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

Denis <denis.ietf@free.fr> Fri, 24 July 2020 08:32 UTC

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From: Denis <denis.ietf@free.fr>
To: Hannes Tschofenig <Hannes.Tschofenig@arm.com>, Rifaat Shekh-Yusef <rifaat.s.ietf@gmail.com>
Cc: Vittorio Bertocci <vittorio.bertocci@auth0.com>, "oauth@ietf.org" <oauth@ietf.org>
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Subject: Re: [OAUTH-WG] Second WGLC on "JSON Web Token (JWT) Profile for OAuth 2.0 Access Tokens"
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Hi Hannes,

This email has been left answered. It raises major issues:

    If the authorization server has no way to know that the client is
    sending a request which implies compliance
    with draft-ietf-oauth-access-token-jwt why should it behave in such
    a way ?


    If the resource server has no way to know that it is checking a JWT
    which is supposed to be compliant
    with draft-ietf-oauth-access-token-jwt why should it behave in such
    a way ?

    IMHO, a token type would be able to easily address the problem;
    otherwise sections 3 and 4 should be deleted.


What is the status of this document ?

Denis

_Note_: A remainder was sent privately to both Hannes and Rifaat on June 
23. It was also left answered. Hereafter is its content:

    Hi Hannes,

    At this time, I have not yet received an answer to the email
    attached below.

    Would you please provide a response on the mailing list ?

    Denis

> Hi  Hannes,
>
> Let us start by the last argument of this email which is copied below:
>
>     Finally, there are still two questions that have been raised but
>     which have not yet been answered at this time:
>
>       * how can a client request a JWT compliant to /this/ profile, and
>       * how can a client be confident that it got a JWT compliant to
>         /this/ profile ?
>
>     [Hannes] Regarding the two questions: It cannot and it was never
>     the intention of this work.
>
> If this document was limited to section 2, it would simply be a 
> description of a specific profile, but it is more than that
> since it includes two additional sections:
>
>     3. Requesting a JWT Access Token
>     4. Validating JWT Access Tokens
>
> In section 3, the text states:
>
>
>    If the request does not include a "resource" parameter, the 
> authorization server *MUST* use in the "aud" claim
>   as default resource indicator.
>
> If the authorization server has no way to know that the client is 
> sending a request which implies compliance
> with draft-ietf-oauth-access-token-jwt why should it behave in such a 
> way ?
>
>
> In section 4 the text states:
>
>
> resource servers receiving a JWT access token *MUST* validate it in 
> the following manner
>
>
> If the resource server has no way to know that it is checking a JWT 
> which is supposed to be compliant
> with draft-ietf-oauth-access-token-jwt why should it behave in such a 
> way ?
>
>
> The responses to these two questions are important before handling the 
> other comments.
>
> IMHO, a token type would be able to easily address the problem; 
> otherwise sections 3 and 4 should be deleted.
>
>
> The remaining of my replies are within the text below prefixed with 
> [Denis].
>
>> Hi Denis,
>>
>> Please see my response below.
>>
>> *From:* Denis <denis.ietf@free.fr>
>> *Sent:* Wednesday, June 3, 2020 12:12 PM
>> *To:* Hannes Tschofenig <Hannes.Tschofenig@arm.com>
>> *Cc:* Rifaat Shekh-Yusef <rifaat.s.ietf@gmail.com>om>; Vittorio Bertocci 
>> <vittorio.bertocci@auth0.com>om>; oauth@ietf.org
>> *Subject:* Re: [OAUTH-WG] Second WGLC on "JSON Web Token (JWT) 
>> Profile for OAuth 2.0 Access Tokens"
>>
>> Hi Hannes,
>>
>> I do appreciate your efforts to attempt to get rid of the "MUST NOT" 
>> in the "Privacy considerations" section.
>>
>> Let us look at the following proposed sentence:
>>
>> While this is technical possible, it is important to note that the 
>> OAuth 2.0 protocol does not aim to expose the content of the access 
>> token
>>     to the client. The access token is therefore, by design, 
>> considered to be opaque to the client".
>> /
>> In the context of this document/, a detailed content of the JWT is 
>> expected and thus, if a client receives a JWT compliant to this profile
>> (and if the token is not encrypted which is most often the case) it 
>> will absolutely be sure to pick up any guaranteed field within the JWT.
>> So, /in the context of this document/, the access token cannot be 
>> considered to be opaque to the client.
>>
>> [Hannes] Here we have a disconnect. The OAuth 2.0 design does not 
>> assume that the client inspects the access tokens if it flies by. 
>> This document could not change that.
>> The purpose of this document is actually quite simple: Those who want 
>> to use JWT as a format for access tokens they can use the claims 
>> described in this document.
>> You are also free to use whatever format you want.
>>
> [Denis] You wrote: "The OAuth 2.0 design does not *assume *that the 
> client inspects the access tokens if it flies by". However, there are 
> cases where
> the client may be interested to know which attributes have been placed 
> into the JWT, there should be some way(s) to be able to do it.
> Using Token Introspection (extended to be used by clients) would be 
> like bringing in an elephant to kill a mouse. Using a local API would 
> be a simple
> solution, however the IETF defines (most often) protocols rather than 
> local APIs.
>
> The user's privacy cannot be fulfilled if the client is unable to know 
> which attributes have been placed into the JWT. A solution to address 
> this issue
> would be to clearly advertise the following in the Privacy 
> Considerations section:
>
>     As the OAuth 2.0 design does not assume that the client inspects
>     the access tokens if it flies by, clients have no way to know
>     which identity
>     attributes have effectively been placed into the JWT. Since these
>     identity attributes may disclose more private information than
>     what is strictly
>     necessary to perform one or more operations, this may be a serious
>     concern for users that care about their privacy.
>
>> About the second paragraph, /in the context of this document 
>> (/besides the case where the JWT is encrypted), it is neither difficult,
>> nor impossible to parse the token/.
>> /
>> About the second paragraph, let us look at the following proposed 
>> sentence/in the context of this document/ :
>>
>>     " Additionally, there is no guarantee that the access token is 
>> conveyed by value and the authorization server implementation may change
>>       the token format at any time ".
>>
>> The argumentation that the token format may change at any point of 
>> time, while being valid in the general case, is invalid /in the 
>> context of this document/.
>> This JWT profile will be stable over time. This means that this 
>> quoted sentence is inappropriate /in the context of this document/.
>>
>> [Hannes] Here is the issue. In a given deployment you do not know how 
>> the access token is encoded nor whether the claims are in this format.
>> You don’t know whether the token is conveyed by reference or by 
>> value. Hence, why should we suddenly even give developers the impression
>> that OAuth Clients should look at the token.
>>
> [Denis] OAuth clients SHOULD only look at the attributes placed into 
> the JWT, when/if they have privacy concerns about the identity attributes
> that have been placed into the JWT. Otherwise, they would have no idea 
> on how they could be traced by the resources servers /and other servers //
> //that don't use OAuth/. In the current situation, it appears 
> necessary to clearly advertise in the Privacy Considerations section 
> that the token opacity
> may be a serious concern for users that care about their privacy.
>
> It is also important to note that the /foundational design assumption 
> /of keeping access tokens opaque to clients (and their users) is 
> closing the door
> to any confidence for clients that their privacy is indeed preserved 
> by the authorization servers.
>
>> The third proposed paragraph is stating :
>>
>>     "In scenarios where it is where it is desirable for the clients 
>> to obtain information transmitted in the access token, OAuth 2.0 
>> token introspection
>>       may provide a useful tool to enable such functionality (proper 
>> authorization assumed) ".
>>
>> RFC 7662 (OAuth 2.0 Token Introspection) is a protocol to be used by 
>> protected resources, but is not a protocol to be used by clients.
>> As indicated, in order to be usable, a "proper authorization" also 
>> needs to be managed. Besides the difficulty to support such a 
>> protocol for clients
>> and to twist its original usage as defined in RFC 7662, it is simpler 
>> to develop the code to examine the content of the JWT, since its 
>> content is guaranteed
>> to be stable over time.
>>
>> [Hannes] While it may be simpler to inspect the access token, the use 
>> of token introspection is a better match for the OAuth architecture.
>> We can talk about updating the token introspection RFC to also 
>> describe this use case, assuming there is interest.
>>
> [Denis] Updating the token introspection RFC would be like bringing a 
> bull in a china shop.
>
>> The question in general will surface why the client should get access 
>> to the content of the access token in the first place.
>> For those cases where information is passed to the client other 
>> mechanisms, such as the identity token in OIDC, have been developed.
>>
> [Denis] The reason has been explained above: it has to do with 
> correlation of the users by different resources servers,
> but also by other servers when "globally unique identifiers" are being 
> used in the "sub" claim.
>
>> The last proposed paragraph is the following:
>>
>>    " Since the content of the access token is accessible to the 
>> resource server it is important to evaluate whether the resource 
>> server gained the proper entitlement
>>       to have access to any content received in form of claims, /for 
>> example through user consent in some form, policies and agreements 
>> with the organization running /
>> /      the authorization servers, and so on/. The policies and the 
>> user interfaces to enable this user consent are, however, part of a 
>> specific deployment and therefore
>>       outside the scope of this document ".
>>
>> The sentence "for example through user consent in some form, policies 
>> and agreements with the organization running the authorization 
>> servers, and so on"
>> should be removed, since this example lets believe that the consent 
>> is handled by the authorizations servers while it might be handled by 
>> the resource servers.
>>
>> [Hannes] The information is disclosed by the authorization server and 
>> hence the consent has to be with the authorization server.
>>
>> The last proposed paragraph would be solution neutral if the example 
>> were removed. This would lead to the following sentence:
>>
>> Since the content of the access token is accessible to the resource 
>> server it is important to evaluate whether the resource server gained 
>> the proper entitlement
>> to have access to any content received in form of claims. The 
>> policies and the user interfaces to enable this user consent are, 
>> however, part of a specific deployment
>> and therefore outside the scope of this document.
>>
> [Denis] A resource server may say: "In order to perform this operation 
> , I need your date of birth". If the user agrees, then the client will 
> ask to the authorization
> server to insert the date of birth of the user into the JWT. In this 
> way, the consent is given by the client when talking to the resource 
> server. The authorization
> server is not involved with a consent given by the user. Obviously, 
> this is one scenario and other scenarios exist, but such a scenario 
> should not be prevented.
>
> The AS does not need to know which operation will be performed by the 
> user, it only needs to know that the user is willing his birth date to 
> be included into the JWT.
> However, at the moment, since there is no RFC supporting such a 
> possibility, asking for specific standardized attributes is not (yet) 
> possible.
>
>>
>> Finally, there are still two questions that have been raised but 
>> which have not yet been answered at this time:
>>
>>   * how can a client request a JWT compliant to /this/ profile, and
>>   * how can a client be confident that it got a JWT compliant to
>>     /this/ profile ?
>>
>> [Hannes] Regarding the two questions: It cannot and it was never the 
>> intention of this work.
>>
> [Denis] This point has been addressed at the top of this email. 
> However, I would like to add one point.
>
> Let us suppose that a token type would be added both in the token 
> request and within the token itself,
> and if, at the minimum, the client would  be allowed to access to this 
> token type, saying "This token is conformant to RFC XXX",
> then the client would be able to call a local API able to disclose the 
> content of the token.
>
> This would be like using a piece of cheese to catch the mouse.
>
> Denis
>
>> Ciao
>>
>> Hannes
>>
>>
>> Denis
>>
>>     Let me try to jump in here in order to make a proposal for the
>>     text in the privacy consideration section:
>>
>>     FROM:
>>
>>     *6*
>>     <https://tools.ietf.org/html/draft-ietf-oauth-access-token-jwt-04#section-6>*. 
>>     Privacy Considerations*
>>
>>        As JWT access tokens carry information by value, it now becomes
>>
>>        possible for requestors and receivers to directly peek inside the
>>
>>        token claims collection. The client MUST NOT inspect the
>>     content of
>>
>>        the access token: the authorization server and the resource server
>>
>>        might decide to change token format at any time (for example by
>>
>>        switching from this profile to opaque tokens) hence any logic
>>     in the
>>
>>        client relying on the ability to read the access token content
>>     would
>>
>>        break without recourse. Nonetheless, authorization servers should
>>
>>        not assume that clients will comply with the above.  Whenever
>>     client
>>
>>        access to the access token content presents privacy issues for a
>>
>>        given scenario, the authorization server should take explicit
>>     steps
>>
>>        to prevent it as described below.
>>
>>        In scenarios in which JWT access tokens are accessible to the end
>>
>>        user, it should be evaluated whether the information can be
>>     accessed
>>
>>        without privacy violations (for example, if an end user would
>>     simply
>>
>>        access his or her own personal information) or if steps must
>>     be taken
>>
>>        to enforce cofidentiality. Possible measures include:
>>     encrypting the
>>
>>        access token, encrypting the sensitive claims, omitting the
>>     sensitive
>>
>>        claims or not using this profile, falling back on opaque access
>>
>>        tokens.
>>
>>        In every scenario, the content of the JWT access token will
>>
>>        eventually be accessible to the resource server.  It's
>>     important to
>>
>>        evaluate whether the resource server gained the proper
>>     entitlement to
>>
>>        have access to any content received in form of claims, for example
>>
>>        through user consent in some form, policies and agreements
>>     with the
>>
>>        organization running the authorization servers, and so on.
>>
>>     TO:
>>
>>     *6
>>     <https://tools.ietf.org/html/draft-ietf-oauth-access-token-jwt-04#section-6>.
>>     Privacy Considerations*
>>
>>        The design of OAuth 2.0 envisions that access tokens are
>>     created by
>>
>>        authorization servers and consumed by resource servers.
>>
>>        As JWT access tokens, as described in this document, carry
>>     information by value, it is
>>
>>        possible for OAuth clients to peek inside the access token.
>>
>>        While this is technical possible, it is important to note that the
>>
>>        OAuth 2.0 protocol does not aim to expose the content of the
>>
>>        access token to the client. The access token is therefore, by
>>     design, considered to be
>>
>>        opaque to the client.
>>
>>        A number of cases may make it difficult or impossible for
>>     clients to
>>
>>        inspect the token, for example, the access token may be
>>     encrypted,
>>
>>        the access token may contain vendor-specific claims that have
>>     not been
>>
>>        standardized or have been standardized in other consortia
>>     making parsing
>>
>>        of the token difficult. Additionally, there is no guarantee
>>     that the
>>
>>        access token is conveyed by value and the authorization server
>>     implementation
>>
>>        may change the token format at any time.
>>
>>        In scenarios where it is desirable for the clients to obtain
>>     information
>>
>>        transmitted in the access token, OAuth 2.0 token introspection
>>     may provide
>>
>>        a useful tool to enable such functionality (proper
>>     authorization assumed).
>>
>>        In scenarios where the content of the access token must not be
>>     readable
>>
>>        by clients, encrypting the content of the access token is
>>     RECOMMENDED.
>>
>>        Since the content of the access token is accessible to the
>>     resource server
>>
>>        it is important to
>>
>>        evaluate whether the resource server gained the proper
>>     entitlement to
>>
>>        have access to any content received in form of claims, for example
>>
>>        through user consent in some form, policies and agreements
>>     with the
>>
>>        organization running the authorization servers, and so on. The
>>     policies
>>
>>        and the user interfaces to enable this user consent are,
>>     however, part
>>
>>        of a specific deployment and therefore outside the scope of
>>     this document.
>>
>>     How does this sound?
>>
>>     Ciao
>>
>>     Hannes
>>
>>     *From:* OAuth <oauth-bounces@ietf.org>
>>     <mailto:oauth-bounces@ietf.org> *On Behalf Of *Rifaat Shekh-Yusef
>>     *Sent:* Thursday, May 14, 2020 8:03 PM
>>     *To:* Denis <denis.ietf@free.fr> <mailto:denis.ietf@free.fr>
>>     *Cc:* Vittorio Bertocci <vittorio.bertocci@auth0.com>
>>     <mailto:vittorio.bertocci@auth0.com>; oauth@ietf.org
>>     <mailto:oauth@ietf.org>
>>     *Subject:* Re: [OAUTH-WG] Second WGLC on "JSON Web Token (JWT)
>>     Profile for OAuth 2.0 Access Tokens"
>>
>>     Denis,
>>
>>     You are rehashing the same issues that you have already discussed
>>     on the mailing list multiple times,
>>
>>     You could not get the WG to agree with your points, because the
>>     WG believe that this issue is outside the scope of this document.
>>
>>     The best the chairs can do at this stage is to capture your point
>>     in the shepherd write-up to the IESG.
>>
>>     We think this document has the support of the WG and is ready to
>>     move forward.
>>
>>     Regards,
>>
>>      Rifaat
>>
>>     On Thu, May 14, 2020 at 12:29 PM Denis <denis.ietf@free.fr
>>     <mailto:denis.ietf@free.fr>> wrote:
>>
>>         Hi Vittorio,
>>
>>         I am referring to the email you sent on April the 29 th which
>>         is copied below.
>>
>>         1) You wrote:
>>
>>             /> targeting of access tokens/
>>
>>             Let me think about that a bit longer.
>>
>>             I acknowledge that the decision of including an audience
>>             has the effect of letting the AS track when the client
>>             accesses a particular resource,
>>             but at the same time that’s completely mainstream and
>>             very much by design in a very large number of cases. As
>>             such, I find the language
>>             you are suggesting to be potentially confusing, as it
>>             positions this as an exception vs a privacy protecting
>>             mainstream that is in fact not common,
>>             and ascribes to the client more latitude than I believe
>>             is legitimate to expect or grant.
>>
>>             *I’ll try to come up with concise language that clarifies
>>             to the reader that the current mechanism does allow AS
>>             tracking*.
>>
>>         Since the last draft has been published on the 27 th, you
>>         have not proposed any "concise language that clarifies to the
>>         reader
>>         that the current mechanism does allow AS tracking".
>>
>>         2) You also wrote about the "sub" uniqueness:
>>
>>             As long as an identifier identifies one resource only, it
>>             satisfies uniqueness. It doesn’t have to be a singleton.
>>
>>         RFC 7519 defines in section 4.1.2 the semantics of the "sub"
>>         claim using the following sentence:
>>
>>             The subject value MUST either be scoped to be locally
>>             unique in the context of the issuer or be globally unique.
>>
>>         The text does NOT say that the subject value "MUST be scoped
>>         to be locally unique in the context of the *resource server*".
>>
>>         Changing the semantics of an already defined claim is not
>>         permitted. If you would like to have such a semantics available,
>>         a new claim should be defined (and it would be very nice to
>>         have it !).
>>
>>         3) The text is the privacy considerations section states:
>>
>>            Although the ability to correlate requests might be
>>         required by design in many scenarios, there are scenarios
>>         where the authorization
>>            server might want to prevent correlation to preserve the
>>         desired level of privacy.
>>
>>         In the real world, it is also clients or end-users which
>>         would like to prevent correlation to preserve their desired
>>         level of privacy.
>>
>>         A better sentence would be:
>>
>>            Although the ability to correlate requests might be
>>         required by design in many scenarios, there are scenarios
>>         where the authorization
>>            server *or the client* might want to prevent correlation
>>         to preserve the desired level of privacy.
>>
>>         4) The text continues with:
>>
>>            Authorization servers should choose how to assign "sub"
>>         values according to the level of privacy required by each
>>            situation.  For instance: if a solution requires
>>         preventing tracking  principal activities across multiple
>>         resource servers,
>>            the  authorization server should ensure that JWT access
>>         tokens meant for different resource servers have distinct "sub"
>>            values that cannot be correlated in the event of resource
>>         servers collusion.
>>
>>         Authorization servers are not necessarily able to choose the
>>         level of privacy required by each situation. When there are
>>         different
>>         situations for the same resource server, the scope is
>>         (unfortunately at the moment) the only way to select the
>>         "level of privacy that is required".
>>
>>         The example ("For instance:") is only an example that
>>         provides a vague recommendation for the ASs which is NOT
>>         conformant
>>         with the semantics of the "sub" claim as defined in RFC 7519.
>>
>>         What should be discussed here are not "examples" or what an
>>         authorization server should do, but explanations about the
>>         implications
>>         for the end-user or for the client for the various values
>>         that can be placed into the "sub" claim by an AS. The problem
>>         is wider that simply
>>         a collusion between resource servers, but also with other
>>         servers that DO NOT participate in any OAuth exchange.
>>
>>         RFC 6973 (Privacy Considerations) states in section 7 :
>>         Guidelines
>>
>>             This section provides guidance for document authors in
>>             the form of a questionnaire about a protocol being designed.
>>             The questionnaire may be useful at any point in the
>>             design process, particularly after document authors have
>>             developed
>>             a high-level protocol model as described in [RFC4101].
>>
>>         One of the questions is:
>>
>>             f. *Correlation*. Does the protocol allow for correlation
>>             of identifiers ?  Are there expected ways that
>>             information exposed
>>             by the protocol will be combined or *correlated with
>>             information obtained outside the protocol* ?
>>
>>         It is important to provide an answer to these two questions.
>>
>>         Hereafter is some text that is fully conformant with RFC 7519
>>         which should be incorporated into the privacy considerations
>>         section
>>         which explains the implications of the two (and only two)
>>         flavours of the "sub" claim.
>>
>>             When the sub claim contains a locally unique identifier
>>             in the context of the issuer, this allows the tracking of
>>             principal activities
>>             across multiple resource servers.
>>
>>             When the sub claim contains a globally unique identifier,
>>             this allows to correlate principal activities across
>>             multiple resource
>>             servers, while in addition, this globally unique
>>             identifier may also allow to correlate the principal
>>             activities on servers where
>>             no access has been performed by the principals to these
>>             servers but where the same globally unique identifiers
>>             are being used
>>             by these servers.
>>
>>         Denis
>>
>>             Thanks Denis for the thorough commentary.
>>
>>             /> The title of this spec./
>>
>>             Fixed, thanks!
>>
>>             /> The client MUST NOT inspect the content of the access
>>             token/
>>
>>             This is really a sticky point. I really want to
>>             acknowledge your PoV on this, but at the same time I
>>             found this to be one of the biggest sources of issues in
>>             the use of JWT for access tokens hence I feel we really
>>             need to give solid guidance here. Let me expand further
>>             on the reasoning behind it, and perhaps we can get to
>>             language that satisfies both PoVs.
>>
>>             To me the key point is that clients should not write
>>             /code/ that inspects access tokens. Taking a dependency
>>             on the ability to do so is ignoring fundamental
>>             information about the architecture and relationships
>>             between OAuth roles, and suggests an ability of the
>>             client to understand the semantic of the content that
>>             cannot be assumed in the general case. I expanded on the
>>             details in my former reply to you on this topic, I would
>>             recommend referring to it. Clients violating this simple
>>             principle has been one of the most common sources of
>>             production issues I had to deal with in the past few
>>             years, and one of the hardest to remediate given that
>>             clients are hard to update and sometimes the things they
>>             relied on were irremediably lost. This is why I am
>>             inclined to put in here strong language.
>>
>>             That said: I have nothing against client developers
>>             examining a network trace and drawing conclusions based
>>             on the content of what they see. That doesn’t create any
>>             hard dependencies and has no implications in respect to
>>             changes in the solution behavior. However I am not sure
>>             how to phrase that in the specification, given that
>>             referring to the client inevitably refers to its code. I
>>             am open to suggestions.
>>
>>             >  3)…
>>
>>             I have a pretty hard time following the chain of
>>             reasoning in this section. Let me attempt to tackle it to
>>             the best of my understanding.
>>
>>             I think the key might be
>>
>>             /> a client should be able to choose whether it wishes
>>             the sub claim to contain [..]/
>>
>>             I don’t think that should be a choice left to the client.
>>             In business systems, my experience is that the type of
>>             identifiers to be used (when the IdP gives any choice at
>>             all)  is established at resource provisioning time. I am
>>             not aware of mechanisms thru which a client signals the
>>             nature of the identifier to be used, nor that would be
>>             fully feasible (the resource knows what it needs to
>>             perform its function).
>>
>>             Furthermore:
>>
>>             /> which has nothing to do with uniqueness since the
>>             value changes for every generated token./
>>
>>             Again, this is something that was touched on in my former
>>             reply to your message. As long as an identifier
>>             identifies one resource only, it satisfies uniqueness. It
>>             doesn’t have to be a singleton.
>>
>>             Finally, the scope is optional (for good reasons: 1^st
>>             party and non delegation scenarios don’t require it)
>>             hence it cannot be relied upon for properties that should
>>             hold in every scenario.
>>
>>             In summary: per the preceding thread on this topic, the
>>             consensus was that varying the sub content was a
>>             satisfactory way of protecting against correlation. I
>>             don’t a gree that clients should have a mechanism to
>>             request different sub flavors, as that decision should be
>>             done out of band by the AS and RS; and the scope isn’t
>>             always available anyway.
>>
>>             /> targeting of access tokens/
>>
>>             Let me think about that a bit longer.
>>
>>             I acknowledge that the decision of including an audience
>>             has the effect of letting the AS track when the client
>>             accesses a particular resource, but at the same time
>>             that’s completely mainstream and very much by design in a
>>             very large number of cases. As such, I find the language
>>             you are suggesting to be potentially confusing, as it
>>             positions this as an exception vs a privacy protecting
>>             mainstream that is in fact not common, and ascribes to
>>             the client more latitude than I believe is legitimate to
>>             expect or grant.
>>
>>             I’ll try to come up with concise language that clarifies
>>             to the reader that the current mechanism does allow AS
>>             tracking.
>>
>>             *From: *OAuth <oauth-bounces@ietf.org>
>>             <mailto:oauth-bounces@ietf.org> on behalf of Denis
>>             <denis.ietf@free.fr> <mailto:denis.ietf@free.fr>
>>             *Date: *Wednesday, April 29, 2020 at 09:12
>>             *To: *"oauth@ietf.org" <mailto:oauth@ietf.org>
>>             <oauth@ietf.org> <mailto:oauth@ietf.org>
>>             *Subject: *Re: [OAUTH-WG] Second WGLC on "JSON Web Token
>>             (JWT) Profile for OAuth 2.0 Access Tokens"
>>
>>             You will find four comments numbered 1) to 4).
>>
>>             *1) *The title of this spec. is:
>>
>>             JSON Web Token (JWT) Profile for OAuth *2.0* Access Tokens
>>
>>             So, this spec. is supposed to be targeted to OAuth *2.0.
>>             * However, the header at the top of the page omits to
>>             mention it.
>>
>>             Currently, it is :
>>
>>             Internet-Draft OAuth Access Token JWT Profile          
>>             April 2020
>>
>>             It should rather be:
>>
>>             Internet-Draft OAuth *2.0* Access Token JWT
>>             Profile           April 2020
>>
>>             *2)* The following text is within section 6.
>>
>>             The client MUST NOT inspect the content of
>>             the access token: the authorization server and the
>>             resource server
>>             might decide to change token format at any time (for
>>             example by
>>             switching from this profile to opaque tokens) hence any
>>             logic in the
>>             client relying on the ability to read the access token
>>             content would
>>             break without recourse.
>>             Nonetheless, authorization servers should
>>             not assume that clients will comply with the above.
>>
>>             It is of a primary importance that clients MAY be able to
>>             inspect tokens before transmitting them.
>>             The "MUST NOT" is not acceptable.
>>
>>             The above text should be replaced with:
>>
>>             Reading the access token content may be useful for the
>>             user to verify that
>>             the access token content matches with its expectations. 
>>             However,
>>             the authorization server and the resource server might
>>             decide to change the
>>             token format at any time.  Thus, the client should not
>>             expect to always be
>>             in a position to read the access token content.
>>
>>             The remaining of the text about this topic is fine.
>>
>>
>>             *3) *The next topic is about the sub claim.
>>
>>             The text states:
>>
>>             Although the ability to correlate requests might be
>>             required by
>>             design in many scenarios, there are scenarios where the
>>             authorization
>>             server might want to prevent correlation to preserve the
>>             desired
>>             level of privacy. Authorization servers should choose how
>>             to assign
>>             sub values according to the level of privacy required by each
>>             situation.
>>
>>             I have a set of questions:
>>
>>              1. How can authorization servers choose how to assign
>>                 sub values according to the level of privacy required
>>                 "by each situation" ?
>>              2. How can authorization servers know the level of
>>                 privacy required "by each situation" ?
>>              3. How can the users be informed of the level of privacy
>>                 required "by each situation" ?
>>              4. How can the users *consent* with the level of privacy
>>                 required "by each situation" ?
>>
>>             Currently, the request MUST include either a resource
>>             parameter or an aud claim parameter, while it MAY include
>>             a scope parameter.
>>
>>             The syntax of the scope parameter is a list of
>>             space-delimited, case-sensitive strings (RFC 6749). It is
>>             thus subject to private agreements
>>             between clients and Authorization Servers. Since the
>>             scope is being returned, it is a primary importance that
>>             the returned scope matches
>>             with its expectations before transmitting the token to a
>>             Resource Server.
>>
>>             In theory, a client should be able to choose whether it
>>             wishes the sub claim to contain :
>>
>>               * a global unique identifier for all ASs ("globally
>>                 unique"),
>>               * a unique identifier for each AS ("locally unique in
>>                 the context of the issuer"),
>>               * a different pseudonym for each RS, or
>>               * a different pseudonym for each authorization token
>>                 request.
>>
>>             The only variable parameter that it can use for this
>>             purpose in the token request is the scope parameter.
>>
>>             RFC 7519 states is section 4.1.2:
>>
>>             The subject value MUST either be scoped to be locally
>>             unique in the context of the issuer
>>             or be globally unique.
>>
>>             It is quite hard to recognize that the sub claim is able
>>             to carry a different pseudonym for each RS, i.e. for case
>>             (c), or
>>             a different pseudonym for each authorization token
>>             request, i.e. for case (d), which has nothing to do with
>>             uniqueness
>>             since the value changes for every generated token.
>>
>>             This has implications about the following text:
>>
>>             For instance: if a solution requires preventing tracking
>>             principal activities across multiple resource servers, the
>>             authorization server should ensure that JWT access tokens
>>             meant for
>>             different resource servers have distinct sub values that
>>             cannot be
>>             correlated in the event of resource servers collusion.
>>
>>             Since it addresses case (c).
>>
>>             and also about the following text:
>>
>>             4.b) Similarly: if a solution requires preventing a
>>             resource server from
>>             correlating the principal’s activity within the resource
>>             itself, the
>>             authorization server should assign different sub values
>>             for every JWT
>>             access token issued.
>>
>>             Since it addresses case (d).
>>
>>             This means that the current text placed in the privacy
>>             considerations section was a good attempt to address the
>>             case,
>>             but that the text needs to be revised.
>>
>>             Proposed text replacement for all the previously quoted
>>             sentences:
>>
>>             According to 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.
>>
>>             When the sub claim contains a globally unique identifier,
>>             this allows to correlate principal activities across
>>             multiple resource servers, while in addition,
>>             this globally unique identifier may also allow to
>>             correlate the principal activities on servers where no
>>             access has been performed by the principals
>>             to these servers but where the same globally unique
>>             identifiers are being used by these servers.
>>
>>             When the sub claim contains a locally unique identifier
>>             in the context of the issuer, this also allows the
>>             tracking of principal activities across multiple resource
>>             servers.
>>
>>             The scope request parameter is the only way to influence
>>             on the content of the sub claim parameter. Its meaning is
>>             subject to a private agreement
>>             between the client and the AS, which means that the use
>>             of the scope parameter is the only way to choose between
>>             a locally unique identifier
>>             in the context of the issuer or a globally unique identifier.
>>
>>             Since the scope parameter is being returned, it is a
>>             primary importance that the returned scope matches with
>>             the expectations of the client before transmitting
>>             the token to a Resource Server.
>>
>>             However, there are other cases where the client would
>>             like to be able to choose whether it wishes the sub claim
>>             to contain :
>>                 - a different pseudonym for each RS so that different
>>             resource servers will be unable to correlate its
>>             activities, or
>>                 - a different pseudonym for each authorization token
>>             request, so that the same resource server cannot
>>             correlate its activities performed at different instant
>>             of time.
>>
>>             Considering the semantics of the sub claim, these two
>>             cases cannot be currently supported.
>>
>>
>>             *4) *The next topic is about the targeting of access tokens
>>
>>             Text had been proposed before the last conference call.
>>             Then, the topic has been presented at the very end of the
>>             last conference call, but no text has been included
>>             in the next draft.
>>
>>             Here is a revised text be included in the privacy
>>             considerations section:
>>
>>             For security reasons, some clients may be willing to
>>             target their access tokens but, for privacy reasons, may
>>             be unwilling to disclose to Authorization Servers
>>             an identification of the Resource Servers they are going
>>             to access, so that Authorization Servers will be unable
>>             to know which resources servers are being accessed.
>>             The disclosure of the Resource Servers names allows the
>>             Authorization Servers to list all the Resource Servers
>>             being access by all its users and in addition to list pairs
>>             of (Principal, Resource Servers) which allow to trace all
>>             the users accesses to Resource Servers performed through
>>             a given Authorization Server. When a token is targeted,
>>             this profile does not contain provisions to address these
>>             two threats.
>>
>>             Denis
>>
>>                 Hi all,
>>
>>                 This is a second working group last call for "JSON
>>                 Web Token (JWT) Profile for OAuth 2.0 Access Tokens".
>>
>>                 Here is the document:
>>
>>                 https://tools.ietf.org/html/draft-ietf-oauth-access-token-jwt-06
>>
>>                 Please send your comments to the OAuth mailing list
>>                 by April 29, 2020.
>>
>>                 Regards,
>>
>>                  Rifaat & Hannes
>>
>>                 _______________________________________________
>>
>>                 OAuth mailing list
>>
>>                 OAuth@ietf.org  <mailto:OAuth@ietf.org>
>>
>>                 https://www.ietf.org/mailman/listinfo/oauth
>>
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