Re: [Cfrg] Primes vs. hardware side channels

David Leon Gil <coruus@gmail.com> Fri, 17 October 2014 01:25 UTC

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From: David Leon Gil <coruus@gmail.com>
Date: Thu, 16 Oct 2014 21:25:30 -0400
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To: "cfrg@irtf.org" <cfrg@irtf.org>, "D. J. Bernstein" <djb@cr.yp.to>
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Subject: Re: [Cfrg] Primes vs. hardware side channels
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On Thu, Oct 16, 2014 at 8:55 PM, D. J. Bernstein <djb@cr.yp.to>; wrote:
> In the case of Brainpool's 256-bit curve, the defense under discussion
> consists of obscuring the bits of a 256-bit scalar s by adding a small
> multiple of the 256-bit group order to s, typically a 32-bit multiple.
> This seems safe if the bits are further obscured by a considerable level
> of physical noise. However, for plausibly smaller noise levels this is
> breakable, as illustrated by the recent paper by Schindler and Wiemers.

For folks less familiar with this issue, page 85 et seq. of (the
slides for) Marc Joye's FDTC 2013 keynote present a very readable
explanation of why additive blinding, as described above, doesn't
work:

http://conferenze.dei.polimi.it/FDTC13/shared/FDTC-2013-keynote-2.pdf