A system has an image store, a digital hashing unit, and a watermark
encoder. A digital image hashing unit computes a hash value
representative of a digital image in such a manner that visually similar
images hash to the same hash value and visually distinct images hash to
different values. The hash value is stored in an image hash table and is
associated via the table with the original image. This image hash table
can be used to index the image storage. A watermark encoder computes a
watermark based on the hash value and a secret. Using both values renders
the watermark resistant to BORE (Break Once, Run Everywhere) attacks
because even if the global watermark secret is discovered, an attacker
still needs the hash value of each image to successfully attack the
image. The system can be configured to police the Internet to detect
pirated copies. The system randomly collects images from remote Web sites
and hashes the images using the same hashing function. The system then
compares the image hashes to hashes of the original images. If the hashes
match, the collected image is suspected as being a copy of the original.