multi: introduce and use new TapTweak and SignMethod fields

In this commit, we add a new field `TapTweak` to be used for key path
spends. Before this commit, we'd overload the existing `WitnessScript`
field to pass this information to the signing context. This was
confusing as for tapscript spends, this was the leaf script, which
mirrors the other script based spending types.

With this new filed, users need to set this to the script root for
keypath spends where the output key commits to a real merkle root, and
nothing when bip 86 spending is being used.

To make the signing even more explicit, we also add a new field called
sign_method with an enum type that differentiates between the different
segwit v0 and v1 signing methods.

Fixes https://github.com/lightningnetwork/lnd/issues/6446.

lntest/itest/lnd_taproot_test.go

@@ -26,6 +26,9 @@ import (
 const (
 	testTaprootKeyFamily = 77
 	testAmount           = 800_000
+	signMethodBip86      = signrpc.SignMethod_SIGN_METHOD_TAPROOT_KEY_SPEND_BIP0086
+	signMethodRootHash   = signrpc.SignMethod_SIGN_METHOD_TAPROOT_KEY_SPEND
+	signMethodTapscript  = signrpc.SignMethod_SIGN_METHOD_TAPROOT_SCRIPT_SPEND
 )
 
 var (
@@ -193,6 +196,7 @@ func testTaprootSignOutputRawScriptSpend(ctxt context.Context, t *harnessTest,
 				KeyDesc:       keyDesc,
 				Sighash:       uint32(txscript.SigHashDefault),
 				WitnessScript: leaf2.Script,
+				SignMethod:    signMethodTapscript,
 			}},
 		},
 	)
@@ -202,6 +206,27 @@ func testTaprootSignOutputRawScriptSpend(ctxt context.Context, t *harnessTest,
 			"input 0 is missing its previous outpoint information",
 	)
 
+	// We also want to make sure we get an error when we don't specify the
+	// correct signing method.
+	_, err = alice.SignerClient.SignOutputRaw(
+		ctxt, &signrpc.SignReq{
+			RawTxBytes: buf.Bytes(),
+			SignDescs: []*signrpc.SignDescriptor{{
+				Output:        utxoInfo[0],
+				InputIndex:    0,
+				KeyDesc:       keyDesc,
+				Sighash:       uint32(txscript.SigHashDefault),
+				WitnessScript: leaf2.Script,
+			}},
+			PrevOutputs: utxoInfo,
+		},
+	)
+	require.Error(t.t, err)
+	require.Contains(
+		t.t, err.Error(), "selected sign method witness_v0 is not "+
+			"compatible with given pk script 5120",
+	)
+
 	// Do the actual signing now.
 	signResp, err := alice.SignerClient.SignOutputRaw(
 		ctxt, &signrpc.SignReq{
@@ -212,6 +237,7 @@ func testTaprootSignOutputRawScriptSpend(ctxt context.Context, t *harnessTest,
 				KeyDesc:       keyDesc,
 				Sighash:       uint32(txscript.SigHashDefault),
 				WitnessScript: leaf2.Script,
+				SignMethod:    signMethodTapscript,
 			}},
 			PrevOutputs: utxoInfo,
 		},
@@ -307,12 +333,12 @@ func testTaprootSignOutputRawKeySpendBip86(ctxt context.Context,
 		ctxt, &signrpc.SignReq{
 			RawTxBytes: buf.Bytes(),
 			SignDescs: []*signrpc.SignDescriptor{{
-				Output:          utxoInfo[0],
-				InputIndex:      0,
-				KeyDesc:         keyDesc,
-				SingleTweak:     dummyKeyTweak[:],
-				Sighash:         uint32(txscript.SigHashDefault),
-				TaprootKeySpend: true,
+				Output:      utxoInfo[0],
+				InputIndex:  0,
+				KeyDesc:     keyDesc,
+				SingleTweak: dummyKeyTweak[:],
+				Sighash:     uint32(txscript.SigHashDefault),
+				SignMethod:  signMethodBip86,
 			}},
 			PrevOutputs: utxoInfo,
 		},
@@ -405,13 +431,13 @@ func testTaprootSignOutputRawKeySpendRootHash(ctxt context.Context,
 		ctxt, &signrpc.SignReq{
 			RawTxBytes: buf.Bytes(),
 			SignDescs: []*signrpc.SignDescriptor{{
-				Output:          utxoInfo[0],
-				InputIndex:      0,
-				KeyDesc:         keyDesc,
-				SingleTweak:     dummyKeyTweak[:],
-				Sighash:         uint32(txscript.SigHashDefault),
-				WitnessScript:   rootHash[:],
-				TaprootKeySpend: true,
+				Output:      utxoInfo[0],
+				InputIndex:  0,
+				KeyDesc:     keyDesc,
+				SingleTweak: dummyKeyTweak[:],
+				Sighash:     uint32(txscript.SigHashDefault),
+				TapTweak:    rootHash[:],
+				SignMethod:  signMethodRootHash,
 			}},
 			PrevOutputs: utxoInfo,
 		},