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IEEE Transactions on Computers

A secure fault-tolerant conference-key agreement protocol

Wen-Guey Tzeng

Published Apr 1, 2002• 90 citations
Publisher

Abstract

When a group of people want to communicate securely over an open network, they run a conference-key protocol to establish a common conference key K such that all their communications thereafter are encrypted with the key K. In this paper, we propose a provably secure fault-tolerant conference-key agreement protocol under the authenticated broadcast channel model. We show that a passive adversary gets zero knowledge about the conference key established by the honest participants under the assumption of a variant Diffie-Hellman (1976) decision problem. We also show that the honest participants can agree on a common conference key no matter how many participants are malicious. Furthermore, we show that even if the broadcast channel is not authenticated, our protocol is secure against impersonators under the random oracle model.

DOI
10.1109/12.995447

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When a group of people want to communicate securely over an open network, they run a conference-key protocol to establish a common conference key K such that all their communications thereafter are encrypted with the key K. In this paper, we propose a provably secure fault-tolerant conference-key agreement protocol under the authenticated broadcast channel model. We show that a passive adversary gets zero knowledge about the conference key established by the honest participants under the assumption of a variant Diffie-Hellman (1976) decision problem. We also show that the honest participants can agree on a common conference key no matter how many participants are malicious. Furthermore, we show that even if the broadcast channel is not authenticated, our protocol is secure against impersonators under the random oracle model.
An original approach to establish a computationally secure and robust conference key between multiple users is presented, which is built on known secret sharing schemes and requires the authenticated and encrypted point-to-point channels between servers and users. By running of the protocol, every honest user of a given conference can get a common key, even if a minority of the servers malfunction or misbehave. This scheme does not rely on any unproven cryptographic assumptions or on the availability of any tamper-proof hardware. By using zero knowledge proof, any corrupted information and incorrect results can be detected. And by distributing the sensitive security information across several servers and never reconstructing any key at a single location, the compromise of a few servers will not compromise the privacy of any key. Analysis shows that under the assumption of a Diffie-Hellman decisional problem, a passive adversary gets zero knowledge about the conference key, and in the random oracle model, an active adversary cannot impersonate successfully. We have implemented the scheme in a distributed environment. By conducting a number of experiments in the fault-free case and various fault scenarios, we show that it has an acceptable performance of practicability.
This paper proposes a computationally secure and fault-tolerant conference key distribution scheme, which only requires the authenticated and encrypted point-to-point channels between each server and each user. By the combined use of knowledge proof and verifiable secret sharing, the scheme has the properties of simple structure and high security. Analysis shows that every honest user of a conference can get a common key after the running of the protocol, even if a minority of the servers malfunction or misbehave. We also show that on the assumption of a Diffie-Hellman decisional problem, a passive adversary gets zero knowledge about the conference key, and the active adversary cannot impersonate someone successfully. Because the knowledge proof method we adopt is non-interactive, both operation and communication overheads are reduced, thus making the schemes very efficient.
By its very nature, a non-authenticated multi-party key agreement protocol cannot provide participant and message authentication, so it must rely on an authenticated network channel. This paper presents the inability of two famous multi-party key agreement protocols to withstand malicious participant attacks, even though their protocols are based on the authenticated network channel. This attack involves a malicious participant disrupting the multi-party key agreement among honest participants. In this case, other honest participants do not correctly agree on a common key. Obviously, the malicious participant cannot obtain the common key either, and the communication confidentiality among participants is not breached. However, in some emergency situations or applications, a multi-party key agreement protocol design that is resistant to malicious participants is useful. Therefore, in this paper, a non-authenticated multi-party key agreement protocol resistant to malicious participants is proposed. The proposed robust protocol requires constant rounds to establish a common key. Each participant broadcasts a constant number of messages. Under the assumption of the Decision Diffie–Hellman problem and the random oracle model, we will show that the proposed protocol is provably secure against passive adversaries and malicious participants.
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.
A computationally secure and robust conference key distribution scheme is presented, which requires only the authenticated and encrypted point-to-point channels between each server and each user. By using it, every honest user of a conference can get a common key after running of the protocol, even if some minority of the servers malfunction or misbehave. Analysis shows that a passive adversary gets zero knowledge about the conference key, and the active adversary cannot impersonate successfully. Because this scheme is noninteractive, both operation and communication overhead are reduced, this make it very efficient.
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.