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

Eric Mill <eric@konklone.com> Thu, 13 August 2015 05:31 UTC

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From: Eric Mill <eric@konklone.com>
Date: Thu, 13 Aug 2015 01:30:22 -0400
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To: Richard Barnes <rlb@ipv.sx>
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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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> That seems like a great way to simplify the protocol.  On the other
> hand, Jacob's /.well-known/certificate/acme-account-keys.json idea is
> also quite nice.

This is only tangentially on-topic, but since the idea's been reinforced,
just a note that some deployment setups will find it challenging to deploy
and maintain special content at HTTP URIs that aren't part of the app's
business logic. I'm thinking of platform-as-a-service hosts like Heroku or
Cloud Foundry, which map hostnames entirely to "apps" (often with a code
repository as the sole content source for URIs).

Right now, /.well-known is only used for the Simple HTTP validation
mechanism, where the ability to put special metadata at HTTP URIs within
the validated hostname is already a prerequisite. Making that ability a
requirement for the overall ACME protocol would add friction for a class of
users.

-- Eric

On Thu, Aug 13, 2015 at 1:21 AM, Richard Barnes <rlb@ipv.sx>; wrote:

> On Wed, Aug 12, 2015 at 4:04 PM, Andrew Ayer <agwa@andrewayer.name>; wrote:
> > On Tue, 11 Aug 2015 22:52:05 -0700
> > Richard Barnes <rlb@ipv.sx>; wrote:
> >
> >> I'm not 100% sure I agree that there are non-trivial cases where RSA
> >> allows one to find a key that will verify a given (message, signature)
> >> pair.  (I would note, however, that you don't even need to find the
> >> modular inverse d of e -- you just need n and e such that s^e == m mod
> >> n.)  It's even less clear for ECDSA.  I'm not sure we even need to get
> >> a clear answer to that question, though.  There are protocol ways to
> >> hack around it, as you suggest.
> >
> > Thanks to Trevor Perrin, I now know that this attack is called
> > "duplicate-signature key selection," and found a paper which presents
> > a general way to construct a non-trivial RSA key (pages 4-5):
> >
> > http://eprint.iacr.org/2011/343
> >
> > It also presents an attack with ECDSA, though it depends on the
> > attacker picking their own base point, and I believe that the
> > commonly-used curves (P-256, P-384, and so on) specify a fixed base
> > point.
>
> Nice find!  (Thanks, Trevor!)
>
> I would note, though that in practice, e=65537 pretty much always, and
> the attack would almost never produce that value.  So this could still
> be prevented by checks on account public keys.
>
> Still doesn't change the conclusion, though :)
>
> --Richard
>
>
> >
> >> I think you're on the right track that we really should just not use
> >> signatures here.  I had added those in response to concerns about a
> >> CDNs in front of the ACME server, but in retrospect, they're solving
> >> the wrong problem.  The risk posed by a CDN is that it swaps the keys
> >> out, much like this situation.  So it's enough for the statement by
> >> the domain owner (i.e., the validation object) to include an
> >> indication of which account key he intends to authorize.
> >>
> >> This actually adds some symmetry to the challenges.  I had thought
> >> that proofOfPossession seemed like the odd one out, since the account
> >> key was being signed instead of doing the signing.  Your observation
> >> that the domain holder needs to assert the key basically says that the
> >> other challenges should follow the proofOfPossession model and have
> >> the domian owner make a statement about the account key.  It's just
> >> that in the other cases, the authenticity of the statement won't be
> >> shown with a signature, but with its being provisioned in a particular
> >> place.
> >
> > Exactly.
> >
> >> We will probably want to bind some more stuff into the validation
> >> object besides the public key, though, in order to bound replay
> >> opportunities.  At the very least, there needs to be a token that the
> >> CA can use to associate the validation object with things like which
> >> identifier is being authorized, and what type of challenge it goes
> >> with (to prevent replay for different domains, or in different
> >> channels).
> >
> > I may be overlooking something, but I'm skeptical of the value of
> > including extra information.  If an attacker can replay a validation
> > object for another domain or in the context of a different challenge,
> > that means the attacker effectively controls that domain/validation
> > channel, and can just contact the ACME server, start a new challenge,
> > and complete it "normally" without any need for replays.
> >
> >> In light of the above, ISTM that the right tactical move is probably
> >> to define a standard validation object that many challenges can use.
> >> Then the proofOfPossession challenge can sign the validation object,
> >> and the "put it here" challenges can provision a digest of the
> >> validation object.
> >
> > That seems like a great way to simplify the protocol.  On the other
> > hand, Jacob's /.well-known/certificate/acme-account-keys.json idea is
> > also quite nice.
> >
> > Regards,
> > Andrew
>
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