'A' has a lock and its key. 'B' has a and its key. A needs to send a letter to B in a Box. How do they do it if they don't want to share their keys with each other?

Ques:- 'A' has a lock and its key. 'B' has a and its key. A needs to send a letter to B in a Box. How do they do it if they don't want to share their keys with each other?

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