Hi Christian and Watson,
Hi Watson,
Sorry for the late response. I would fully agree that challenge would
be the better term here and there is a bit of ambiguity for the term
nonce in OAuth imho.
+1 Yes, for a nonce sent by the authorization server, the term
"challenge" would be a better term.
While a nonce generally speaking would usually mean a random input, in
the context of OAuth, it is afaik interpreted as an opaque challenge
that should be sufficiently random (so it cannot be guessed).
A nonce in this context could just be a server-signed/HMACed JWT
containing things like the server time when the nonce was generated
for which the term challenge would probably be a better fit
- If I recall correctly, that behaviour/usage is also explained in the
DPoP spec.
See current IANA definition:
Claim Name: nonce
Claim Description: Value used to associate a Client session with an
ID Token (MAY also be used for nonce values in other applications of JWTs)
The IANA definition of the claim nonce is rather vague and in an client-
server paradigm, does not say
whether the nonce is generated by the client or by the server .
The current draft is using the term "nonce" when it is generated by the
(authorization) server.
In general, two mechanisms are able to *detect* replay attacks:
- either using "unique numbers" that are generated and sent by a
client to a server,
- or using "challenges" that are generated and sent by a server to
a client.
Using "unique numbers" does not require any Preflight request, but for
practical implementation reasons,
mandates the use of a local clock loosely synchronized with UTC. In
practice, a "unique number" will be composed
of the iat claim and of a random number (rnd claim). This eliminates the
need of a *challenge* fetching mechanism
for Attestation-Based Client Authentication.
Using "challenges" mandates to previously obtain a challenge from the
server which means a Preflight request.
This mandates the need of a *challenge* fetching mechanism for
Attestation-Based Client Authentication and
thus mandates to identify an end-point to request it.
The question are now as follows: should this document describe:
1) a replay detection mechanism using only "unique numbers" ?
2) a replay detection mechanism using only "challenges" ?
3) a replay detection mechanism using either "unique numbers" or
"challenges" ?
In the context of SD-JWT, I have posted a full description of these two
replay detection methods in:
https://mailarchive.ietf.org/arch/msg/oauth/JEk5Jtyw2G042LFRA5-r0J9i2EI/
See comment 6 under the topic of REPLAY DETECTION OF a **Key Binding JWT**.
Best regards,
Denis
Best Regards,
Christian
*From: *Watson Ladd <watsonbl...@gmail.com>
*Date: *Sunday, 6. April 2025 at 02:27
*To: *Christian Bormann <chris.borm...@gmx.de>
*Cc: *oauth <oauth@ietf.org>
*Subject: *Re: [OAUTH-WG] Nonce fetching mechanism for
Attestation-Based Client Authentication
On Sat, Apr 5, 2025, 8:17 AM Christian Bormann
<chris.bormann=40gmx...@dmarc.ietf.org<mailto:40gmx...@dmarc.ietf.org>>
wrote:
Hi All,
We had a discussion about a nonce fetching mechanism for the
Attestation-Based Client Authentication draft at the
IETF 122 session. Since we didn’t really reach a consensus there,
we’d like to continue the discussion on the mailing list.
I alas haven't had a chance to watch all the discussion even now so
forgive me if I'm being redundant. What you want is not a nonce for a
few reasons:
1:nonces can be predictable. Ideally we'd bind the proof of possession
to the one who needs it to prevent an attacker relaying the request or
issuing one for the same predicted value.
2: we probably want some degree of licenses
3: counting is hard in big systems
All of this makes me think we just want to sign a server provided
challenge with some rules on construction rather than specifically a
nonce.
To summarize the problem briefly: The draft specifies a proof of
possession that optionally signs over a server-provided
nonce to guarantee freshness of said proof of possession. Since we
expect this specification to be used in some contexts
where creating a PoP might be expensive (e.g., require a user
interaction), we were searching for a mechanism where
the nonce is not provided via an error (as is the case for DPoP -
which would often require the generation of 2 PoPs),
but in a way that guarantees that we have a fresh nonce before
creating a PoP.
We were thinking about either
* a dedicated nonce endpoint (within the scope of an AS or RS)
* or a mechanism to explicitly ask for a nonce in a request to
an existing OAuth endpoint (e.g., the PAR endpoint).
After some discussion at OAuth Security Workshop, we proposed to
use a dedicated header to signal a request for a new
nonce. This could work at any existing OAuth endpoint that wishes
to use an attestation-based client authentication. Brian
rightfully mentioned that only adding one header field and
completely changing the behaviour of said endpoint does not
sound like a good idea and proposed to use a different HTTP
method. Brian initially proposed HEAD and after some more
discussion we ended with an OPTIONS request as the seemingly best
idea.
The idea as currently document in the draft is to use an OPTIONS
request with a specific header field to request a nonce.
The current proposal would mean that for a request to a PAR endpoint
1. The client discovers via metadata that the PAR endpoint
requires attestation-based client authentication with a nonce
2. The client sends an OPTIONS request:
OPTIONS /as/par HTTP/1.1
Host: as.example.com<http://as.example.com/>
attestation-nonce-request: true
3. The client receives a nonce in the response:
HTTP/1.1 200 OK
Host: as.example.com<http://as.example.com/>
attestation-nonce: AYjcyMzY3ZDhiNmJkNTZ
4. The client does the “real” request to the PAR endpoint
including the client authentication (via header fields):
POST /as/par HTTP/1.1
Host: as.example.com<http://as.example.com/>
Content-Type: application/x-www-form-urlencoded
OAuth-Client-Attestation: eyJ0eXAiOiJvYXV0aC…
OAuth-Client-Attestation-PoP: eyJhbGciOiJFUzI…
response_type=code&state=af0ifjsldkj&client_id=s6BhdRkqt3
&redirect_uri=https%3A%2F%2Fclient.example.org<http://2fclient.example.org/>%2Fcb
&code_challenge=K2-ltc83acc4h0c9w6ESC_rEMTJ3bww-uCHaoeK1t8U
&code_challenge_method=S256&scope=account-information
At the IETF 122 session, Filip voiced concerns that since OPTIONS
is used for CORS preflight requests, it would mean that at
least for frontend clients, this mechanism would result in several
OPTIONS requests. For JavaScript clients, the CORS preflight
requests cannot be used or modified and the client would then
manually create another OPTIONS request to get the nonce.
From a simple OPTIONS (preflight) and POST requests (normal
request to PAR), we would get to OPTIONS (preflight),
OPTIONS (nonce fetch), OPTIONS (preflight), POST requests.
We tried to capture those concerns in this issue:
https://github.com/oauth-wg/draft-ietf-oauth-attestation-based-client-auth/issues/102<https://github.com/oauth-wg/draft-ietf-oauth-attestation-based-client-auth/issues/102>
and would like to pick that discussion up again to find some
consensus what the best option would be to for a nonce request.
Would people be more comfortable if we instead point to an
endpoint that can be used to request a nonce (dedicated endpoint
that could for example, be discoverable via metadata), or is it
fine to cause more requests and we should move ahead with the
current variant based on OPTIONS?
Best Regards,
Christian, Paul, Tobias
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