Re: [CFRG] Small subgroup question for draft-irtf-cfrg-hash-to-curve

rsw@cs.stanford.edu Sat, 10 April 2021 21:56 UTC

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Date: Sat, 10 Apr 2021 17:56:40 -0400
From: rsw@cs.stanford.edu
To: "Hao, Feng" <Feng.Hao@warwick.ac.uk>, CFRG <cfrg@irtf.org>
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Subject: Re: [CFRG] Small subgroup question for draft-irtf-cfrg-hash-to-curve
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Hello Feng,

"Hao, Feng" <Feng.Hao@warwick.ac.uk> wrote:
> That current map-to-curve functions don’t preclude low-order points
> is a known fact, and acknowledged by the authors in their papers.

The function that is being standardized is hash_to_curve, which has
the properties I listed in my prior email. Conforming implementations
of the hash-to-curve draft do not have the issue you have raised
in this thread, period, because no conforming implementation uses
one of the deterministic maps in a standalone way.

Further discussion of whether the deterministic maps described in the
hash-to-curve draft do or do not return low-order points is not a good
use of time. Yes, they do. We all know that. But that is irrelevant,
because they should never be used on their own.

Regards,

-=rsw