Secret Mail Puzzle - Solution

The Puzzle:

Secret Mail
You want to send a secret message to your friend Alex in the mail.

But Sam, who you don't trust, has access to all your mail.

So you put your message in a box with a lock.

But you are not allowed to send a key!

How can you get your message through securely?

Our Solution:

Send the box with the lock to Alex.

Alex can't open it, but can put another lock on the box.

Alex sends this box with the 2 locks back to you, you unlock your lock and send it back to Alex again.

So there is just Alex's lock on the box and Alex can now open it.

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