ssl,dtls: fix message hash for extended_master_secret

The handshake_messages for extended master secret start at Client Hello
and end at ClientKeyExchange. Previously all handshake messages in a
record would be hashed, possibly covering those after CKE. Now it will
hash each handshake message separately.

For DTLS, the CH and HVR messages must also be ignored. Additionally,
DTLS needs special treatment to merge multiple fragments.

Bug: 13704
Change-Id: I0c8f19b4c352c0195848ded319b6991d76542720
Reviewed-on: https://code.wireshark.org/review/21625
Reviewed-by: Peter Wu <peter@lekensteyn.nl>
Petri-Dish: Peter Wu <peter@lekensteyn.nl>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Alexis La Goutte <alexis.lagoutte@gmail.com>

1 files changed, 36 insertions, 2 deletions

diff --git a/epan/dissectors/packet-dtls.c b/epan/dissectors/packet-dtls.c
index f9e2539ebd..63caaa7b8c 100644
--- a/epan/dissectors/packet-dtls.c
+++ b/epan/dissectors/packet-dtls.c
@@ -825,8 +825,6 @@ dissect_dtls_record(tvbuff_t *tvb, packet_info *pinfo,
     }
   case SSL_ID_HANDSHAKE:
     {
-      ssl_calculate_handshake_hash(ssl, tvb, offset, record_length);
-
       /* try to retrieve and use decrypted handshake record, if any. */
       if (decrypted) {
         dissect_dtls_handshake(decrypted, pinfo, dtls_record_tree, 0,
@@ -1055,6 +1053,7 @@ dissect_dtls_handshake(tvbuff_t *tvb, packet_info *pinfo,
       tvbuff_t      *new_tvb  = NULL;
       const gchar   *frag_str = NULL;
       gboolean       fragmented;
+      guint32        hs_offset = offset;
 
       /* add a subtree for the handshake protocol */
       ti = proto_tree_add_item(tree, hf_dtls_handshake_protocol, tvb, offset, -1, ENC_NA);
@@ -1246,6 +1245,29 @@ dissect_dtls_handshake(tvbuff_t *tvb, packet_info *pinfo,
           sub_tvb = tvb_new_subset_length(tvb, offset, fragment_length);
         }
 
+        /*
+         * Add handshake message (including type, length, etc.) to hash (for
+         * Extended Master Secret). The computation must however happen as if
+         * the message was sent in a single fragment (RFC 6347, section 4.2.6).
+         */
+        if (fragment_offset == 0) {
+          /* Unfragmented packet. */
+          ssl_calculate_handshake_hash(ssl, tvb, hs_offset, 12 + fragment_length);
+        } else {
+          /*
+           * Handshake message was fragmented over multiple messages, fake a
+           * single fragment and add reassembled data.
+           */
+          /* msg_type (1), length (3), message_seq (2) */
+          ssl_calculate_handshake_hash(ssl, tvb, hs_offset, 6);
+          /* fragment_offset (3) equals to zero. */
+          ssl_calculate_handshake_hash(ssl, NULL, 0, 3);
+          /* fragment_length (3) equals to length. */
+          ssl_calculate_handshake_hash(ssl, tvb, hs_offset + 1, 3);
+          /* actual handshake data */
+          ssl_calculate_handshake_hash(ssl, sub_tvb, 0, length);
+        }
+
         /* now dissect the handshake message, if necessary */
         switch ((HandshakeType) msg_type) {
           case SSL_HND_HELLO_REQUEST:
@@ -1268,6 +1290,18 @@ dissect_dtls_handshake(tvbuff_t *tvb, packet_info *pinfo,
             break;
 
           case SSL_HND_HELLO_VERIFY_REQUEST:
+            /*
+             * The initial ClientHello and HelloVerifyRequest are not included
+             * in the calculation of the handshake_messages
+             * (https://tools.ietf.org/html/rfc6347#page-18). This is also
+             * important for correct calculation of Extended Master Secret.
+             */
+            if (ssl && ssl->handshake_data.data_len) {
+              ssl_debug_printf("%s erasing previous handshake_messages: %d\n", G_STRFUNC, ssl->handshake_data.data_len);
+              wmem_free(wmem_file_scope(), ssl->handshake_data.data);
+              ssl->handshake_data.data = NULL;
+              ssl->handshake_data.data_len = 0;
+            }
             dissect_dtls_hnd_hello_verify_request(&dissect_dtls_hf, sub_tvb, pinfo,
                                                   ssl_hand_tree, 0, length);
             break;