Re: [OAUTH-WG] Web Authorization Protocol (oauth) WG Virtual Meeting: 2020-04-13

[Denis <[e-mail]>]

Vittorio, some comments in line:

 > An SDK is going to support "sub" wether it is required or optional.
> When I think about support for sub in this case, I am not thinking 
> about just parsing the sub value if it’s present or not surfacing it 
> in an object model if it’s not- i think about reliably offering the 
> higher level jobs to be done that are made possible by the sub 
> presence (such as the correlation with the calling subject and local 
> information, exactly as in your example all the movies a user watched 
> thru that particular API).
>
> > the resource is learning that the same user is doing different 
> transactions
> Thanks for the clarification, I didn’t realize that you were not just 
> considering correlation across resources, but across calls to the same 
> API. And you are right that in that case, an API would be able to 
> correlate the user across subsequent API calls.
> Per the above, I think that the ability of performing that correlation 
> inter-API is a goal on most systems. More below.
>
> > But NOT every token. So there were use cases where it was not required.
> Not that it proves anything conclusively given that I only sampled 7 
> system, but sub was actually used by all of them- with one of them 
> omitting it only in a particular circumstance, for tokens issued via 
> client credentials grant, and leveraging its absence as a way of 
> deducing the nature of the token (user or app identity). During 
> earlier drafts we debated at length the issue of providing mechanics 
> to perform that distinction and the outcome was that there was no 
> interest in providing it; and in the same debate it was established 
> that sub applies to the authenticated entity, hence it can describe 
> the app principal as well.
> Again, the fact that the main analyzed systems don’t make a strong 
> case for the absence of sub doesn’t imply that no such case exist; but 
> I believe it is a strong indicator of sub usefulness in real world 
> scenarios. In fact, together with iss and aud, sub is the only claim 
> appearing in every provider JWT AT examined (Tho both sub and aud have 
> each one exception due to special circumstances).
>
> I am fine with explicitly calling out that the presence of a mandatory 
> sub makes it possible for an API to correlate subsequent calls on the 
> same entity.

I agree with you that the ability of performing correlation inter-API is 
a goal on most systems. However, this correlation *alone *CANNOT be 
achieved
using the sub claim using the definition of sub as defined in RFC 7519 
(4.1.2):

         The subject value MUST either be scoped to be *locally unique 
in the context of the issuer or be globally unique*.

Using the sub claim for inter-API correlation purposes would necessarily 
allow correlation across different resources.

In the "Privacy considerations" section, whether sub is REQUIRED or 
OPTIONAL, the reader should be warned that the presence of the "sub" 
(subject) claim
in a JWT makes it possible for different resources to correlate 
different calls, as well as for the same resource to correlate between 
inter-API calls.

> I am ready to bet it won’t be a surprise for people using proprietary 
> JWT ATs, also thanks to the widespread practice of using slightly 
> modified ID token validation SDKs
> (when ID token themselves aren’t directly used in lieu of ATs for API 
> calls, as done by the likes of Kubernetes and other mainstream 
> products, which hopefully will
> eventually consider this profile instead).
>
> As a last resort, nothing forces the AS to stop at varying sub values 
> only per resource; it could go farther and supply a different sub 
> value at every token issuance
> for the same authenticating principal if it so chooses, and still 
> adhere to the letter of this profile while preventing cross operation 
> correlation.

Unfortunately, this is not possible since RFC 7519 (4.1.2) states:

         The subject value MUST either be scoped to be *locally unique 
in the context of the issuer or be globally unique*.

> That would adhere to the letter of this interop spec if not the the 
> spirit, given that layout would be respected and one can creatively 
> define principals in its own system.
> If Apple can assign anti correlation emails, nothing prevents an AS 
> doing the same with sub at different granularity.
> That would break the job to be done that sub is meant to enable, but 
> would prevent correlation in practice AND it would allow for us to 
> include a piece of info
> that proved to be useful for a large portion of known cases. But to 
> reiterate, even if this workaround would not be possible,
> I would still be OK with admitting cross operation correlation within 
> the same resource as by design.

...  and I hope you are also OK for admitting cross correlation of 
subjects between different resources. The question whether this was also 
done "by design" (or not) is left open.

Similar considerations apply for the client_id (client identifier) since 
RFC 6749 (2.2) states:

       The client identifier [i.e. a unique string representing the 
registration information provided by the client] is *unique to the 
authorization server*.

Denis

>
>
> On Mon, Apr 13, 2020 at 18:05 Dick Hardt <[e-mail] 
> <mailto:[e-mail]>> wrote:
>
>
>
>
>     An SDK is going to support "sub" wether it is required or optional.
>
>
>
>     On Mon, Apr 13, 2020 at 1:40 PM Vittorio Bertocci
>     <[e-mail] <mailto:[e-mail]>> wrote:
>
>         “Ide rockers” is iPhone autocorrect jargon for “identifiers”,
>         of course :P
>
>         On Mon, Apr 13, 2020 at 13:13 Vittorio Bertocci
>         <[e-mail] <mailto:[e-mail]>> wrote:
>
>             It’s certainly possible to conceive ATs without subs, but
>             I think the profile would be way less useful for SDK
>             developers.
>             On the objections:
>             The sub doesn’t have to be a user, if you look at the
>             earlier discussions the case in which the token has been
>             issued for an application via client creds (hence non
>             user) has been debated at length.
>             Knowing the subject does not necessarily lead to API being
>             able to collide and track users, given that the sub can be
>             a PPID that is different for every resource even if the
>             authenticated subject was the same.
>
>
>     The privacy correlation I described is not solved by a per
>     resource directed identifier as the resource is learning that the
>     same user is doing different transactions -- or per my example,
>     the resource is learning all the movies the user is seeing.
>
>
>             The sub is mandatory here because it was present in nearly
>             every token among the ones observed,
>
>
>     But NOT every token. So there were use cases where it was not
>     required.
>
>             and although one should not overindex on those scenarios,
>             I took that as strong indication that it is a frequently
>             used field and guaranteeing its presence facilitates
>             embedding in SDKs lots of downstream features, such as
>             correlating with locally stored attributes, which would
>             otherwise left as exercise to the reader.
>
>
>     NOT correlating all the different actions by the user may be the
>     desired privacy goal by a deployment.
>
>             Per the points above, there are no obvious downsides in
>             requiring the presence of the sub and significant upsides.
>             Again, the AS is in full control of the sub content hence
>             none of the privacy concerns mentioned here are mandated
>             by the sheer presence of the sub claim. If you feel the
>             privacy section should be stronger in warning an AS
>             against the possibility of collusion if global ide rockers
>             are used, I am happy to reword accordingly
>
>
>     Per Aaron's comment, if this profile is NOT intended to support
>     use cases where the RS should not be able to correlate a user
>     between resource accesses, then it is fine for "sub" to be
>     required. If so, then the document should strongly point that out.
>
>
>             On Mon, Apr 13, 2020 at 10:16 Aaron Parecki
>             <aaron@parecki..com <mailto:[e-mail]>> wrote:
>
>                 This is a good point, I often use the hotel key
>                 analogy as well. The room door is the RS, the key is
>                 the access token, the door does not need to know who
>                 the user is in order to know if it’s okay to unlock
>                 given a particular key.
>
>                 If sub is required, then this profile is limited in
>                 use to cases where user identifiers are part of the
>                 system, and there will be situations in which it’s not
>                 appropriate to use this profile for access tokens. If
>                 that’s okay, this should be clarified in the overview
>                 section to describe when this profile should be used.
>
>                 Aaron
>
>
>
>                 On Mon, Apr 13, 2020 at 10:08 AM Dick Hardt
>                 <[e-mail] <mailto:[e-mail]>>
>                 wrote:
>
>                     Why does the "sub" need to be required?
>
>                     An access token is to prove authorization. The RS
>                     may not need "sub" to constrain fulfilling the
>                     client request.
>
>                     For example, it the access token has the same
>                     properties as a movie ticket, the RS does not need
>                     to have any identifier for who purchased the movie
>                     ticket.
>
>                     The privacy implication is the RS can correlate
>                     across API calls to know it is the same subject.
>
>
>
>
>                     On Mon, Apr 13, 2020 at 8:16 AM Denis
>                     <[e-mail] <mailto:[e-mail]>>
>                     wrote:
>
>                         Hello,
>
>                         More on privacy about "JWT Profile for Access
>                         Tokens".
>
>                         The current document REQUIRES the claim names
>                         *sub* and *client_id*.
>
>                           * sub  REQUIRED - as defined in section
>                             4.1.2 of [RFC7519].
>                           * client_id  REQUIRED - as defined in
>                             section 4.3 of [RFC8693]
>
>                         *1) **sub REQUIRED*
>
>                         RFC 7519 states:
>
>                             4.1.2.  "sub" (Subject) Claim
>                                The "sub" (subject) claim identifies
>                             the principal that is the
>                                subject of the JWT. The claims in a JWT
>                             are normally statements
>                                about the subject.  The subject value
>                             MUST either be scoped to**
>                             *be locally unique in the context of the
>                             issuer or be globally unique*.
>                                The processing of this claim is
>                             generally application specific.  The
>                                "sub" value is a case-sensitive string
>                             containing a StringOrURI
>                                value. *Use of this claim is OPTIONAL.*
>
>                         If *sub *is *REQUIRED *for this profile, then
>                         this allows all resource servers to link
>                         accesses made by the same client on different
>                         servers.
>
>                         *2) client_id  REQUIRED*
>
>                         RFC 8693 states:
>
>                             4.3. "client_id" (Client Identifier) Claim
>                             The client_id claim carries the client
>                             identifier of the OAuth 2.0 [RFC 6749]
>                             client that requested the token.
>
>                         RFC 6749 states:
>
>                             2.2.  Client Identifier
>                                The authorization server issues the
>                             registered client a client
>                                identifier -- a unique string
>                             representing the registration
>                                information provided by the client. 
>                             The client identifier is not a
>                                secret; it is exposed to the resource
>                             owner and MUST NOT be used
>                                alone for client authentication. *The
>                             client identifier is unique to**
>                             **   the authorization server.*
>
>                         If *client_id* is REQUIRED for this profile,
>                         this also allows all resource servers to link
>                         accesses made by the same client on different
>                         servers.
>
>                         *Both claim names should be OPTIONAL *to allow
>                         to support the privacy principle of unlinkability.
>
>                         Would both names remain *REQUIRED*, the
>                         Privacy considerations section should mention
>                         that such a linkage by different resource servers
>                         will always be possible when using this profile.
>
>                         Denis
>
>>                         I have uploaded the second presentation for
>>                         today's session, the JWT Profile for Access
>>                         Tokens.
>>                         https://datatracker.ietf.org/meeting/interim-2020-oauth-04/session/oauth
>>
>>
>>                         Regards,
>>                          Rifaat
>>
>
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>                 -- 
>                 ----
>                 Aaron Parecki
>                 aaronparecki.com <http://aaronparecki.com>
>                 @aaronpk <http://twitter.com/aaronpk>
>
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