preimages: enforce a merchant-set price per preimage batch

Invoice creation is unauthenticated, so buyers could name their own
price for a queued preimage (create the usePreimage invoice with
amount: 1). POST /preimages now takes an optional price (sats) that is
stored per entry and overrides the requested amount when the preimage
is consumed; omitting it keeps the old any-amount behavior. Also gate
the stored-preimage reveal in credit() on the invoice being fully paid,
since internal sends can settle partial amounts. Bare-hex entries
queued before this change still parse (treated as priceless).

Co-Authored-By: Claude Fable 5 <[email protected]>

2 files changed, 32 insertions, 8 deletions

diff --git a/lib/invoices.ts b/lib/invoices.ts
index 434a2375..c3722ef5 100644
--- a/lib/invoices.ts
+++ b/lib/invoices.ts
@@ -13,6 +13,16 @@ import { PaymentType } from "$lib/types";
 const bc = rpc(config.bitcoin);
 const lq = rpc(config.liquid);
 
+// Preimage queue entries are {preimage, price} JSON, but entries queued
+// before prices existed are bare hex strings — treat those as priceless
+export const parseEntry = (e) => {
+  try {
+    const parsed = JSON.parse(e);
+    if (parsed?.preimage) return { price: null, ...parsed };
+  } catch {}
+  return { preimage: e, price: null };
+};
+
 export const generate = async ({ invoice, user }) => {
   let {
     address_type,
@@ -89,7 +99,6 @@ export const generate = async ({ invoice, user }) => {
       expiry ||= 60 * 60 * 24 * 30;
 
       const args = {
-        amount_msat: amount ? `${amount + tip}sat` : "any",
         label: `${id} ${user.username} ${Date.now()}`,
         description: memo || "",
         expiry,
@@ -103,10 +112,19 @@ export const generate = async ({ invoice, user }) => {
         // merchant chose. Never fall back to a random preimage — a buyer would
         // be paying for a secret that unlocks nothing.
         const queue = `${user.id}:preimages`;
-        let preimage;
-        while ((preimage = await db.lPop(queue))) {
+        let entry;
+        while ((entry = await db.lPop(queue))) {
+          const { preimage, price } = parseEntry(entry);
+          // The invoice creator is unauthenticated, so a merchant-set price
+          // must win over the amount in the request — otherwise a buyer
+          // could name their own price for the secret
+          if (price) amount = price;
           try {
-            r = await ln.invoice({ ...args, preimage });
+            r = await ln.invoice({
+              ...args,
+              amount_msat: amount ? `${amount + tip}sat` : "any",
+              preimage,
+            });
             // Keep the preimage recoverable for internal settlements, where
             // cln never pays the invoice and so never reveals it — credit()
             // picks this up as the payment ref
@@ -121,13 +139,16 @@ export const generate = async ({ invoice, user }) => {
               warn("discarding unusable preimage", user.username, e.message);
               continue;
             }
-            await db.lPush(queue, preimage);
+            await db.lPush(queue, entry);
             throw e;
           }
         }
         if (!r) fail("no preimages available");
       } else {
-        r = await ln.invoice(args);
+        r = await ln.invoice({
+          ...args,
+          amount_msat: amount ? `${amount + tip}sat` : "any",
+        });
       }
     }
 
diff --git a/lib/payments.ts b/lib/payments.ts
index 01de2eff..df2c8857 100644
--- a/lib/payments.ts
+++ b/lib/payments.ts
@@ -293,10 +293,13 @@ export const credit = async ({
   // preimage:${paymentHash} (see generate()). Prefer it as the settlement
   // ref: external settlements already carry the same value, but internal
   // ones pass ref=sender.id, which would otherwise be surfaced as the
-  // payment preimage and lose the secret the buyer paid for.
+  // payment preimage and lose the secret the buyer paid for. Only once the
+  // invoice is fully paid, though — internal sends can settle partial
+  // amounts, and an underpayment must not reveal the secret.
   if (
     inv.paymentHash &&
-    ![PaymentType.bitcoin, PaymentType.liquid].includes(type)
+    ![PaymentType.bitcoin, PaymentType.liquid].includes(type) &&
+    inv.received + amount >= inv.amount
   ) {
     const stored = await db.get(`preimage:${inv.paymentHash}`);
     if (stored) ref = stored;