Implement Taproot key-path wallet signing

Author: mswilkison

go.mod

@@ -61,6 +61,7 @@ require (
 
 require (
 	github.com/davecgh/go-spew v1.1.1 // indirect
+	github.com/decred/dcrd/crypto/blake256 v1.0.1 // indirect
 	github.com/go-task/slim-sprig/v3 v3.0.0 // indirect
 	github.com/munnerz/goautoneg v0.0.0-20191010083416-a7dc8b61c822 // indirect
 	github.com/pion/datachannel v1.5.10 // indirect

pkg/bitcoin/script.go

@@ -19,6 +19,7 @@ const (
 	P2WPKHScript
 	P2SHScript
 	P2WSHScript
+	P2TRScript
 )
 
 func (st ScriptType) String() string {
@@ -31,6 +32,8 @@ func (st ScriptType) String() string {
 		return "P2SH"
 	case P2WSHScript:
 		return "P2WSH"
+	case P2TRScript:
+		return "P2TR"
 	default:
 		return "NonStandard"
 	}
@@ -147,8 +150,21 @@ func PayToScriptHash(scriptHash [20]byte) (Script, error) {
 		Script()
 }
 
+// PayToTaproot constructs a P2TR script for the provided 32-byte x-only
+// Taproot output key. The function assumes the provided output key is valid.
+func PayToTaproot(outputKey [32]byte) (Script, error) {
+	return txscript.NewScriptBuilder().
+		AddOp(txscript.OP_1).
+		AddData(outputKey[:]).
+		Script()
+}
+
 // GetScriptType gets the ScriptType of the given Script.
 func GetScriptType(script Script) ScriptType {
+	if isPayToTaproot(script) {
+		return P2TRScript
+	}
+
 	switch txscript.GetScriptClass(script) {
 	case txscript.PubKeyHashTy:
 		return P2PKHScript
@@ -163,6 +179,12 @@ func GetScriptType(script Script) ScriptType {
 	}
 }
 
+func isPayToTaproot(script Script) bool {
+	return len(script) == 34 &&
+		script[0] == txscript.OP_1 &&
+		script[1] == txscript.OP_DATA_32
+}
+
 // ExtractPublicKeyHash extracts the public key hash from a P2WPKH or P2PKH
 // script.
 func ExtractPublicKeyHash(script Script) ([20]byte, error) {
@@ -189,3 +211,15 @@ func ExtractPublicKeyHash(script Script) ([20]byte, error) {
 
 	return publicKeyHash, nil
 }
+
+// ExtractTaprootKey extracts the x-only output key from a P2TR script.
+func ExtractTaprootKey(script Script) ([32]byte, error) {
+	if GetScriptType(script) != P2TRScript {
+		return [32]byte{}, fmt.Errorf("not a P2TR script")
+	}
+
+	var outputKey [32]byte
+	copy(outputKey[:], script[2:])
+
+	return outputKey, nil
+}

pkg/bitcoin/script_test.go

@@ -334,6 +334,32 @@ func TestPayToScriptHash(t *testing.T) {
 	testutils.AssertBytesEqual(t, expectedResult, result[:])
 }
 
+func TestPayToTaproot(t *testing.T) {
+	outputKeyBytes, err := hex.DecodeString(
+		"1b84c5567b126440995d3ed5aaba0565d71e1834604819ff9c17f5e9d5dd078f",
+	)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	var outputKey [32]byte
+	copy(outputKey[:], outputKeyBytes)
+
+	result, err := PayToTaproot(outputKey)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	expectedResult, err := hex.DecodeString(
+		"51201b84c5567b126440995d3ed5aaba0565d71e1834604819ff9c17f5e9d5dd078f",
+	)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	testutils.AssertBytesEqual(t, expectedResult, result[:])
+}
+
 func TestGetScriptType(t *testing.T) {
 	fromHex := func(hexString string) []byte {
 		bytes, err := hex.DecodeString(hexString)
@@ -363,6 +389,10 @@ func TestGetScriptType(t *testing.T) {
 			script:       fromHex("002086a303cdd2e2eab1d1679f1a813835dc5a1b65321077cdccaf08f98cbf04ca96"),
 			expectedType: P2WSHScript,
 		},
+		"p2tr script": {
+			script:       fromHex("51201b84c5567b126440995d3ed5aaba0565d71e1834604819ff9c17f5e9d5dd078f"),
+			expectedType: P2TRScript,
+		},
 		"non-standard script": {
 			script: fromHex(
 				"14934b98637ca318a4d6e7ca6ffd1690b8e77df6377508f9f0c90d0003" +
@@ -387,6 +417,59 @@ func TestGetScriptType(t *testing.T) {
 	}
 }
 
+func TestExtractTaprootKey(t *testing.T) {
+	fromHex := func(hexString string) []byte {
+		bytes, err := hex.DecodeString(hexString)
+		if err != nil {
+			t.Fatal(err)
+		}
+		return bytes
+	}
+
+	var outputKey [32]byte
+	copy(
+		outputKey[:],
+		fromHex("1b84c5567b126440995d3ed5aaba0565d71e1834604819ff9c17f5e9d5dd078f"),
+	)
+
+	var tests = map[string]struct {
+		script            Script
+		expectedOutputKey [32]byte
+		expectedErr       error
+	}{
+		"P2TR script": {
+			script:            fromHex("51201b84c5567b126440995d3ed5aaba0565d71e1834604819ff9c17f5e9d5dd078f"),
+			expectedOutputKey: outputKey,
+		},
+		"other script": {
+			script:      fromHex("00148db50eb52063ea9d98b3eac91489a90f738986f6"),
+			expectedErr: fmt.Errorf("not a P2TR script"),
+		},
+	}
+
+	for testName, test := range tests {
+		t.Run(testName, func(t *testing.T) {
+			actualOutputKey, err := ExtractTaprootKey(test.script)
+
+			if !reflect.DeepEqual(test.expectedErr, err) {
+				t.Errorf(
+					"unexpected error\nexpected: %+v\nactual:   %+v\n",
+					test.expectedErr,
+					err,
+				)
+			}
+
+			if test.expectedOutputKey != actualOutputKey {
+				t.Errorf(
+					"unexpected taproot output key\nexpected: 0x%x\nactual:   0x%x\n",
+					test.expectedOutputKey,
+					actualOutputKey,
+				)
+			}
+		})
+	}
+}
+
 func TestExtractPublicKeyHash(t *testing.T) {
 	fromHex := func(hexString string) []byte {
 		bytes, err := hex.DecodeString(hexString)