The Relationship Between Password-Authenticated Key Exchange and Other Cryptographic Primitives

We consider the problem of password-authenticated key exchange (PAK) also known as session-key generation using passwords: constructing session-key generation protocols that are secure against active adversaries (person-in-the-middle) and only require the legitimate parties to share a low-entropy password (e.g. coming from a dictionary of size poly(

n

)).

We study the relationship between PAK and other cryptographic primitives. The main result of this paper is that password-authenticated key exchange and public-key encryption are

incomparable

under black-box reductions. In addition, we strengthen previous results by Halevi and Krawczyk [14] and Boyarsky [5] and show how to build key agreement and semi-honest oblivious transfer from any PAK protocol that is secure for the Goldreich-Lindell (GL) definition [11].

We highlight the difference between two existing definitions of PAK, namely the indistinguishability-based definition of Bellare, Pointcheval and Rogaway (BPR) [1] and the simulation-based definition of Goldreich and Lindell [11] by showing that there exists a PAK protocol that is secure for the BPR definition and only assumes the existence of one-way functions in the case of exponential-sized dictionaries. Hence, unlike the GL definition, the BPR definition does not imply semi-honest oblivious transfer for exponental-sized dictionaries under black-box reductions.