encryption for the layman

i received a newsletter today from grand illusions. they are one of the places i get all my geek toys from, as well as thinkgeek. at the end of the newsletter, it had a "puzzle". i recognized the puzzle immediately as it is a real-life example of how public- and private-key encryption works (that stuff your web browser uses all the time so you aren't thieved of your credit card info). typically, the story involves Alice (A) trying to contact Bob (B) without Eve (Eavesdropper) finding out the message. this time they used john and mary and left eve to be understood as the general populous. the first answer is an example of private-key encryption, the second is public-key.

John and Mary have fallen in love, via the internet, and John wishes to send Mary a ring. Unfortunately they live in a country where anything sent through the mail will be stolen unless it is enclosed in a padlocked box.

John and Mary each have plenty of padlocks, but none to which the other has a key. How can John get the ring safely into Mary's hands?

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1. John puts the ring in a box, padlocks the box, and sends it in the post to Mary. Mary attaches a padlock of her own, so there are now 2 padlocks on the box, and sends the box back to John. John uses his key to remove the original padlock, and sends the box back to Mary. Mary uses her key, removes the remaining padlock, opens the box and retrieves the ring.

2. John sends Mary an empty box that has been locked with a padlock. Attached to the hasp of the padlock is a key. John then sends a second box to Mary; this box can be opened using the key that was attached to box 1. Inside box 2 is the ring.

see, computers aren't hard. i thought that was cool.