Validate the public/private halves of EC keys on import. (#4241)

* Validate the public/private halves of EC keys on import.

OpenSSL's API is a little finicky. If one sets the public key before the
private key, it does not validate that they match. If set in the other
order, it does validate this.

In particular, KASValidityTest_ECCStaticUnified_NOKC_ZZOnly_init.fax
describes error code 7 as:

  Result = F (7 - IUT's Static private key d changed-prikey validity)

Reordering the two operations makes those tests to fail on key import,
which is what CAVP appears to have intended.

* Wrap to 79 rather than 80 columns

2 files changed, 7 insertions, 6 deletions

diff --git a/src/cryptography/hazmat/backends/openssl/backend.py b/src/cryptography/hazmat/backends/openssl/backend.py
index 9cf969cd..0b7550e5 100644
--- a/src/cryptography/hazmat/backends/openssl/backend.py
+++ b/src/cryptography/hazmat/backends/openssl/backend.py
@@ -1355,14 +1355,15 @@ class Backend(object):
         self.openssl_assert(ec_cdata != self._ffi.NULL)
         ec_cdata = self._ffi.gc(ec_cdata, self._lib.EC_KEY_free)
 
-        ec_cdata = self._ec_key_set_public_key_affine_coordinates(
-            ec_cdata, public.x, public.y)
-
         private_value = self._ffi.gc(
             self._int_to_bn(numbers.private_value), self._lib.BN_clear_free
         )
         res = self._lib.EC_KEY_set_private_key(ec_cdata, private_value)
         self.openssl_assert(res == 1)
+
+        ec_cdata = self._ec_key_set_public_key_affine_coordinates(
+            ec_cdata, public.x, public.y)
+
         evp_pkey = self._ec_cdata_to_evp_pkey(ec_cdata)
 
         return _EllipticCurvePrivateKey(self, ec_cdata, evp_pkey)
diff --git a/tests/hazmat/primitives/test_ec.py b/tests/hazmat/primitives/test_ec.py
index 1b491a10..59e4c995 100644
--- a/tests/hazmat/primitives/test_ec.py
+++ b/tests/hazmat/primitives/test_ec.py
@@ -1068,9 +1068,9 @@ class TestECDH(object):
                 ec._CURVE_TYPES[vector['curve']]()
             )
         )
-        # Errno 5 and 6 indicates a bad public key, this doesn't test the ECDH
-        # code at all
-        if vector['fail'] and vector['errno'] in [5, 6]:
+        # Errno 5-7 indicates a bad public or private key, this doesn't test
+        # the ECDH code at all
+        if vector['fail'] and vector['errno'] in [5, 6, 7]:
             with pytest.raises(ValueError):
                 private_numbers.private_key(backend)
             return