Modify DH/ECDH examples to be explicit for DHE/ECDHE (#3622)

* Modify DH/ECDH examples to be explicit for DHE/ECDHE

Also add note to DH docs that you should probably use ECDH

* give a reason

2 files changed, 32 insertions, 3 deletions

diff --git a/docs/hazmat/primitives/asymmetric/dh.rst b/docs/hazmat/primitives/asymmetric/dh.rst
index 58ff663d..f4cae1c3 100644
--- a/docs/hazmat/primitives/asymmetric/dh.rst
+++ b/docs/hazmat/primitives/asymmetric/dh.rst
@@ -5,6 +5,11 @@ Diffie-Hellman key exchange
 
 .. currentmodule:: cryptography.hazmat.primitives.asymmetric.dh
 
+.. note::
+    For security and performance reasons we suggest using
+    :class:`~cryptography.hazmat.primitives.asymmetric.ec.ECDH` instead of DH
+    where possible.
+
 
 `Diffie-Hellman key exchange`_ (D–H) is a method that allows two parties
 to jointly agree on a shared secret using an insecure channel.
@@ -20,16 +25,28 @@ derivation function.
 
     >>> from cryptography.hazmat.backends import default_backend
     >>> from cryptography.hazmat.primitives.asymmetric import dh
+    >>> # Generate some parameters. These can be reused.
     >>> parameters = dh.generate_parameters(generator=2, key_size=2048,
     ...                                     backend=default_backend())
+    >>> # Generate a private key for use in the exchange.
     >>> private_key = parameters.generate_private_key()
+    >>> # In a real handshake the peer_public_key will be received from the
+    >>> # other party. For this example we'll generate another private key and
+    >>> # get a public key from that. Note that in a DH handshake both peers
+    >>> # must agree on a common set of parameters.
     >>> peer_public_key = parameters.generate_private_key().public_key()
     >>> shared_key = private_key.exchange(peer_public_key)
+    >>> # For the next handshake we MUST generate another private key, but
+    >>> # we can reuse the parameters.
+    >>> private_key_2 = parameters.generate_private_key()
+    >>> peer_public_key_2 = parameters.generate_private_key().public_key()
+    >>> shared_key_2 = private_key_2.exchange(peer_public_key_2)
 
 DHE (or EDH), the ephemeral form of this exchange, is **strongly
-preferred** over simple DH and provides `forward secrecy`_ when used.
-You must generate a new private key using :func:`~DHParameters.generate_private_key` for
-each :meth:`~DHPrivateKey.exchange` when performing an DHE key exchange.
+preferred** over simple DH and provides `forward secrecy`_ when used.  You must
+generate a new private key using :func:`~DHParameters.generate_private_key` for
+each :meth:`~DHPrivateKey.exchange` when performing an DHE key exchange. This
+is demonstrated in the previous example.
 
 To assemble a :class:`~DHParameters` and a :class:`~DHPublicKey` from
 primitive integers, you must first create the
diff --git a/docs/hazmat/primitives/asymmetric/ec.rst b/docs/hazmat/primitives/asymmetric/ec.rst
index 46f2f5ac..55146175 100644
--- a/docs/hazmat/primitives/asymmetric/ec.rst
+++ b/docs/hazmat/primitives/asymmetric/ec.rst
@@ -210,13 +210,25 @@ Elliptic Curve Key Exchange algorithm
 
         >>> from cryptography.hazmat.backends import default_backend
         >>> from cryptography.hazmat.primitives.asymmetric import ec
+        >>> # Generate a private key for use in the exchange.
         >>> private_key = ec.generate_private_key(
         ...     ec.SECP384R1(), default_backend()
         ... )
+        >>> # In a real handshake the peer_public_key will be received from the
+        >>> # other party. For this example we'll generate another private key
+        >>> # and get a public key from that.
         >>> peer_public_key = ec.generate_private_key(
         ...     ec.SECP384R1(), default_backend()
         ... ).public_key()
         >>> shared_key = private_key.exchange(ec.ECDH(), peer_public_key)
+        >>> # For the next handshake we MUST generate another private key.
+        >>> private_key_2 = ec.generate_private_key(
+        ...     ec.SECP384R1(), default_backend()
+        ... )
+        >>> peer_public_key_2 = ec.generate_private_key(
+        ...     ec.SECP384R1(), default_backend()
+        ... ).public_key()
+        >>> shared_key_2 = private_key_2.exchange(ec.ECDH(), peer_public_key_2)
 
     ECDHE (or EECDH), the ephemeral form of this exchange, is **strongly
     preferred** over simple ECDH and provides `forward secrecy`_ when used.