Re: [Acme] lack of nonce for external account binding (was Re: Benjamin Kaduk's Discuss on draft-ietf-acme-acme-14:) (with DISCUSS and COMMENT)

[Richard Barnes <[e-mail]>]

On Wed, Oct 3, 2018 at 11:54 AM Benjamin Kaduk <[e-mail]> wrote:

> On Sat, Sep 15, 2018 at 08:51:55PM -0500, Benjamin Kaduk wrote:
> > On Fri, Sep 14, 2018 at 03:59:07PM -0400, Daniel McCarney wrote:
> > > > My co-author Daniel McCarney is working on the COMMENT comments.
> > >
> > > > IMPORTANT: I don't think I understand why "nonce" MUST NOT be
> present in
> > > > the external-binding JWS object, though I think I understand why one
> is
> > > not
> > > > needed in order to bind the MAC to the current transaction.  (That
> is,
> > > this
> > > > is in effect a "triply nested" construct, where a standalone MAC that
> > > > certifies an ACME account (public) key as being authorized by the
> > > > external-account holder to act on behal of that external account.
> But
> > > this
> > > > standalone MAC will only be accepted by the ACME server in the
> context of
> > > > the outer JWS POST, that must be signed by the ACME account key,
> which is
> > > > assumed to be kept secure by the ACME client, ensuring that both
> > > > key-holding entities agree to the account linkage.)  Proof of
> freshness of
> > > > the commitment from the external account holder to authorize the ACME
> > > > account key would only be needed if there was a scenario where the
> > > external
> > > > account holder would revoke that association, which does not seem to
> be a
> > > > workflow supported by this document.  Any need to effectuate such a
> > > > revocation seems like it would involve issuing a new MAC key for the
> > > > external account (and invalidating the old one), and
> revoking/deactivating
> > > > the ACME account, which is a somewhat heavy hammer but perhaps
> reasonable
> > > > for such a scenario.
> > > > Account key rollover just says that the nonce is NOT REQUIRED, and
> also
> > > > uses some nicer (to me) language about "outer JWS" and "inner JWS".
> It
> > > > might be nice to synchronize these two sections.
> > >
> > > I defer on this to the other authors/people that want
> > > externalAccountBinding to
> > > be a thing.
> >
> > Okay.  I would like to avoid having needless normative requirements if
> > there is in fact no reason for this requirement.
>
> My apologies if I missed it when it went by, but did we ever hear more
> about this requirement from the proponents of externalAccountBinding?
>

Picking this up...

I actually have the opposite inclination to you here -- if a field is not
used by the protocol, then it should be forbidden, in the spirit of [1].
By that logic, we should also forbid the use of the "nonce" field in
roll-over.  I think it was just an oversight that we didn't.  The security
analysis that Bhargavan et al. did long ago did not presume any use of
it.   I've made a PR making it a MUST NOT:

https://github.com/ietf-wg-acme/acme/pull/464

[1] https://tools.ietf.org/html/draft-iab-protocol-maintenance-00


>
> Thanks,
>
> Benjamin
>
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