Abstract
We present the first constant-round, tree-based, group key exchange protocol based on SIDH with logarithmic-order communication and memory complexity, where the only previous isogeny-based group key exchange, SIBD, has linear-order communication and memory complexity. We call our protocol the supersingular isogeny tree-based group key exchange (SIT). We show that our protocol satisfies post-quantum security through a reduction to the supersingular decisional Diffie–Hellman (SSDDH) problem in the security model of Manulis, Suzuki, and Ustaoglu. We also construct a peer-to-peer (sequential) version of SIT. Finally, we present a compiler that turns SIT into an authenticated group key exchange while maintaining the same complexity and security as SIT, resulting in the authenticated SIT group key exchange (A-SIT).
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Notes
Original TikZ code courtesy of De Feo via GitHub [6].
The authors claim that their scheme may also easily be constructed for odd n; however, their indexing method does not allow for odd n.
Although the original BDII scheme has no such concept, we introduce a ‘score’ for each party, needed in order to alternate SIDH bases so as to avoid the attack outlined in Note 1.
\(\mathcal {D}\) needs at most \((2n-4)t_{isog}\) additional time to generate all the isogenies for the transcript, where \(t_{isog}\) is the amount of time needed to do a single SIDH isogeny computation.
This s may not be the same for each party in the protocol instance. See Sect. 2.1.
\(\mathcal {P}_i|sID|0|r_i|\sigma \) can just be sent to all parties to eliminate the need to store a list of descendants, which in the case of \(\mathcal {P}_0\) and \(\mathcal {P}_1\) has size O(n).
At most this requires one re-signing of a message when a party has to send the SIT protocol x value to a child (with which it has already exchanged more than one previous message) and its other descendants (with which it has sent at most one previous message).
In doing so, we assume that broadcasting/multicasting a message does not depend on the number of receivers but that receiving l messages means that the receiver incurs a cost of l, even if all messages are received in a single round. The reason for this is that it takes into account that receiving messages requires being online and also storing said messages while broadcasting/multicasting is usually a one-time operation.
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This work is partially supported by enPiT (Education Network for Practical Information Technologies) at MEXT, and Innovation Platform for Society 5.0 at MEXT.
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Hougaard, H.B., Miyaji, A. Authenticated logarithmic-order supersingular isogeny group key exchange. Int. J. Inf. Secur. 21, 207–221 (2022). https://doi.org/10.1007/s10207-021-00549-4
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DOI: https://doi.org/10.1007/s10207-021-00549-4