Swaps: don't reuse a burned swap index (deterministic-preimage collision)

A reverse swap's preimage is sha256(swapNode(idx).privateKey) — deterministic per
index. nextSwapIndex was derived only from *saved* swaps, so an attempt that
created the swap at Boltz but threw before saving (e.g. the lockup-address-mismatch
bug) never advanced the index; the next try reused the same preimage and Boltz
rejected it ('a swap with this preimage hash exists already').

Make nextSwapIndex a persisted, monotonic counter (reserves the index on every
call, seeded from saved swaps), and have startReverse retry with the next index
when Boltz reports the preimage already exists — which also recovers wallets
already stuck on a burned index.

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

2 files changed, 31 insertions, 8 deletions

diff --git a/src/swap.js b/src/swap.js
index 2627068..129712d 100644
--- a/src/swap.js
+++ b/src/swap.js
@@ -276,13 +276,25 @@ export class SwapManager {
   // (bolt11) the user has someone pay; hal then claims the lockup on-chain.
   async startReverse(amountSat) {
     const w = this.wallet, net = this.network;
-    const idx = w.nextSwapIndex();
-    const node = w.swapNode(idx);
-    const preimage = sha256(node.privateKey);
-    const res = await boltzReq(this.getApi(), '/v2/swap/reverse', {
-      invoiceAmount: amountSat, from: 'BTC', to: 'BTC',
-      claimPublicKey: bytesToHex(node.publicKey), preimageHash: bytesToHex(sha256(preimage)),
-    });
+    let idx, node, preimage, res;
+    // Our preimage is deterministic per swap index. If a prior attempt burned this
+    // index at Boltz ("swap with this preimage exists"), nextSwapIndex has advanced,
+    // so just retry with the next one.
+    for (let attempt = 0; ; attempt++) {
+      idx = w.nextSwapIndex();
+      node = w.swapNode(idx);
+      preimage = sha256(node.privateKey);
+      try {
+        res = await boltzReq(this.getApi(), '/v2/swap/reverse', {
+          invoiceAmount: amountSat, from: 'BTC', to: 'BTC',
+          claimPublicKey: bytesToHex(node.publicKey), preimageHash: bytesToHex(sha256(preimage)),
+        });
+        break;
+      } catch (e) {
+        if (attempt < 5 && /preimage|already exist/i.test(String(e?.message || e))) continue;
+        throw e;
+      }
+    }
     const d = swapP2TR({ kind: 'reverse', preimageHash160: hash160(preimage), boltzPub33: hexToBytes(res.refundPublicKey), halPub33: node.publicKey, lockTime: res.timeoutBlockHeight, network: net });
     if (d.address !== res.lockupAddress) throw new Error('lockup address mismatch — not paying');
     if (bytesToHex(decodeBolt11(res.invoice).paymentHash) !== bytesToHex(sha256(preimage))) throw new Error('invoice hash mismatch');
diff --git a/src/wallet.js b/src/wallet.js
index b3e3d40..c16bd66 100644
--- a/src/wallet.js
+++ b/src/wallet.js
@@ -1537,8 +1537,19 @@ export class Wallet {
   saveSwaps(list) {
     try { localStorage.setItem(this._swapKey(), JSON.stringify(list)); } catch {}
   }
+  _swapIdxKey() { return this._cacheKey() + ':swapidx'; }
+  // Reserve the next swap index: monotonic + persisted on every call. A reverse/
+  // submarine attempt that fails AFTER Boltz created the swap (a later validation
+  // throw, a dropped response) still burns its index, so a retry can't reuse the
+  // same deterministic preimage (Boltz rejects "swap with this preimage exists").
+  // Seeded from saved swaps so existing wallets don't regress.
   nextSwapIndex() {
-    return this.loadSwaps().reduce((m, s) => Math.max(m, (s.swapIndex ?? -1) + 1), 0);
+    const fromSaved = this.loadSwaps().reduce((m, s) => Math.max(m, (s.swapIndex ?? -1) + 1), 0);
+    let stored = 0;
+    try { stored = parseInt(localStorage.getItem(this._swapIdxKey()) || '0', 10) || 0; } catch {}
+    const idx = Math.max(fromSaved, stored);
+    try { localStorage.setItem(this._swapIdxKey(), String(idx + 1)); } catch {}
+    return idx;
   }
   // Deterministic swap keypair on a dedicated chain (2) so swap keys never
   // collide with receive (0) / change (1). Returns the HDKey node (has