Small updates

src/index.ts

@@ -23,7 +23,7 @@ export type Target = {
 
 export type SilentPaymentGroup = {
   Bscan: Buffer;
-  BmValues: Array<[Buffer, number | undefined]>;
+  BmValues: Array<[Buffer, number | undefined, number]>;
 };
 
 function taggedHash(tag: string, data: Buffer): Buffer {
@@ -45,12 +45,13 @@ export class SilentPayment {
    * Numeric values (if present) for targets are passed through.
    */
   createTransaction(utxos: UTXO[], targets: Target[]): Target[] {
-    const ret: Target[] = [];
+    const ret: Target[] = new Array(targets.length);
 
     const silentPaymentGroups: Array<SilentPaymentGroup> = [];
-    for (const target of targets) {
+    for (let i = 0; i < targets.length; i++) {
+      const target = targets[i];
       if (!target.address?.startsWith("sp1")) {
-        ret.push(target); // passthrough
+        ret[i] = target; // passthrough
         continue;
       }
 
@@ -66,11 +67,11 @@ export class SilentPayment {
       // Addresses with the same Bscan key all belong to the same recipient
       const recipient = silentPaymentGroups.find((group) => Buffer.compare(group.Bscan, Bscan) === 0);
       if (recipient) {
-        recipient.BmValues.push([Bm, target.value]);
+        recipient.BmValues.push([Bm, target.value, i]);
       } else {
         silentPaymentGroups.push({
           Bscan: Bscan,
-          BmValues: [[Bm, target.value]],
+          BmValues: [[Bm, target.value, i]],
         });
       }
     }
@@ -84,10 +85,10 @@ export class SilentPayment {
     for (const group of silentPaymentGroups) {
       // Bscan * a * outpoint_hash
       const ecdh_shared_secret_step1 = Buffer.from(ecc.privateMultiply(outpoint_hash, a) as Uint8Array);
-      const ecdh_shared_secret = ecc.pointMultiply(group.Bscan, ecdh_shared_secret_step1);
+      const ecdh_shared_secret = Buffer.from(ecc.getSharedSecret(ecdh_shared_secret_step1, group.Bscan) as Uint8Array);
 
       let k = 0;
-      for (const [Bm, amount] of group.BmValues) {
+      for (const [Bm, amount, i] of group.BmValues) {
         const tk = taggedHash("BIP0352/SharedSecret", Buffer.concat([ecdh_shared_secret!, SilentPayment._ser32(k)]));
 
         // Let Pmk = tk·G + Bm
@@ -97,7 +98,7 @@ export class SilentPayment {
         const address = SilentPayment.pubkeyToAddress(Pmk.slice(1).toString("hex"));
         const newTarget: Target = { address };
         newTarget.value = amount;
-        ret.push(newTarget);
+        ret[i] = newTarget;
         k += 1;
       }
     }

src/noble_ecc.ts

@@ -58,6 +58,10 @@ const ecc = {
       return { parity, xOnlyPubkey: P.slice(1) };
     }),
 
+  getSharedSecret: (sk: Uint8Array, pk: Uint8Array, compressed?: boolean): Uint8Array => {
+    return necc.getSharedSecret(sk, pk, defaultTrue(compressed));
+  },
+
   pointFromScalar: (sk: Uint8Array, compressed?: boolean): Uint8Array | null => throwToNull(() => necc.getPublicKey(sk, defaultTrue(compressed))),
 
   pointCompress: (p: Uint8Array, compressed?: boolean): Uint8Array => {

tests/silent-payment.test.ts

@@ -118,6 +118,56 @@ it("2 inputs - 0 SP outputs (just a passthrough)", () => {
   );
 });
 
+it("2 inputs - 1 SP output, 1 legacy, 1change (should not rearrange order of inputs )", () => {
+  const sp = new SilentPayment();
+  assert.deepStrictEqual(
+    sp.createTransaction(
+      [
+        {
+          txid: "f4184fc596403b9d638783cf57adfe4c75c605f6356fbc91338530e9831e9e16",
+          vout: 0,
+          wif: ECPair.fromPrivateKey(Buffer.from("1cd5e8f6b3f29505ed1da7a5806291ebab6491c6a172467e44debe255428a192", "hex")).toWIF(),
+          utxoType: "p2wpkh",
+        },
+        {
+          txid: "a1075db55d416d3ca199f55b6084e2115b9345e16c5cf302fc80e9d5fbf5d48d",
+          vout: 0,
+          wif: ECPair.fromPrivateKey(Buffer.from("7416ef4d92e4dd09d680af6999d1723816e781c030f4b4ecb5bf46939ca30056", "hex")).toWIF(),
+          utxoType: "p2wpkh",
+        },
+      ],
+      [
+        {
+          address: "3FiYaHYHQTmD8n2SJxVYobDeN1uQKvzkLe",
+          value: 11_111,
+        },
+        {
+          address: "sp1qqgste7k9hx0qftg6qmwlkqtwuy6cycyavzmzj85c6qdfhjdpdjtdgqjuexzk6murw56suy3e0rd2cgqvycxttddwsvgxe2usfpxumr70xc9pkqwv",
+          value: 22_222,
+        },
+        {
+          // no address, which should be interpreted as change
+          value: 33_333,
+        },
+      ]
+    ),
+    [
+      {
+        address: "3FiYaHYHQTmD8n2SJxVYobDeN1uQKvzkLe",
+        value: 11_111,
+      },
+      {
+        address: "bc1pszgngkje7t5j3mvdw8xc5l3q7n28awdwl8pena6hrvxgg83lnpmsme6u6j", // unwrapped from SP
+        value: 22_222,
+      },
+      {
+        // no address, which should be interpreted as change
+        value: 33_333,
+      },
+    ]
+  );
+});
+
 it("SilentPayment._outpointHash() works", () => {
   const A = ECPair.fromWIF("L4cJGJp4haLbS46ZKMKrjt7HqVuYTSHkChykdMrni955Fs3Sb8vq").publicKey;
   assert.deepStrictEqual(