Forward-Security under Continual Leakage

Current signature and encryption schemes secure against continual leakage fail completely if the key in any time period is fully exposed. We suggest forward security as a second line of defense, so that in the event of full exposure of the current secret key, at least uses of keys prior to this remain secure, a big benefit in practice. (For example if the signer is a certificate authority, full exposure of the current secret key would not invalidate certificates signed under prior keys.) We provide definitions for signatures and encryption that are forward-secure under continual leakage. Achieving these definitions turns out to be challenging, and we make initial progress with some constructions and transforms

Optimal Channel Security Against Fine-Grained State Compromise: The Safety of Messaging

We aim to understand the best possible security of a (bidirectional) cryptographic channel against an adversary that may arbitrarily and repeatedly learn the secret state of either communicating party. We give a formal security definition and a proven-secure construction. This construction provides better security against state compromise than the Signal Double Ratchet Algorithm or any other known channel construction. To facilitate this we define and construct new forms of public-key encryption and digital signatures that update their keys over time

Secure Messaging: From Systems to Theory

The standard view of cryptography is that secure systems should be built by implementing known primitives whose theoretical security guarantees are well understood. In this work we take the opposite approach, taking inspiration from existing systems and providing the theoretical basis with which to understand their security goals. Our particular inspirations are modern secure messaging apps (e.g. Signal, WhatsApp) which have deployed new techniques with the goal of maintaining some security against attackers which sometimes gain temporary access to honest users’ devices.We propose that these security goals should be studied in a modular manner where distinct cryptographic components are studied in isolation. Towards this we separately provide formal models for understanding the initial exchange of cryptographic secrets and their later use for the exchange of messages in this setting. We provide provable secure constructions of these separate components (often achieving better security than what is currently deployed by messaging apps) and a composition result which generically proves security when these isolated components are used together