How can you arrange for an image or any other file uploaded to a general website to become inaccessible after a set expiry date?
It seems impossible without modifying how the site works but a team of German researchers has devised an auto-expiry mechanism targeted at photos on social networking sites.
The fact that images once posted to the Internet stay there indefinitely is often perceived as a problem. And it is a problem that Professor Michael Backes of the Information Security and Cryptography department at Saarland University and a Max Planck Fellow of the Max Planck Institute for Software Systems decided to tackle.
He was the initiator of X-Pire, software that gives images an expiration date by tagging them with an encrypted key.
X-Pire creates encrypted copies of images and requires an expiration date (either one month, six months, 1 year or a specific date) when you post them online. To view the encrypted images you need the X-Pire browser add-on (which has a German language interface) is currently is only available under Firefox. When the viewer encounters an encrypted image it sends off a request for a key to unlock it. This key will only be sent, and the image become viewable, if the expiration date has not been passed. Images given an expiration date with X-Pire have been successfully uploaded to Flickr, Facebook and many other websites.
The whole algorithm depends on the simple fact that you can upload an encrypted bitmap as long as it is still formatted correctly. The encryption can be detected and a browser add-on can do the job of decrypting it. This means that even though there is no facility built into the site to delete, let alone time delete an image or file, access to the file can be time limited. A little thought suggests that this is a mechanism that could be generalised to other situations including non-time based restrictions to files posted on public websites.
Although you can download X-Pire for free, using it to upload images costs 2 euros a month. If your subscription lapses images will not suddenly become viewable but you will not be able to put expiration dates on new images.