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vault1317/signal-dakez: An authenticated key exchange protocol with a public key concealing and a participation deniability designed for secure messaging.

Richard B. Riddick

Published Jan 1, 2020• 0 citations
OpenAlex

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Abstract A deniable authenticated key exchange (DAKE) protocol establishes a secure channel without producing cryptographic evidence of communication. A DAKE offersstrong deniabilityif transcripts provide no evidence even if long-term key material is compromised (offline deniability) and no outsider can obtain evidence even when interactively colluding with an insider (online deniability). Unfortunately, existing strongly deniable DAKEs have not been adopted by secure messaging tools due to security and deployability weaknesses. In this work, we propose three new strongly deniable key exchange protocols—DAKEZ, ZDH, and XZDH—that are designed to be used in modern secure messaging applications while eliminating the weaknesses of previous approaches. DAKEZ offers strong deniability in synchronous network environments, while ZDH and XZDH can be used to construct asynchronous secure messaging systems with offline and partial online deniability. DAKEZ and XZDH provide forward secrecy against active adversaries, and all three protocols can provide forward secrecy against future quantum adversaries while remaining classically secure if attacks against quantum-resistant cryptosystems are found. We seek to reduce barriers to adoption by describing our protocols from a practitioner’s perspective, including complete algebraic specifications, cryptographic primitive recommendations, and prototype implementations. We evaluate concrete instantiations of our DAKEs and show that they are the most efficient strongly deniable schemes; with all of our classical security guarantees, our exchanges require only 1 ms of CPU time on a typical desktop computer and at most 464 bytes of data transmission. Our constructions are nearly as efficient as key exchanges with weaker deniability, such as the ones used by the popular OTR and Signal protocols.
There is proposed a new method for deniable encryption based on commutative transformations. The method has been used to design the deniable encryption protocol resistant to the passive coercive attacks, which uses no pre-shared secret keys and no pre-exchanged public keys. The protocol begins with the stage at which the sender and receiver exchange their single-use public keys and compute the single-use shared secret key. Then, it is performed pseudo-probabilistic three-pass protocol with simultaneous commutative encryption of the fake and secret messages. Resistance of the proposed protocol to coercive attacks is provided by its computational indistinguishability from probabilistic no-key three-pass protocol used to send securely the fake message. To perform commutative encryption, it used exponentiation cipher. To provide security against active coercer, the protocol is to be complemented with procedure for authenticating the sent messages.
Key-exchange, in particular Diffie-Hellman key-exchange (DHKE), is among the core cryptographic mechanisms for ensuring network security. For key-exchange over the Internet, both security and privacy are desired. In this paper, we develop a family of privacy-preserving authenticated DHKE protocols named deniable Internet key-exchange (DIKE), both in the traditional PKI setting and in the identity-based setting. The newly developed DIKE protocols are of conceptual clarity and practical (online) efficiency. They provide useful privacy protection to both protocol participants, and add novelty and new value to the IKE standard. To the best of our knowledge, our protocols are the first provably secure DHKE protocols that additionally enjoy all the following privacy protection advantages: 1) forward deniability, actually concurrent non-malleable statistical zero-knowledge, for both protocol participants simultaneously; 2) the session transcript and session-key can be generated merely from DH-exponents (together with some public values), which thus cannot be traced to the pair of protocol participants; and 3) exchanged messages do not bear peer's identity, and do not explicitly bear player role information.