Communications - July 2011

the TPM. The number of private call minutes available to the platform can be controlled through software in the platform, with the software certified by remote attestation. If need be, the private call minutes can be prepaid.

The potential for considering the
private mode as a prepaid service may
have a significant advantage with re-
spect to CALEA, as CALEA does not
currently cover prepaid cellular tele-
phones. In the U.S. and many other
countries, one may buy and use a pre-
paid cellular telephone without associ-
ating one’s name with the phone.w The
proposed privacy overlay would thus
provide postpaid cellular telephone
users with the privacy benefits of pre-
paid cellular.x
Other problems remain to be ad-
dressed, of course. For example,
Cortes, Pregibon, and Volinsky have
shown that it is possible to identify
fraudulent users of a cellular system
by using call data to construct dy-
namic graphs, and then performing
a comparative analysis of subgraphs
that form “communities of interest.” 7
A similar comparative analysis can be
used for deanonymizing users of the
proposed system unless the random
tag is changed fairly frequently.

Conclusion

We have seen that cellular telephony is a surveillance technology. Cellular networks were designed, however unintentionally, to collect personal data, thus creating an extremely attractive source of information for law enforcement agencies and marketers. The impact of this surveillance on the users and uses of the cellular platform is becoming increasingly important as the platform plays a prominent role in so- w According to the UPI, many of the cell phones used to coordinate action in the Philippine uprisings against former President Estrada were unregistered, prepaid phones. See http://www. upiasia.com/Politics/2008/01/21/texting_as_ an_activist_tool/6075/.

cial, economic, and political contexts. It remains possible, however, to secure cellular networks against surveillance. The private cellular overlay proposed here would serve this purpose while potentially putting the subscriber in control of his or her personal information. Legal issues remain and legislation may be necessary before a private cellular system can be made available to the public, but a public discussion as to whether we want a technology as important as cellular to be open to covert surveillance would be a good and highly democratic idea.

Acknowledgments This work was funded in part by the National Science Foundation TRUST Science and Technology Center and the NSF Trustworthy Computing Program. The author gratefully acknowledges the technical and editorial assistance of Sarah Hale, Lee Humphries, and Jeff Pool. He also extends thanks to the anonymous reviewers for their extensive and insightful comments.

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