Evaluating the privacy properties of telephone metadata
aSecurity Laboratory, Department of Computer Science, Stanford University, Stanford, CA 94305;
bStanford Law School, Stanford University, Stanford, CA 94305
Edited by Cynthia Dwork, Microsoft Research Silicon Valley, Mountain View, CA, and approved March 1, 2016 (received for review April 27, 2015)
Privacy protections against government surveillance are often scoped to communications content and exclude communications metadata. In the United States, the National Security Agency operated a particularly controversial program, collecting bulk telephone metadata nationwide. We investigate the privacy properties of telephone metadata to assess the impact of policies that distinguish between content and metadata. We find that telephone metadata is densely interconnected, can trivially be reidentified, enables automated location and relationship inferences, and can be used to determine highly sensitive traits.
Since 2013, a stream of disclosures has prompted reconsideration of surveillance law and policy. One of the most controversial principles, both in the United States and abroad, is that communications metadata receives substantially less protection than communications content. Several nations currently collect telephone metadata in bulk, including on their own citizens. In this paper, we attempt to shed light on the privacy properties of telephone metadata. Using a crowdsourcing methodology, we demonstrate that telephone metadata is densely interconnected, can trivially be reidentified, and can be used to draw sensitive inferences.
Communications privacy law, in the United States and many other nations, draws a distinction between “content” and “metadata” ( 1 ). The former category reflects the substance of an electronic communication; the latter includes all other information about the communication, such as parties, time, and duration ( 2 ). *
When a government agency compels disclosure of content, the agency must usually comply with extensive substantive and procedural safeguards. Demands for metadata, by contrast, are often left to the near-total discretion of authorities. In the United States, for instance, a law enforcement officer can request telephone calling records with merely a subpoena—essentially a formal letter from the investigating agency ( 3 ). An intelligence program by the National Security Agency (NSA) has drawn particular criticism; under the business records provision of the USA PATRIOT Act ( 4 ), the agency acquired a substantial share of all domestic telephone metadata ( 5 ). †
In this paper, we empirically investigate factual assumptions that undergird policies of differential treatment for content and metadata. Using crowdsourced telephone logs and social networking information, we find that telephone metadata is densely interconnected, susceptible to reidentification, and enables highly sensitive inferences. ‡
The balance of the paper is organized into three parts. First, we discuss our data collection methodology and properties of our participant population. We next present our results. Finally, we discuss implications for policy and future quantitative social science research. Additional methodological detail and figures are available in the Supporting Information .
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