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

"Scott Fluhrer (sfluhrer)" <sfluhrer@cisco.com> Fri, 09 April 2021 18:26 UTC

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From: "Scott Fluhrer (sfluhrer)" <sfluhrer@cisco.com>
To: "Scott Fluhrer (sfluhrer)" <sfluhrer=40cisco.com@dmarc.ietf.org>, "Mike Hamburg" <mike@shiftleft.org>, "Hao, Feng" <Feng.Hao@warwick.ac.uk>
CC: CFRG <cfrg@irtf.org>
Thread-Topic: [CFRG] Small subgroup question for draft-irtf-cfrg-hash-to-curve
Thread-Index: AQHXLUacM7sBZ3ZdE0a+hIjdr1H+CqqsQ0gAgAADtICAABZvgIAAC3EAgAATDWCAAARIcA==
Date: Fri, 9 Apr 2021 18:25:53 +0000
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Subject: Re: [CFRG] Small subgroup question for draft-irtf-cfrg-hash-to-curve
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Correction: Opaque does use a hash-to-curve operation (used to translate the password into an elliptic curve point); if it happens to translate a specific password to a low order point, then that specific password is easy to test for; however there are no other implications…

From: CFRG <cfrg-bounces@irtf.org> On Behalf Of Scott Fluhrer (sfluhrer)
Sent: Friday, April 9, 2021 2:17 PM
To: Mike Hamburg <mike@shiftleft.org>rg>; Hao, Feng <Feng.Hao@warwick.ac.uk>
Cc: CFRG <cfrg@irtf.org>
Subject: Re: [CFRG] Small subgroup question for draft-irtf-cfrg-hash-to-curve

Opaque doesn’t use a hash-to-curve operation.

CPace does; it also automatically aborts (fails) if the hash-to-curve operation happens to return a low order point (that is, a point that, after multiplying by the cofactor, is the neutral element).

From: CFRG <cfrg-bounces@irtf.org<mailto:cfrg-bounces@irtf.org>> On Behalf Of Mike Hamburg
Sent: Friday, April 9, 2021 1:00 PM
To: Hao, Feng <Feng.Hao@warwick.ac.uk<mailto:Feng.Hao@warwick.ac.uk>>
Cc: CFRG <cfrg@irtf.org<mailto:cfrg@irtf.org>>
Subject: Re: [CFRG] Small subgroup question for draft-irtf-cfrg-hash-to-curve

I don’t know if the same holds for OPAQUE or CPace: for all I know, they may have specification holes and/or end in failure in that case.