Secret Mail Problem

Puzzle:

A wants to send a secret message to his friend B in the mail.
But C (A’s Friend), who A don’t trust, has access to all A’s mail. So A put his message in a box with a lock. But A is not allowed to send a key!

How can A send his message through securely?

Puzzle Solution:

Send the box with the lock to B.

B can’t open it, but can put another lock on the box.

B sends this box with the 2 locks back to A, A unlock his lock and send it back to B again.

So there is just B’s lock on the box and B can now open it.

One Thought on “Secret Mail Problem

  1. Brian on May 10, 2016 at 6:26 pm said:

    I came up with an alternative solution, A could hide the key somewhere that B would know (they’re friends, so they know each other well), A could send a message to B with a hint as to where the key is, that only B would know. e.g. A send to B, “remember that time I got a flat tire? I hid the key under a rock there”…

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