Deniable encryption, authentication, and key exchange

We present some foundational ideas related to deniable encryption, message authentication, and key exchange in classical cryptography. We give detailed proofs of results that were previously only sketched in the literature. In some cases, we reach the same conclusions as in previous papers; in other cases, the focus on rigorous proofs leads us to different formulations of the results

Real World Deniability in Messaging

This work discusses 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

Online Deniability for Multiparty Protocols with Applications to Externally Anonymous Authentication

In the problem of anonymous authentication (Boneh et al. CCS 1999), a sender wishes to authenticate a message to a given recipient in a way that preserves anonymity: the recipient does not know the identity of the sender and only is assured that the sender belongs to some authorized set. Although solutions for the problem exist (for example, by using ring signatures, e.g. Naor, Crypto 2002), they provide no security when the anonymity set is a singleton. This work is motivated by the question of whether there is any type of anonymity possible in this scenario. It turns out that we can still protect the identity of all senders (authorized or not) if we shift our concern from preventing the identity information be revealed to the recipient to preventing it could be revealed to an external entity, other than the recipient. We define a natural functionality which provides such guarantees and we denote it by F_{eaa} for externally anonymous authenticated channel. We argue that any realization of F_{eaa} must be deniable in the sense of Dodis et al. TCC 2009. To prove the deniability of similar primitives, previous work defined ad hoc notions of deniability for each task, and then each notion was showed equivalent to realizing the primitive in the Generalized Universal Composability framework (GUC, Canetti et al. TCC 2007). Instead, we put forward the question of whether deniability can be defined independently from any particular task. We answer this question in the affirmative providing a natural extension of the definition of Dodis et al. for arbitrary multiparty protocols. Furthermore, we show that a protocol satisfies this definition if an only if it realizes the ideal functionality F_{eaa} in the GUC framework. This result enables us to prove that most GUC functionalities we are aware of (and their realizations) are deniable. We conclude by applying our results to the construction of a deniable protocol that realizes F_{eaa}

Can the government compel decryption? Don't trust -- verify

If a court knows that a respondent knows the password to a device, can the court compel the respondent to enter that password into the device? In this work, we propose a new approach to the foregone conclusion doctrine from Fisher v US that governs the answer to this question. The Holy Grail of this line of work would be a framework for reasoning about whether the testimony implicit in any action is already known to the government. In this paper we attempt something narrower. We introduce a framework for specifying actions for which all implicit testimony is, constructively, a foregone conclusion. Our approach is centered around placing the burden of proof on the government to demonstrate that it is not "rely[ing] on the truthtelling" of the respondent. Building on original legal analysis and using precise computer science formalisms, we propose demonstrability as a new central concept for describing compelled acts. We additionally provide a language for whether a compelled action meaningfully entails the respondent to perform in a manner that is 'as good as' the government's desired goal. Then, we apply our definitions to analyze the compellability of several cryptographic primitives including decryption, multifactor authentication, commitment schemes, and hash functions. In particular, our framework reaches a novel conclusion about compelled decryption in the setting that the encryption scheme is deniable: the government can compel but the respondent is free to use any password of her choice.CNS-1915763 - National Science Foundation; HR00112020021 - Department of Defense/DARPA; CNS-1718135 - National Science Foundation; CNS-1801564 - National Science Foundation; CNS-1931714 - National Science FoundationAccepted manuscrip