One-Round Deniable Key Exchange with Perfect Forward Security

In response to the need for secure one-round authenticated key exchange protocols providing both perfect forward secrecy and full deniability, we put forward a new paradigm for constructing protocols from a Diffie-Hellman type protocol plus a non-interactive designated verifier proof of knowledge (DV-PoK) scheme. We define the notion of DV-PoK which is a variant of non-interactive zero-knowledge proof of knowledge, and provide an efficient DV-PoK scheme as a central technical building block of our protocol. The DV-PoK scheme possesses nice properties such as unforgeability and symmetry which help our protocol to achieve perfect forward secrecy and full deniability respectively. Moreover, the security properties are formally proved in the Canetti-Krawczyk model under the Gap Diffie-Hellman assumption. In sum, our protocol offers a remarkable combination of salient security properties and efficiency, and the notion of DV-PoK is of independent interests

Revisiting Deniability in Quantum Key Exchange via Covert Communication and Entanglement Distillation

We revisit the notion of deniability in quantum key exchange (QKE), a topic that remains largely unexplored. In the only work on this subject by Donald Beaver, it is argued that QKE is not necessarily deniable due to an eavesdropping attack that limits key equivocation. We provide more insight into the nature of this attack and how it extends to other constructions such as QKE obtained from uncloneable encryption. We then adopt the framework for quantum authenticated key exchange, developed by Mosca et al., and extend it to introduce the notion of coercer-deniable QKE, formalized in terms of the indistinguishability of real and fake coercer views. Next, we apply results from a recent work by Arrazola and Scarani on covert quantum communication to establish a connection between covert QKE and deniability. We propose DC-QKE, a simple deniable covert QKE protocol, and prove its deniability via a reduction to the security of covert QKE. Finally, we consider how entanglement distillation can be used to enable information-theoretically deniable protocols for QKE and tasks beyond key exchange.Comment: 16 pages, published in the proceedings of NordSec 201

Authentication and Key Management Automation in Decentralized Secure Email and Messaging via Low-Entropy Secrets

We revisit the problem of entity authentication in decentralized end-to-end encrypted email and secure messaging to propose a practical and self-sustaining cryptographic solution based on password-authenticated key exchange (PAKE). This not only allows users to authenticate each other via shared low-entropy secrets, e.g., memorable words, without a public key infrastructure or a trusted third party, but it also paves the way for automation and a series of cryptographic enhancements; improves security by minimizing the impact of human error and potentially improves usability. First, we study a few vulnerabilities in voice-based out-of-band authentication, in particular a combinatorial attack against lazy users, which we analyze in the context of a secure email solution. Next, we propose solving the problem of secure equality test using PAKE to achieve entity authentication and to establish a shared high-entropy secret key. Our solution lends itself to offline settings, compatible with the inherently asynchronous nature of email and modern messaging systems. The suggested approach enables enhancements in key management such as automated key renewal and future key pair authentications, multi-device synchronization, secure secret storage and retrieval, and the possibility of post-quantum security as well as facilitating forward secrecy and deniability in a primarily symmetric-key setting. We also discuss the use of auditable PAKEs for mitigating a class of online guess and abort attacks in authentication protocols

Deniable Key Establishment Resistance against eKCI Attacks

In extended Key Compromise Impersonation (eKCI) attack against authenticated key establishment (AKE) protocols the adversary impersonates one party, having the long term key and the ephemeral key of the other peer party. Such an attack can be mounted against variety of AKE protocols, including 3-pass HMQV. An intuitive countermeasure, based on BLS (Boneh–Lynn–Shacham) signatures, for strengthening HMQV was proposed in literature. The original HMQV protocol fulfills the deniability property: a party can deny its participation in the protocol execution, as the peer party can create a fake protocol transcript indistinguishable from the real one. Unfortunately, the modified BLS based version of HMQV is not deniable. In this paper we propose a method for converting HMQV (and similar AKE protocols) into a protocol resistant to eKCI attacks but without losing the original deniability property. For that purpose, instead of the undeniable BLS, we use a modification of Schnorr authentication protocol, which is deniable and immune to ephemeral key leakages

DABKE: Secure deniable attribute-based key exchange framework

National Research Foundation (NRF) Singapor

Deniable Key Exchanges for Secure Messaging

Despite our increasing reliance on digital communication, much of our online discourse lacks any security or privacy protections. Almost no email messages sent today provide end-to-end security, despite privacy-enhancing technologies being available for decades. Recent revelations by Edward Snowden of government surveillance have highlighted this disconnect between the importance of our digital communications and the lack of available secure messaging tools. In response to increased public awareness and demand, the market has recently been flooded with new applications claiming to provide security and privacy guarantees. Unfortunately, the urgency with which these tools are being developed and marketed has led to inferior or insecure products, grandiose claims of unobtainable features, and widespread confusion about which schemes can be trusted. Meanwhile, there remains disagreement in the academic community over the definitions and desirability of secure messaging features. This incoherent vision is due in part to the lack of a broad perspective of the literature. One of the most contested properties is deniability—the plausible assertion that a user did not send a message or participate in a conversation. There are several subtly different definitions of deniability in the literature, and no available secure messaging scheme meets all definitions simultaneously. Deniable authenticated key exchanges (DAKEs), the primary cryptographic tool responsible for deniability in a secure messaging scheme, are also often unsuitable for use in emerging applications such as smartphone communications due to unreasonable resource or network requirements. In this thesis, we provide a guide for a practitioner seeking to implement deniable secure messaging systems. We examine dozens of existing secure messaging protocols, both proposed and implemented, and find that they achieve mixed results in terms of security. This systematization of knowledge serves as a resource for understanding the current state-of-the-art approaches. We survey formalizations of deniability in the secure messaging context, as well as the properties of existing DAKEs. We construct several new practical DAKEs with the intention of providing deniability in modern secure messaging environments. Notably, we introduce Spawn, the first non-interactive DAKE that offers forward secrecy and achieves deniability against both offline and online judges; Spawn can be used to improve the deniability properties of the popular TextSecure secure messaging application. We prove the security of our new constructions in the generalized universal composability (GUC) framework. To demonstrate the practicality of our protocols, we develop and compare open-source instantiations that remain secure without random oracles

Hecate: abuse reporting in secure messengers with sealed sender

End-to-end encryption provides strong privacy protections to billions of people, but it also complicates efforts to moderate content that can seriously harm people. To address this concern, Tyagi et al. [CRYPTO 2019] introduced the concept of asymmetric message franking (AMF), which allows people to report abusive content to a moderator, while otherwise retaining end-to-end privacy by default and even compatibility with anonymous communication systems like Signal’s sealed sender. In this work, we provide a new construction for asymmetric message franking called Hecate that is faster, more secure, and introduces additional functionality compared to Tyagi et al. First, our construction uses fewer invocations of standardized crypto primitives and operates in the plain model. Second, on top of AMF’s accountability and deniability requirements, we also add forward and backward secrecy. Third, we combine AMF with source tracing, another approach to content moderation that has previously been considered only in the setting of non-anonymous networks. Source tracing allows for messages to be forwarded, and a report only identifies the original source who created a message. To provide anonymity for senders and forwarders, we introduce a model of "AMF with preprocessing" whereby every client authenticates with the moderator out-of-band to receive a token that they later consume when sending a message anonymously.CNS-1718135 - National Science Foundation; CNS-1801564 - National Science Foundation; OAC-1739000 - National Science Foundation; CNS-1931714 - National Science Foundation; CNS-1915763 - National Science Foundation; HR00112020021 - Department of Defense/DARPA; 000000000000000000000000000000000000000000000000000000037211 - SRI Internationalhttps://www.usenix.org/system/files/sec22-issa.pdfPublished versio