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In the meantime, here’s a simple logic puzzle about the mail, which I found on the wonderful puzzle site MindCipher:
Alice and Bob live very far apart. Alice wants to exchange mail via the postal service with Bob, but they both know their mail carrier is corrupt and opens their letters.
Alice has a box that can be secured with one or more padlocks, and the mail carrier cannot open the box when it is locked. Alice and Bob each have a keyed padlock that can secure the box, but Alice’s key cannot open Bob’s padlock, and Bob’s key cannot open Alice’s padlock.
How can Alice send a letter to Bob without it being read by the mail carrier?
[su_spoiler title=”Solution 1″]Alice puts her letter in the box, locks the box with her padlock, and mails the box to Bob. Bob puts his padlock on the box and mails the box with both locks on it back to Alice. Alice unlocks her padlock and mails the box back to Bob. Bob unlocks his padlock, opens the box, and reads the letter. [/su_spoiler]
[su_spoiler title=”Solution 2″]Bob sends his unlocked padlock to Alice. Alice puts the letter and her unlocked padlock in the box, locks the box with Bob’s padlock, and sends the box to Bob. Bob unlocks his padlock, opens the box, and reads the letter.
He can now send a reply to Alice by putting his letter and his open padlock in the box, then locking the box with Alice’s padlock.[/su_spoiler]