feat: add DER encoding methods and update key type alias in key_utils

Signed-off-by: MonaaEid <monaa_eid@hotmail.com>

Author: MonaaEid

src/hiero_sdk_python/utils/key_utils.py

@@ -7,47 +7,30 @@
 
 from __future__ import annotations
 
-from hiero_sdk_python.crypto.key import Key as SdkKey
-from hiero_sdk_python.crypto.private_key import PrivateKey
-from hiero_sdk_python.crypto.public_key import PublicKey
+from hiero_sdk_python.crypto.key import Key
 from hiero_sdk_python.hapi.services import basic_types_pb2
 
 
-# Type alias for keys that can be either PrivateKey or PublicKey
-Key = PrivateKey | PublicKey | SdkKey
-
-
 def key_to_proto(key: Key | None) -> basic_types_pb2.Key | None:
     """
-    Helper function to convert a key (PrivateKey or PublicKey) to protobuf Key format.
+    Helper function to convert an SDK key to protobuf Key format.
 
-    This function handles the conversion of SDK key types to protobuf format:
-    - If a PrivateKey is provided, its corresponding public key is extracted and converted.
-    - If a PublicKey is provided, it is converted directly to protobuf.
-    - If None is provided, None is returned.
+    This function handles any concrete subclass of Key by delegating to its
+    to_proto_key() implementation. If None is provided, None is returned.
 
     Args:
-        key (Optional[Key]): The key to convert (PrivateKey or PublicKey), or None
+        key (Optional[Key]): The key to convert, or None
 
     Returns:
         basic_types_pb2.Key (Optional): The protobuf key or None if key is None
 
     Raises:
-        TypeError: If the provided key is not a PrivateKey, PublicKey, or None.
+        TypeError: If the provided key is not a Key instance or None.
     """
     if key is None:
         return None
 
-    # If it's a PrivateKey, get the public key first, then convert to proto
-    if isinstance(key, PrivateKey):
-        return key.public_key()._to_proto()
-
-    # If it's a PublicKey, convert directly to proto
-    if isinstance(key, PublicKey):
-        return key._to_proto()
-
-    # Any other SDK Key implementation (e.g., KeyList / ThresholdKey wrapper)
-    if isinstance(key, SdkKey):
+    if isinstance(key, Key):
         return key.to_proto_key()
 
-    raise TypeError("Key must be of type PrivateKey or PublicKey, or another SDK Key implementation")
+    raise TypeError("Key must be of type PrivateKey or PublicKey")

tests/unit/key_utils_test.py

@@ -4,9 +4,10 @@
 
 import pytest
 
+from hiero_sdk_python.crypto.key import Key
 from hiero_sdk_python.crypto.private_key import PrivateKey
 from hiero_sdk_python.hapi.services import basic_types_pb2
-from hiero_sdk_python.utils.key_utils import Key, key_to_proto
+from hiero_sdk_python.utils.key_utils import key_to_proto
 
 
 pytestmark = pytest.mark.unit
@@ -75,18 +76,16 @@ def test_key_to_proto_with_invalid_string_raises_error():
     with pytest.raises(TypeError) as e:
         key_to_proto("this is not a key")
 
-    assert "Key must be of type PrivateKey or PublicKey" in str(e.value)
+    assert "Key must be an instance" in str(e.value)
 
 
-def test_key_type_alias():
-    """Tests that the Key type alias works correctly."""
+def test_key_type_inheritance():
+    """Tests that Key typing accepts PrivateKey and PublicKey instances."""
     private_key = PrivateKey.generate_ed25519()
     public_key = private_key.public_key()
 
-    # Test that both PrivateKey and PublicKey can be assigned to Key type
     key1: Key = private_key
     key2: Key = public_key
 
-    # Both should work with key_to_proto
     assert key_to_proto(key1) is not None
     assert key_to_proto(key2) is not None