Re: [xacml] Does the XACML 2.0 and 3.0 Subject have some inconsistenciesin Core and Multi Profile?

[Erik Rissanen <erik@axiomatics.com>]

Hi Rich,

See responses inline.

Rich.Levinson wrote:
> Hi Erik and TC,
>
> Been looking at how the Subject evolved in 2.0 to 3.0 plus a possible
> problem in 2.0 that may or may not carry over to 3.0.
>
> First the possible problem:
>
>     * XACML 2.0 section 2.4 describes the "intent" of "multiple
>       subjects", which I think can be summed up as saying multiple
>       identified Subject entities can be involved in a single access
>       decision request. If there are multiple Subject elements in a
>       Request then they are all potentially relevant to a single
>       authorization decision and have a subject-category attribute
>       that identifies what their "role" might be in the overall
>       decision context.
>     * Appendix B.2 enumerates 5 possible values of this
>       subject-category attribute, and there may be additional ones
>       defined by "users" (section 2.4).
>     * Of the 5 values in section B.2, 2 of them,
>       "intermediary-subject" and "codebase" have the statement "There
>       may be more than one." in their defn, which I take to mean that
>       a single Request element could have multiple Subject elements
>       that contain one of these two categories.
>     * The "problem" that I see is that section 6.2 lines 2921-2923 say
>       that if there is more than one Subject element with the same
>       SubjectCategory, they will be treated as if they were in the
>       same Subject element
>           o This appears to be a problem because, for example, if
>             there are more than one "intermediary-subjects", then the
>             attributes describing each will be mixed up together and
>             one will not be able to tell which attribute in the
>             Subject element pertains to which intermediary.
>

This has already been discussed on the list and there is an issue number
for it on the issues list. The issue was closed with no action since
"fixing" it introduces a huge amount of complexity since everything
retrieved from the request context then would become bags of bags of
attributes. You can find the discussion on the list. (Try to look for
emails around the date when the issue was closed.)

> The above "problem" may or may not go away in 3.0. It appears that the
> semantics of section 6.2 from 2.0 are "lost" in 3.0, because all the
> "categories are now Attributes and the designators for Subject,
> Resource, Action, Environment no longer exist, except as they are
> buried in identifiers, and as 3.0 refers to them as "RECOMMENDED" in
> XACML 3.0 section B.2.
>
> Assuming that is ok (although I am becoming increasingly uneasy the
> generalizing "Subject, Resource, Action, Environment" to "Attributes",
> and allowing "Attributes" to go beyond those four conceptual entities,
> may cause us to lose a lot of implicit semantics carried by the XACML
> 2.0 designations of these four entities), I noticed what might be
> considered to be "creeping functionality" in the XACML 3.0 Multiple
> Resource Profile. In particular:
>
>     * The following sentence was removed from the 2.0 Multi-Profile
>       from section 2.3.3 lines 232-234:
>           o "Note that the semantics for multiple <Resource> elements
>             are very different from the semantics for multiple
>             <Subject> elements in a request context as described in
>             the XACML core specification [XACML]."
>     * In addition, the following sentence introducing section 2.3 in
>       the 2.0 Profile:
>           o "This Section describes use of multiple <Resource>
>             elements in a request context to specify a request for
>             access to multiple resources."
>     * has been changed in the 3.0 Multi Profile to be:
>           o "This Section describes use of multiple <Attributes>
>             elements with the same category in a request context to
>             specify a request for access to multiple resources *or
>             requests for access by multiple subjects*."
>
>
> The bottom line is that I don't think the access by multiple subjects
> will work in the 3.0 profile, at least because there will now be no
> way to determine which members of one subject-category are associated
> with the members of the other subject-categories.

This was a decision made by the TC some time ago, to generalize the
multiple resource profile so it works the same way with other attribute
categories than the resource.

> My recommendation is that at the very least, we pull back the multiple
> subject notion from the multi-resource profile for now.

I think we can keep it. It's perhaps not as useful as the multiple
resource case, but we won't be able to do the "distinct subjects"
functionality anyway.

> Also, I suggest that we put an errata in for 2.0 removing the lines
> 2921-2923 as they seem to contradict the intentions specified
> elsewhere in the 2.0 document, which have also been carried forward to
> 3.0.
>
>     Thanks,
>     Rich
>