Deniable Protocol Research Articles

Circumventing Cryptographic Deniability with Remote Attestation

Abstract Deniable messaging protocols allow two parties to have ‘off-the-record’ conversations without leaving any record that can convince external verifiers about what either of them said during the conversation. Recent events like the Podesta email dump underscore the importance of deniable messaging to politicians, whistleblowers, dissidents and many others. Consequently, messaging protocols like Signal and OTR are designed with cryptographic mechanisms to ensure deniable communication, irrespective of whether the communications partner is trusted. Many commodity devices today support hardware-assisted remote attestation which can be used to convince a remote verifier of some property locally observed on the device. We show how an adversary can use remote attestation to undetectably generate a non-repudiable transcript from any deniable protocol (including messaging protocols) providing sender authentication, proving to skeptical verifiers what was said. We describe a concrete implementation of the technique using the Signal messaging protocol. We then show how to design protocols that are deniable even against an adversary capable of attestation, and in particular how attestation itself can be used to restore deniability by thwarting realistic classes of adversary.

Fully Deniable Message Authentication Protocols Preserving Confidentiality

Although the objective of secure communication can be achieved by using cryptographic tools, the undeniability that results from cryptographic properties may create a potential threat to the sender of the message. Unfortunately, most existing deniable protocols only provide 1-out-of-2 deniability. When both parties (the sender and the receiver) are allowed to deny generating the message, a dispute might occur between these two parties. The 1-out-of-2 deniable protocol can result in an unfair resolution of the dispute. Therefore, we propose a new model of deniability, called 1-out-of-∞ deniability, that can provide full deniability. The 1-out-of-∞ deniability protocol allows the originator of the message to deny that he or she generated the message, since there are an infinite number of possible message generators; at the same time, all transmitted messages can be protected and authenticated between the sender and the intended receiver. Our design can be implemented by using any public-key cryptography technique. We also analyze the correctness of the proposed protocols based on logical rules, and two practical examples are given to illustrate our design.

A novel deniable authentication protocol using generalized ElGamal signature scheme

A deniable authentication protocol enables a receiver to identify the true source of a given message, but not to prove the identity of the sender to a third party. This property is very useful for providing secure negotiation over the Internet. Consequently, many interactive and non-interactive deniable authentication protocols have been proposed. However, the interactive manner makes deniable protocols inefficient. In addition, a security hole is generated in deniable protocols that use the non-interactive manner if a session secret is compromised. Thus, there is no secure and efficient deniable authentication protocol as of now. In this paper, a new protocol based on the non-interactive manner is proposed to efficiently and securely achieve deniable authentication. This protocol can furthermore replace the underlying signature scheme in order to retain a secure status even if the previously used signature method is broken.