itest: add itest for MuSig2 CombineKey

Before we change anything in the MuSig2 API, we first add an integration
test that makes sure the current version of the API combines keys
according to the v0.4.0 test vectors.

lntest/itest/lnd_taproot_test.go

@@ -35,11 +35,14 @@ const (
 )
 
 var (
-	dummyInternalKeyBytes, _ = hex.DecodeString(
+	hexDecode = func(keyStr string) []byte {
+		keyBytes, _ := hex.DecodeString(keyStr)
+		return keyBytes
+	}
+	dummyInternalKey, _ = btcec.ParsePubKey(hexDecode(
 		"03464805f5468e294d88cf15a3f06aef6c89d63ef1bd7b42db2e0c74c1ac" +
 			"eb90fe",
-	)
-	dummyInternalKey, _ = btcec.ParsePubKey(dummyInternalKeyBytes)
+	))
 )
 
 // testTaproot ensures that the daemon can send to and spend from taproot (p2tr)
@@ -61,6 +64,7 @@ func testTaproot(ht *lntemp.HarnessTest) {
 	testTaprootMuSig2KeySpendRootHash(ht, ht.Alice)
 	testTaprootMuSig2ScriptSpend(ht, ht.Alice)
 	testTaprootMuSig2CombinedLeafKeySpend(ht, ht.Alice)
+	testMuSig2CombineKey(ht, ht.Alice)
 
 	testTaprootImportTapscriptFullTree(ht, ht.Alice)
 	testTaprootImportTapscriptPartialReveal(ht, ht.Alice)
@@ -1856,3 +1860,41 @@ func testTaprootCoopClose(ht *lntemp.HarnessTest) {
 	require.False(ht, assertTaprootDeliveryUsed(closingTxid),
 		"taproot addr shouldn't be used!")
 }
+
+// testMuSig2CombineKey makes sure that combining a key with MuSig2 returns the
+// correct result according to the MuSig2 version specified.
+func testMuSig2CombineKey(ht *lntemp.HarnessTest, alice *node.HarnessNode) {
+	testVector040Key1 := hexDecode(
+		"F9308A019258C31049344F85F89D5229B531C845836F99B08601F113BCE0" +
+			"36F9",
+	)
+	testVector040Key2 := hexDecode(
+		"DFF1D77F2A671C5F36183726DB2341BE58FEAE1DA2DECED843240F7B502B" +
+			"A659",
+	)
+	testVector040Key3 := hexDecode(
+		"3590A94E768F8E1815C2F24B4D80A8E3149316C3518CE7B7AD338368D038" +
+			"CA66",
+	)
+
+	resp := alice.RPC.MuSig2CombineKeys(&signrpc.MuSig2CombineKeysRequest{
+		AllSignerPubkeys: [][]byte{
+			testVector040Key1, testVector040Key2,
+			testVector040Key3,
+		},
+		TaprootTweak: &signrpc.TaprootTweakDesc{
+			KeySpendOnly: true,
+		},
+	})
+
+	expectedFinalKey := hexDecode(
+		"5b257b4e785d61157ef5303051f45184bd5cb47bc4b4069ed4dd453645" +
+			"9cb83b",
+	)
+	expectedPreTweakKey := hexDecode(
+		"d70cd69a2647f7390973df48cbfa2ccc407b8b2d60b08c5f1641185c79" +
+			"98a290",
+	)
+	require.Equal(ht, expectedFinalKey, resp.CombinedKey)
+	require.Equal(ht, expectedPreTweakKey, resp.TaprootInternalKey)
+}