Wire SP foundations into the wallet: configurable indexer + keys/address

api.js: per-network SP indexer config (presets + custom, stored per network) with
getSpIndexerConfig/spIndexerUrl/spReceiveEnabled — regtest defaults to the local
indexer, others to custom; a hosted mainnet default slots in once we run one.

wallet.js: silentPaymentKeys() derives BIP-352 scan/spend keys from the master
seed (needs the mnemonic / depth-0 xprv), and silentPaymentAddress() returns the
sp1…/tsp1… address. Verified it matches the vector-validated e2e address.

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

2 files changed, 57 insertions, 1 deletions

diff --git a/src/api.js b/src/api.js
index 2095d51..5287e13 100644
--- a/src/api.js
+++ b/src/api.js
@@ -106,6 +106,40 @@ function netDefaults(net) { return NET_DEFAULTS[net] || NET_DEFAULTS.mainnet; }
 // get a ":<net>" suffix.
 function nk(base, net) { return net === 'mainnet' ? base : `${base}:${net}`; }
 
+// Silent-payment (BIP-352) tweak indexer — a separate, optional endpoint used
+// only for SP receiving. It serves public tweak/UTXO data (the scan key never
+// leaves the device), but a public one can observe which blocks hold your
+// outputs, so self-hosting is the privacy-max option. Per network; an empty URL
+// means SP receiving is unavailable on that network.
+const SP_INDEXER_PRESETS_BY_NET = {
+  mainnet: [{ id: 'custom', label: 'Custom', url: '' }], // a hosted default goes here once we run one
+  testnet: [{ id: 'custom', label: 'Custom', url: '' }],
+  mutinynet: [{ id: 'custom', label: 'Custom', url: '' }],
+  regtest: [{ id: 'local', label: 'Local (localhost:8888)', url: 'http://localhost:8888' }, { id: 'custom', label: 'Custom', url: '' }],
+};
+const SP_INDEXER_DEFAULT = { mainnet: 'custom', testnet: 'custom', mutinynet: 'custom', regtest: 'local' };
+const SP_INDEXER_KEY = 'btc-wallet-sp-indexer';
+export function spIndexerPresets(net = getNetwork()) { return SP_INDEXER_PRESETS_BY_NET[net] || SP_INDEXER_PRESETS_BY_NET.mainnet; }
+export function getSpIndexerConfig(net = getNetwork()) {
+  try {
+    const c = JSON.parse(localStorage.getItem(nk(SP_INDEXER_KEY, net)) || 'null');
+    if (c && c.server) return { server: c.server, url: c.url || '' };
+  } catch {}
+  return { server: SP_INDEXER_DEFAULT[net] || 'custom', url: '' };
+}
+export function setSpIndexerConfig({ server, url }, net = getNetwork()) {
+  try { localStorage.setItem(nk(SP_INDEXER_KEY, net), JSON.stringify({ server, url: url || '' })); } catch {}
+}
+// The resolved indexer base URL for a network, or '' if none is configured.
+export function spIndexerUrl(net = getNetwork()) {
+  const cfg = getSpIndexerConfig(net);
+  if (cfg.server === 'custom') return cfg.url.trim().replace(/\/+$/, '');
+  const p = spIndexerPresets(net).find((x) => x.id === cfg.server);
+  return p && p.url ? p.url.replace(/\/+$/, '') : '';
+}
+// SP receiving is available only when an indexer is configured for this network.
+export function spReceiveEnabled(net = getNetwork()) { return !!spIndexerUrl(net); }
+
 // Resolve the configured explorer to the host list the Api tries in order.
 function resolveHosts(net) {
   const cfg = getExplorerConfig(net);
diff --git a/src/wallet.js b/src/wallet.js
index 46b9f5c..219b0f5 100644
--- a/src/wallet.js
+++ b/src/wallet.js
@@ -24,7 +24,7 @@ import { concatBytes } from '@scure/btc-signer/utils.js';
 import { Api, pool, getBackend, explorerWeb, electrumCandidates } from './api.js';
 import { ElectrumApi } from './electrum.js';
 import { NostrSync, getSyncConfig } from './nostr.js';
-import { isSilentPaymentAddress, decodeSilentPaymentAddress, silentPaymentScripts, silentPaymentPlaceholder } from './silentpay.js';
+import { isSilentPaymentAddress, decodeSilentPaymentAddress, silentPaymentScripts, silentPaymentPlaceholder, deriveSilentPaymentKeys, encodeSilentPaymentAddress, silentPaymentScan } from './silentpay.js';
 
 // No look-ahead: stop scanning a chain at the first unused address. This wallet
 // only ever exposes ONE unused address at a time (freshReceive = first unused;
@@ -255,6 +255,28 @@ export class Wallet {
     return this.account().publicExtendedKey;
   }
 
+  // BIP-352 scan/spend keys, derived from the master seed (m/352' paths) — so
+  // they need the mnemonic (or a depth-0 master xprv); watch-only and account-
+  // level imported keys can't do silent payments. Cached per account.
+  silentPaymentKeys() {
+    if (this.watchOnly) return null;
+    const coin = NETS[this.netName].coin;
+    const ck = `sp|${this.netName}|${this.mnemonic}|${this.passphrase}|${this.xprv}`;
+    if (this._spKeys && this._spKeysKey === ck) return this._spKeys;
+    let master = null;
+    if (this.mnemonic) master = HDKey.fromMasterSeed(mnemonicToSeedSync(this.mnemonic, this.passphrase));
+    else if (this.xprv) { const n = HDKey.fromExtendedKey(this.xprv); if (n.depth === 0) master = n; }
+    this._spKeys = master ? deriveSilentPaymentKeys(master, coin) : null;
+    this._spKeysKey = ck;
+    return this._spKeys;
+  }
+
+  // Our silent-payment address (sp1…/tsp1…), or null if this wallet can't do SP.
+  silentPaymentAddress() {
+    const k = this.silentPaymentKeys();
+    return k ? encodeSilentPaymentAddress(k.scanPub, k.spendPub, { testnet: this.netName !== 'mainnet' }) : null;
+  }
+
   node(chain, index) {
     // Relative derivation from the account node (chain/index are non-hardened).
     return this.account().deriveChild(chain).deriveChild(index);