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Re: [Cfrg] I-D Action: draft-irtf-cfrg-spake2-09.txt

Greg Hudson <ghudson@mit.edu> Sun, 06 October 2019 02:05 UTC

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From: Greg Hudson <ghudson@mit.edu>
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Subject: Re: [Cfrg] I-D Action: draft-irtf-cfrg-spake2-09.txt
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I have two clarity concerns and two copy-editing notes:

* This version of the draft refers to "the quotient group" in two
places.  If I look up "quotient group" I find that it's the set of all
cosets of a given normal subgroup in G.  Is that really applicable here?
 RFC 7748 just refers to "the prime-order subgroup" and I assume that's
what we mean here (aka the subset of elements of G which have order 1 or p).

* This version defines new names pA, pB, cA, and cB for protocol
messages.  It is easy to misread these names as scalars multiplied by
group elements.  I don't have a great suggestion for improving clarity
here, unfortunately.

* Section 4 begins with "The protocol transcript TT, as defined in
Section Section 3.3" which has a repeated word.

* Section 6 contains "... can lead to attacks It is..." which is missing
punctuation.

This version also removes SPAKE2+, removes the undesirable check for S
and T elements of small order, and adds a multiplication by h in the
computation of K.  I think the draft 7 approach to the cofactor was fine
(it was consistent with X25519), but the new approach does have the
advantage of breaking interoperability if the multiplication is omitted.
So I have no blocking substantive concerns.

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