International Association
for Cryptologic Research

It is well known that a trusted setup allows one to solve the Byzantine agreement problem in the presence of t < n/2 corruptions, bypassing the setup-free t < n/3 barrier. Alas, the overwhelming majority of protocols in the literature have the caveat that their security crucially hinges on the security of the cryptography and setup, to the point where if the cryptography is broken, even a single corrupted party can violate the security of the protocol. Thus these protocols provide higher corruption resilience (n/2 instead of n/3) for the price of increased assumptions. Is this trade-off necessary? We further the study of _crypto-agnostic_ Byzantine agreement among n parties that answers this question in the negative. Specifically, let t_s and t_i denote two parameters such that (1) 2t_i + t_s < n, and (2) t_i <= t_s < n/2. Crypto-agnostic Byzantine agreement ensures agreement among honest parties if (1) the adversary is computationally bounded and corrupts up to t_s parties, or (2) the adversary is computationally unbounded and corrupts up to t_i parties, and is moreover given all secrets of all parties established during the setup. We propose a compiler that transforms any pair of resilience-optimal Byzantine agreement protocols in the authenticated and information-theoretic setting into one that is crypto-agnostic. Our compiler has several attractive qualities, including using only O(lambda n^2) bits over the two underlying Byzantine agreement protocols, and preserving round and communication complexity in the authenticated setting. In particular, our results improve the state-of-the-art bit complexity by at least two factors of n and provide either early stopping (deterministic) or expected constant round complexity (randomized). We therefore provide fallback security for authenticated Byzantine agreement _for free_ for t_i <= n/4.

Ratcheted key exchange captures the heart of modern secure messaging, wherein protocol participants continuously update their secret material to protect against full state exposure through forward security (protecting past secrets and messages) and post-compromise security (recovering from compromise). However, many practical attacks only provide the adversary with partial information about the secret state of a given party, an attack vector that has been extensively studied under the umbrella of leakage resilience. Existing models of ratcheted key exchange or messaging therefore provide less-than-optimal guarantees under partial leakage due to inherent limitations in security under full state exposure that are exacerbated by relaxations in security made by many practical protocols for performance reasons. In this work, we initiate the study of leakage-resilient ratcheted key exchange that provides typical guarantees under full state exposure and additional guarantees under partial state exposure between ratchets of the protocol. We consider unidirectional ratcheted key exchange (URKE) where one party acts as the sender and the other as receiver. Starting from the notions of Balli et al. introduced at ASIACRYPT 2020, we formalise a key indistinguishability game under randomness manipulation and bounded leakage (KIND), which in particular enables the adversary to continually leak a bounded amount of the sender's state between honest send calls. We construct a corresponding protocol from a key-updatable key encapsulation mechanism (kuKEM) and a leakage-resilient one-time MAC. By instantiating this MAC in the random oracle model (ROM), results from Balli et al. imply that in the ROM, kuKEM and KIND-secure URKE are equally powerful, i.e., can be built from each other. As a second step, given the strong limitations that key indistinguishability imposes on the adversary, we formalise a one-wayness game that also permits leakage on the receiver. We then propose a corresponding construction from leakage-resilient kuKEM, which we introduce, and a leakage-resilient one-time MAC. Furthermore, we show that leakage-resilient kuKEM and one-way-secure URKE can be built from each other in the ROM, highlighting the increased cost that strong one-way security entails. Our work opens exciting directions for developing practical, leakage-resilient messaging protocols.

Continuous Group Key Agreement (CGKA) is a primitive with which members of a group can continuously establish shared keys. With every interaction, these members also update their individual, local secrets such that temporary corruptions of these secrets only affect the security of shared keys established shortly before (Forward Security; FS) and after the corruption (Post-Compromise Security; PCS). Due to these interactive updates–possibly enriched by dynamic group membership changes–, CGKA is a very powerful but also very complex primitive. In this work, we limit the power of CGKA to identify and analyze its core components. More concretely, we consider the case that all members of a group are always either senders or receivers. Thus, the interaction is strictly unidirectional from the former to the latter: a group of senders Alice establishes shared keys with a group of receivers Bob. With every shared key, Alice updates her local state to achieve FS and PCS; when receiving an established key, each Bob also updates their local state to achieve FS. This notion naturally lifts the so called Unidirectional Ratcheted Key Exchange concept (Bellare et al., Crypto 2017; Poettering and Rösler, Crypto 2018) to the group setting and, thereby, captures and generalizes Signal's Sender Key Mechanism, which is the core of WhatsApp and Signal's group chat protocols. We modularize this concept of Group Unidirectional RKE (GURKE) by considering either single or multiple senders, single or multiple receivers, and static or dynamic membership on each of both sides of the group. To instantiate these new primitives, we develop a building block called Updatable Broadcast KEM (UB-KEM). Using UB-KEM, our GURKE constructions for static groups only use standard Key Encapsulation Mechanisms (KEMs) and induce only a constant communication overhead. Our GURKE constructions for dynamic groups are based on general Non-Interactive Key Exchange (NIKE) and offer a constant communication overhead as long as the set of members is unchanged; only for adding and removing users, a communication overhead logarithmic in the group size is induced. We discuss the benefits of replacing the Sender Key Mechanism in Signal and WhatsApp with our constructions, and demonstrate their practicality with a performance evaluation of our proof of concept UB-KEM implementation.

The widely used Signal protocol provides protection against state exposure attacks through forward security (protecting past messages) and post-compromise security (for restoring security). It supports immediate decryption, allowing messages to be re-ordered or dropped at the protocol level without affecting correctness. In this work, we consider strong active attack detection for secure messaging with immediate decryption, where parties are able to immediately detect active attacks under certain conditions. We first consider in-band active attack detection, where participants who have been actively compromised but are still able to send a single message to their partner can detect the compromise. We propose two complementary notions to capture security, and present a compiler that provides security with respect to both notions. Our notions generalise existing work (RECOVER security) which only supported in-order messaging. We also study the related out-of-band attack detection problem by considering communication over out-of-band, authenticated channels and propose analogous security notions. We prove that one of our two notions in each setting imposes a linear communication overhead in the number of sent messages and security parameter using an information-theoretic argument. This implies that each message must information-theoretically contain all previous messages and that our construction, that essentially attaches the entire message history to every new message, is asymptotically optimal. We then explore ways to bypass this lower bound and highlight the feasibility of practical active attack detection compatible with immediate decryption.

Distributed key generation (DKG) protocols are an essential building block for threshold cryptosystems. Many DKG protocols tolerate up to t_s<n/2 corruptions assuming a well-behaved synchronous network, but become insecure as soon as the network delay becomes unstable. On the other hand, solutions in the asynchronous model operate under arbitrary network conditions, but only tolerate t_a<n/3 corruptions, even when the network is well-behaved. In this work, we ask whether one can design a protocol that achieves security guarantees in either scenario. We show a complete characterization of _network-agnostic_ DKG protocols, showing that the tight bound is t_a + 2t_s < n. As a second contribution, we provide an optimized version of the network-agnostic MPC protocol by Blum, Liu-Zhang and Loss [CRYPTO'20] which improves over the communication complexity of their protocol by a linear factor. Moreover, using our DKG protocol, we can instantiate our MPC protocol in the _plain PKI model_, i.e., without the need to assume an expensive trusted setup. Our protocols incur comparable communication complexity as state-of-the-art DKG and MPC protocols with optimal resilience in their respective purely synchronous and asynchronous settings, thereby showing that network-agnostic security comes (almost) _for free_.

Developing end-to-end encrypted instant messaging solutions for group conversations is an ongoing challenge that has garnered significant attention from practitioners and the cryptographic community alike. Notably, industry-leading messaging apps such as WhatsApp and Signal Messenger have adopted the Sender Keys protocol, where each group member shares their own symmetric encryption key with others. Despite its widespread adoption, Sender Keys has never been formally modelled in the cryptographic literature, raising the following natural question: What can be proven about the security of the Sender Keys protocol, and how can we practically mitigate its shortcomings? In addressing this question, we first introduce a novel security model to suit protocols like Sender Keys, deviating from conventional group key agreement-based abstractions. Our framework allows for a natural integration of two-party messaging within group messaging sessions that may be of independent interest. Leveraging this framework, we conduct the first formal analysis of the Sender Keys protocol, and prove it satisfies a weak notion of security. Towards improving security, we propose a series of efficient modifications to Sender Keys without imposing significant performance overhead. We combine these refinements into a new protocol that we call Sender Keys+, which may be of interest both in theory and practice.

This work discuss real world deniability in messaging. We highlight how the different models for cryptographic deniability do not ensure practical deniability. To overcome this situation, we propose a model for real world deniability that takes into account the entire messaging system. We then discuss how deniability is (not) used in practice and the challenges arising from the design of a deniable system. We propose a simple, yet powerful solution for deniability: applications should enable direct modification of local messages; we discuss the impacts of this strong deniability property.