Offline count-limited certificates

In this paper, we present the idea of offline count-limited certificates (or clics for short), and show how these can be implemented using minimal trusted hardware functionality already widely available today. Offline count-limited certificates are digital certificates that: (1) specify usage conditions that depend on irreversible counters, and (2) are used in a protocol that guarantees that any attempt to use them in violation of these usage conditions will be detected even if the user of the certificate and the verifying party have no contact at all with the outside world at the time of the transaction. Such certificates enable many interesting applications not possible with traditional (unlimited use) certificates, including count-limited delegation and access, offline commerce and trading using cashlike migratable certificates, and others. We show how all these applications can be made possible by using only a simple trusted timestamping device (TTD), which can in turn be implemented using existing trusted hardware devices such as smartcards, and the Trusted Platform Module (TPM) chips embedded in PCs available today. Significantly, our solutions do not require trust in any other components in the host machines aside from the TTD itself; they remain tamper-evident as long as the TTD is not compromised, even if the entire host system, including the BIOS, CPU, OS and memory, is compromised. This not only provides better security by minimizing the required trusted computing base, but also makes implementation possible on present-day machines without requiring a particular kind of OS. We demonstrate all these ideas by implementing a prototype application that runs under both Linux and Windows, and presenting experimental performance results.