The top and bottom halves of this image look rather similar.  The reason: the message is encoded in the pixels which are either in the top or bottom half, but not both.  Overlapping the two halves using an XOR filter (or by hand :) allows us to read "ENJOY 'PLEXUS PROBLEMS DAILY!"

filtered image on imageshack

Incidently, this is the same encoding method proposed to allow paper receipts for electronic voting.  There were great articles in November (I believe) Scientific American and Popular Science.

Edit: Actually the proposal was to use the simpler AND filter, but the principle is the same.  A voter would recieve a receipt with one half of the coded message, and the other half would be posted online.  The voter could then combine the public receipt with his own to verify his or her vote, but neither receipt would contain enough information on its own for a third party to determine a person's vote.

Edited on January 17, 2005, 3:45 am

Edited on January 17, 2005, 3:45 am

Posted by Jay Schamel on 2005-01-16 02:09:11