The Strongbox

A wealthy, anonymous eccentric has offered to purchase the fabulous Icosahedron Diamond, a prize from one of your recent adventures. To preserve his identity, you and he have been communicating by a series of notes left in one of the public strongboxes in the Puzzlanian Library. These boxes are free for Puzzlanians to use for temporary storage. If you want security, you must provide your own lock.

The boxes, made of unobtanium, are nearly indestructible and cannot be removed. The eccentric has proposed that you purchase a lock and chain, and seal the diamond in the strongbox for him to later retrieve.

After you purchased a suitable lock and chain, a flaw with this plan occurred to you: The purchaser will have no way of removing the lock without the key. If you leave the key in the box first, a cunning thief might copy the key and return it, only to open the box himself before the eccentric or his henchmen arrive.

Can you find a way for the wealthy collector's plan to deliver the diamond safely?