You want to send a secret message to your friend by mail but another untrustworthy friend has access to your mail. You can put the message in a locked box but cannot send the key. To securely send the message, you must find a way to let your friend know the key without directly sending it through the mail.

1. Secret mail puzzle

2. Puzzle Question:
You (A) want to send a secret message to u
r friend (B) by mail. But u r another friend
(C) who u don’t trust him, has all access to
your mail. So u can put u r message in a
box with a lock. Only those who know the
key can open it. But u r not allowed to send
the key. How can u send the message
through securely?