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?