Add ECC curve selection order config in TLS server

As per RFC 4492 Sec 5.1, the preferred order of selection of named
curves is based on client preferences.

Currently, the SSL application only picks entries according to the
absolute order of entries as tracked in a hardcoded list in code.

This patch changes things so that the client-specified order is
preferred. It also allows a mode where the server can be configured to
override the client's preferred order with its own, although the chosen
ECC must still be within both lists.

The configuration is done through the following options:

- `eccs`, shared by clients and servers alike, allows the specification
  of the supported named curves, in their preferred order, and may
  eventually support more values for explicit primes and so on.
- `honor_ecc_order`, a server-only option, is similar to
  `honor_cipher_order` and will, by default let the server pick the
  client-preferred ECC, and otherwise pick the server-preferred one.

The default value for `eccs` is the same as before, although the
server-chosen ECC now defaults to the client rather than previous
choice.

A function `ssl:eccs()` has been added that returns the highest
supported ECCs for the library.

1 files changed, 30 insertions, 15 deletions

diff --git a/lib/ssl/src/ssl_handshake.erl b/lib/ssl/src/ssl_handshake.erl
index 5b51ac0916..4acc745c5f 100644
--- a/lib/ssl/src/ssl_handshake.erl
+++ b/lib/ssl/src/ssl_handshake.erl
@@ -70,7 +70,7 @@
 %% Extensions handling
 -export([client_hello_extensions/6,
 	 handle_client_hello_extensions/9, %% Returns server hello extensions
-	 handle_server_hello_extensions/9, select_curve/2
+	 handle_server_hello_extensions/9, select_curve/2, select_curve/3
 	]).
 
 %% MISC
@@ -120,11 +120,13 @@ server_hello_done() ->
     #server_hello_done{}.
 
 client_hello_extensions(Host, Version, CipherSuites, 
-			#ssl_options{signature_algs = SupportedHashSigns, versions = AllVersions} = SslOpts, ConnectionStates, Renegotiation) ->
+			#ssl_options{signature_algs = SupportedHashSigns,
+				     eccs = SupportedECCs,
+				     versions = AllVersions} = SslOpts, ConnectionStates, Renegotiation) ->
     {EcPointFormats, EllipticCurves} =
 	case advertises_ec_ciphers(lists:map(fun ssl_cipher:suite_definition/1, CipherSuites)) of
 	    true ->
-		client_ecc_extensions(tls_v1, Version);
+		client_ecc_extensions(SupportedECCs);
 	    false ->
 		{undefined, undefined}
 	end,
@@ -1169,8 +1171,9 @@ select_session(SuggestedSessionId, CipherSuites, HashSigns, Compressions, Port,
 	    {resumed, Resumed}
     end.
 
-supported_ecc({Major, Minor} = Version) when ((Major == 3) and (Minor >= 1)) orelse (Major > 3) ->
-    Curves = tls_v1:ecc_curves(Version),
+%% Deprecated?
+supported_ecc({Major, Minor}) when ((Major == 3) and (Minor >= 1)) orelse (Major > 3) ->
+    Curves = tls_v1:ecc_curves(Minor),
     #elliptic_curves{elliptic_curve_list = Curves};
 supported_ecc(_) ->
     #elliptic_curves{elliptic_curve_list = []}.
@@ -1454,12 +1457,12 @@ srp_user(#ssl_options{srp_identity = {UserName, _}}) ->
 srp_user(_) ->
     undefined.
 
-client_ecc_extensions(Module, Version) ->
+client_ecc_extensions(SupportedECCs) ->
     CryptoSupport = proplists:get_value(public_keys, crypto:supports()),
     case proplists:get_bool(ecdh, CryptoSupport) of
 	true ->
 	    EcPointFormats = #ec_point_formats{ec_point_format_list = [?ECPOINT_UNCOMPRESSED]},
-	    EllipticCurves = #elliptic_curves{elliptic_curve_list = Module:ecc_curves(Version)},
+	    EllipticCurves = SupportedECCs,
 	    {EcPointFormats, EllipticCurves};
 	_ ->
 	    {undefined, undefined}
@@ -1493,22 +1496,34 @@ advertises_ec_ciphers([{ecdh_anon, _,_,_} | _]) ->
     true;
 advertises_ec_ciphers([_| Rest]) ->
     advertises_ec_ciphers(Rest).
-select_curve(#elliptic_curves{elliptic_curve_list = ClientCurves}, 
-	     #elliptic_curves{elliptic_curve_list = ServerCurves}) -> 
-    select_curve(ClientCurves, ServerCurves);
-select_curve(undefined, _) ->
+
+select_curve(Client, Server) ->
+    select_curve(Client, Server, false).
+
+select_curve(#elliptic_curves{elliptic_curve_list = ClientCurves},
+	     #elliptic_curves{elliptic_curve_list = ServerCurves},
+	     ServerOrder) ->
+    case ServerOrder of
+        false ->
+            select_shared_curve(ClientCurves, ServerCurves);
+        true ->
+            select_shared_curve(ServerCurves, ClientCurves)
+    end;
+select_curve(undefined, _, _) ->
     %% Client did not send ECC extension use default curve if 
     %% ECC cipher is negotiated
-    {namedCurve, ?secp256r1};
-select_curve(_, []) ->
+    {namedCurve, ?secp256r1}.
+
+select_shared_curve([], _) ->
     no_curve;
-select_curve(Curves, [Curve| Rest]) ->
+select_shared_curve([Curve | Rest], Curves) ->
     case lists:member(Curve, Curves) of
 	true ->
 	    {namedCurve, Curve};
 	false ->
-	    select_curve(Curves, Rest)
+	    select_shared_curve(Rest, Curves)
     end.
+
 %% RFC 6066, Section 3: Currently, the only server names supported are
 %% DNS hostnames
 sni(_, disable) ->