I'm managing a client that distributes their users images and files through one source. My background is heavily in development as opposed to security, and I'm of course familiar with some tricks that tickle the development side (MD5, etc) but nothing serious or in good practices. Hence, question:
What is the best method of tagging images for a group of users so that if one user leaks the information we can tell who leaked it originally (a unique user tag of some sort generated when someone downloads)?
Manual distribution is not an option. Further, our client's service is based on the honor system, so a secure viewer is not an option either. We want to retroactively find leaks-- preventing them entirely is impossible without user inconvenience in this case, anyway.
Thanks in advance!