Paper 2018/114

Offline Assisted Group Key Exchange

Colin Boyd and Gareth T. Davies and Kristian Gjøsteen and Yao Jiang

Abstract

We design a group key exchange protocol where most of the participants remain offline until they wish to compute the key. This is well suited to a cloud storage environment where users are often offline, but have online access to the server which can assist in key exchange. We define and instantiate a new primitive, a blinded KEM, which we show can be used in a natural way as part of our generic protocol construction. Our new protocol has a security proof based on a well-known model for group key exchange. Our protocol provides a restricted form of forward secrecy which we argue is as strong as can be achieved in practice. Our protocol is efficient, requiring Diffie--Hellman with a handful of standard public key operations per user in our concrete instantiation.

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