add ec.private_key_from_secret_and_curve (#3225)

* finish https://github.com/pyca/cryptography/pull/1973

* change API & add test

Function will now return an instance of EllipticCurvePrivateKey, as that
is the users' ultimate goal anyway.

* fix test

* improve coverage

* complete coverage

* final fix

* centos fix

* retry

* cleanup asserts

* use openssl_assert

* skip unsupported platforms

* change API name to derive_private_key

* change version added

* improve description of `secret` param

* separate successful and failure test cases

* simplify successful case

* add docs for derive_elliptic_curve_public_point

* add period

1 files changed, 16 insertions, 0 deletions

diff --git a/tests/hazmat/backends/test_multibackend.py b/tests/hazmat/backends/test_multibackend.py
index 1cd87336..319edf7d 100644
--- a/tests/hazmat/backends/test_multibackend.py
+++ b/tests/hazmat/backends/test_multibackend.py
@@ -27,6 +27,12 @@ class DummyBackend(object):
     pass
 
 
+@utils.register_interface(ec.EllipticCurve)
+class DummyCurve(object):
+    name = "dummy-curve"
+    key_size = 1
+
+
 @utils.register_interface(CipherBackend)
 class DummyCipherBackend(object):
     def __init__(self, supported_ciphers):
@@ -179,6 +185,10 @@ class DummyEllipticCurveBackend(object):
             self.elliptic_curve_supported(curve)
         )
 
+    def derive_elliptic_curve_public_point(self, private_value, curve):
+        if not self.elliptic_curve_supported(curve):
+            raise UnsupportedAlgorithm(_Reasons.UNSUPPORTED_ELLIPTIC_CURVE)
+
 
 @utils.register_interface(PEMSerializationBackend)
 class DummyPEMSerializationBackend(object):
@@ -501,6 +511,12 @@ class TestMultiBackend(object):
             ec.ECDH(), ec.SECT163K1()
         )
 
+        with pytest.raises(UnsupportedAlgorithm):
+            backend.derive_elliptic_curve_public_point(123, DummyCurve())
+
+        assert backend.derive_elliptic_curve_public_point(
+            123, ec.SECT283K1()) is None
+
     def test_pem_serialization_backend(self):
         backend = MultiBackend([DummyPEMSerializationBackend()])