If you use a supermarket affinity card, there is no limit to what that business can do with the information about your purchases. Robert Rivera says that when he sued Vons markets they threatened to use information about his alcohol purchases against him in court. If you have any kind of health or life insurance, you will almost certainly have given permission to the insurer to see just about any piece of medical information. This is not merely a cautionary book - Garfinkel has thoughts about what should be done, and his prescriptions aren't what you might expect from one of the digerati. Paradoxical as it might sound, he thinks on the whole that the best protector of your personal information is not encryption but government regulation.
Cryptography guarantees the confidentiality of the transmission.
But if a prosecutor subpoenas your purchases from the website at the other end of the transmission and then publicizes the names of the books you've purchased, your privacy has still been violated - even though the data itself was safely encrypted while in transit. He suggests establishing a federal government agency for privacy. Some privacy activists scoff at the idea of using government to assure our privacy. Legal approaches work because the US government usually follows its own laws. He goes on to briefly mention the very different privacy environment in Europe - and this is one of the book's weakest points, its lack of historical or geographical perspective.