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Re: [Cfrg] I-D Action: draft-turner-thecurve25519function-00.txt

Sean Turner <TurnerS@ieca.com> Wed, 30 July 2014 17:32 UTC

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From: Sean Turner <TurnerS@ieca.com>
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To: Ilari Liusvaara <ilari.liusvaara@elisanet.fi>
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Subject: Re: [Cfrg] I-D Action: draft-turner-thecurve25519function-00.txt
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On Jul 29, 2014, at 16:41, Ilari Liusvaara <ilari.liusvaara@elisanet.fi> wrote:

> On Tue, Jul 29, 2014 at 04:05:04PM -0400, Sean Turner wrote:
>> 
>> Please review and provide comments if needed.
>> 
>>> 	Filename        : draft-turner-thecurve25519function-00.txt
> 
>  To compute the conditional swap in constant time (independent of s_t)
>  use dummy = s_t*(x_2-x_3) x_2 = x_2 - dummy x_3 = x_3 + dummy where
>  s_t is 1 or 0, or dummy = s_t & (x_2 XOR x_3) x_2 = x_2 XOR x_3 x_3 =
>  x_3 XOR x_2 where s_t is regarded as the all-1 word of 255 bits.  The
>  latter version is more efficient on most architectures.
> 
> The latter formulas: Where is the dummy used? And s_t can also be 0
> (no swap)?
> 
> Should those formulas be:
> dummy = s_t & (x_2 XOR x_3)
> x_2 <- x_2 XOR dummy
> x_3 <- x_3 XOR dummy
> 
> ?
> 
> And that paragraph could perhaps use some reformatting.
> 
> -Ilari

Let me fix up the formatting first and then see if it’s clearer.

spt

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