You don't always need to share a key to send something securely. Sometimes it is enough to use each other's locks in the correct order.
Trunks and padlocks without shared key
Riddle statement
You and another person are far away and can only send each other a trunk by courier.
Each of you has your own lock and key. Neither can open the other's lock.
How can you send the contents of the trunk safely, without sharing keys and without the trunk ever traveling open?
Show solution
Solution
Answer: It can be done in three trips of the trunk.
- You put the contents in the trunk, close it with your padlock and send it away.
- The other person adds their own lock to the trunk and hands it back to you—without being able to open it.
- You remove your lock and send it again.
- The other person receives the trunk closed only with its lock, opens it with his key and accesses the contents.
The trunk never travels open: first it is protected by your lock, then both at the same time, finally only that of the other person.
No key is shared. Each one removes only his own lock.