ssh: add extra random length padding in packets

A new experimental option 'max_random_length_padding', not documented so it might change...

5 files changed, 27 insertions, 6 deletions

diff --git a/lib/ssh/src/ssh.erl b/lib/ssh/src/ssh.erl
index 5b2e0a988c..132de71aed 100644
--- a/lib/ssh/src/ssh.erl
+++ b/lib/ssh/src/ssh.erl
@@ -397,6 +397,8 @@ handle_option([{id_string, _ID} = Opt|Rest], SocketOptions, SshOptions) ->
     handle_option(Rest, SocketOptions, [handle_ssh_option(Opt) | SshOptions]);
 handle_option([{profile, _ID} = Opt|Rest], SocketOptions, SshOptions) ->
     handle_option(Rest, SocketOptions, [handle_ssh_option(Opt) | SshOptions]);
+handle_option([{max_random_length_padding, _Bool} = Opt|Rest], SocketOptions, SshOptions) ->
+    handle_option(Rest, SocketOptions, [handle_ssh_option(Opt) | SshOptions]);
 handle_option([Opt | Rest], SocketOptions, SshOptions) ->
     handle_option(Rest, [handle_inet_option(Opt) | SocketOptions], SshOptions).
 
@@ -515,6 +517,9 @@ handle_ssh_option({id_string, random}) ->
     {id_string, {random,2,5}}; %% 2 - 5 random characters
 handle_ssh_option({id_string, ID} = Opt) when is_list(ID) ->
     Opt;
+handle_ssh_option({max_random_length_padding, Value} = Opt) when is_integer(Value),
+								 Value =< 255 ->
+    Opt;
 handle_ssh_option({profile, Value} = Opt) when is_atom(Value) ->
     Opt;
 handle_ssh_option(Opt) ->
diff --git a/lib/ssh/src/ssh.hrl b/lib/ssh/src/ssh.hrl
index 8df5ee820c..462c98f503 100644
--- a/lib/ssh/src/ssh.hrl
+++ b/lib/ssh/src/ssh.hrl
@@ -124,6 +124,7 @@
 	  recv_sequence = 0,
 	  keyex_key,
 	  keyex_info,
+	  random_length_padding = 255, % From RFC 4253 section 6.
 	  
 	  %% User auth
 	  user,
diff --git a/lib/ssh/src/ssh_connection_handler.erl b/lib/ssh/src/ssh_connection_handler.erl
index c059834b27..180698d741 100644
--- a/lib/ssh/src/ssh_connection_handler.erl
+++ b/lib/ssh/src/ssh_connection_handler.erl
@@ -1187,7 +1187,10 @@ init_ssh(client = Role, Vsn, Version, Options, Socket) ->
 	 opts = Options,
 	 userauth_supported_methods = AuthMethods,
 	 peer = {PeerName, PeerAddr},
-	 available_host_keys = supported_host_keys(Role, KeyCb, Options)
+	 available_host_keys = supported_host_keys(Role, KeyCb, Options),
+	 random_length_padding = proplists:get_value(max_random_length_padding, 
+						     Options, 
+						     (#ssh{})#ssh.random_length_padding)
 	};
 
 init_ssh(server = Role, Vsn, Version, Options, Socket) ->
@@ -1207,7 +1210,10 @@ init_ssh(server = Role, Vsn, Version, Options, Socket) ->
 	 userauth_methods = AuthMethodsAsList,
 	 kb_tries_left = 3,
 	 peer = {undefined, PeerAddr},
-	 available_host_keys = supported_host_keys(Role, KeyCb, Options)
+	 available_host_keys = supported_host_keys(Role, KeyCb, Options),
+	 random_length_padding = proplists:get_value(max_random_length_padding, 
+						     Options, 
+						     (#ssh{})#ssh.random_length_padding)
 	 }.
 
 supported_host_keys(client, _, Options) ->
diff --git a/lib/ssh/src/ssh_transport.erl b/lib/ssh/src/ssh_transport.erl
index 38a0b7ec7c..9ed6c85ff7 100644
--- a/lib/ssh/src/ssh_transport.erl
+++ b/lib/ssh/src/ssh_transport.erl
@@ -745,13 +745,20 @@ ssh_packet(Msg, Ssh) ->
 
 pack(Data0, #ssh{encrypt_block_size = BlockSize, 
 		 send_sequence = SeqNum, send_mac = MacAlg,
-		 send_mac_key = MacKey} 
+		 send_mac_key = MacKey,
+		 random_length_padding = RandomLengthPadding} 
      = Ssh0) when is_binary(Data0) ->
     {Ssh1, Data} = compress(Ssh0, Data0),
     PL = (BlockSize - ((4 + 1 + size(Data)) rem BlockSize)) rem BlockSize,
-    PaddingLen = if PL <  4 -> PL + BlockSize;
-		    true -> PL
-		 end,
+    MinPaddingLen = if PL <  4 -> PL + BlockSize;
+		       true -> PL
+		    end,
+    PadBlockSize =  max(BlockSize,4),
+    MaxExtraBlocks = (max(RandomLengthPadding,MinPaddingLen) - MinPaddingLen) div PadBlockSize,
+    ExtraPaddingLen = try crypto:rand_uniform(0,MaxExtraBlocks)*PadBlockSize
+		      catch _:_ -> 0
+		      end,
+    PaddingLen = MinPaddingLen + ExtraPaddingLen,
     Padding = ssh_bits:random(PaddingLen),
     PacketLen = 1 + PaddingLen + size(Data),
     PacketData = <<?UINT32(PacketLen),?BYTE(PaddingLen), 
diff --git a/lib/ssh/test/ssh_basic_SUITE.erl b/lib/ssh/test/ssh_basic_SUITE.erl
index f30e86f193..6dfff945ac 100644
--- a/lib/ssh/test/ssh_basic_SUITE.erl
+++ b/lib/ssh/test/ssh_basic_SUITE.erl
@@ -466,6 +466,7 @@ rekey_limit(Config) ->
     DataFile = filename:join(UserDir, "rekey.data"),
 
     {Pid, Host, Port} = ssh_test_lib:daemon([{system_dir, SystemDir},
+					     {max_random_length_padding, 0},
     					     {user_dir, UserDir},
     					     {user_passwords,
     					      [{"simon", "says"}]}]),
@@ -475,6 +476,7 @@ rekey_limit(Config) ->
     					    {user, "simon"},
     					    {password, "says"},
     					    {rekey_limit, 2500},
+					    {max_random_length_padding, 0},
     					    {user_interaction, false},
     					    {silently_accept_hosts, true}]),