Re: [Cfrg] PAKE review

Björn Haase <bjoern.m.haase@web.de> Sat, 17 August 2019 13:09 UTC

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Subject: Re: [Cfrg] PAKE review
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Dear Brian,

>The magic-wormhole protocol currently uses SPAKE2 (a symmetric PAKE) on
>an Ed25519 group, with an additional form of symmetry: the two sides do
>not have to decide ahead of time which role they are playing. In SPAKE2
>terms, the "M" and "N" elements are equal, which saves a roundtrip.

IIUC the proofs for SPAKE2 assume that "M != N" but this most probably could be addressed by a somewhat modified security proof. IMO it should suffice to blind one of the Diffie-Hellman points: IIRC this is also what is done in the PAK-Family of protocols.

When using this approach in a product, I'd recommend you to have a look on the patent applications/patents on the PAK protocol family. Using distinct M and N and blinding two points might have been one component necessary/helpful for the patent circumvention strategy that was in mind when designing the SPAKE2 construction.

Yours,

Björn.