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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