From Ephemerizer to Timed-Ephemerizer: Achieve Assured Lifecycle Enforcement for Sensitive Data

The concept of Ephemerizer, proposed by Perlman, is a cryptographic primitive for assured data deletion. With an Ephemerizer protocol, data in persistent storage devices will always be encrypted simultaneously using an ephemeral public key of the Ephemerizer (an entity which will publish a set of ephemeral public keys and periodically delete the expired ones) and the long-term public key of a user. An Ephemerizer protocol enables the user to securely decrypt the encrypted data without leaking any information to the Ephemerizer. So far, no security model has ever been proposed for this primitive and existing protocols have not been studied formally. Not surprisingly, we show that some existing Ephemerizer protocols possess security vulnerabilities. In this paper, we introduce the notion of Timed-Ephemerizer, which can be regarded as a hybrid primitive by combining Ephemerizer and Timed-Release Encryption. Compared with an Ephemerizer protocol, a Timed-Ephemerizer protocol further guarantees that data will only be released after a pre-defined disclosure time. Moreover, we propose a security model for Timed-Ephemerizer and formalize relevant security properties. We also propose a new Timed-Ephemerizer protocol and prove its security in the security model.