Abstract. We extend the definitional work of Dwork, Naor and Sahai from deniable authentication to deniable key-exchange protocols. We then use these definitions to prove the deniability features of SKEME and SIGMA, two natural and efficient protocols which serve as basis for the Internet Key Exchange (IKE) protocol. The two protocols require distinct approaches to their deniability analysis, hence highlighting important definitional issues as well as necessitating different tools in the analysis. SKEME is an encryption-based protocol for which we prove full deniability based on the plaintext awareness of the underlying encryption scheme. Interestingly SKEME’s deniability is possibly the first “natural ” application which essentially requires plaintext awareness (until now this notion has been mainly used as a tool for proving chosen-ciphertext security); in particular this use of plaintext awareness is not tied to the random oracle model. SIGMA, on the other hand, uses non-repudiable signatures for authentication and hence cannot be proven to be fully deniable. Yet we are able to prove a weaker, but meaningful, “partial deniability ” property: a party may not be able to deny that it was “alive ” at some point in time but can fully deny the contents of its communications and the identity of its interlocutors. We remark that the deniability of SKEME and SIGMA holds in a concurrent setting and does not essentially rely on the random oracle model.

Jan 1, 20062 citations

Improved Strongly Deniable Authenticated Key Exchanges for Secure Messaging

Nik Unger, Ian Goldberg

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.

Proceedings on Privacy Enhancing TechnologiesJan 1, 201838 citations