extended master secret support: only resume an extended_ms session (o…

…therwise, do full handshake), always send extended_ms in clienthello, use in serverhello if sent by client

lib/handshake_client.ml

@@ -28,15 +28,15 @@ let default_client_hello config =
     | TLS_1_2           -> cs
   and sessionid =
     match config.session_cache None with
-    | None -> None
-    | Some { session_id ; _ } -> Some session_id
+    | Some { session_id ; extended_ms ; _ } when extended_ms = true -> Some session_id
+    | _ -> None
   in
   let ch = {
     version      = Supported version ;
     random       = Rng.generate 32 ;
     sessionid    = sessionid ;
     ciphersuites = List.map Ciphersuite.ciphersuite_to_any_ciphersuite ciphers ;
-    extensions   = host @ signature_algos
+    extensions   = host @ signature_algos @ [ExtendedMasterSecret]
   }
   in
   (ch , version)
@@ -64,7 +64,9 @@ let answer_server_hello state ch (sh : server_hello) raw log =
   validate_reneg (get_secure_renegotiation sh.extensions) >|= fun () ->
 
   let epoch_matches (epoch : epoch_data) =
-    epoch.ciphersuite = sh.ciphersuites && epoch.protocol_version = sh.version
+    epoch.ciphersuite = sh.ciphersuites &&
+    epoch.protocol_version = sh.version &&
+    epoch.extended_ms = List.mem ExtendedMasterSecret sh.extensions
   in
 
   match state.config.session_cache sh.sessionid with
@@ -85,12 +87,17 @@ let answer_server_hello state ch (sh : server_hello) raw log =
     let machina =
       let cipher = sh.ciphersuites in
       let session_id = match sh.sessionid with None -> Cstruct.create 0 | Some x -> x in
+      let extended_ms =
+        let ems = List.mem ExtendedMasterSecret in
+        ems ch.extensions && ems sh.extensions
+      in
       let session = { empty_session with
                       client_random    = ch.random ;
                       client_version   = ch.version ;
                       server_random    = sh.random ;
                       ciphersuite      = cipher ;
                       session_id ;
+                      extended_ms ;
                     }
       in
       Ciphersuite.(match ciphersuite_kex cipher with

lib/handshake_server.ml

@@ -173,6 +173,10 @@ let server_hello session version reneg =
   and secren = match reneg with
     | None            -> SecureRenegotiation (Cstruct.create 0)
     | Some (cvd, svd) -> SecureRenegotiation (cvd <+> svd)
+  and ems = if session.extended_ms then
+      [ExtendedMasterSecret]
+    else
+      []
   and session_id =
     match Cstruct.len session.session_id with
     | 0 -> Rng.generate 32
@@ -183,7 +187,7 @@ let server_hello session version reneg =
         random       = server_random ;
         sessionid    = Some session_id ;
         ciphersuites = session.ciphersuite ;
-        extensions   = secren :: host }
+        extensions   = secren :: host @ ems }
   in
   (* Tracing.sexpf ~tag:"handshake-out" ~f:sexp_of_tls_handshake sh ; *)
   (Writer.assemble_handshake sh,
@@ -208,6 +212,8 @@ let answer_client_hello_common state reneg ch raw =
         | Some _ -> fail (`Error (`NoConfiguredCiphersuite cciphers))
         | None -> fail (`Fatal (`NoCiphersuite ch.ciphersuites)) ) >|= fun cipher ->
 
+    let extended_ms = List.mem ExtendedMasterSecret ch.extensions in
+
     (* Tracing.sexpf ~tag:"cipher" ~f:Ciphersuite.sexp_of_ciphersuite cipher ; *)
 
     { empty_session with
@@ -216,7 +222,8 @@ let answer_client_hello_common state reneg ch raw =
       ciphersuite      = cipher ;
       own_certificate  = chain ;
       own_private_key  = priv ;
-      own_name         = host }
+      own_name         = host ;
+      extended_ms      = extended_ms }
 
   and server_cert session =
     match session.own_certificate with
@@ -314,14 +321,16 @@ let answer_client_hello state (ch : client_hello) raw =
 
 
   and resume ch state =
-    let epoch_matches (epoch : Core.epoch_data) version ciphers =
+    let epoch_matches (epoch : Core.epoch_data) version ciphers extensions =
       let cciphers = filter_map ~f:Ciphersuite.any_ciphersuite_to_ciphersuite ciphers in
       List.mem epoch.ciphersuite cciphers &&
-      version = epoch.protocol_version
+      version = epoch.protocol_version &&
+      List.mem ExtendedMasterSecret extensions = epoch.extended_ms &&
+      epoch.extended_ms
     in
 
     match state.config.session_cache ch.sessionid with
-    | Some epoch when epoch_matches epoch state.protocol_version ch.ciphersuites ->
+    | Some epoch when epoch_matches epoch state.protocol_version ch.ciphersuites ch.extensions ->
       Some { session_of_epoch epoch with
              client_random = ch.random ;
              client_version = ch.version ;