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 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. © Springer-Verlag Berlin Heidelberg 2007.