6 files changed, 56 insertions, 0 deletions
diff --git a/src/cryptography/exceptions.py b/src/cryptography/exceptions.py
index 29be22be..3bf8a75b 100644
--- a/src/cryptography/exceptions.py
+++ b/src/cryptography/exceptions.py
@@ -20,6 +20,7 @@ class _Reasons(Enum):
     UNSUPPORTED_ELLIPTIC_CURVE = 6
     UNSUPPORTED_SERIALIZATION = 7
     UNSUPPORTED_X509 = 8
+    UNSUPPORTED_EXCHANGE_ALGORITHM = 9
 
 
 class UnsupportedAlgorithm(Exception):
diff --git a/src/cryptography/hazmat/backends/interfaces.py b/src/cryptography/hazmat/backends/interfaces.py
index d93968cf..92d9653a 100644
--- a/src/cryptography/hazmat/backends/interfaces.py
+++ b/src/cryptography/hazmat/backends/interfaces.py
@@ -215,6 +215,12 @@ class EllipticCurveBackend(object):
         Return an EllipticCurvePrivateKey provider using the given numbers.
         """
 
+    @abc.abstractmethod
+    def elliptic_curve_exchange_algorithm_supported(self, algorithm, curve):
+        """
+        Returns whether the exchange algorithm is supported by this backend.
+        """
+
 
 @six.add_metaclass(abc.ABCMeta)
 class PEMSerializationBackend(object):
diff --git a/src/cryptography/hazmat/backends/multibackend.py b/src/cryptography/hazmat/backends/multibackend.py
index 9db32aa5..c4d2c133 100644
--- a/src/cryptography/hazmat/backends/multibackend.py
+++ b/src/cryptography/hazmat/backends/multibackend.py
@@ -271,6 +271,12 @@ class MultiBackend(object):
             _Reasons.UNSUPPORTED_ELLIPTIC_CURVE
         )
 
+    def elliptic_curve_exchange_algorithm_supported(self, algorithm, curve):
+        return any(
+            b.elliptic_curve_exchange_algorithm_supported(algorithm, curve)
+            for b in self._filtered_backends(EllipticCurveBackend)
+        )
+
     def load_pem_private_key(self, data, password):
         for b in self._filtered_backends(PEMSerializationBackend):
             return b.load_pem_private_key(data, password)
diff --git a/src/cryptography/hazmat/backends/openssl/backend.py b/src/cryptography/hazmat/backends/openssl/backend.py
index 0d3b3dd4..f86c3aa1 100644
--- a/src/cryptography/hazmat/backends/openssl/backend.py
+++ b/src/cryptography/hazmat/backends/openssl/backend.py
@@ -1671,6 +1671,13 @@ class Backend(object):
 
         return _EllipticCurvePublicKey(self, ec_cdata, evp_pkey)
 
+    def elliptic_curve_exchange_algorithm_supported(self, algorithm, curve):
+        return (
+            self.elliptic_curve_supported(curve) and
+            self._lib.Cryptography_HAS_ECDH == 1 and
+            isinstance(algorithm, ec.ECDH)
+        )
+
     def _ec_cdata_to_evp_pkey(self, ec_cdata):
         evp_pkey = self._lib.EVP_PKEY_new()
         self.openssl_assert(evp_pkey != self._ffi.NULL)
diff --git a/src/cryptography/hazmat/backends/openssl/ec.py b/src/cryptography/hazmat/backends/openssl/ec.py
index 939a3f90..cfd559ae 100644
--- a/src/cryptography/hazmat/backends/openssl/ec.py
+++ b/src/cryptography/hazmat/backends/openssl/ec.py
@@ -171,6 +171,31 @@ class _EllipticCurvePrivateKey(object):
                 "Unsupported elliptic curve signature algorithm.",
                 _Reasons.UNSUPPORTED_PUBLIC_KEY_ALGORITHM)
 
+    def exchange(self, algorithm, peer_public_key):
+        if not (
+            self._backend.elliptic_curve_exchange_algorithm_supported(
+                algorithm, self.curve
+            )
+        ):
+            raise UnsupportedAlgorithm(
+                "This backend does not support the ECDH algorithm.",
+                _Reasons.UNSUPPORTED_EXCHANGE_ALGORITHM
+            )
+
+        group = self._backend._lib.EC_KEY_get0_group(self._ec_key)
+        z_len = (self._backend._lib.EC_GROUP_get_degree(group) + 7) // 8
+        self._backend.openssl_assert(z_len > 0)
+        z_buf = self._backend._ffi.new("uint8_t[]", z_len)
+        peer_key = self._backend._lib.EC_KEY_get0_public_key(
+            peer_public_key._ec_key
+        )
+
+        r = self._backend._lib.ECDH_compute_key(
+            z_buf, z_len, peer_key, self._ec_key, self._backend._ffi.NULL
+        )
+        self._backend.openssl_assert(r > 0)
+        return self._backend._ffi.buffer(z_buf)[:z_len]
+
     def public_key(self):
         group = self._backend._lib.EC_KEY_get0_group(self._ec_key)
         self._backend.openssl_assert(group != self._backend._ffi.NULL)
diff --git a/src/cryptography/hazmat/primitives/asymmetric/ec.py b/src/cryptography/hazmat/primitives/asymmetric/ec.py
index f1d39eed..c6f83667 100644
--- a/src/cryptography/hazmat/primitives/asymmetric/ec.py
+++ b/src/cryptography/hazmat/primitives/asymmetric/ec.py
@@ -44,6 +44,13 @@ class EllipticCurvePrivateKey(object):
         """
 
     @abc.abstractmethod
+    def exchange(self, algorithm, peer_public_key):
+        """
+        Performs a key exchange operation using the provided algorithm with the
+        provided peer's public key.
+        """
+
+    @abc.abstractmethod
     def public_key(self):
         """
         The EllipticCurvePublicKey for this private key.
@@ -302,3 +309,7 @@ class EllipticCurvePrivateNumbers(object):
 
     def __ne__(self, other):
         return not self == other
+
+
+class ECDH(object):
+    pass