Illustration by David Plunkert

Your doctor needs access to all the medical data in your file, but you don't want the insurance clerks seeing your blood results; besides, that could violate federal privacy laws. Billions of documents--from banking records to personnel files--present similar concerns, which makes for "a pretty gnarly security problem," says ­Jessica ­Staddon, manager of security research at the Palo Alto Research Center (PARC), the Xerox subsidiary in California.

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