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Subject: Re: [xacml] Open issue 66

Hi Erik,

I don't think your comments are addressing the specifics that were
raised in the issue, which I am copying below for quick reference.

To summarize the issue:

 Part 1. is basically saying that we need some context about the example
    in order to look at the details in a consistent manner. Basically, my reading
    of the example is that when rule 1 is evaluated, the Target determines that
    the Rule is applicable, and presumably the Rule is evaluated and it should
    produce an Indeterminate result because there is no Subject "patient-number"
    as required on line 1134 when the Condition is evaluated.

    Based on this Rule I expect that the Indeterminate result causes us to
    look at the prescribed processing in section 7.15.3, which says that
    a missing-attribute status code could/should be returned.

    To me, this provides an excellent example of how a PEP is informed
    of what attribute needs to be supplied.

Before I go further into the details, I'd like to get your response to that.

Also, I'd like to note that while I realize that this issue has been floating
there for a long time, one reason is that no one has really wanted to
discuss it in detail. Prior responses were more of the nature of there
will be some other mechanism for getting the attribute. In general, when
we are considering attributes that need to come from users or from
resource attributes, I believe there is a non-trivial issue about how
those attributes get brought into the picture. The first problem is
knowing that they are needed. The fact that they are needed is
driven by the existence of an applicable policy that needs it, which
clearly is a state that can come and go as an administrator adds or
removes a particular attribute from the policy.

So, the basic use case here is: an admin creates or changes a policy
and decides to add an attr as part of the policy eval. Given this
change, how does this attr become part of the request.

 (Note: I'd actually prefer to focus on resource attrs rather than
    subject attrs, but since this particular situation w Rule 1
    appeared obvious to me as a typical example, it happened
    that the missing attr was subject-based (there is also an
    assoc resource attr, but the req mysteriously already had
    supplied that one)

My reading of the spec is that 7.15.3 provides a general purpose
mechanism for accomplishing this by returning an identifier specifying
what attr is needed, based on which the PEP and appl can do
whatever they want to get the attr then resubmit the request with
the attribute included.

This seems to me to be pretty essential functionality. Let me know
if you think I am missing some important point here. I will be happy
to help "fix" the spec w the detail I think is needed, but before we
get to that point there needs to be some agreement that changes
are even appropriate.


66. Missing attributes may be underspecified

I did a somewhat detailed analysis of "Example two" in the core spec from the point of view of understanding how fine grained authorization (fga) (applying resource attrs to az decision) was implemented and came across a number of items that I think need to be addressed especially in potential interoperability situations. I will put all in one issue initially, we can decide if it needs to be broken out later.

1. line 1090-91 describing ResourceContent. In both the core spec and the sample messages, the ResourceContent contains the following:

While I recognize that the example itself is not intended to be perfect, it provides a convenient context for raising the following questions/issues, especially wrt fga.

  1. If this is a first request from a PEP, why is the PEP supplying patient-number on line 1056? This looks like a required attr to evaluate Rule 1 (line 1141), if the requestor is the patient, but this example the requestor is the physician.
    • The physician-id is supplied in the request (line 1044), but the only rule it appears in is rule 3 (line 1522). This rule only allows "write" access (line 1507), so I expect this request would probably fail as it is currently set up. i.e. we would need to add a "read" action to rule 3 or add a physician-id test to rule 1.
    b. Assuming the above request fails, let's consider what might be done. There was a "read" request issued (line 1072), so that would mean that rule 1 (line 1182), rule 2 (line 1347), or rule 4 (line 1668) could be applied.

Rule 1 requires a Subject attribute patient-number (line 1134) to match the patient-number in the requested resource record (line 1141). Presumably a <MissingAttributeDetail> could be returned, somehow identifying these 2 attributes to the PEP.

Rule 2 requires a patientDoB resource attr (line 1297), a parent-guardian-id subject attr (line 1363), and a parentGuardianId resource attr (line 1371). Similarly a <MissingAttributeDetail> could be returned requesting these.

  • Assuming this to be the case, one question I have is how does the

    <MissingAttributesDetail> tell the PEP whether the attributes that are missing should be resubmitted as part of the Subject or as part of the Resource? This info is provided in the Request from the xml structure, however, the <MissingAttributeDetail> does not have equivalent structure to make such distinctions.

The above is intended just to give an example of questions that occur for this particular example, but it is my opinion that it is symptomatic of a general problem of how PEPs are supposed to know how to construct the proper RequestContext necessary, in general, for complex scenarios that require substantive fga attrs.

In these more complex fga scenarios it is likely that <MissingAttributeDetail> will be typically needed to collect all the required attributes. Therefore, I believe some more robust mechanisms, possibly using MissingAttributeDetail as a good starting point will be needed to adequately define operation in this area.

In this context as well, it is likely that xpaths are probably not the way to go since they are only applicable to certain types of resources (xml-based) and those resource structures are likely to change in time, and these changes should not percolate into the enterprise Policy arena. Therefore, mechanisms such as vocabularies should be recommended usage here with the PEP being responsible for mapping the vocabulary item to the particular resource physical access path such as the xpath.

Status: OPEN


Erik Rissanen wrote:
48C76772.3060402@axiomatics.com" type="cite">All,

I propose that issue 66, "Missing attributes may be underspecified", to be closed without action. The issue has been up there a long time with no proposals for a solution. In addition to that, I believe it is technically impossible to provide a solution. The reason someone gets a not-applicable from the PEP is because "there is no policy which applies". In general there is no way to describe in the form of "missing attributes" what the PEP needs to provide for the policy to apply. Policies can be much too complex for this. In particular, a policy could be NotApplicable because an attribute is present, for example. Or it might require that three particular integer attributes form a pythagorean tripple. How do you express that as "missing attributes"?!

And we demonstrated in the RSA interop that obligations can be used to handle simple use cases where some attributes can be expected to be missing. An obligation can be used to mark the part of the policy which required an attribute, and the obligation can then be returned by the PDP if the attribute is missing.

Best regards,

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