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

[Benjamin Kaduk <[e-mail]>]

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?

Thanks,

Benjamin