International Association
for Cryptologic Research

End-to-End (E2E) encrypted messaging, which prevents even the service provider from learning communication contents, is gaining popularity. Since users care about maintaining access to their data even if their devices are lost or broken or just replaced, these systems are often paired with cloud backup solutions: typically, the user will encrypt its messages with a fixed key, and upload the ciphertexts to the server. Unfortunately, this naive solution has many drawbacks. First, it often undermines the fancy security guarantees of the core application, such as forward-secrecy (FS) and post-compromise security (PCS), in case the single backup key is compromised. Second, they are wasteful for backing up conversations in large groups, where many users are interested in backing up the same sequence of messages. Instead, we formalize a new primitive called Compact Key Storage (CKS) as the "right" solution to this problem. Such CKS scheme allows a mutable set of parties to delegate to a server storage of an increasing set of keys, while each client maintains only a small state. Clients update their state as they learn new keys (maintaining PCS), or whenever they want to forget keys (achieving FS), often without the need to interact with the server. Moreover, access to the keys (or some subset of them) can be efficiently delegated to new group members, who all efficiently share the same server's storage. We carefully define syntax, correctness, privacy, and integrity of CKS schemes, and build two efficient schemes provably satisfying these notions. Our line scheme covers the most basic "all-or-nothing" flavor of CKS, where one wishes to compactly store and delegate the entire history of past secrets. Thus, new users enjoy the efficiency and compactness properties of the CKS only after being granted access to the entire history of keys. In contrast, our interval scheme is only slightly less efficient but allows for finer-grained access, delegation, and deletion of past keys.

In May 2020, Zoom Video Communications, Inc. (Zoom) announced a multi-step plan to comprehensively support end-to-end encrypted (E2EE) group video calls and subsequently rolled out basic E2EE support to customers in October 2020. In this work we provide the first formal security analysis of Zoom's E2EE protocol, and also lay foundation to the general problem of E2EE group video communication. We observe that the vast security literature analyzing asynchronous messaging does not translate well to synchronous video calls. Namely, while strong forms of forward secrecy and post compromise security are less important for (typically short-lived) video calls, various liveness properties become crucial. For example, mandating that participants quickly learn of updates to the meeting roster and key, media streams being displayed are recent, and banned participants promptly lose any access to the meeting. Our main results are as follows: 1. Propose a new notion of leader-based continuous group key agreement with liveness, which accurately captures the E2EE properties specific to the synchronous communication scenario. 2. Prove security of the core of Zoom's E2EE meetings protocol in the above well-defined model. 3. Propose ways to strengthen Zoom's liveness properties by simple modifications to the original protocol, which have since been deployed in production.

Recently, the area of Key Transparency (KT) has received a lot of attention, as it allows the service provider to provide auditable and verifiable proofs regarding authenticity of public keys used by various participants. Moreover, it is highly preferable to do it in a privacy-preserving ways, so that users and auditors do not learn anything beyond what is necessary to keep the service provider accountable. Abstractly, the problem of building such systems reduces to constructing so called append-only Zero-Knowledge Sets (aZKS). Unfortunately, none of the previous aZKS constructions adequately addressed the problem of key rotation, which would provide Post-Compromise Security (PCS) in case the server in compromised. In this work we address this concern, and refine an extension of aZKS called Rotatable ZKS (RZKS). In addition to addressing the PCS concern, our notion of RZKS has several other attractive features, such as stronger soundness notion (called extractability), and the ability for a stale communication party to quickly catch up with the current epoch, while ensuring the the server did not erase any of the past data. Of independent interest, we also introduce and build a new primitive called Rotatable Verifiable Random Function (VRF), and show how to build RZKS in a modular fashion from rotatable VRF, ordered accumulators and append-only vector commitment schemes.