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

Watson Ladd <watsonbladd@gmail.com> Wed, 20 July 2016 14:49 UTC

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From: Watson Ladd <watsonbladd@gmail.com>
Date: Wed, 20 Jul 2016 07:48:59 -0700
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To: Jim Schaad <ietf@augustcellars.com>
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Subject: Re: [Cfrg] draft-irtf-cfrg-eddsa - Implementation failure for 448
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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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