How does a machine know who is using it? Current systems authenticate their users infrequently, and assume the user's identity does not change. Such persistent authentication is inappropriate for mobile and ubiquitous systems, where associations between people and devices are fluid and unpredictable. We solve this problem with Transient Authentication, in which a small hardware token continuously authenticates the user's presence over a short-range, wireless link. We present the four principles

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... underlying Transient Authentication, and describe two techniques for securing applications. Applications can be protected transparently by encrypting in-memory state when the user departs and decrypting this state when the user returns. This technique is effective, requiring just under 10 seconds to protect and restore an entire machine, but indiscriminate. Instead, applications can utilize an API for Transient Authentication, protecting only sensitive state. We describe our ports of three applications-PGP, SSH, and Mozilla-to this API. Mozilla, the most complicated application we have ported, suffers less than 4% overhead in page loads in the worst case, and in typical use can be protected in less than 250 milliseconds.