Mail a ring safely when you can't share keys

Question

Alice wants to mail a valuable ring to Bob. The postal service is crooked: any parcel that arrives unlocked is opened and the contents stolen. A locked parcel is safe, a thief can't force it, but Alice and Bob each own padlocks that only their own key opens, and they have never shared keys. Mailing a key won't help either, since a key travels in an unlocked-to-the-thief way and would be copied. They agree a plan in advance. How does Alice get the ring to Bob so that it is never in transit unlocked?

Accepted Answer

The clever part is that the box can carry two padlocks at once on the same hasp, so ownership of the lock can change hands without any key ever traveling. Three trips: 1. Alice → Bob. Alice puts the ring in the box, snaps her own padlock on it, and mails it. In transit it is locked (with Alice's lock), so the thief can't open it. Bob receives it but cannot open it, he has no key for Alice's lock. 2. Bob → Alice. Bob adds his own padlock alongside Alice's (the box now has two locks) and mails it back. Still locked the whole way. 3. Alice → Bob. Alice removes her padlock with her own key, leavi…

Mail a ring safely when you can't share keys

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