X448 support (#4580)

* x448 support

This work was originally authored by derwolfe

* update docs to have a more useful derived key length

* error if key is not a valid length in from_public_bytes

* one more

* switch to using evp_pkey_keygen_gc for x448 keygen

* review feedback

* switch to using evp_pkey_derive

* nit fix

1 files changed, 61 insertions, 0 deletions

diff --git a/src/cryptography/hazmat/primitives/asymmetric/x448.py b/src/cryptography/hazmat/primitives/asymmetric/x448.py
new file mode 100644
index 00000000..69bfa408
--- /dev/null
+++ b/src/cryptography/hazmat/primitives/asymmetric/x448.py
@@ -0,0 +1,61 @@
+# This file is dual licensed under the terms of the Apache License, Version
+# 2.0, and the BSD License. See the LICENSE file in the root of this repository
+# for complete details.
+
+from __future__ import absolute_import, division, print_function
+
+import abc
+
+import six
+
+from cryptography.exceptions import UnsupportedAlgorithm, _Reasons
+
+
+@six.add_metaclass(abc.ABCMeta)
+class X448PublicKey(object):
+    @classmethod
+    def from_public_bytes(cls, data):
+        from cryptography.hazmat.backends.openssl.backend import backend
+        if not backend.x448_supported():
+            raise UnsupportedAlgorithm(
+                "X448 is not supported by this version of OpenSSL.",
+                _Reasons.UNSUPPORTED_EXCHANGE_ALGORITHM
+            )
+
+        return backend.x448_load_public_bytes(data)
+
+    @abc.abstractmethod
+    def public_bytes(self):
+        """
+        The serialized bytes of the public key.
+        """
+
+
+@six.add_metaclass(abc.ABCMeta)
+class X448PrivateKey(object):
+    @classmethod
+    def generate(cls):
+        from cryptography.hazmat.backends.openssl.backend import backend
+        if not backend.x448_supported():
+            raise UnsupportedAlgorithm(
+                "X448 is not supported by this version of OpenSSL.",
+                _Reasons.UNSUPPORTED_EXCHANGE_ALGORITHM
+            )
+        return backend.x448_generate_key()
+
+    @classmethod
+    def _from_private_bytes(cls, data):
+        from cryptography.hazmat.backends.openssl.backend import backend
+        return backend.x448_load_private_bytes(data)
+
+    @abc.abstractmethod
+    def public_key(self):
+        """
+        The serialized bytes of the public key.
+        """
+
+    @abc.abstractmethod
+    def exchange(self, peer_public_key):
+        """
+        Performs a key exchange operation using the provided peer's public key.
+        """