Re: [Cfrg] draft-irtf-cfrg-eddsa - Implementation failure for 448

Jim Schaad <ietf@augustcellars.com> Wed, 20 July 2016 15:08 UTC

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From: Jim Schaad <ietf@augustcellars.com>
To: 'Watson Ladd' <watsonbladd@gmail.com>
References: <00de01d1e280$e31c0290$a95407b0$@augustcellars.com> <20160720124904.GA22541@LK-Perkele-V2.elisa-laajakaista.fi> <006401d1e294$63418310$29c48930$@augustcellars.com> <CACsn0c=BZamd8h6hczY32pvgKodsEmhT+F0N=4RbCgY1YnT3qw@mail.gmail.com>
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Date: Wed, 20 Jul 2016 17:07:55 +0200
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Subject: Re: [Cfrg] draft-irtf-cfrg-eddsa - Implementation failure for 448
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Maybe because this is an a=-1 curve?  (Guess on my part).

Jim


> -----Original Message-----
> From: Watson Ladd [mailto:watsonbladd@gmail.com]
> Sent: Wednesday, July 20, 2016 4:49 PM
> To: Jim Schaad <ietf@augustcellars.com>
> Cc: Ilari Liusvaara <ilariliusvaara@welho.com>; cfrg@ietf.org; draft-irtf-cfrg-
> eddsa@ietf.org
> Subject: Re: [Cfrg] draft-irtf-cfrg-eddsa - Implementation failure for 448
> 
> Why aren't we using the formulas from EFD?
> https://hyperelliptic.org/EFD/g1p/auto-twisted-extended-1.html
> 
> We know these are right due to a Magma script checking them.
> 
> On Wed, Jul 20, 2016 at 7:38 AM, Jim Schaad <ietf@augustcellars.com> wrote:
> > OK - Another one for you.
> >
> > H = (X1+X2)*(Y1+Y2)
> > X3 = A*G*(H-C-D)
> >
> > Vs code
> >
> >         tmp.x=zcp*F*((self.x+self.y)*(y.x+y.y)-xcp-ycp)
> >
> > note that in once case the two X are being added and in the other x and y are
> being added.
> >
> > Jim
> >
> >
> >> -----Original Message-----
> >> From: ilariliusvaara@welho.com [mailto:ilariliusvaara@welho.com]
> >> Sent: Wednesday, July 20, 2016 2:49 PM
> >> To: Jim Schaad <ietf@augustcellars.com>
> >> Cc: draft-irtf-cfrg-eddsa@ietf.org; cfrg@ietf.org
> >> Subject: Re: draft-irtf-cfrg-eddsa - Implementation failure for 448
> >>
> >> On Wed, Jul 20, 2016 at 02:18:52PM +0200, Jim Schaad wrote:
> >> > There is a difference between the document and the code for point
> >> > addition of 448.  Which is the correct one?
> >> >
> >> > Text in section 5.2.4
> >> >
> >> >     X3 = A*G*(H-C-D)
> >> >     Y3 = A*G*(D-C)
> >> >
> >> > Python Code
> >> >         tmp.x=zcp*F*((self.x+self.y)*(y.x+y.y)-xcp-ycp)
> >> >         tmp.y,tmp.z=zcp*G*(ycp-xcp),F*G
> >> >
> >> > Note that the first uses A*G the section uses A*F in computing the
> >> > new x value.
> >>
> >> Good catch: It should be A*F (as in code). Fixed in Editor's Copy.
> >>
> >>
> >> -Ilari
> >
> > _______________________________________________
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> > Cfrg@irtf.org
> > https://www.irtf.org/mailman/listinfo/cfrg
> 
> 
> 
> --
> "Man is born free, but everywhere he is in chains".
> --Rousseau.