password authenticated key exchange using hidden smooth subgroups
Existing techniques for designing efficient password authenticated key exchange (PAKE) protocols all can be viewed as variations of a small number of fundamental paradigms, and all are based on either the Diffie-Hellman or RSA assumptions. In this paper we propose a new technique for the design of PAKE protocols that does not fall into any of those paradigms, and which is based on a different assumption. In our technique, the server uses the password to construct a multiplicative group with a (hidden) smooth order subgroup, where the group order depends on the password. The client uses its knowledge of the password to generate a root extraction problem instance in the server's group and a discrete logarithm problem instance in the (smooth order) subgroup. If the server constructed its group correctly based on the password, the server can use its knowledge of the group order to solve the root extraction problem, and can solve the discrete logarithm problem by leveraging the smoothness of the hidden subgroup.The resulting scheme is provably secure (in the random oracle model) under the "decision subgroup assumption." The scheme can be efficiently instantiated using composite modulus groups, in which case the client and server each perform the equivalent of a small number of modular exponentiations, and the security reduces to a simple variant of the "?-hiding" assumption. We provide preliminary implementation results of this instantiation.
password authentication key exchange cryptography