Secure memory deallocation (the eager clearing of deallocated memory) and secure file deletion do not completely solve the problem because they do not address the issue of a user’s data remaining in long-lived shared servers (including the OS) on that user’s machine. Even when applications such as Web browsers explicitly support “private” or “incognito” mode, intended to leave no evidence of users’ activities on the host machine, they fail to achieve their objective because traces are kept by system components outside the application’s control. Traces of users’ activities remain in application and OS memory, file systems (through both direct and indirect channels such as OS swap), device drivers, memories of peripheral devices, etc. None of the above are possible in modern computers. You, the reader, may wish to browse a medical, adult, or some other sensitive website without your machine keeping a record of the visit. A biomedical researcher may want to read a patient’s file or run a data-mining computation on a database of clinical histories and then erase all traces of the sensitive data from his computer. A political dissident may want to upload text and photos to a social media site, watch a forbidden video, or have a voice-over-IP conversation without leaving incriminating evidence on her laptop. Computers keep memories of users’ activities-whether users want it or not.

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