Re: [OAUTH-WG] draft-ietf-oauth-access-token-jwt-07

[Denis <[e-mail]>]

Hi Dick and Hannes,

1)  While reading RFC 7519, no reader may be able to figure out that 
there are more than two flavours of the "sub" claim.
      This draft is introducing two new other favours of the semantics 
of the "sub" claim which are not present in RFC 7519.
      When an element has been defined, its semantics cannot be changed 
... unless making an Errata to RFC 7519
      which would be a clean way to proceed.

2) The argument about "changing the token format at any time" does not 
apply in the context of this future RFC.
     This sentence should be either removed or modified  This means that 
the following sentence which is a derivative
     of this sentence should also be either removed or modified:

    Hence, any logic in the client relying on the ability to read the
    access token content would break without recourse.

3) The following questions have still not been answered:

    Some questions raised during the WGLC have not been answered: How
    can a client request an access token compliant to this profile ?
    Which parameter(s) allow it to ask an access token compliant to this
    profile ? How can the AS know that it got a call for the issuance
    of an access token compliant to this profile ?

Denis

> Denis
>
>
> The objective of this document is to standardize the token the AS 
> shares with the RS. It is not to standardize how the client can read 
> the token. Just because the user is using the client, that does not 
> mean the user wants the client to see any claims about themselves. 
> Letting the client see the contents of the token may be a privacy 
> violation.
>
> client != user
> ᐧ
>
> On Tue, Sep 8, 2020 at 9:10 AM Denis <[e-mail] 
> <mailto:[e-mail]>> wrote:
>
>     Hi Hannes,
>
>     Two comments between the lines.
>
>>     Hi Victorio, Hi all,
>>
>>     I am doing my shepherd write-up for
>>     draft-ietf-oauth-access-token-jwt-07. Reading through the draft I
>>     have a few minor suggestions:
>>
>>     Section 2:
>>
>>     I would delete this sentence "JWT access tokens are regular JWTs
>>     complying with the requirements described in this section."
>>
>>     Reason: You pretty much make the same statement on the previous
>>     page (see terminology section).
>>
>>     Section 2.1
>>
>>     s/asymmetric algorithms/asymmetric cryptography
>>
>>     (same replacement in Section 4)
>>
>>     s/   This specification registers the "application/at+jwt" media
>>     type,
>>
>>        which can be used to indicate that the content is an access
>>     token./This specification registers the "application/at+jwt"
>>     media type,
>>
>>        which can be used to indicate that the content is a JWT access
>>     token.
>>
>>     Use capitalized "Section" when a section number is indicated,
>>     such as in Section 2.2.
>>
>>     Section 2.2
>>
>>     s/""aud"/"aud"
>>
>>     2.2.1
>>
>>     s/   auth_time  OPTIONAL - as defined in section 2 of
>>     [OpenID.Core]./   auth_time OPTIONAL - as defined in Section 2 of
>>     [OpenID.Core].
>>
>>     s/   acr, amr  OPTIONAL - as defined in section 2 of
>>     [OpenID.Core]./   acr, amr OPTIONAL - as defined in Section 2 of
>>     [OpenID.Core].
>>
>>     s/Please see/See
>>
>>     s/For example:/For example,
>>
>>     Section 4
>>
>>     You write:
>>
>>     "Authorization servers SHOULD implement OAuth 2.0 Authorization
>>     Server Metadata [RFC8414] ... "
>>
>>     Are you sure you mean "implement" and not "use"? The paragraph
>>     gives me the impression that you talk about "ASs using RFC 8414"
>>
>>     s/Please see section Section 5 for further guidance on security
>>     implications./Please see Section 5 for further guidance on
>>     security implications.
>>
>>     This sentence sounds strange to me:
>>
>>     "
>>
>>        When invoked as described in OAuth 2.0 Bearer Token Usage
>>     [RFC6750],
>>
>>        resource servers receiving a JWT access token MUST validate it
>>     in the
>>
>>        following manner.
>>
>>     "
>>
>>     How about:
>>
>>     "
>>
>>        Resource servers receiving a JWT access token MUST validate it
>>     in the
>>
>>        following manner.
>>
>>     "
>>
>>     Question: If you refer to RFC 6750 and then list the steps are
>>     you just repeating the steps from RFC 6750 or are you augmenting
>>     them?
>>
>>     You write:
>>
>>     "
>>
>>     If the JWT access token includes authorization claims as described in
>>
>>        the authorization claims section, the resource server SHOULD
>>     use them
>>
>>        in combination with any other contextual information available to
>>
>>        determine whether the current call should be authorized or
>>     rejected.
>>
>>     "
>>
>>     Include a reference to the authorization claims section
>>
>>     s/ For more
>>
>>        details on cross-JWT confusion please refer to 2.8 of
>>     [RFC8725]./ For more
>>
>>        details on cross-JWT confusion please refer to Section 2.8 of
>>     [RFC8725].
>>
>>     You write:
>>
>>     "
>>
>>        Authorization servers should not rely on the use of different keys
>>
>>        for signing OpenID Connect ID Tokens and JWT tokens as a method to
>>
>>        safeguard against the consequences of leaking specific keys.
>>
>>     "
>>
>>     The phrase "leaking keys" is probably not the best term to
>>     describe what follows afterwards in the text.
>>
>>     You write:
>>
>>     "
>>
>>     The client MUST NOT inspect the content of
>>
>>        the access token
>>
>>     "
>>
>>     This RFC 2119 language is not really enforceable in terms of
>>     interoperability. Maybe you could rephrase a bit. Something like
>>     the following would work:
>>
>>     "
>>
>>        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. The OAuth 2.0 framework assumes that
>>     access tokens
>>
>>        are treated opaque by clients.
>>
>>        Administrators of authorization servers should also take into
>>     account that
>>
>>        the content of an access token is visible to the client.
>>     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.
>>
>>     "
>>
>>
>     /In the general case, /the OAuth 2.0 framework assumes that access
>     tokens are treated as opaque by clients.
>     However, with this coming RFC, we are not in the general case:
>     since the client gets back an access token conformant to _this_
>     RFC, then it knows
>     exactly its detailed structure. The argument about "changing the
>     token format at any time" does not apply. In this case, the client
>     is quite sure
>     that it would be able to understand most of its content (at least
>     all the standard claims). The above text proposal would need to be
>     reconsidered.
>
>     Hiding (by encrypting it) the content of the access token to the
>     client is odd when an access token contains claims about a
>     human-user :
>     these claims are personal data and the human-user is usually
>     allowed to have access to his own personal data.
>
>     Encryption is nice in theory but complicated in practice, since a
>     key management system must put in place. Whenever possible, it
>     should be avoided.
>
>     BTW, some questions raised during the WGLC have not been answered:
>     How can a client request an access token compliant to this profile ?
>     Which parameter(s) allow it to ask an access token compliant to
>     this profile ? How can the AS know that it got a call for the
>     issuance of an access token
>     compliant to this profile ?
>
>     Another comment follows.
>
>>     You wrote:
>>
>>     "
>>
>>        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 confidentiality. 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.
>>
>>     "
>>
>>     The first sentence is a repetition of the previous paragraph. I
>>     would suggest to delete
>>
>>     the first sentence in this paragraph and to move the second
>>     sentence to the previous paragraph.
>>
>>     You wrote:
>>
>>     "
>>
>>        This profile mandates the presence of the "sub" claim in every JWT
>>
>>        access token, making it possible for resource servers to rely
>>     on that
>>
>>        information for performing tasks such as correlating incoming
>>
>>        requests with data stored locally for the authenticated principal.
>>
>>        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.  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 tht
>>     cannot be
>>
>>        correlated in the event of resource servers collusion. 
>>     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.  In turn, the client should obtain a new JWT access
>>
>>        token for every call to the resource server, to ensure that the
>>
>>        resource server receives different "sub" and "jti" values at every
>>
>>        call, thus preventing correlation between distinct requests.
>>
>>     "
>>
>>     The above paragraph suggests that there are different levels of
>>     privacy. What you are
>>
>>     talking about in the text is unlinkability and identification.
>>     Ways to deal with such
>>
>>     privacy threats are described in Section 6 of RFC 6973.
>>
>>     Hence, I would suggest to slightly rephrase the paragraph to
>>     something like:
>>
>>     "
>>
>>        This profile mandates the presence of the "sub" claim in every JWT
>>
>>        access token, making it possible for resource servers to rely
>>     on that
>>
>>        information for correlating incoming
>>
>>        requests with data stored locally for the authenticated principal.
>>
>>        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. The "sub" claim
>>     should be
>>
>>        populated by the authorization servers according to a privacy
>>     impact
>>
>>        assessment. 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.
>>
>     While the idea is really nice, the use of the "sub" claim in this
>     context is not compatible with the definition of the "sub" claim
>     as defined in RFC 7519:
>
>          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.
>
>     There are two options and two options only:
>
>         "locally unique in the context of the issuer" means that it is
>         the same for all RSs.
>         "globally unique" means that it is the same not only for all
>         the RSs but also for servers that have nothing to do with
>         OAuth (e.g. an email address).
>
>
>>         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.  In turn, the client should obtain a new JWT access
>>
>>        token for every call to the resource server, to ensure that the
>>
>>        resource server receives different "sub" and "jti" values at every
>>
>>        call, thus preventing correlation between distinct requests.
>>
>     The proposed text describes two different cases where the sub
>     claim is either unique for an AS/RS pair orunique for each access
>     token.
>
>     These two cases are not included in the definition found in RFC 7519.
>
>     In the general case, an identifier can be:
>
>      1. locally unique in the context of the issuer (i.e. the same for
>         all RSs),
>      2. globally unique (i.e. the same not only for all the RSs but
>         also for servers that have nothing to do with OAuth),
>      3. unique for an AS/RS pair, or
>      4. unique for each access token.
>
>     I see different ways to solve this problem:
>
>         1° Stick to the definition of RFC 7519 and (unfortunately)
>         remove these possibilities.
>         2° Define two new claims which would support the two cases
>         where the sub claim would be either unique for an AS/RS pair
>         orunique for one access token.
>         3° Define four new claims which would support the four above
>         cases.
>
>     Denis
>
>>     "
>>
>>     Section 7.2
>>
>>     s/   Section Section 2.2.3.1 of this specification refers to the
>>
>>        attributes "roles", "groups", "entitlements" defined in
>>     [RFC7643] to
>>
>>        express authorization information in JWT access tokens.
>>
>>     /   Section 2.2.3.1 of this specification refers to the
>>
>>        attributes "roles", "groups", "entitlements" defined in
>>     [RFC7643] to
>>
>>        express authorization information in JWT access tokens.
>>
>>     References
>>
>>     RFC 7519 has to be a normative reference:
>>
>>        [RFC7519]  Jones, M., Bradley, J., and N. Sakimura, "JSON Web
>>     Token
>>
>>                   (JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015,
>>
>>     <https://www.rfc-editor.org/info/rfc7519>
>>     <https://www.rfc-editor.org/info/rfc7519>.
>>
>>     RFC 7644 is an unused reference:
>>
>>        [RFC7644]  Hunt, P., Ed., Grizzle, K., Ansari, M., Wahlstroem, E.,
>>
>>                   and C. Mortimore, "System for Cross-domain Identity
>>
>>     Management: Protocol", RFC 7644, DOI 10.17487/RFC7644,
>>
>>     September 2015, <https://www.rfc-editor.org/info/rfc7644>
>>     <https://www.rfc-editor.org/info/rfc7644>.
>>
>>     The same is true for RFC 3986:
>>
>>        [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter,
>>     "Uniform
>>
>>                   Resource Identifier (URI): Generic Syntax", STD 66,
>>
>>                   RFC 3986, DOI 10.17487/RFC3986, January 2005,
>>
>>     <https://www.rfc-editor.org/info/rfc3986>
>>     <https://www.rfc-editor.org/info/rfc3986>.
>>
>>     Ciao
>>
>>     Hannes
>>
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>>
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