An Efficient Password-Only Two-Server Authenticated Key Exchange System
One of the prominent advantages of password-only two-server authenticated key exchange is that the user password will remain secure against offline dictionary attacks even after one of the servers has been compromised. The first system of this type was pr
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Department of Computer Science City University of Hong Kong Hong Kong, China [email protected] 2 Department of Computer Science University of Science and Technology of China China [email protected], [email protected]
Abstract. One of the prominent advantages of password-only two-server authenticated key exchange is that the user password will remain secure against offline dictionary attacks even after one of the servers has been compromised. The first system of this type was proposed by Yang, Deng and Bao in 2006. The system is efficient with a total of eight communication rounds in one protocol run. However, the security assumptions are strong. It assumes that one particular server cannot be compromised by an active adversary. It also assumes that there exists a secure communication channel between the two servers. Recently, a new protocol has been proposed by the same group of researchers. The new one removes these assumptions, but in return pays a very high price on the communication overhead. It takes altogether ten rounds to complete one protocol run and requires more computation. Therefore, the question remains is whether it is possible to build a protocol which can significantly reduce the number of communication rounds without introducing additional security assumptions or computational complexity. In this paper, we give an affirmative answer by proposing a very efficient protocol with no additional assumption introduced. The protocol requires only six communication rounds without increasing the computational complexity.
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Introduction
Password-only authenticated key exchange is a scheme which allows a user who holds only a low-entropy password to conduct authentication and key exchange with a server. Comparing with related types of authenticated key exchange schemes, for example, schemes based on cryptographic keys, password-only authenticated key exchange is very practical with high usability, because users only need to memorize a short password which is already used commonly in existing authentication systems.
The work was supported by CityU grants (Project Nos. 7001844, 7001959, 7002001).
S. Qing, H. Imai, and G. Wang (Eds.): ICICS 2007, LNCS 4861, pp. 44–56, 2007. c Springer-Verlag Berlin Heidelberg 2007
An Efficient PTAKE System
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A Password-only Two-Server Authenticated Key Exchange (PTAKE) scheme [18,17] is an extension of the conventional single-server setting [13]. Besides a user and a server, PTAKE also has an additional server. The existing, front-end, server that the user is communicating with is called the Service Server, SS, and the additional, back-end, server which communicates only with SS is called the Control Server, CS. In a conventional single-server scheme, the server has a database of the users’ passwords or some verification data of the passwords. If the server is compromised, an adversary can obtain the passwords of all the users directly from the database, or be able to launch offline dictionary attacks against all users’ passwords as the database provides enough information
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