How can person 'A' send his message through securely?

Question

Person ‘A’ wants to send a secret message to his friend ‘B’ through a mail, but 'C' (A’s friend), whom 'A' does not trust has access to all A’s mails. So 'A' puts his message in a box with a lock but 'A' is not allowed to send a key. How can 'A' send his message through securely?

Answer 1

I think it would take a bit of back and forth but I have a solution to this.

A will lock the box and send it to B.
B will lock another lock on to it of which he has the key and sends it back to A.
A will remove his lock and send it back to B.
Now B can read it by unlocking his lock with his key.

This process reverses if B has to send a message to A and vice versa.

Answer 2

Send the box without the lock to B.

B can’t open it, but can put another lock on the box.

B sends this box with the 2 locks back to A, A unlock his lock and send it back to B again.

So there is just B’s lock on the box and B can now open it.