Re: [Acme] Signature misuse vulnerability in draft-barnes-acme-04

Phillip Hallam-Baker <phill@hallambaker.com> Thu, 13 August 2015 23:19 UTC

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Date: Thu, 13 Aug 2015 19:18:59 -0400
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From: Phillip Hallam-Baker <phill@hallambaker.com>
To: Simon Josefsson <simon@josefsson.org>
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Cc: Richard Barnes <rlb@ipv.sx>, Tony Arcieri <bascule@gmail.com>, "acme@ietf.org" <acme@ietf.org>, Ilari Liusvaara <ilari.liusvaara@elisanet.fi>, Andrew Ayer <agwa@andrewayer.name>, Stephen Farrell <stephen.farrell@cs.tcd.ie>
Subject: Re: [Acme] Signature misuse vulnerability in draft-barnes-acme-04
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On Thu, Aug 13, 2015 at 6:42 PM, Simon Josefsson <simon@josefsson.org>;
wrote:

> > The caveat I gave was that protocol designers shouldn't assume those
> > sharp edges aren't present. I think you'll find most digital
> > signature algorithms break under the assumption that two keys can't
> > produce the same digital signature.
>
> Exactly -- there is no cryptographic requirement on a public-key
> signature system that the above property holds.  Protocol designers
> using crypto primitives should only require the properties that are
> actually promised.  Further, before using any non-standard property,
> you should demand that there is a security proof reducing that property
> into a common computational problem that people believe is difficult to
> solve.


Yep, I certainly don't advise anyone relying on the signature scheme not
resisting this sort of thing. If has to be belt and braces.