diff --git a/hdkeychain/extendedkey.go b/hdkeychain/extendedkey.go
index fb09d7c..d937113 100644
--- a/hdkeychain/extendedkey.go
+++ b/hdkeychain/extendedkey.go
@@ -359,6 +359,16 @@ func (k *ExtendedKey) Address(net *chaincfg.Params) (*btcutil.AddressPubKeyHash,
 	return btcutil.NewAddressPubKeyHash(pkHash, net)
 }
 
+// paddedAppend appends the src byte slice to dst, returning the new slice.
+// If the length of the source is smaller than the passed size, leading zero
+// bytes are appended to the dst slice before appending src.
+func paddedAppend(size uint, dst, src []byte) []byte {
+	for i := 0; i < int(size)-len(src); i++ {
+		dst = append(dst, 0)
+	}
+	return append(dst, src...)
+}
+
 // String returns the extended key as a human-readable base58-encoded string.
 func (k *ExtendedKey) String() string {
 	if len(k.key) == 0 {
@@ -380,7 +390,7 @@ func (k *ExtendedKey) String() string {
 	serializedBytes = append(serializedBytes, k.chainCode...)
 	if k.isPrivate {
 		serializedBytes = append(serializedBytes, 0x00)
-		serializedBytes = append(serializedBytes, k.key...)
+		serializedBytes = paddedAppend(32, serializedBytes, k.key)
 	} else {
 		serializedBytes = append(serializedBytes, k.pubKeyBytes()...)
 	}
diff --git a/hdkeychain/extendedkey_test.go b/hdkeychain/extendedkey_test.go
index 95ce51a..37e90e1 100644
--- a/hdkeychain/extendedkey_test.go
+++ b/hdkeychain/extendedkey_test.go
@@ -245,6 +245,134 @@ tests:
 	}
 }
 
+// TestPrivateDerivation tests several vectors which derive private keys from
+// other private keys works as intended.
+func TestPrivateDerivation(t *testing.T) {
+	// The private extended keys for test vectors in [BIP32].
+	testVec1MasterPrivKey := "xprv9s21ZrQH143K3QTDL4LXw2F7HEK3wJUD2nW2nRk4stbPy6cq3jPPqjiChkVvvNKmPGJxWUtg6LnF5kejMRNNU3TGtRBeJgk33yuGBxrMPHi"
+	testVec2MasterPrivKey := "xprv9s21ZrQH143K31xYSDQpPDxsXRTUcvj2iNHm5NUtrGiGG5e2DtALGdso3pGz6ssrdK4PFmM8NSpSBHNqPqm55Qn3LqFtT2emdEXVYsCzC2U"
+
+	tests := []struct {
+		name     string
+		master   string
+		path     []uint32
+		wantPriv string
+	}{
+		// Test vector 1
+		{
+			name:     "test vector 1 chain m",
+			master:   testVec1MasterPrivKey,
+			path:     []uint32{},
+			wantPriv: "xprv9s21ZrQH143K3QTDL4LXw2F7HEK3wJUD2nW2nRk4stbPy6cq3jPPqjiChkVvvNKmPGJxWUtg6LnF5kejMRNNU3TGtRBeJgk33yuGBxrMPHi",
+		},
+		{
+			name:     "test vector 1 chain m/0",
+			master:   testVec1MasterPrivKey,
+			path:     []uint32{0},
+			wantPriv: "xprv9uHRZZhbkedL37eZEnyrNsQPFZYRAvjy5rt6M1nbEkLSo378x1CQQLo2xxBvREwiK6kqf7GRNvsNEchwibzXaV6i5GcsgyjBeRguXhKsi4R",
+		},
+		{
+			name:     "test vector 1 chain m/0/1",
+			master:   testVec1MasterPrivKey,
+			path:     []uint32{0, 1},
+			wantPriv: "xprv9ww7sMFLzJMzy7bV1qs7nGBxgKYrgcm3HcJvGb4yvNhT9vxXC7eX7WVULzCfxucFEn2TsVvJw25hH9d4mchywguGQCZvRgsiRaTY1HCqN8G",
+		},
+		{
+			name:     "test vector 1 chain m/0/1/2",
+			master:   testVec1MasterPrivKey,
+			path:     []uint32{0, 1, 2},
+			wantPriv: "xprv9xrdP7iD2L1YZCgR9AecDgpDMZSTzP5KCfUykGXgjBxLgp1VFHsEeL3conzGAkbc1MigG1o8YqmfEA2jtkPdf4vwMaGJC2YSDbBTPAjfRUi",
+		},
+		{
+			name:     "test vector 1 chain m/0/1/2/2",
+			master:   testVec1MasterPrivKey,
+			path:     []uint32{0, 1, 2, 2},
+			wantPriv: "xprvA2J8Hq4eiP7xCEBP7gzRJGJnd9CHTkEU6eTNMrZ6YR7H5boik8daFtDZxmJDfdMSKHwroCfAfsBKWWidRfBQjpegy6kzXSkQGGoMdWKz5Xh",
+		},
+		{
+			name:     "test vector 1 chain m/0/1/2/2/1000000000",
+			master:   testVec1MasterPrivKey,
+			path:     []uint32{0, 1, 2, 2, 1000000000},
+			wantPriv: "xprvA3XhazxncJqJsQcG85Gg61qwPQKiobAnWjuPpjKhExprZjfse6nErRwTMwGe6uGWXPSykZSTiYb2TXAm7Qhwj8KgRd2XaD21Styu6h6AwFz",
+		},
+
+		// Test vector 2
+		{
+			name:     "test vector 2 chain m",
+			master:   testVec2MasterPrivKey,
+			path:     []uint32{},
+			wantPriv: "xprv9s21ZrQH143K31xYSDQpPDxsXRTUcvj2iNHm5NUtrGiGG5e2DtALGdso3pGz6ssrdK4PFmM8NSpSBHNqPqm55Qn3LqFtT2emdEXVYsCzC2U",
+		},
+		{
+			name:     "test vector 2 chain m/0",
+			master:   testVec2MasterPrivKey,
+			path:     []uint32{0},
+			wantPriv: "xprv9vHkqa6EV4sPZHYqZznhT2NPtPCjKuDKGY38FBWLvgaDx45zo9WQRUT3dKYnjwih2yJD9mkrocEZXo1ex8G81dwSM1fwqWpWkeS3v86pgKt",
+		},
+		{
+			name:     "test vector 2 chain m/0/2147483647",
+			master:   testVec2MasterPrivKey,
+			path:     []uint32{0, 2147483647},
+			wantPriv: "xprv9wSp6B7cXJWXZRpDbxkFg3ry2fuSyUfvboJ5Yi6YNw7i1bXmq9QwQ7EwMpeG4cK2pnMqEx1cLYD7cSGSCtruGSXC6ZSVDHugMsZgbuY62m6",
+		},
+		{
+			name:     "test vector 2 chain m/0/2147483647/1",
+			master:   testVec2MasterPrivKey,
+			path:     []uint32{0, 2147483647, 1},
+			wantPriv: "xprv9ysS5br6UbWCRCJcggvpUNMyhVWgD7NypY9gsVTMYmuRtZg8izyYC5Ey4T931WgWbfJwRDwfVFqV3b29gqHDbuEpGcbzf16pdomk54NXkSm",
+		},
+		{
+			name:     "test vector 2 chain m/0/2147483647/1/2147483646",
+			master:   testVec2MasterPrivKey,
+			path:     []uint32{0, 2147483647, 1, 2147483646},
+			wantPriv: "xprvA2LfeWWwRCxh4iqigcDMnUf2E3nVUFkntc93nmUYBtb9rpSPYWa8MY3x9ZHSLZkg4G84UefrDruVK3FhMLSJsGtBx883iddHNuH1LNpRrEp",
+		},
+		{
+			name:     "test vector 2 chain m/0/2147483647/1/2147483646/2",
+			master:   testVec2MasterPrivKey,
+			path:     []uint32{0, 2147483647, 1, 2147483646, 2},
+			wantPriv: "xprvA48ALo8BDjcRET68R5RsPzF3H7WeyYYtHcyUeLRGBPHXu6CJSGjwW7dWoeUWTEzT7LG3qk6Eg6x2ZoqD8gtyEFZecpAyvchksfLyg3Zbqam",
+		},
+
+		// Custom tests to trigger specific conditions.
+		{
+			// Seed 000000000000000000000000000000da.
+			name:     "Derived privkey with zero high byte m/0",
+			master:   "xprv9s21ZrQH143K4FR6rNeqEK4EBhRgLjWLWhA3pw8iqgAKk82ypz58PXbrzU19opYcxw8JDJQF4id55PwTsN1Zv8Xt6SKvbr2KNU5y8jN8djz",
+			path:     []uint32{0},
+			wantPriv: "xprv9uC5JqtViMmgcAMUxcsBCBFA7oYCNs4bozPbyvLfddjHou4rMiGEHipz94xNaPb1e4f18TRoPXfiXx4C3cDAcADqxCSRSSWLvMBRWPctSN9",
+		},
+	}
+
+tests:
+	for i, test := range tests {
+		extKey, err := hdkeychain.NewKeyFromString(test.master)
+		if err != nil {
+			t.Errorf("NewKeyFromString #%d (%s): unexpected error "+
+				"creating extended key: %v", i, test.name,
+				err)
+			continue
+		}
+
+		for _, childNum := range test.path {
+			var err error
+			extKey, err = extKey.Child(childNum)
+			if err != nil {
+				t.Errorf("err: %v", err)
+				continue tests
+			}
+		}
+
+		privStr := extKey.String()
+		if privStr != test.wantPriv {
+			t.Errorf("Child #%d (%s): mismatched serialized "+
+				"private extended key -- got: %s, want: %s", i,
+				test.name, privStr, test.wantPriv)
+			continue
+		}
+	}
+}
+
 // TestPublicDerivation tests several vectors which derive public keys from
 // other public keys works as intended.
 func TestPublicDerivation(t *testing.T) {