Add regtest SP tweak indexer; accept Core's array-witness in inputPubKey

tools/sp-indexer.js: a minimal BIP-352 oracle (Bun) that walks Core and serves,
per block, one entry per eligible tx — the tweak (input_hash*A) plus that tx's
taproot outputs — for the wallet to scan on-device. Shares src/silentpay.js so
the tweak math is the vector-validated code. Added to the regtest compose as the
'sp' service (oven/bun, pointed at bc:18443).

inputPubKey now accepts a witness as either serialized hex (vectors) or an array
of stack items (Core getblock RPC). Verified end-to-end on regtest: a real SP
payment is indexed and the scan recovers the output, value, and spend tweak.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

1 files changed, 118 insertions, 0 deletions

diff --git a/tools/sp-indexer.js b/tools/sp-indexer.js
new file mode 100644
index 0000000..12f76f7
--- /dev/null
+++ b/tools/sp-indexer.js
@@ -0,0 +1,118 @@
+// Minimal BIP-352 silent-payment tweak indexer (an "oracle") for regtest dev.
+//
+// It walks the chain from a Bitcoin Core node and, per block, emits one entry per
+// silent-payment-eligible transaction: the tweak (input_hash·A over the tx's
+// eligible inputs) plus that transaction's taproot outputs. A wallet fetches
+// these, computes ecdh = scan_priv·tweak on-device, derives P_k = B_spend + t_k·G,
+// and matches it against the entry's outputs — the scan key never leaves the
+// client. Discovered outputs are ordinary taproot UTXOs, so spends/confirmations
+// are tracked by the normal Electrum backend; this server only does discovery.
+//
+// Shares the SP crypto with the wallet (../src/silentpay.js) so the tweak math is
+// the same code validated against the BIP-352 vectors. In-memory, regtest-sized;
+// re-indexes on restart. Run with bun.
+
+import { inputPubKey, silentPaymentTweak } from '../src/silentpay.js';
+
+const RPC = process.env.CORE_RPC || 'http://127.0.0.1:18443';
+const RPC_USER = process.env.RPC_USER || 'admin1';
+const RPC_PASS = process.env.RPC_PASS || '123';
+const PORT = Number(process.env.SP_PORT || 8888);
+const START = Number(process.env.START_HEIGHT || 0);
+const POLL_MS = Number(process.env.POLL_MS || 2000);
+
+const auth = 'Basic ' + Buffer.from(`${RPC_USER}:${RPC_PASS}`).toString('base64');
+let rpcId = 0;
+async function rpc(method, params = []) {
+  const res = await fetch(RPC, {
+    method: 'POST',
+    headers: { 'content-type': 'application/json', authorization: auth },
+    body: JSON.stringify({ jsonrpc: '1.0', id: ++rpcId, method, params }),
+  });
+  if (!res.ok) throw new Error(`rpc ${method} HTTP ${res.status}`);
+  const j = await res.json();
+  if (j.error) throw new Error(`rpc ${method}: ${j.error.message}`);
+  return j.result;
+}
+
+// height -> { hash, items: [{ txid, tweak(hex), outputs: [{ vout, value(sat), xonly(hex) }] }] }
+const blocks = new Map();
+let indexed = -1; // highest indexed height
+
+// Is a scriptPubKey (hex) a P2TR output? OP_1 PUSH32 <x-only> = 5120<64 hex>.
+const taprootXonly = (hex) => (hex && hex.length === 68 && hex.startsWith('5120') ? hex.slice(4) : null);
+
+function scanTx(tx) {
+  // Eligible-input set: every input with its recovered pubkey (null if ineligible
+  // or coinbase); the tweak needs all outpoints but only eligible pubkeys.
+  if (tx.vin.some((i) => i.coinbase)) return null; // coinbase is never a silent payment
+  const inputs = tx.vin.map((i) => ({
+    txid: i.txid,
+    vout: i.vout,
+    pubkey: inputPubKey({
+      scriptSig: (i.scriptSig && i.scriptSig.hex) || '',
+      witness: i.txinwitness || [],
+      scriptPubKey: (i.prevout && i.prevout.scriptPubKey && i.prevout.scriptPubKey.hex) || '',
+    }) || undefined,
+  }));
+  const outputs = [];
+  for (const o of tx.vout) {
+    const x = taprootXonly(o.scriptPubKey && o.scriptPubKey.hex);
+    if (x) outputs.push({ vout: o.n, value: Math.round(o.value * 1e8), xonly: x });
+  }
+  if (!outputs.length) return null; // no taproot output → can't be an SP payment
+  let tweak;
+  try { tweak = silentPaymentTweak(inputs); } catch { return null; }
+  if (!tweak) return null; // no eligible inputs
+  return { txid: tx.txid, tweak: Buffer.from(tweak).toString('hex'), outputs };
+}
+
+async function indexBlock(height) {
+  const hash = await rpc('getblockhash', [height]);
+  const block = await rpc('getblock', [hash, 3]); // verbosity 3: full txs with prevouts
+  const items = [];
+  for (const tx of block.tx) { const it = scanTx(tx); if (it) items.push(it); }
+  blocks.set(height, { hash, items });
+  indexed = height;
+}
+
+async function sync() {
+  try {
+    const tip = await rpc('getblockcount', []);
+    for (let h = Math.max(START, indexed + 1); h <= tip; h++) {
+      await indexBlock(h);
+      if (h % 100 === 0 || h === tip) console.log(`indexed height ${h}/${tip} (${blocks.get(h).items.length} sp-tx)`);
+    }
+  } catch (e) {
+    console.error('sync error:', e.message);
+  }
+}
+
+const CORS = { 'access-control-allow-origin': '*', 'access-control-allow-headers': '*' };
+const json = (obj, status = 200) => new Response(JSON.stringify(obj), { status, headers: { 'content-type': 'application/json', ...CORS } });
+
+Bun.serve({
+  port: PORT,
+  async fetch(req) {
+    const url = new URL(req.url);
+    const p = url.pathname;
+    if (req.method === 'OPTIONS') return new Response(null, { headers: CORS });
+    if (p === '/height') return json({ height: indexed });
+    let m;
+    if ((m = p.match(/^\/block\/(\d+)$/))) {
+      const h = Number(m[1]);
+      const b = blocks.get(h);
+      if (!b) return json({ error: 'not indexed', height: h, tip: indexed }, 404);
+      return json({ height: h, hash: b.hash, items: b.items });
+    }
+    if ((m = p.match(/^\/tweaks\/(\d+)$/))) {
+      const b = blocks.get(Number(m[1]));
+      return json(b ? b.items.map((i) => i.tweak) : []);
+    }
+    return json({ error: 'not found' }, 404);
+  },
+});
+
+console.log(`sp-indexer: core=${RPC} start=${START} serving :${PORT}`);
+await sync();
+setInterval(sync, POLL_MS);