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:: Message and Locks ::

Two people live far apart and need to secretly communicate via
exchanged letter messages. The messages can only be sent in a
padlocked box. Anything sent without it being in a padlocked box
will be kept by the postal service and confiscated. (So, you
cannot put the message into a padlocked box and send the key
together with, or separately from, the box). A padlock can only
have one key which the person placing the lock owns (no duplicate
keys allowed). How can you send messages without the postal
service reading the messages and keeping it.

Hint: You can use more than one lock on the box.

Answer: Trick to this is multiple locking. One person writes the message and puts it into the box locking it with their own lock (which only they have the keys to) and sends it to their friend. The friend puts a second lock on the box and sends it back. The first lock is remove and sent again to the friend. The friend now has the keys to their own lock and opens the box to read the message.

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