On key agreement and conference key agreement

Chih-Hung Li, Josef Pieprzyk

Colin Boyd, Juan Manuel González Nieto

Federico Grasselli

Yuh‐Min Tseng

Recently, Tzeng proposed a provably secure and fault-tolerant conference-key agreement protocol. It requires only a constant number of rounds to establish a conference key among all honest participants. This article will show that Tzeng’s protocol does not offer forward secrecy. We say that a conference-key agreement protocol offers forward secrecy if the long-term secret key of any participant is compromised and will not result in the compromise of the previously established conference keys. This property is important and has been included in most key agreement protocols and standards. In this paper, an improvement based on Tzeng’s protocol is proposed and it achieves forward secrecy. Under the Diffie–Hellman decision problem assumption and the random oracle model, we show that the proposed protocol can withstand passive attacks and is secure against impersonator’s attacks. The improved protocol requires a constant number of rounds to compute a conference key. The improved protocol provides fault-tolerance.

Colin Boyd, Anish Mathuria

Xun Yi

Lots of conference key agreement protocols have been suggested to secure computer network conference. Most of them operate only when all conferees are honest, but do not work when some conferees are malicious and attempt to delay or destruct the conference. Recently, Tzeng proposed a conference key agreement protocol with fault tolerance in terms that a common secret conference key among honest conferees can be established even if malicious conferees exist. In the case where a conferee can broadcast different messages in different subnetworks, Tzeng's protocol is vulnerable to a "different key attack" from malicious conferees. In addition, Tzeng's protocol requires each conferee to broadcast to the rest of the group and receive n - 1 message in a single round (where n stands for the number of conferees). Moreover, it has to handle n simultaneous broadcasts in one round. In this paper, we propose a fault-tolerant conference key agreement protocol, in which each conferee only needs to send one message to a "semitrusted" conference bridge and receive one broadcast message. Our protocol is an identity-based key agreement, built on elliptic curve cryptography. It is resistant to the different key attack from malicious conferees and needs less communication cost than Tzeng's protocol.