hdkeychain: extend API with ChainCode(), ChildNum()

No way to access them otherwise. Handy for interoperability.

hdkeychain/extendedkey.go

@@ -191,6 +191,21 @@ func (k *ExtendedKey) ParentFingerprint() uint32 {
 	return binary.BigEndian.Uint32(k.parentFP)
 }
 
+// ChainCode returns the chain code part of this extended key.
+//
+// It is identical for both public and private extended keys.
+func (k *ExtendedKey) ChainCode() []byte {
+	return append([]byte{}, k.chainCode...)
+}
+
+// ChildNum returns the index at which the child extended key was derived.
+//
+// Extended keys with ChildNum value between 0 and 2^31-1 are normal child
+// keys, and those with a value between 2^31 and 2^32-1 are hardened keys.
+func (k *ExtendedKey) ChildNum() uint32 {
+	return k.childNum
+}
+
 // Child returns a derived child extended key at the given index.  When this
 // extended key is a private extended key (as determined by the IsPrivate
 // function), a private extended key will be derived.  Otherwise, the derived

hdkeychain/extendedkey_test.go

@@ -562,6 +562,8 @@ func TestExtendedKeyAPI(t *testing.T) {
 		extKey     string
 		isPrivate  bool
 		parentFP   uint32
+		chainCode  []byte
+		childNum   uint32
 		privKey    string
 		privKeyErr error
 		pubKey     string
@@ -572,6 +574,8 @@ func TestExtendedKeyAPI(t *testing.T) {
 			extKey:    "xprv9s21ZrQH143K3QTDL4LXw2F7HEK3wJUD2nW2nRk4stbPy6cq3jPPqjiChkVvvNKmPGJxWUtg6LnF5kejMRNNU3TGtRBeJgk33yuGBxrMPHi",
 			isPrivate: true,
 			parentFP:  0,
+			chainCode: []byte{135, 61, 255, 129, 192, 47, 82, 86, 35, 253, 31, 229, 22, 126, 172, 58, 85, 160, 73, 222, 61, 49, 75, 180, 46, 226, 39, 255, 237, 55, 213, 8},
+			childNum:  0,
 			privKey:   "e8f32e723decf4051aefac8e2c93c9c5b214313817cdb01a1494b917c8436b35",
 			pubKey:    "0339a36013301597daef41fbe593a02cc513d0b55527ec2df1050e2e8ff49c85c2",
 			address:   "15mKKb2eos1hWa6tisdPwwDC1a5J1y9nma",
@@ -581,6 +585,8 @@ func TestExtendedKeyAPI(t *testing.T) {
 			extKey:     "xpub6D4BDPcP2GT577Vvch3R8wDkScZWzQzMMUm3PWbmWvVJrZwQY4VUNgqFJPMM3No2dFDFGTsxxpG5uJh7n7epu4trkrX7x7DogT5Uv6fcLW5",
 			isPrivate:  false,
 			parentFP:   3203769081,
+			chainCode:  []byte{4, 70, 107, 156, 200, 225, 97, 233, 102, 64, 156, 165, 41, 134, 197, 132, 240, 126, 157, 200, 31, 115, 93, 182, 131, 195, 255, 110, 199, 177, 80, 63},
+			childNum:   2147483650,
 			privKeyErr: ErrNotPrivExtKey,
 			pubKey:     "0357bfe1e341d01c69fe5654309956cbea516822fba8a601743a012a7896ee8dc2",
 			address:    "1NjxqbA9aZWnh17q1UW3rB4EPu79wDXj7x",
@@ -610,6 +616,20 @@ func TestExtendedKeyAPI(t *testing.T) {
 			continue
 		}
 
+		chainCode := key.ChainCode()
+		if !bytes.Equal(chainCode, test.chainCode) {
+			t.Errorf("ChainCode #%d (%s): want %v, got %v", i,
+				test.name, chainCode, test.chainCode)
+			continue
+		}
+
+		childNum := key.ChildNum()
+		if childNum != test.childNum {
+			t.Errorf("ChildNum #%d (%s): want %d, got %d", i,
+				test.name, childNum, test.childNum)
+			continue
+		}
+
 		serializedKey := key.String()
 		if serializedKey != test.extKey {
 			t.Errorf("String #%d (%s): mismatched serialized key "+