implement AES KW with padding (RFC 5649) (#3880)

* implement AES KW with padding (RFC 5649)

fixes #3791

* oops, 2.2

* make sure this is the right valueerror

* more match

* make key padding easier to read

* review feedback

* review feedback

4 files changed, 166 insertions, 0 deletions

diff --git a/CHANGELOG.rst b/CHANGELOG.rst
index 2f8924fd..912a368f 100644
--- a/CHANGELOG.rst
+++ b/CHANGELOG.rst
@@ -18,6 +18,11 @@ Changelog
   :meth:`~cryptography.fernet.MultiFernet.rotate`.
 * Fixed a memory leak in
   :func:`~cryptography.hazmat.primitives.asymmetric.ec.derive_private_key`.
+* Added support for AES key wrapping with padding via
+  :func:`~cryptography.hazmat.primitives.keywrap.aes_key_wrap_with_padding`
+  and
+  :func:`~cryptography.hazmat.primitives.keywrap.aes_key_unwrap_with_padding`
+  .
 
 .. _v2-1-4:
 
diff --git a/docs/hazmat/primitives/keywrap.rst b/docs/hazmat/primitives/keywrap.rst
index 95cae8d7..1c15f9d1 100644
--- a/docs/hazmat/primitives/keywrap.rst
+++ b/docs/hazmat/primitives/keywrap.rst
@@ -50,6 +50,45 @@ protections offered by key wrapping are also offered by using authenticated
     :raises cryptography.hazmat.primitives.keywrap.InvalidUnwrap: This is
         raised if the key is not successfully unwrapped.
 
+.. function:: aes_key_wrap_with_padding(wrapping_key, key_to_wrap, backend)
+
+    .. versionadded:: 2.2
+
+    This function performs AES key wrap with padding as specified in
+    :rfc:`5649`.
+
+    :param bytes wrapping_key: The wrapping key.
+
+    :param bytes key_to_wrap: The key to wrap.
+
+    :param backend: A
+        :class:`~cryptography.hazmat.backends.interfaces.CipherBackend`
+        instance that supports
+        :class:`~cryptography.hazmat.primitives.ciphers.algorithms.AES`.
+
+    :return bytes: The wrapped key as bytes.
+
+.. function:: aes_key_unwrap_with_padding(wrapping_key, wrapped_key, backend)
+
+    .. versionadded:: 2.2
+
+    This function performs AES key unwrap with padding as specified in
+    :rfc:`5649`.
+
+    :param bytes wrapping_key: The wrapping key.
+
+    :param bytes wrapped_key: The wrapped key.
+
+    :param backend: A
+        :class:`~cryptography.hazmat.backends.interfaces.CipherBackend`
+        instance that supports
+        :class:`~cryptography.hazmat.primitives.ciphers.algorithms.AES`.
+
+    :return bytes: The unwrapped key as bytes.
+
+    :raises cryptography.hazmat.primitives.keywrap.InvalidUnwrap: This is
+        raised if the key is not successfully unwrapped.
+
 Exceptions
 ~~~~~~~~~~
 
diff --git a/src/cryptography/hazmat/primitives/keywrap.py b/src/cryptography/hazmat/primitives/keywrap.py
index 702a6932..3b531318 100644
--- a/src/cryptography/hazmat/primitives/keywrap.py
+++ b/src/cryptography/hazmat/primitives/keywrap.py
@@ -68,6 +68,63 @@ def _unwrap_core(wrapping_key, a, r, backend):
     return a, r
 
 
+def aes_key_wrap_with_padding(wrapping_key, key_to_wrap, backend):
+    if len(wrapping_key) not in [16, 24, 32]:
+        raise ValueError("The wrapping key must be a valid AES key length")
+
+    aiv = b"\xA6\x59\x59\xA6" + struct.pack(">i", len(key_to_wrap))
+    # pad the key to wrap if necessary
+    pad = (8 - (len(key_to_wrap) % 8)) % 8
+    key_to_wrap = key_to_wrap + b"\x00" * pad
+    if len(key_to_wrap) == 8:
+        # RFC 5649 - 4.1 - exactly 8 octets after padding
+        encryptor = Cipher(AES(wrapping_key), ECB(), backend).encryptor()
+        b = encryptor.update(aiv + key_to_wrap)
+        assert encryptor.finalize() == b""
+        return b
+    else:
+        r = [key_to_wrap[i:i + 8] for i in range(0, len(key_to_wrap), 8)]
+        return _wrap_core(wrapping_key, aiv, r, 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")
+
+    if len(wrapping_key) not in [16, 24, 32]:
+        raise ValueError("The wrapping key must be a valid AES key length")
+
+    if len(wrapped_key) == 16:
+        # RFC 5649 - 4.2 - exactly two 64-bit blocks
+        decryptor = Cipher(AES(wrapping_key), ECB(), backend).decryptor()
+        b = decryptor.update(wrapped_key)
+        assert decryptor.finalize() == b""
+        a = b[:8]
+        data = b[8:]
+        n = 1
+    else:
+        r = [wrapped_key[i:i + 8] for i in range(0, len(wrapped_key), 8)]
+        encrypted_aiv = r.pop(0)
+        n = len(r)
+        a, r = _unwrap_core(wrapping_key, encrypted_aiv, r, backend)
+        data = b"".join(r)
+
+    # 1) Check that MSB(32,A) = A65959A6.
+    # 2) Check that 8*(n-1) < LSB(32,A) <= 8*n.  If so, let
+    #    MLI = LSB(32,A).
+    # 3) Let b = (8*n)-MLI, and then check that the rightmost b octets of
+    #    the output data are zero.
+    (mli,) = struct.unpack(">I", a[4:])
+    b = (8 * n) - mli
+    if (
+        not bytes_eq(a[:4], b"\xa6\x59\x59\xa6") or not
+        8 * (n - 1) < mli <= 8 * n or not bytes_eq(data[-b:], b"\x00" * b)
+    ):
+        raise InvalidUnwrap()
+
+    return data[:-b]
+
+
 def aes_key_unwrap(wrapping_key, wrapped_key, backend):
     if len(wrapped_key) < 24:
         raise ValueError("Must be at least 24 bytes")
diff --git a/tests/hazmat/primitives/test_keywrap.py b/tests/hazmat/primitives/test_keywrap.py
index f41baedb..8311c2a4 100644
--- a/tests/hazmat/primitives/test_keywrap.py
+++ b/tests/hazmat/primitives/test_keywrap.py
@@ -114,3 +114,68 @@ class TestAESKeyWrap(object):
         # Keys to unwrap must be a multiple of 8 bytes
         with pytest.raises(ValueError):
             keywrap.aes_key_unwrap(b"sixteen_byte_key", b"\x00" * 27, backend)
+
+
+@pytest.mark.supported(
+    only_if=lambda backend: backend.cipher_supported(
+        algorithms.AES(b"\x00" * 16), modes.ECB()
+    ),
+    skip_message="Does not support AES key wrap (RFC 5649) because AES-ECB"
+                 " is unsupported",
+)
+@pytest.mark.requires_backend_interface(interface=CipherBackend)
+class TestAESKeyWrapWithPadding(object):
+    @pytest.mark.parametrize(
+        "params",
+        _load_all_params(
+            os.path.join("keywrap", "kwtestvectors"),
+            ["KWP_AE_128.txt", "KWP_AE_192.txt", "KWP_AE_256.txt"],
+            load_nist_vectors
+        )
+    )
+    def test_wrap(self, backend, params):
+        wrapping_key = binascii.unhexlify(params["k"])
+        key_to_wrap = binascii.unhexlify(params["p"])
+        wrapped_key = keywrap.aes_key_wrap_with_padding(
+            wrapping_key, key_to_wrap, backend
+        )
+        assert params["c"] == binascii.hexlify(wrapped_key)
+
+    @pytest.mark.parametrize(
+        "params",
+        _load_all_params(
+            os.path.join("keywrap", "kwtestvectors"),
+            ["KWP_AD_128.txt", "KWP_AD_192.txt", "KWP_AD_256.txt"],
+            load_nist_vectors
+        )
+    )
+    def test_unwrap(self, backend, params):
+        wrapping_key = binascii.unhexlify(params["k"])
+        wrapped_key = binascii.unhexlify(params["c"])
+        if params.get("fail") is True:
+            with pytest.raises(keywrap.InvalidUnwrap):
+                keywrap.aes_key_unwrap_with_padding(
+                    wrapping_key, wrapped_key, backend
+                )
+        else:
+            unwrapped_key = keywrap.aes_key_unwrap_with_padding(
+                wrapping_key, wrapped_key, backend
+            )
+            assert params["p"] == binascii.hexlify(unwrapped_key)
+
+    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'):
+            keywrap.aes_key_unwrap_with_padding(
+                b"sixteen_byte_key", b"\x00" * 15, backend
+            )
+
+    def test_wrap_invalid_key_length(self, backend):
+        with pytest.raises(ValueError, match='must be a valid AES key length'):
+            keywrap.aes_key_wrap_with_padding(b"badkey", b"\x00", backend)
+
+    def test_unwrap_invalid_key_length(self, backend):
+        with pytest.raises(ValueError, match='must be a valid AES key length'):
+            keywrap.aes_key_unwrap_with_padding(
+                b"badkey", b"\x00" * 16, backend
+            )