Change the exception we raise in keywrap unwrapping on invalid length (…

…pyca#4337)

I believe this can reasonably be considered backwards compatible since other invalid inputs already lead to InvalidUnwrap, and clients shouldn't be distinguishing between these two conditions, and ValueError wasn't documented anyways.

CHANGELOG.rst

@@ -19,6 +19,10 @@ Changelog
 * The :class:`~cryptography.x509.RelativeDistinguishedName` class now
   preserves the order of attributes. Duplicate attributes now raise an error
   instead of silently discarding duplicates.
+* :func:`~cryptography.hazmat.primitives.keywrap.aes_key_unwrap` and
+  :func:`~cryptography.hazmat.primitives.keywrap.aes_key_unwrap_with_padding`
+  now raise :class:`~cryptography.hazmat.primitives.keywrap.InvalidUnwrap` if
+  the wrapped key is an invalid length, instead of ``ValueError``.
 
 .. _v2-2-2:
 

src/cryptography/hazmat/primitives/keywrap.py

@@ -89,7 +89,7 @@ def aes_key_wrap_with_padding(wrapping_key, key_to_wrap, backend):
 
 def aes_key_unwrap_with_padding(wrapping_key, wrapped_key, backend):
     if len(wrapped_key) < 16:
-        raise ValueError("Must be at least 16 bytes")
+        raise InvalidUnwrap("Must be at least 16 bytes")
 
     if len(wrapping_key) not in [16, 24, 32]:
         raise ValueError("The wrapping key must be a valid AES key length")
@@ -132,10 +132,10 @@ def aes_key_unwrap_with_padding(wrapping_key, wrapped_key, backend):
 
 def aes_key_unwrap(wrapping_key, wrapped_key, backend):
     if len(wrapped_key) < 24:
-        raise ValueError("Must be at least 24 bytes")
+        raise InvalidUnwrap("Must be at least 24 bytes")
 
     if len(wrapped_key) % 8 != 0:
-        raise ValueError("The wrapped key must be a multiple of 8 bytes")
+        raise InvalidUnwrap("The wrapped key must be a multiple of 8 bytes")
 
     if len(wrapping_key) not in [16, 24, 32]:
         raise ValueError("The wrapping key must be a valid AES key length")

tests/hazmat/primitives/test_keywrap.py

@@ -108,11 +108,11 @@ def test_wrap_invalid_key_to_wrap_length(self, backend):
 
     def test_unwrap_invalid_wrapped_key_length(self, backend):
         # Keys to unwrap must be at least 24 bytes
-        with pytest.raises(ValueError):
+        with pytest.raises(keywrap.InvalidUnwrap):
             keywrap.aes_key_unwrap(b"sixteen_byte_key", b"\x00" * 16, backend)
 
         # Keys to unwrap must be a multiple of 8 bytes
-        with pytest.raises(ValueError):
+        with pytest.raises(keywrap.InvalidUnwrap):
             keywrap.aes_key_unwrap(b"sixteen_byte_key", b"\x00" * 27, backend)
 
 
@@ -189,7 +189,9 @@ def test_unwrap_additional_vectors(self, backend, params):
 
     def test_unwrap_invalid_wrapped_key_length(self, backend):
         # Keys to unwrap must be at least 16 bytes
-        with pytest.raises(ValueError, match='Must be at least 16 bytes'):
+        with pytest.raises(
+            keywrap.InvalidUnwrap, match='Must be at least 16 bytes'
+        ):
             keywrap.aes_key_unwrap_with_padding(
                 b"sixteen_byte_key", b"\x00" * 15, backend
             )