Serialization x25519 (#4688)

* modify x25519 serialization to match x448

supports raw and pkcs8 encoding on private_bytes
supports raw and subjectpublickeyinfo on public_bytes

deprecates zero argument call to public_bytes

* add docs

* this is public now

* don't need that

* review feedback

1 files changed, 64 insertions, 0 deletions

diff --git a/tests/hazmat/primitives/test_serialization.py b/tests/hazmat/primitives/test_serialization.py
index 81d372fc..2bc49078 100644
--- a/tests/hazmat/primitives/test_serialization.py
+++ b/tests/hazmat/primitives/test_serialization.py
@@ -1297,3 +1297,67 @@ class TestX448Serialization(object):
         assert public_key.public_bytes(
             encoding, PublicFormat.SubjectPublicKeyInfo
         ) == data
+
+
+@pytest.mark.supported(
+    only_if=lambda backend: backend.x25519_supported(),
+    skip_message="Requires OpenSSL with X25519 support"
+)
+class TestX25519Serialization(object):
+    def test_load_der_private_key(self, backend):
+        data = load_vectors_from_file(
+            os.path.join("asymmetric", "X25519", "x25519-pkcs8-enc.der"),
+            lambda derfile: derfile.read(),
+            mode="rb"
+        )
+        unencrypted = load_vectors_from_file(
+            os.path.join("asymmetric", "X25519", "x25519-pkcs8.der"),
+            lambda derfile: derfile.read(),
+            mode="rb"
+        )
+        key = load_der_private_key(data, b"password", backend)
+        assert key.private_bytes(
+            Encoding.DER, PrivateFormat.PKCS8, NoEncryption()
+        ) == unencrypted
+
+    def test_load_pem_private_key(self, backend):
+        data = load_vectors_from_file(
+            os.path.join("asymmetric", "X25519", "x25519-pkcs8-enc.pem"),
+            lambda pemfile: pemfile.read(),
+            mode="rb"
+        )
+        unencrypted = load_vectors_from_file(
+            os.path.join("asymmetric", "X25519", "x25519-pkcs8.pem"),
+            lambda pemfile: pemfile.read(),
+            mode="rb"
+        )
+        key = load_pem_private_key(data, b"password", backend)
+        assert key.private_bytes(
+            Encoding.PEM, PrivateFormat.PKCS8, NoEncryption()
+        ) == unencrypted
+
+    @pytest.mark.parametrize(
+        ("key_path", "encoding", "loader"),
+        [
+            (
+                ["X25519", "x25519-pub.pem"],
+                Encoding.PEM,
+                load_pem_public_key
+            ),
+            (
+                ["X25519", "x25519-pub.der"],
+                Encoding.DER,
+                load_der_public_key
+            ),
+        ]
+    )
+    def test_load_public_key(self, key_path, encoding, loader, backend):
+        data = load_vectors_from_file(
+            os.path.join("asymmetric", *key_path),
+            lambda pemfile: pemfile.read(),
+            mode="rb"
+        )
+        public_key = loader(data, backend)
+        assert public_key.public_bytes(
+            encoding, PublicFormat.SubjectPublicKeyInfo
+        ) == data