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

Simon Josefsson <simon@josefsson.org> Thu, 13 August 2015 22:42 UTC

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Date: Fri, 14 Aug 2015 00:42:06 +0200
From: Simon Josefsson <simon@josefsson.org>
To: Tony Arcieri <bascule@gmail.com>
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Cc: Phillip Hallam-Baker <phill@hallambaker.com>, Richard Barnes <rlb@ipv.sx>, "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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> 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.

/Simon