Freedom to Tinker via Boing Boing:
Andrew Appel has some fascinating analysis of the "guilt by association" algorithm that AT&T uses to help the FBI figure out whose life to ruin with baseless accusations of terrorist involvement. Other phone companies like Verizon refused to help out with these fishing expeditions, but AT&T jumped right in. The thing is, after three hops, your social network encompasses half the planet, including many of its terrorists (or even "terrorists").
What is the communities of interest” technology? It’s spelled out very clearly in a 2001 research paper from AT&T itself, entitled 'Communities of Interest' (by C. Cortes, D. Pregibon, and C. Volinsky). They use high-tech data-mining algorithms to scan through the huge daily logs of every call made on the AT&T network; then they use sophisticated algorithms to analyze the connections between phone numbers: who is talking to whom? The paper literally uses the term “Guilt by Association” to describe what they’re looking for: what phone numbers are in contact with other numbers that are in contact with the bad guys.
Hell, I feel safer. And it's not like they're doing background checks when you buy an assault rifle at a gun show.