Paper 2018/114

Offline Assisted Group Key Exchange

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


We design a group key exchange protocol with forward secrecy 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 is efficient, requiring Diffie-Hellman with a handful of standard public key operations per user in our concrete instantiation.

Note: Formatting changes and typo fixes.

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Publication info
Published elsewhere. MINOR revision.ISC 2018
Authenticated Key ExchangeGroup Key ExchangeForward SecrecyCloud StorageBlinded Key Encapsulation
Contact author(s)
yao jiang @ ntnu no
2018-07-03: last of 3 revisions
2018-01-31: received
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Creative Commons Attribution


      author = {Colin Boyd and Gareth T.  Davies and Kristian Gjøsteen and Yao Jiang},
      title = {Offline Assisted Group Key Exchange},
      howpublished = {Cryptology ePrint Archive, Paper 2018/114},
      year = {2018},
      note = {\url{}},
      url = {}
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