  The Internet and its current information culture of preserving all kinds of
data cause severe problems with privacy. Most of today's Internet users,
especially teenagers, publish various kinds of sensitive information, yet
without recognizing that revealing this information might be detrimental to
their future life and career. Unflattering images that can be openly accessed
now and in the future, e.g., by potential employers, constitute a particularly
important such privacy concern. We have developed a novel, fast, and scalable
system called X-pire! that allows users to set an expiration date for images in
social networks (e.g., Facebook and Flickr) and on static websites, without
requiring any form of additional interaction with these web pages. Once the
expiration date is reached, the images become unavailable. Moreover, the
publishing user can dynamically prolong or shorten the expiration dates of his
images later, and even enforce instantaneous expiration. Rendering the approach
possible for social networks crucially required us to develop a novel technique
for embedding encrypted information within JPEG files in a way that survives
JPEG compression, even for highly optimized implementations of JPEG
post-processing with their various idiosyncrasies as commonly used in such
networks. We have implemented our system and conducted performance measurements
to demonstrate its robustness and efficiency.
