Compact Privacy Protocols from Post-quantum and Timed Classical Assumptions.

While basic lattice-based primitives like encryption and digital signature schemes are already fairly short, more advanced privacy-preserving protocols (e.g. group signatures) that are believed to be post-quantum secure have outputs of at least several hundred kilobytes. In this paper, we propose a framework for building privacy protocols with significantly smaller parameter sizes whose secrecy is based on post-quantum assumptions, but soundness additionally assumes that some classical assumption, e.g., the discrete logarithm problem (DLP), is hard to break within a short amount of time. The main ingredients of our constructions are statistical zero-knowledge proofs of knowledge for certain relations, whose soundness rely on the hardness of solving the discrete logarithm problem for a fresh DLP instance per proof. This notion has recently been described by the term quantum annoyance. Using such proofs, while also enforcing that they be completed in a fixed amount of time, we then show how to construct privacy-preserving primitives such as (dynamic) group signatures and DAA schemes, where soundness is based on the hardness of the "timed" discrete logarithm problem and SIS. The outputs of our schemes are significantly shorter (approximate to 30X) than purely lattice-based schemes.