Confidential mail puzzle
X wants to send a Confidential message to his friend Y in the mail.
But Z (X’s Friend), who X don’t trust, has access to all X’s mail. So X put his message in a box with a lock. But X is not allowed to send a key!
How can X send his message through securely?
Assuming that the box has provision to support at least two locks at a time AND that delay in receiving the letter is not a matter of concern:
Use the ‘double lock’ method. ‘X’ should lock the box and send it to Y (X keeps the key with himself). Y, in turn, should add another lock on the box and send it back (key remains with Y). On receipt of the box, X should open and remove his lock and send it back to Y (with the other lock intact.) Once the box is received, Y can open the box and retrieve the letter by opening his own lock.
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