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

Richard Barnes <rlb@ipv.sx> Wed, 12 August 2015 22:54 UTC

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Date: Wed, 12 Aug 2015 15:54:17 -0700
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From: Richard Barnes <rlb@ipv.sx>
To: yan <yan@eff.org>
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Cc: "acme@ietf.org" <acme@ietf.org>, Andrew Ayer <agwa@andrewayer.name>
Subject: Re: [Acme] Signature misuse vulnerability in draft-barnes-acme-04
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On Wed, Aug 12, 2015 at 1:55 PM, yan <yan@eff.org>; wrote:
>
>
> On 8/11/15 10:52 PM, Richard Barnes wrote:
>
>> Smallest diff change from the current document would be simply to
>> explicitly require validation value bound to account key that created
>> it -- not the one the signs the response.  Since the attack requires
>> that the attacker change keys (using recovery) after receiving the
>> token, the attack only works if the validation is done against the new
>> public key.  This option introduces non-trivial implementation
>> complexity, though, since the server now has to remember what key
>> signed the new-authorization request that caused the challenges to be
>> issued.
>
>
> Doesn't it already have to remember this? The current instructions for
> verifying a DNS challenge says: "1. Verify the validation JWS using the
> account key for which this challenge was issued."
>
> Since the challenge was issued before the attacker initiated account
> recovery to do the key change, the wording implies that the server remembers
> the original key at validation time.

Yeah, good point.  I still think that it would be better to simplify
things in the way Andrew suggested.

--Richard