3PaaS: Privacy-Preserving Post-Compromise Security as a Service

Post-Compromise Security [20] (PCS) expresses that even after a party has been compromised, it may be able to recover (or "heal") security afterwards. PCS has been extensively studied, and many end-to-end secure messaging applications include mechanisms, such as the double ratchet, to achieve it. However, current approaches only work partially, and their healing effects are extremely limited. For example, recent work [27] showed that due to real-world constraints such as resilience against state loss, even highly secure messengers such as Signal may not achieve it in practice. Moreover, since healing is session-specific, healing effects do not carry over to newly spawned sessions, different groups [24] , or different services that use the same identity. In this work, we tackle these issues by designing the first protocol that can provide PCS as a Service for identities through a third party. The major challenge is privacy: achieving PCS requires regular updates among participants, and involving a third party can lead to significant privacy concerns. Moreover, the type of update that PCS requires (updating a user's secret, but only once) seems to require servers to verify the users' identities in a way that contradicts the unlinkability required for privacy: the server should not learn anything about the users' activities. We develop the 3PaaS protocol, including the first efficient zero-knowledge proofs for blind signatures, to achieve our goals, and even allow for revocation, without revealing the identity to the server. We formally analyze our protocol for high assurance, provide an implementation of our novel ZK building blocks, and show how our protocol could be used with a messaging application.