.launch({ executablePath: process.env.CHROME || '/usr/bin/google-chrome', headless: 'new', args: ['--no-sandbox'] });
+const ctx = browser.defaultBrowserContext();
+await ctx.overridePermissions(server.url.origin, ['clipboard-read', 'clipboard-write', 'clipboard-sanitized-write']);
+const page = await browser.newPage();
+const errs = []; page.on('pageerror', (e) => errs.push(String(e)));
+const click = (t) => page.evaluate((x) => { const e = [...document.querySelectorAll('button')].find((n) => n.textContent.trim().toLowerCase() === x || n.textContent.trim().toLowerCase().includes(x)); if (e) { e.click(); return true; } return false; }, t);
+const text = () => page.evaluate(() => document.body.innerText);
+const waitText = async (x, ms = 8000) => { for (let i = 0; i < ms / 200; i++) { if ((await text()).includes(x)) return true; await sleep(200); } return false; };
+const freshSend = async () => {
+  await page.evaluate(() => [...document.querySelectorAll('button')].find((b) => b.textContent.trim() === 'Receive')?.click()); await sleep(300);
+  await page.evaluate(() => [...document.querySelectorAll('button')].find((b) => b.textContent.trim() === 'Send')?.click());
+  await page.waitForSelector('textarea.send-dest', { timeout: 10000 });
+  await page.$eval('textarea.send-dest', (e) => { e.value = ''; e.dispatchEvent(new Event('input', { bubbles: true })); }); await sleep(300);
+  await page.focus('textarea.send-dest');
+};
+const paste = async (s) => { await page.evaluate((v) => navigator.clipboard.writeText(v), s); await page.keyboard.down('Control'); await page.keyboard.press('KeyV'); await page.keyboard.up('Control'); };
+try {
+  await page.setViewport({ width: 390, height: 844 });
+  await page.goto(server.url.origin, { waitUntil: 'domcontentloaded' });
+  await sleep(400);
+  await click('create a new wallet'); await sleep(500); await click('to import'); await sleep(400);
+  await page.waitForSelector('textarea'); await page.type('textarea', generateMnemonic(wordlist)); await click('open wallet');
+  for (let i = 0; i < 80 && !(await page.evaluate(() => [...document.querySelectorAll('button')].some((b) => b.textContent.trim() === 'Send'))); i++) {
+    await page.evaluate(() => [...document.querySelectorAll('button')].find((b) => /^(not now|skip|continue|later)$/i.test(b.textContent.trim()))?.click());
+    await sleep(400);
+  }
+  await sleep(3000);
+  const host = new URL(server.url.origin).hostname;
+
+  console.log('[a login lnurl pasted into Send]');
+  await freshSend();
+  await paste(loginLnurl);
+  check('the wallet asks first, naming the site', await waitText('Log in with Lightning') && (await text()).includes(host + ' is asking you to log in'), (await text()).slice(0, 200));
+  check('…says what it is about to do, and that nothing is paid', /key it keeps for this site alone/.test(await text()) && /No payment is made/.test(await text()));
+  check('…no amount field, no payment flow', !(await page.$('input[type=number]')));
+  check('nothing was sent to the site before the user agreed', logins.length === 0);
+  await click('log in');
+  check('the site accepts the signed challenge', await waitText('Logged in to ' + host, 8000), (await text()).slice(0, 300));
+  check('…it was told once, with a signature that verifies under the key', logins.length === 1 && logins[0].good && logins[0].k1 === k1, JSON.stringify(logins));
+  const key1 = logins[0].key;
+  await click('done');
+  check('Done returns to a blank Send form', await page.waitForSelector('textarea.send-dest', { timeout: 5000 }).then(() => true, () => false) && (await page.$eval('textarea.send-dest', (e) => e.value)) === '');
+
+  console.log('[the same site again: the same key]');
+  await freshSend();
+  await paste(loginLnurl);
+  await waitText('Log in with Lightning'); await click('log in');
+  await waitText('Logged in to ' + host, 8000);
+  check('the wallet is the same user on the site next time', logins.length === 2 && logins[1].key === key1, JSON.stringify(logins.map((l) => l.key.slice(0, 12))));
+  await click('done');
+
+  console.log('[a site that says no]');
+  await freshSend();
+  await paste(closedLnurl);
+  check('a register link asks to create an account', await waitText('asking you to create an account'));
+  await click('create account');
+  check("the site's reason is shown", await waitText('registrations are closed', 8000), (await text()).slice(0, 300));
+  check('…with a way to try again, and back', (await text()).includes('Try again') && (await text()).includes('Back'));
+  await click('back');
+  check('Back leaves the login', await page.waitForSelector('textarea.send-dest', { timeout: 5000 }).then(() => true, () => false));
+  check('no page errors', errs.length === 0, errs.join('; ').slice(0, 300));
+} catch (e) {
+  check('run completed', false, e.stack);
+} finally {
+  await browser.close(); server.stop(true);
+}
+console.log(ok ? '\n✅ a login lnurl logs in' : '\n❌ failures above');
+process.exit(ok ? 0 : 1);
diff --git a/tools/lnurl-auth-unit-test.js b/tools/lnurl-auth-unit-test.js
new file mode 100644
index 0000000..0707a8e
--- /dev/null
+++ b/tools/lnurl-auth-unit-test.js
@@ -0,0 +1,42 @@
+// 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');