Zen and the Art of Breaking Security - Part IIuddenly start to exchange images or, for this matter, even PGP emails (easily identifiable by searching for the -----BEGIN PGP MESSAGE----- header), this may be flagged as "interesting, requires further investigation."
We do not have to limit ourselves to email. Traffic analysis has been used in military intelligence for a long time. By observing the paths of supply trucks or triangulating radio transmitters, the relocation of the military bases can be inferred. Cellular companies use it to detect fraud by flagging unusual call patterns. Banks analyze electronic payments to detect buying patterns. Intrusion detection systems do it to signal unusual packets.
Of course, there will be a lot of false positives. By itself, traffic analysis cannot tell us more, but that's not its job. Traffic analysis is a statistical tool and thus requires large amounts of data. It also requires the computing and logistical means to gather it, including access to points where traffic is concentrated.
The balance of power has also changed: it is no longer the monitoring system against the indiscriminate traffic flow, but the system vs. an entity. And the more information is available, the more expanded in scope traffic analysis becomes, and at some point the term "surveillance" would be better suited. But this is where the scope of the article ends. Perhaps with the ding of a bell.
 Ross Anderson, Markus Kuhn, Tamper Resistance - a Cautionary Note
 D. Boneh, C. Dunworth, and R. Lipton, Breaking DES using a molecular computer
 Leonard M. Adleman, Paul W. K. Rothemund, Sam Roweisy, Erik Winfree, On Applying Molecular Computation To The Data Encryption Standard
 Leonard M. Adleman, Molecular computation of solutions to combinatorial problems
 Fabien A.P. Petitcolas, Ross J. Anderson, Markus G. Kuhn, Attacks on Copyright Marking Systems
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