Re: [Anima] [lamps] Long-lived certificates, but frequently renewed certificates

Nico Williams <nico@cryptonector.com> Thu, 18 March 2021 19:12 UTC

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Date: Thu, 18 Mar 2021 14:12:05 -0500
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From: Nico Williams <nico@cryptonector.com>
To: Michael Richardson <mcr+ietf@sandelman.ca>
Cc: Toerless Eckert <tte@cs.fau.de>, spasm@ietf.org, anima@ietf.org, netconf@ietf.org
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Subject: Re: [Anima] [lamps] Long-lived certificates, but frequently renewed certificates
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On Thu, Mar 18, 2021 at 02:58:08PM -0400, Michael Richardson wrote:
> A pity that EST (and I think SCEP, but I haven't read it all), just returns
> the resulting certificate, and not something more useful, like a JSON dict
> that includes the certificate.

But you can always send this kind of metadata in response headers.

> If the whole thing was more RESTful, then holders could be told to GET their
> certificate from some place, and we could use ETag, and Expiry headers to
> tell the holder that it hasn't changed, and isn't expected to change for
> awhile, so piss off and come back then.

Yes, why not.

A client that wants a certificate renewed too soon is not really a
problem, unless it's abusive.  But how would a _stateless_ CA know a
client is being abusive other than by how soon the client is back?

Still, suppose some supplicant has a valid certificate and some RP balks
and says "come back with a fresher certificate"?  What if this happens
before 50% of the original certificate lifetime is up?

I think "too soon" might best be defined as after 25% or less of the
original certificate lifetime has passed since issuance.  But then, I
don't run massively popular public online CAs.

> I'm much less concerned about the RP here.
> The reason to issue long lifetime certificates is that things don't break
> just because some less critical infrastructure is not alive, or not reachable.
> 
> Our reference example/use case in brski-async-enroll is communication between
> thermostats and furnaces in a (newly built) residence.
> The furnace needs to heat keep the home above zero even when the house is not
> occupied, and possibly has not yet been occupied.

In a small enough network like a home network, I would think that
forever credentials + revocation is the best answer.

> Maybe abuse of the certificate renewal process is the wrong way to accomplish
> transfers of ownership.

I think next time I move I'll rip out all the thermostats and replace
them with new ones to my preference :)  But yes, I suspect there will
come a time when there are many computers (and much firmware) in a home
just as in motor vehicles, many/most/all not serviceable or replaceable
by the "owner".

Nico
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