Hello,

I'm trying to find the name of a concept used in secure communication. Here's how it works:

1. The sender puts a message in a box and locks it with their own lock.
2. The box is sent to the recipient, who can't open it because it's locked with the sender's lock.
3. The recipient adds their own lock to the box and sends it back to the sender.
4. The sender receives the box with two locks (their own and the recipient's lock), removes their own lock, and sends the box back to the recipient.
5. The recipient now receives the box with only their own lock, which they can open to access the message.

This analogy is used to explain how to securely send a message without sharing keys directly. Does anyone know what this concept is called?