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A study by two Stanford graduate students has demonstrated the sheer amount of personal information that can be gleaned from phone call metadata. The students, Jonathan Mayer and Patrick Mutchler, were able to able to determine their subjects’ medical ailments, whether or not one owned a firearm, and that one subject probably planned to grow some pot. They didn’t need to listen in on anyone’s phone calls — they simply needed to know who their subjects were calling and how to access public records.

Mayer and Mutchler write:

At the outset of this study, we shared the same hypothesis as our computer science colleagues—we thought phone metadata could be very sensitive. We did not anticipate finding much evidence one way or the other, however, since the MetaPhone participant population is small and participants only provide a few months of phone activity on average.

We were wrong. We found that phone metadata is unambiguously sensitive, even in a small population and over a short time window.

Mayer and Mutchler learned intimate details about their subjects’ lives. They learned that one subject regularly called a “hotline for a pharmaceutical used solely to treat relapsing multiple sclerosis.” They learned that another ” placed a series of calls to the local Planned Parenthood location.”

The study is another indictment of the United States government’s claims that gathering phone call metadata from millions of Americans is not an invasion of privacy. It doesn’t matter that no-one is listening to those phone calls. Unless the second most well-funded intelligence agency is less capable than two graduate students — which is doubtful, given its ability to surveil millions of people — metadata can be used to infer all kinds of information.

But anyone following the NSA revelations and their ensuing debates already knew that. The American Civil Liberties Union previously published a declaration from Princeton professor Edward Felten, who wrote that “many details of our lives can be gleaned by examining those [metadata] trails, which often yield information more easily than do the actual content of our communications.” The Guardian explained the information shared as metadata when people use their smartphones, search the Web, or access Facebook. The idea that anyone can say that the NSA is “just” collecting metadata with a straight face was proven ludicrous a while ago.

[Image courtesy Ed Yourdon]