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    $\begingroup$ What if Kleptomaniac(K) steals the message in step 1? Jane(J) receives the box from Mike(M), but receives no message. Jane does not know to respond with an answer, therefore step 3 onward do not occur. M--> J (Locked box) and M-->K (message). Mike still has the ring and Jane has a useless box. $\endgroup$
    – Matthew0898
    Commented Apr 3, 2016 at 21:26
  • 2
    $\begingroup$ @Matthew0898: your comment gives a way for K to block the exchange from working, but not a way for K to steal the ring. $\endgroup$
    – Peter LeFanu Lumsdaine
    Commented Apr 3, 2016 at 21:37
  • 2
    $\begingroup$ If Klep copies the instructions and replaces the box with a box with Kleps question, then it is not true that M knows J has M's box. Klep replaces M's key and the passphrase from J with the passphase Klep got out of M's box using M's key. $\endgroup$
    – Ole Tange
    Commented Apr 3, 2016 at 22:41
  • 4
    $\begingroup$ I don't see how step 3 would work. If he sends the key and it's not inside a locked box, the key will be stolen..? $\endgroup$
    – user1429080
    Commented Apr 4, 2016 at 7:56
  • 2
    $\begingroup$ Updated steps 1.5 The middleman keeps Marks box and sends Jane a decoy box and the letter 3.5 Mark sends the key but middleman gets it first. 4.The middleman uses it to open the padlocked box that they kept. 6.The middleman sends the passphrase back to Mark. $\endgroup$
    – ponsfonze
    Commented Apr 4, 2016 at 10:23