From 3e8d8f4595182d0e695cc166d5c6cf4490d17e75 Mon Sep 17 00:00:00 2001
From: Hans Nilsson <hans@erlang.org>
Date: Tue, 4 Aug 2015 16:46:46 +0200
Subject: ssh: options 'dh_gex_groups' and 'dh_gex_limits'

---
 lib/ssh/src/ssh_transport.erl | 54 ++++++++++++++++++++++++++++++++-----------
 1 file changed, 40 insertions(+), 14 deletions(-)

(limited to 'lib/ssh/src/ssh_transport.erl')

diff --git a/lib/ssh/src/ssh_transport.erl b/lib/ssh/src/ssh_transport.erl
index 69ba797faf..dda9192284 100644
--- a/lib/ssh/src/ssh_transport.erl
+++ b/lib/ssh/src/ssh_transport.erl
@@ -404,9 +404,10 @@ handle_kexdh_reply(#ssh_msg_kexdh_reply{public_host_key = HostKey,
 %%% 
 handle_kex_dh_gex_request(#ssh_msg_kex_dh_gex_request{min = Min,
 						      n   = NBits,
-						      max = Max}, Ssh0) when Min=<NBits, NBits=<Max ->
+						      max = Max}, 
+			  Ssh0=#ssh{opts=Opts}) when Min=<NBits, NBits=<Max ->
     %% server
-    {G, P} = dh_gex_group(Min, NBits, Max),
+    {G, P} = dh_gex_group(Min, NBits, Max, proplists:get_value(dh_gex_groups,Opts)),
     {Private, Public} = dh_gen_key(G, P, 1024),
     {SshPacket, Ssh} = 
 	ssh_packet(#ssh_msg_kex_dh_gex_group{p = P, g = G}, Ssh0),
@@ -1236,21 +1237,46 @@ dh_group15() ->
 dh_group16() ->
    {2, 16#FFFFFFFFFFFFFFFFC90FDAA22168C234C4C6628B80DC1CD129024E088A67CC74020BBEA63B139B22514A08798E3404DDEF9519B3CD3A431B302B0A6DF25F14374FE1356D6D51C245E485B576625E7EC6F44C42E9A637ED6B0BFF5CB6F406B7EDEE386BFB5A899FA5AE9F24117C4B1FE649286651ECE45B3DC2007CB8A163BF0598DA48361C55D39A69163FA8FD24CF5F83655D23DCA3AD961C62F356208552BB9ED529077096966D670C354E4ABC9804F1746C08CA18217C32905E462E36CE3BE39E772C180E86039B2783A2EC07A28FB5C55DF06F4C52C9DE2BCBF6955817183995497CEA956AE515D2261898FA051015728E5A8AAAC42DAD33170D04507A33A85521ABDF1CBA64ECFB850458DBEF0A8AEA71575D060C7DB3970F85A6E1E4C7ABF5AE8CDB0933D71E8C94E04A25619DCEE3D2261AD2EE6BF12FFA06D98A0864D87602733EC86A64521F2B18177B200CBBE117577A615D6C770988C0BAD946E208E24FA074E5AB3143DB5BFCE0FD108E4B82D120A92108011A723C12A787E6D788719A10BDBA5B2699C327186AF4E23C1A946834B6150BDA2583E9CA2AD44CE8DBBBC2DB04DE8EF92E8EFC141FBECAA6287C59474E6BC05D99B2964FA090C3A2233BA186515BE7ED1F612970CEE2D7AFB81BDD762170481CD0069127D5B05AA993B4EA988D8FDDC186FFB7DC90A6C08F4DF435C934063199FFFFFFFFFFFFFFFF}.
 
-
 dh_group('diffie-hellman-group1-sha1') -> dh_group1();
 dh_group('diffie-hellman-group14-sha1') -> dh_group14().
     
-
-%%% First try exact match:
-dh_gex_group(_Min, N, _Max) when N==1024 -> dh_group1();
-dh_gex_group(_Min, N, _Max) when N==2048 -> dh_group14();
-dh_gex_group(_Min, N, _Max) when N==3072 -> dh_group15();
-dh_gex_group(_Min, N, _Max) when N==4096 -> dh_group16();
-%%% If not an exact match, select the largest possible:
-dh_gex_group(Min, _N, Max) when Min=<4096, 4096=<Max -> dh_group16();
-dh_gex_group(Min, _N, Max) when Min=<3072, 3072=<Max -> dh_group15();
-dh_gex_group(Min, _N, Max) when Min=<2048, 2048=<Max -> dh_group14();
-dh_gex_group(Min, _N, Max) when Min=<1024, 1024=<Max -> dh_group1().
+dh_gex_default_groups() ->
+    [{1024, dh_group1() },
+     {2048, dh_group14()},
+     {3072, dh_group15()},
+     {4096, dh_group16()}].
+
+
+dh_gex_group(Min, N, Max, undefined) ->
+    dh_gex_group(Min, N, Max, dh_gex_default_groups());
+dh_gex_group(Min, N, Max, Groups) ->
+    %% First try to find an exact match. If not an exact match, select the largest possible.
+    {_,Group} =
+	lists:foldl(
+	  fun(_, {I,G}) when I==N ->
+		  %% If we have an exact match already: use that one
+		  {I,G};
+	     ({I,G}, _) when I==N ->
+		  %% If we now found an exact match: use that very one
+		  {I,G};
+	     ({I,G}, {Imax,_Gmax}) when Min=<I,I=<Max, % a) {I,G} fullfills the requirements
+				        I>Imax ->      % b) {I,G} is larger than current max
+		  %% A group within the limits and better than the one we have
+		  {I,G};
+	     (_, IGmax) ->
+		  %% Keep the one we have
+		  IGmax
+	  end, {-1,undefined}, Groups),
+
+    case Group of
+	undefined ->
+	    throw(#ssh_msg_disconnect{
+		     code = ?SSH_DISCONNECT_PROTOCOL_ERROR,
+		     description = "No possible diffie-hellman-group-exchange group found", 
+		     language = ""});
+	_ ->
+	    Group
+    end.
 
 
 dh_gen_key(G, P, _) ->
-- 
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