Software to find a digital photo?

Hello all,

I am about to write a program that will allow a user to scan (or
otherwise provide) a picture, and then search through a directory
structure of archived digital photos, and to attempt to find the
picture in its original form. Imagine a photographer who prints pics
for customers, and keeps all the images on a backup drive in case the
customer wants another copy. Customer later requests the image (or
more prints), and photographer has to go in and (from the approximate
date when the photos were taken) find the original image. Been there?

Does anyone know of software available that will do this? Basically
that can recognize a photo that is at least reasonably close to an
original?

I'm assuming the customer-provided image is altered in some way -
different resolution, possibly cropped, possibly scanned from a print,
possibly altered in brightness or contrast or color, etc. It should be
possible to write recognition software that will search based on the
layout of colors and patterns of light and dark, etc, and find the
closest matches.

Any comments welcomed. If there is software out there already then I
might not attempt this.

Thanks!

Dean