Repository: pyca/cryptography

Issue:
X25519 support
This is separate from #2968 so I can put it in it's own milestone.

Proposed API:

```py3
class X25519PublicKey(object):
    @classmethod
    def from_public_bytes(cls, bytes: bytes) -> X25519PublicKey:
        pass

    def public_bytes(self) -> bytes:
        pass
    

class X25519PrivateKey(object):
    @classmethod
    def generate(cls) -> X25519PrivateKey:
        pass

    def public_key(self) -> X25519PublicKey:
        pass

    @classmethod
    def _from_private_bytes(cls, bytes: bytes) -> X25519PrivateKey:
        pass

    def private_bytes(self) -> bytes:
        pass

    def exchange(self, public_key: X25519PublicKey) -> bytes:
        pass
```


Discussion:

Resolving patch:
diff --git a/src/cryptography/hazmat/backends/openssl/backend.py b/src/cryptography/hazmat/backends/openssl/backend.py
--- a/src/cryptography/hazmat/backends/openssl/backend.py
+++ b/src/cryptography/hazmat/backends/openssl/backend.py
@@ -43,6 +43,9 @@
 from cryptography.hazmat.backends.openssl.rsa import (
     _RSAPrivateKey, _RSAPublicKey
 )
+from cryptography.hazmat.backends.openssl.x25519 import (
+    _X25519PrivateKey, _X25519PublicKey
+)
 from cryptography.hazmat.backends.openssl.x509 import (
     _Certificate, _CertificateRevocationList,
     _CertificateSigningRequest, _RevokedCertificate
@@ -1772,6 +1775,57 @@ def x509_name_bytes(self, name):
         self.openssl_assert(res > 0)
         return self._ffi.buffer(pp[0], res)[:]
 
+    def x25519_load_public_bytes(self, data):
+        evp_pkey = self._create_evp_pkey_gc()
+        res = self._lib.EVP_PKEY_set_type(evp_pkey, self._lib.NID_X25519)
+        backend.openssl_assert(res == 1)
+        res = self._lib.EVP_PKEY_set1_tls_encodedpoint(
+            evp_pkey, data, len(data)
+        )
+        backend.openssl_assert(res == 1)
+        return _X25519PublicKey(self, evp_pkey)
+
+    def x25519_load_private_bytes(self, data):
+        # OpenSSL only has facilities for loading PKCS8 formatted private
+        # keys using the algorithm identifiers specified in
+        # https://tools.ietf.org/html/draft-ietf-curdle-pkix-03.
+        # This is the standard PKCS8 prefix for a 32 byte X25519 key.
+        # The form is:
+        #    0:d=0  hl=2 l=  46 cons: SEQUENCE
+        #    2:d=1  hl=2 l=   1 prim: INTEGER           :00
+        #    5:d=1  hl=2 l=   5 cons: SEQUENCE
+        #    7:d=2  hl=2 l=   3 prim: OBJECT            :1.3.101.110
+        #    12:d=1  hl=2 l=  34 prim: OCTET STRING      (the key)
+        # Of course there's a bit more complexity. In reality OCTET STRING
+        # contains an OCTET STRING of length 32! So the last two bytes here
+        # are \x04\x20, which is an OCTET STRING of length 32.
+        pkcs8_prefix = b'0.\x02\x01\x000\x05\x06\x03+en\x04"\x04 '
+        bio = self._bytes_to_bio(pkcs8_prefix + data)
+        evp_pkey = backend._lib.d2i_PrivateKey_bio(bio.bio, self._ffi.NULL)
+        self.openssl_assert(evp_pkey != self._ffi.NULL)
+        evp_pkey = self._ffi.gc(evp_pkey, self._lib.EVP_PKEY_free)
+        return _X25519PrivateKey(self, evp_pkey)
+
+    def x25519_generate_key(self):
+        evp_pkey_ctx = self._lib.EVP_PKEY_CTX_new_id(
+            self._lib.NID_X25519, self._ffi.NULL
+        )
+        self.openssl_assert(evp_pkey_ctx != self._ffi.NULL)
+        evp_pkey_ctx = self._ffi.gc(
+            evp_pkey_ctx, self._lib.EVP_PKEY_CTX_free
+        )
+        res = self._lib.EVP_PKEY_keygen_init(evp_pkey_ctx)
+        self.openssl_assert(res == 1)
+        evp_ppkey = self._ffi.new("EVP_PKEY **")
+        res = self._lib.EVP_PKEY_keygen(evp_pkey_ctx, evp_ppkey)
+        self.openssl_assert(res == 1)
+        self.openssl_assert(evp_ppkey[0] != self._ffi.NULL)
+        evp_pkey = self._ffi.gc(evp_ppkey[0], self._lib.EVP_PKEY_free)
+        return _X25519PrivateKey(self, evp_pkey)
+
+    def x25519_supported(self):
+        return self._lib.CRYPTOGRAPHY_OPENSSL_110_OR_GREATER
+
     def derive_scrypt(self, key_material, salt, length, n, r, p):
         buf = self._ffi.new("unsigned char[]", length)
         res = self._lib.EVP_PBE_scrypt(
diff --git a/src/cryptography/hazmat/backends/openssl/x25519.py b/src/cryptography/hazmat/backends/openssl/x25519.py
new file mode 100644
--- /dev/null
+++ b/src/cryptography/hazmat/backends/openssl/x25519.py
@@ -0,0 +1,71 @@
+# 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
+
+from cryptography import utils
+from cryptography.hazmat.primitives.asymmetric.x25519 import (
+    X25519PrivateKey, X25519PublicKey
+)
+
+
+@utils.register_interface(X25519PublicKey)
+class _X25519PublicKey(object):
+    def __init__(self, backend, evp_pkey):
+        self._backend = backend
+        self._evp_pkey = evp_pkey
+
+    def public_bytes(self):
+        ucharpp = self._backend._ffi.new("unsigned char **")
+        res = self._backend._lib.EVP_PKEY_get1_tls_encodedpoint(
+            self._evp_pkey, ucharpp
+        )
+        self._backend.openssl_assert(res == 32)
+        self._backend.openssl_assert(ucharpp[0] != self._backend._ffi.NULL)
+        data = self._backend._ffi.gc(
+            ucharpp[0], self._backend._lib.OPENSSL_free
+        )
+        return self._backend._ffi.buffer(data, res)[:]
+
+
+@utils.register_interface(X25519PrivateKey)
+class _X25519PrivateKey(object):
+    def __init__(self, backend, evp_pkey):
+        self._backend = backend
+        self._evp_pkey = evp_pkey
+
+    def public_key(self):
+        bio = self._backend._create_mem_bio_gc()
+        res = self._backend._lib.i2d_PUBKEY_bio(bio, self._evp_pkey)
+        self._backend.openssl_assert(res == 1)
+        evp_pkey = self._backend._lib.d2i_PUBKEY_bio(
+            bio, self._backend._ffi.NULL
+        )
+        return _X25519PublicKey(self._backend, evp_pkey)
+
+    def exchange(self, peer_public_key):
+        if not isinstance(peer_public_key, X25519PublicKey):
+            raise TypeError("peer_public_key must be X25519PublicKey.")
+
+        ctx = self._backend._lib.EVP_PKEY_CTX_new(
+            self._evp_pkey, self._backend._ffi.NULL
+        )
+        self._backend.openssl_assert(ctx != self._backend._ffi.NULL)
+        ctx = self._backend._ffi.gc(ctx, self._backend._lib.EVP_PKEY_CTX_free)
+        res = self._backend._lib.EVP_PKEY_derive_init(ctx)
+        self._backend.openssl_assert(res == 1)
+        res = self._backend._lib.EVP_PKEY_derive_set_peer(
+            ctx, peer_public_key._evp_pkey
+        )
+        self._backend.openssl_assert(res == 1)
+        keylen = self._backend._ffi.new("size_t *")
+        res = self._backend._lib.EVP_PKEY_derive(
+            ctx, self._backend._ffi.NULL, keylen
+        )
+        self._backend.openssl_assert(res == 1)
+        self._backend.openssl_assert(keylen[0] > 0)
+        buf = self._backend._ffi.new("unsigned char[]", keylen[0])
+        res = self._backend._lib.EVP_PKEY_derive(ctx, buf, keylen)
+        self._backend.openssl_assert(res == 1)
+        return self._backend._ffi.buffer(buf, keylen[0])[:]
diff --git a/src/cryptography/hazmat/primitives/asymmetric/x25519.py b/src/cryptography/hazmat/primitives/asymmetric/x25519.py
new file mode 100644
--- /dev/null
+++ b/src/cryptography/hazmat/primitives/asymmetric/x25519.py
@@ -0,0 +1,54 @@
+# 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 X25519PublicKey(object):
+    @classmethod
+    def from_public_bytes(cls, data):
+        from cryptography.hazmat.backends.openssl.backend import backend
+        if not backend.x25519_supported():
+            raise UnsupportedAlgorithm(
+                "X25519 is not supported by this version of OpenSSL.",
+                _Reasons.UNSUPPORTED_EXCHANGE_ALGORITHM
+            )
+        return backend.x25519_load_public_bytes(data)
+
+    @abc.abstractmethod
+    def public_bytes(self):
+        pass
+
+
+@six.add_metaclass(abc.ABCMeta)
+class X25519PrivateKey(object):
+    @classmethod
+    def generate(cls):
+        from cryptography.hazmat.backends.openssl.backend import backend
+        if not backend.x25519_supported():
+            raise UnsupportedAlgorithm(
+                "X25519 is not supported by this version of OpenSSL.",
+                _Reasons.UNSUPPORTED_EXCHANGE_ALGORITHM
+            )
+        return backend.x25519_generate_key()
+
+    @classmethod
+    def _from_private_bytes(cls, data):
+        from cryptography.hazmat.backends.openssl.backend import backend
+        return backend.x25519_load_private_bytes(data)
+
+    @abc.abstractmethod
+    def public_key(self):
+        pass
+
+    @abc.abstractmethod
+    def exchange(self, peer_public_key):
+        pass


Test patch:
diff --git a/tests/hazmat/primitives/test_x25519.py b/tests/hazmat/primitives/test_x25519.py
new file mode 100644
--- /dev/null
+++ b/tests/hazmat/primitives/test_x25519.py
@@ -0,0 +1,120 @@
+# 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 binascii
+import os
+
+import pytest
+
+from cryptography.exceptions import _Reasons
+from cryptography.hazmat.backends.interfaces import DHBackend
+from cryptography.hazmat.primitives.asymmetric.x25519 import (
+    X25519PrivateKey, X25519PublicKey
+)
+
+from ...utils import (
+    load_nist_vectors, load_vectors_from_file, raises_unsupported_algorithm
+)
+
+
+@pytest.mark.supported(
+    only_if=lambda backend: not backend.x25519_supported(),
+    skip_message="Requires OpenSSL without X25519 support"
+)
+@pytest.mark.requires_backend_interface(interface=DHBackend)
+def test_x25519_unsupported(backend):
+    with raises_unsupported_algorithm(_Reasons.UNSUPPORTED_EXCHANGE_ALGORITHM):
+        X25519PublicKey.from_public_bytes(b"0" * 32)
+
+    with raises_unsupported_algorithm(_Reasons.UNSUPPORTED_EXCHANGE_ALGORITHM):
+        X25519PrivateKey.generate()
+
+
+@pytest.mark.supported(
+    only_if=lambda backend: backend.x25519_supported(),
+    skip_message="Requires OpenSSL with X25519 support"
+)
+@pytest.mark.requires_backend_interface(interface=DHBackend)
+class TestX25519Exchange(object):
+    @pytest.mark.parametrize(
+        "vector",
+        load_vectors_from_file(
+            os.path.join("asymmetric", "X25519", "rfc7748.txt"),
+            load_nist_vectors
+        )
+    )
+    def test_rfc7748(self, vector, backend):
+        private = binascii.unhexlify(vector["input_scalar"])
+        public = binascii.unhexlify(vector["input_u"])
+        shared_key = binascii.unhexlify(vector["output_u"])
+        private_key = X25519PrivateKey._from_private_bytes(private)
+        public_key = X25519PublicKey.from_public_bytes(public)
+        computed_shared_key = private_key.exchange(public_key)
+        assert computed_shared_key == shared_key
+
+    def test_rfc7748_1000_iteration(self, backend):
+        old_private = private = public = binascii.unhexlify(
+            b"090000000000000000000000000000000000000000000000000000000000"
+            b"0000"
+        )
+        shared_key = binascii.unhexlify(
+            b"684cf59ba83309552800ef566f2f4d3c1c3887c49360e3875f2eb94d9953"
+            b"2c51"
+        )
+        private_key = X25519PrivateKey._from_private_bytes(private)
+        public_key = X25519PublicKey.from_public_bytes(public)
+        for _ in range(1000):
+            computed_shared_key = private_key.exchange(public_key)
+            private_key = X25519PrivateKey._from_private_bytes(
+                computed_shared_key
+            )
+            public_key = X25519PublicKey.from_public_bytes(old_private)
+            old_private = computed_shared_key
+
+        assert computed_shared_key == shared_key
+
+    # These vectors are also from RFC 7748
+    # https://tools.ietf.org/html/rfc7748#section-6.1
+    @pytest.mark.parametrize(
+        ("private_bytes", "public_bytes"),
+        [
+            (
+                binascii.unhexlify(
+                    b"77076d0a7318a57d3c16c17251b26645df4c2f87ebc0992ab177fba"
+                    b"51db92c2a"
+                ),
+                binascii.unhexlify(
+                    b"8520f0098930a754748b7ddcb43ef75a0dbf3a0d26381af4eba4a98"
+                    b"eaa9b4e6a"
+                )
+            ),
+            (
+                binascii.unhexlify(
+                    b"5dab087e624a8a4b79e17f8b83800ee66f3bb1292618b6fd1c2f8b2"
+                    b"7ff88e0eb"
+                ),
+                binascii.unhexlify(
+                    b"de9edb7d7b7dc1b4d35b61c2ece435373f8343c85b78674dadfc7e1"
+                    b"46f882b4f"
+                )
+            )
+        ]
+    )
+    def test_public_bytes(self, private_bytes, public_bytes, backend):
+        private_key = X25519PrivateKey._from_private_bytes(private_bytes)
+        assert private_key.public_key().public_bytes() == public_bytes
+        public_key = X25519PublicKey.from_public_bytes(public_bytes)
+        assert public_key.public_bytes() == public_bytes
+
+    def test_generate(self, backend):
+        key = X25519PrivateKey.generate()
+        assert key
+        assert key.public_key()
+
+    def test_invalid_type_exchange(self, backend):
+        key = X25519PrivateKey.generate()
+        with pytest.raises(TypeError):
+            key.exchange(object())