[Rats] Re: Freshness with Nonces
Hannes Tschofenig <Hannes.Tschofenig@gmx.net> Mon, 24 June 2024 06:49 UTC
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CC: Carl Wallace <carl@redhoundsoftware.com>, "Tschofenig, Hannes" <hannes.tschofenig=40siemens.com@dmarc.ietf.org>, "spasm@ietf.org" <spasm@ietf.org>, rats <rats@ietf.org>
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Subject: [Rats] Re: Freshness with Nonces
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Von: "Henk Birkholz" <henk.birkholz@ietf.contact>
An: "Carl Wallace" <carl@redhoundsoftware.com>, "Tschofenig, Hannes" <hannes.tschofenig=40siemens.com@dmarc.ietf.org>, "spasm@ietf.org" <spasm@ietf.org>, "rats" <rats@ietf.org>
Betreff: [Rats] Re: Freshness with Nonces
hi Carl,
a generic comment first and then some comments in-line.
The subject says freshness and your first sentence then goes directly to
"Evidence" and "detecting replay attacks". Nonces couple recentness
checks with freshness checks and conveniently also improve replay
detection when used in conveyance of Evidence (and I am assuming object
security and not a secure channel here). But, for example, signatures
can be used for replay attack detection, too. I think you do not need
nonces for that at all. Nonces are very useful fore checking recentness
and if recentness is your only way to check for freshness, well... then
it's also your source for freshness.
To be honest, the questions below all arise, because I might miss
something pretty obvious here. What problem are you trying to solve
exactly? Maybe that is actually my most relevant question.
On 19.06.24 11:57, Carl Wallace wrote:
> Inline…
>
> *From: *"Tschofenig, Hannes"
> <hannes.tschofenig=40siemens.com@dmarc.ietf.org>
> *Date: *Monday, June 17, 2024 at 8:33 AM
> *To: *"spasm@ietf.org" <spasm@ietf.org>, rats <rats@ietf.org>
> *Subject: *[Rats] Freshness with Nonces
>
> Hi all,
>
> In the RATS architecture the Evidence is processed by the Verifier. For
> a given Verifier to check for replays timestamps, nonces, and epochs
> have been introduced. I only talk about nonces here.
>
> The nature of nonces is that they are randomly selected by the party
> that checks against replays, the verifier in our case. Section 10.2 of
> RFC 9334 talks about nonce-based freshness.
To my knowledge, the nature of nonces is that they are provided (not
necessarily produced) by a challenger in a challenge-response
interaction model, are intended to check recentness of the corresponding
response and that they cannot be predicted by an attacker so it is
harder to become an imposter responder (or in some scenarios maybe even
an imposter challenger) successfully, right?
>
> No problem so far. However, when we integrate CSR attestation (which
> carries the evidence) into a certificate management protocol like EST or
> CMP we must request the nonce in advance before the attester is able to
> include the nonce in the signed evidence. >
> [CW] I don’t think this “we must request the nonce” is correct, at least
> where “we” includes the attester. In some cases, the attester is acting
> upon instructions provided to it. Those instructions may include a
> nonce. An example of this arrangement is a SCEP payload in the iOS OTA
> protocol. How the MDM (or whatever prepared the instructions) obtained
> the nonce is irrelevant to the attester and, in my experience, need not
> be signaled in the subsequent request.
If you use a challenge-response model, yes, the Atterster requires a
nonce generated by the Verifier. If you use other interaction models,
the "nonce"-like value may come from other actors, as Carl points out.
>
> This raises questions about how the relying party (in the background
> check model) obtains that nonce without conveying any extra information
> from the attester to the relying party about which verifier to select.
Why should the Relying Party need to know a nonce embedded in Evidence?
It processes Attestation Results. I seems not really elegant to me, but
if the Relying Party is a challenger for Attestation Results in
challenge-response interaction models, then the Relying Party generates
it, and it will be included in the response (again assuming object
security and not a secure channel).
>
> What information should be used by the attester and subsequently by the
> relying party to determine the verifier before transmitting the evidence?
>
> [CW] Your question raises questions about how the attester knows where
> to obtain that nonce without having been provided any extra information.
I actually am having a hard time following that question. Is this now a
completely different topic and we left the nonce topic?
Let me re-state so I do not get it wrong. What value should be conveyed
from Attester to Verifier and then from Verifier to Relying Party before
Evidence can be conveyed from the Verifier to the Relying Party?
I really would not know. Not a nonce? Maybe a MUD URL pointing to a RATS
MUD file? I started something ages ago, but it move down on the priority
list: https://datatracker.ietf.org/doc/html/draft-birkholz-rats-mud-00" target="_blank" rel="nofollow">https://datatracker.ietf.org/doc/html/draft-birkholz-rats-mud-00
That is not an IETF document and was early work that we plan to pick up
on again in the next 12 months.
>
> Ideally, the RATS architecture should have provided an answer to this
> question but unfortunately it does not.
I am not sure what "this" is. It seems not to be freshness, but it also
seems not to be about replay attack protection anymore?
>
> [CW] I think this discussion is hinting at a desire for some attestation
> verification protocol. It may be that sticking an extensible field where
> the hint is now is the thing to do, to facilitate the relatively
> abstract current hint mechanism or a future, more concrete, link to a
> verification service.
Very simple CBOR-based request-response messages are outlined in
https://www.ietf.org/archive/id/draft-ietf-rats-reference-interaction-models-09.html#appendix-A" target="_blank" rel="nofollow">https://www.ietf.org/archive/id/draft-ietf-rats-reference-interaction-models-09.html#appendix-A
A CoAP-based reference implementation exists in:
https://github.com/Fraunhofer-SIT/charra" target="_blank" rel="nofollow">https://github.com/Fraunhofer-SIT/charra
>
> Ciao
> Hannes
>
> PS: We (Hendrik and I) thought that the hint introduced in the CSR
> attestation would have been a good candidate for this determination. In
> our mental model the hint would be something like an FQDN because in the
> passport model of the RATS architecture the attester also needs to have
> the FQDN (or even a URL) of the verifier to get the communication working.
If all RATS role should be aware about the same "nonce"'ish value in a
freshness period, I think that is an Epoch Marker? Nonces are not
intended to achieve that by themselves, I think.
Viele Grüße,
Henk
>
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- [Rats] Freshness with Nonces Tschofenig, Hannes
- [Rats] Re: Freshness with Nonces Mike Ounsworth
- [Rats] Re: Freshness with Nonces Muhammad Usama Sardar
- [Rats] Re: [EXTERNAL] Re: Re: Freshness with Nonc… Mike Ounsworth
- [Rats] Re: [EXTERNAL] Re: Re: Freshness with Nonc… Orie Steele
- [Rats] Re: [EXTERNAL] Re: Re: Freshness with Nonc… Mike Ounsworth
- [Rats] Re: [EXTERNAL] Re: Re: Freshness with Nonc… Mike Ounsworth
- [Rats] Re: Freshness with Nonces Carl Wallace
- [Rats] Re: [EXTERNAL] Re: Re: Freshness with Nonc… Muhammad Usama Sardar
- [Rats] Re: [EXTERNAL] Re: Re: Freshness with Nonc… Michael Richardson
- [Rats] Re: Freshness with Nonces Henk Birkholz
- [Rats] Re: Freshness with Nonces Muhammad Usama Sardar
- [Rats] Re: [lamps] Re: Re: Freshness with Nonces Henk Birkholz
- [Rats] Re: [lamps] Re: Re: Freshness with Nonces Muhammad Usama Sardar
- [Rats] Re: Freshness with Nonces Carl Wallace
- [Rats] Re: [lamps] Re: Re: Freshness with Nonces Orie Steele
- [Rats] Re: [lamps] Re: Re: Freshness with Nonces Henk Birkholz
- [Rats] Re: Freshness with Nonces Hannes Tschofenig
- [Rats] Re: Freshness with Nonces Carl Wallace
- [Rats] Re: Freshness with Nonces Henk Birkholz
- [Rats] Re: Freshness with Nonces Hannes Tschofenig
- [Rats] Re: Freshness with Nonces Henk Birkholz
- [Rats] Re: Freshness with Nonces Carl Wallace
- [Rats] Re: Freshness with Nonces Michael Richardson
- [Rats] Re: Freshness with Nonces John Kemp