improved bip32 derivation (#112)

* improved bip32 derivation

* save redundant EC multiplication

* cleaned up code

btclib/bip32/bip32.py

@@ -309,43 +309,27 @@ def __init__(
             self.assert_valid()
 
 
-def __prv_key_derivation(xkey: _BIP32KeyData, index: int) -> None:
-    xkey.index = index
-    Q_bytes = bytes_from_point(mult(xkey.prv_key_int))
-    xkey.parent_fingerprint = hash160(Q_bytes)[:4]
-    hmac_ = (
-        hmac.new(
-            xkey.chain_code,
-            xkey.key + index.to_bytes(4, byteorder="big", signed=False),
-            "sha512",
-        ).digest()
-        if xkey.is_hardened
-        else hmac.new(
-            xkey.chain_code,
-            Q_bytes + index.to_bytes(4, byteorder="big", signed=False),
-            "sha512",
-        ).digest()
+def __prv_key_derivation(xkey: _BIP32KeyData, index: int, pub_key: bytes) -> None:
+    xb = (
+        xkey.key
+        if index >= 0x80000000
+        else pub_key or bytes_from_point(mult(xkey.prv_key_int))
     )
+    xb += index.to_bytes(4, byteorder="big", signed=False)
+    hmac_ = hmac.new(xkey.chain_code, xb, "sha512").digest()
     xkey.chain_code = hmac_[32:]
     offset = int.from_bytes(hmac_[:32], byteorder="big", signed=False)
     xkey.prv_key_int = (xkey.prv_key_int + offset) % ec.n
     xkey.key = b"\x00" + xkey.prv_key_int.to_bytes(32, byteorder="big", signed=False)
-    xkey.pub_key_point = INF
 
 
 def __pub_key_derivation(xkey: _BIP32KeyData, index: int) -> None:
-    xkey.index = index
-    xkey.parent_fingerprint = hash160(xkey.key)[:4]
-    hmac_ = hmac.new(
-        xkey.chain_code,
-        xkey.key + index.to_bytes(4, byteorder="big", signed=False),
-        "sha512",
-    ).digest()
+    xb = xkey.key + index.to_bytes(4, byteorder="big", signed=False)
+    hmac_ = hmac.new(xkey.chain_code, xb, "sha512").digest()
     xkey.chain_code = hmac_[32:]
     offset = int.from_bytes(hmac_[:32], byteorder="big", signed=False)
     xkey.pub_key_point = ec.add(xkey.pub_key_point, mult(offset))
     xkey.key = bytes_from_point(xkey.pub_key_point)
-    xkey.prv_key_int = 0
 
 
 def _derive(
@@ -382,14 +366,22 @@ def _derive(
             raise BTClibValueError(err_msg)
         xkey.version = fversion
 
-    if xkey.is_private:
-        for index in indexes:
-            __prv_key_derivation(xkey, index)
-    else:
-        if any(index >= 0x80000000 for index in indexes):
-            raise BTClibValueError("invalid hardened derivation from public key")
-        for index in indexes:
-            __pub_key_derivation(xkey, index)
+    if indexes:
+        if xkey.is_private:
+            for index in indexes[:-1]:
+                __prv_key_derivation(xkey, index, b"")
+            pub_key = bytes_from_point(mult(xkey.prv_key_int))
+            xkey.parent_fingerprint = hash160(pub_key)[:4]
+            __prv_key_derivation(xkey, indexes[-1], pub_key)
+        else:
+            if any(index >= 0x80000000 for index in indexes):
+                raise BTClibValueError("invalid hardened derivation from public key")
+            for index in indexes[:-1]:
+                __pub_key_derivation(xkey, index)
+            xkey.parent_fingerprint = hash160(xkey.key)[:4]
+            __pub_key_derivation(xkey, indexes[-1])
+
+        xkey.index = indexes[-1]
 
     return xkey
 

tests/bip32/test_bip32.py

@@ -16,7 +16,7 @@
 
 import pytest
 
-from btclib import base58, hashes
+from btclib import base58
 from btclib.b58 import p2pkh  # FIXME why it is needed here
 from btclib.bip32 import (
     BIP32KeyData,
@@ -29,6 +29,7 @@
 from btclib.bip32.bip32 import _derive
 from btclib.bip32.der_path import _indexes_from_bip32_path_str
 from btclib.exceptions import BTClibValueError
+from btclib.hashes import hash160
 from btclib.to_pub_key import pub_keyinfo_from_key
 
 
@@ -78,7 +79,7 @@ def test_assert_valid2() -> None:
         xkey_data.assert_valid()
 
     xkey_data = BIP32KeyData.b58decode(xkey)
-    xkey_data.depth = tuple()  # type: ignore[assignment]
+    xkey_data.depth = ()  # type: ignore[assignment]
     with pytest.raises(TypeError):
         xkey_data.assert_valid()
 
@@ -103,7 +104,7 @@ def test_assert_valid2() -> None:
         xkey_data.assert_valid()
 
     xkey_data = BIP32KeyData.b58decode(xkey)
-    xkey_data.index = tuple()  # type: ignore[assignment]
+    xkey_data.index = ()  # type: ignore[assignment]
     with pytest.raises(TypeError):
         xkey_data.assert_valid()
 
@@ -224,7 +225,7 @@ def test_derive_exceptions() -> None:
     assert rootmxprv == derive(xprv, "m")
     assert rootmxprv == derive(xprv, "")
 
-    fingerprint = hashes.hash160(pub_keyinfo_from_key(xprv)[0])[:4]
+    fingerprint = hash160(pub_keyinfo_from_key(xprv)[0])[:4]
     assert fingerprint == _derive(xprv, bytes.fromhex("80000000")).parent_fingerprint
 
     for der_path in ("/1", "800000", "80000000"):
@@ -362,3 +363,17 @@ def test_bips_pr905() -> None:
     assert derive(xroot, der_path) == xprv
     xpub = "xpub6CpsfWjghR6XdCB8yDq7jQRpRKEDP2LT3ZRUgURF9g5xevB7YoTpogkFRqq5nQtVSN8YCMZo2CD8u4zCaxRv85ctCWmzEi9gQ5DBhBFaTNo"
     assert xpub_from_xprv(xprv) == xpub
+
+
+def test_pub_key_derivation() -> None:
+    parent_xpub = "xpub6CpsfWjghR6XdCB8yDq7jQRpRKEDP2LT3ZRUgURF9g5xevB7YoTpogkFRqq5nQtVSN8YCMZo2CD8u4zCaxRv85ctCWmzEi9gQ5DBhBFaTNo"
+    proper_child = "xpub6FCCuDg6j52SWRVZ1TugkjrnGkqPcDuNNKDzohU2mmd4dxiGJypZa535iqYT8KcN2oouRF7A6tXEGAX6HCSjQe7HVSDR4LQ4yUT3HwF1Tqi"
+    assert derive(parent_xpub, "m/0") == proper_child
+    parent_key = BIP32KeyData.b58decode(parent_xpub).key
+    parent_fingerprint = hash160(parent_key)[:4]
+    assert BIP32KeyData.b58decode(proper_child).parent_fingerprint == parent_fingerprint
+
+    orphan_child_key = BIP32KeyData.b58decode(proper_child)
+    orphan_child_key.parent_fingerprint = b"\x00" * 4
+    orphan_child = "xpub6DXuQW1FgeHbhsSchbuDWE9Bj8mPiPUpiroAmAvRdRqYbGHXHTyEkttkxSvtCac64QzpasL1Tvd5Znvn5GQMswQUrpRBsPRz7npvyZ8ExWi"
+    assert orphan_child_key.b58encode() == orphan_child

tests/mnemonic/test_entropy.py

@@ -158,7 +158,7 @@ def test_exceptions() -> None:
         bin_str_entropy_from_entropy(bytes_entropy216, 224)
 
     with pytest.raises(BTClibValueError, match=err_msg):
-        bin_str_entropy_from_entropy(tuple())  # type: ignore[arg-type]
+        bin_str_entropy_from_entropy(())  # type: ignore[arg-type]
 
     with pytest.raises(ValueError):
         bin_str_entropy_from_int("not an int")