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During school I studied some cryptography and creating a story about it, I was able to create this riddle! I hope you like it <3

In one country all the inhabitants are thieves.

Items cannot be transported on the street without being stolen immediately.

The only way, for example, to send something without being stolen is to put it in a safe locked with a padlock, which is the only thing that is not stolen in that country (while open safes and locks are stolen).

At birth, therefore, each inhabitant receives a safe and a padlock of which he has the only copy of the key. The key is not transferable and cannot be taken out to the house, as it would be immediately stolen.

Furthermore, it is not possible in any way to make a copy of the keys.

Given these conditions, how can a resident of this country send a birthday present to a friend, preventing from being stolen?

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I put the gift in the box, lock it with my lock and send it. The other one put his own lock on it then send it back. I remove my lock and send it back again to him. He can remove his lock and have the gift

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  1. Send your box with gift inside locked with your padlock to the recepient.
  2. He will send the same back locked with yours as well as his padlock.
  3. Remove your padlock and send it back with only his padlock.
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