This month's IEEE GlobalSpec challenge question is:
Joe and Maura live far from each other and have fallen in love via online dating, and Joe wishes to mail her a ring. Unfortunately, they both live in Pilferland, where all packages will be stolen unless enclosed in a padlocked box. Joe and Maura each have plenty of padlocks, but none to which the other has a key. How can Joe get the ring safely into Maura’s hands?
And the answer is:
This puzzle came from Caroline Calderbank, daughter of mathematicians Ingrid Daubechies and Rob Calderbank. In the solution she had in mind, Joe sends Maura a box with the ring in it and one of his padlocks on it. Maura then affixes her own padlock to box and mails it back with both padlocks on it. When Joe gets it he removes his padlock and sends the box back to Maura.
This idea is fundamental to Diffie-Hellman key exchange, an historic breakthrough in cryptography.
Other solutions are possible as well, depending on starting assumptions. One requires that Joe find a padlock whose key has a large hole, or at least a hole which can be sufficiently enlarged by drilling, so that the key can be hooked onto a second padlock’s hasp.
Joe uses this second padlock, with the aforementioned key hooked on its hasp, to lock a small empty box which he then sends to Maura. When enough time has passed for it to get there (perhaps he awaits an email acknowledgment from Maura) he sends the ring in another box, locked by the first padlock. When Maura gets the ring box, she picks up the whole first box and uses the key affixed to it to access her ring.
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