Merge branch 'verify-handoff-count' into 'master'

Verify the handoff count on handoff-receive

See merge request spritely/goblins!40

goblins/ocapn/captp.rkt

@@ -25,6 +25,7 @@
          (submod "../core.rkt" for-captp)
          "../message.rkt"
          "../vat.rkt"
+         "../actor-lib/cell.rkt"
          "../actor-lib/methods.rkt"
          "../actor-lib/swappable.rkt"
          "../actor-lib/common.rkt"
@@ -937,10 +938,8 @@
   ;; made in this session to prevent replay attacks.
   ;; every time a *request* is made, this should be incremented.
   (define our-handoff-count 0)
-  ;; TODO TODO TODO: We need to make use of this and also check the
-  ;;   session listed on the receive certificate to prevent a replay
-  ;;   attack
-  (define remote-handoff-count 0)
+  (define remote-handoff-count
+    (spawn ^cell 0))
 
   (define handoff-pubkey (pk-key->public-only-key handoff-privkey))
 
@@ -1135,7 +1134,7 @@
                                              ;; with the gifter, but with the receiver)
                                              (? bytes? _handoff-session)
                                              (? bytes? _handoff-session-side)
-                                             (? integer? _this-handoff-count)
+                                             (? integer? this-handoff-count)
                                              signed-handoff-give))
                                        (? gcrypt-signature? receive-sig))
         signed-handoff-receive)
@@ -1149,9 +1148,18 @@
       (define give-recipient-key
         (datum->pk-key give-recipient-encoded-key 'rkt-public))
 
-      (and (give-handoff-legit? signed-handoff-give)
-           (pk-verify give-recipient-key encoded-handoff-receive
-                      (gcrypt->racket/signature receive-sig))))
+      (define valid-handoff?
+        (and (give-handoff-legit? signed-handoff-give)
+             (>= this-handoff-count ($ remote-handoff-count))
+             (pk-verify give-recipient-key encoded-handoff-receive
+                        (gcrypt->racket/signature receive-sig))))
+
+      ;; If it is in fact a valid handoff, lets increment the count so
+      ;; it can't be replayed.
+      (when valid-handoff?
+        ($ remote-handoff-count (+ this-handoff-count 1)))
+
+      valid-handoff?)
 
     (methods
      [(get-remote-side-name) remote-side-name]