Re: [OAUTH-WG] WGLC on "JSON Web Token (JWT) Profile for OAuth 2.0 Access Tokens"

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Brian, there are plenty of ways in which an RS can surprise you with odd behavior- for example, developers might see that you used a key for signing an IDtoken and use that for init all their validation middleware for ATs as well, say because the library only supports one key at a time, and then end up failing at runtime when/if the assumption ceases to apply in the future.

Would that be legitimate of them to take such a dependency, even without warning text? No. However I am not looking at this from the “lawyering up” perspective, but from the useful guidance standpoint as well. I am well aware that being concise is a feature, but I am also not crazy about making every specification into an intelligence test for the reader. If a 16 words sentence can help prevent a likely misstep, I’d be inclined to keep it. But if the consensus is that the sentence is confusing, I can also take it out.

 

Brian & George, in the spirit of keeping things simple, and given that this was more of a “just in case” warning rather than a security feature clamored for- if the language is problematic I’d be more inclined to take the sentence out rather than complicating the guidance further.

 

From: Brian Campbell <bcampbell=[e-mail]> 
Sent: Wednesday, March 25, 2020 11:21 AM
To: George Fletcher <[e-mail]>
Cc: Brian Campbell <[e-mail]>; Vittorio Bertocci <[e-mail]>; oauth <[e-mail]>
Subject: Re: [OAUTH-WG] WGLC on "JSON Web Token (JWT) Profile for OAuth 2.0 Access Tokens"

 

I don't think you are missing anything, George (except that, to be pedantic, `kid` is a header rather than a claim). 

 

The question gave me pause, however, and makes me think that maybe the draft, with the aim of improved interoperability, should have some more explicit text about the use of the 'kid' header in a JWT AT and how it references the verification key in the content at the jwks_uri. 

 

 

 

 

 

On Wed, Mar 25, 2020 at 11:54 AM George Fletcher <gffletch=[e-mail] <mailto:[e-mail]> > wrote:

Can we not use the 'kid' claim to inform the RS as to which key is being used? What am I missing?

On 3/25/20 1:51 PM, Brian Campbell wrote:

I think, even without that statement in the draft, that ASes already have
license to use different keys if they so choose. And maybe I'm not creative
enough but I can't think of what problematic assumptions RSes might make
that would prevented by it. So perhaps just removing that whole sentence,
"An authorization server MAY elect to use different keys to sign id_tokens
and JWT access tokens."? Just a thought anyway.
 
On Wed, Mar 25, 2020 at 10:11 AM <vittorio.bertocci=
[e-mail] <mailto:[e-mail]> > wrote:
 

Thank you for the perspective- I guessed something similar (“there would
be no way for the RS to know what key is used for what").
 
As stated below, the intent wasn’t to prevent substitution/confusion, but
mostly to give ASes license to use different keys if they choose to (for
the reasons listed below, or any other reason they might have) and a
headsup to RSes so that they don’t make assumptions.
 
 
 
*From:* Brian Campbell  <mailto:bcampbell=[e-mail]> <bcampbell=[e-mail]>
*Sent:* Wednesday, March 25, 2020 8:48 AM
*To:* Vittorio Bertocci  <mailto:[e-mail]> <[e-mail]>
*Cc:* Richard Backman, Annabelle  <mailto:[e-mail]> <[e-mail]>; oauth <
[e-mail] <mailto:[e-mail]> >
*Subject:* Re: [OAUTH-WG] WGLC on "JSON Web Token (JWT) Profile for OAuth
2.0 Access Tokens"
 
 
 
I'm gonna go out on a limb and guess/suggest that implicit in Annabelle's
comment was an assumption that signing ATs and ID Tokens with different
keys would be done to prevent token substitution/confusion. And there's not
really a practical way to achieve that with the mechanics of the jwks_uri..
 
 
 
On Wed, Mar 25, 2020 at 3:53 AM Vittorio Bertocci <vittorio.bertocci=
[e-mail] <mailto:[e-mail]> > wrote:
 
*>§4 p3: The only practical way for the AS to sign ATs and ID Tokens with
different keys is to publish the keys in two different JWK sets. This only
way to do this today is by publishing separate OAuth 2.0 authorization
server metadata and OIDC Discovery metadata files, where the JWK set in the
former applies to access tokens and the JWK set in the latter applies to ID
Tokens.*
 
Hmm, I don’t follow. The OIDC jwks_uri can contain multiple keys, and they
all can be used for signing. What prevents the AS to use one key from that
list for IDtokens and another for ATs? Separate discovery docs shouldn’t be
necessary. Sure, there would be no way for the RS to know what key is used
for what- but similar mechanisms are already in place today for handling
signing key rotation: e.g. the discovery doc lists the current key and the
future key, but uses only the current- and the RS has no way of
distinguishing between the two. The situation here can be analogous, any
key in the discovery doc should be considered valid by the RS, and in fact
there’s no requirement about selecting specific keys in the validation
section. That doesn’t mean this is useless, an AS might elect to use
different keys for its own purposes (eg separation of concerns for
forensics, different strengths, different lifecycles, and so on).
 
 
 
 
 
 
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