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Subject: Re: How to get attributes from other categories

Hi Rich,

On 30/07/2013 3:00 PM, rich levinson wrote:
Hi Steven,

Apologies for taking so long to get back - as I indicated at the mtg,
I was on vacation when this email arrived and simply missed it.

I don't think I agree w your analysis/interpretation of my proposal.
Let me try doing it more explicitly using your example as a basis.

In the policy, the attribute designator contains, as you have suggested:


I think where my differs begins in the attribute directly targeted by
the designator, which is "organization" in the "access-subject" category.
I think the attribute would look like this:

       <Attribute AttributeId="organization">

where the only difference is I have provided the Attribute envelope
w the AttributeId, plus I have changed the XPathCategory to something
that looks more like a category than what you had there (for which I
did not understand the motivation?: did you intend that there be
a category called "http://foo.com";? this did not seem to make sense
to me.).

I intended that there be another entity represented by an Attributes
element and identified by the URI "http://foo.com";. This is a specific
organization associated with the access subject, which is neither the
access-subject nor the resource. I could have conjured up a new,
predefined category called "organization", but this assumes that
the access-subject can only be associated with one organization. The
"Attributes of Relations" thread is, in part, about how to deal with
multiple instances of the same kind of entity. In this case, association
with more than one organization. If each such organization's Attributes
element used the same CategoryId, then this would be interpreted as a
request for multiple decisions, which we don't want. So to allow more
than one organization entity in the request context, they each need
to have a unique CategoryId, which is really an entity identifier in
their case, but I don't have the luxury of changing the core schema
so CategoryId has to do double duty.

Now, if you are only going to use your proposal to reference from one
predefined category into another predefined category, then I don't see
the value in it. The attribute designator could just be written to
directly access the other category instead, e.g.:


In any event, given the way this example is set up, we would now
look in the resource category "Attributes" element for an attribute
w AttributeId="non-profit-organization" (again, I am just following
the example, although the naming does not seem to make sense,
which may have something to do w our differing views).

    <Attributes Category="resource">
       <Attribute AttributeId="non-profit-organization">
         <AttributeValue DataType="boolean"

So, basically, the motivation here, in the context of this example,
is that for some reason, the <Attributes Category="resource">
contains an attribute that the preparer of the
     <Attributes Category="subject-access">
wants to refer to. Maybe this is just the way they want to
set up their policies w the subject-access containing attributes
that are associated w other "entities" than the subject-access
itself. However, let's leave the motivation until we have a
common understanding of the proposed mechanism and
then see if it addresses the attributes of relations use case,
which would then be the motivation, if appropriate.

Now, to address the issues you raised:

First the summary statement would say

    "then the attribute designator would fetch the boolean value(s) of the
      non-profit-organization attribute in the resource Attributes category
      of the Request"

So the statement begins the same, but now indicates that the actual value
is obtained from another Attributes category element.

That was what I intended. The CategoryId of that Attributes element
being "http://foo.com";.

Now, the issues:

  * "The content of the attribute value in the access subject (a URI)
       doesn't conform to it's nominated syntax (boolean)."

      o You are correct that the DataType in the AttributeValue
           in the Attribute w AttributeId "organization" does not
           match the "DataType" of the AttributeValue.

        However, the reason is that the "XPathCategory" xml attribute
          of the AttributeValue semantically changes the DataType
          xml attribute to apply to the target AttributeValue in the
          Attributes element w the category specified in the
          current value. i.e. the AttributeValue of the referencing
          Attribute, which is the one referenced by the
          AttributeDesignator contains the AttributeId of the
          Attribute to be found in the Attributes w
          the category specified by XPathCategory.
        i.e. The presence of the XPathCategory changes the
          AttributeValue element from being a value container
          to being a value "referencer".

I understand that the presence of the XPathCategory XML attribute changes
the actual data-type, but it's ugly to have the value proclaim it is one
type when it is really a different type. I also realize that you can't
fix that without breaking something else.

  * "It's not obvious from the attribute designator what attribute values
       are being fetched."

      o I disagree. The AttributeDesignator is pointing to the
        "access-subject" Attributes element, w metadata
          restrictions of AttributeId and DataType.

        Because the Attribute contains an XPathCategory this
          indicates that the target value is not here, but in
          another Attributes element.

Well that's kind of my point. The AttributeDesignator says get the organization
attribute from the access-subject, but the values that are returned are actually
from the non-profit-organization attribute in a different category. Or they
might actually be from a boolean organization attribute in the access-subject
because the values didn't carry the XPathCategory XML attribute. Or they might
be from some other attribute entirely because the PEP or PIP put in something
other than what you intended. We can't look at an AttributeDesignator and be
certain where the values it fetches will be coming from. Control has passed from
the policy writer to the PEP and PIP.

        This appears to me to be the same as is done w
          the XPathExpression, where, if XPathCategory
          is set to anything other than that of the current
          Attributes element, then the value is retrieved
          from a different Attributes element.

        In that case, the XPathExpression, itself, acts
          as a kind of AttributeId, by pointing to a specific
          location in an external xml document, namely,
          the Content "document element", wherever it
          may be in the overall Request.

        i.e. in the XPathExpression case, the AttributeDesignator
          also does not "know" in which Attributes element that
          the AttributeValue will be found.

Actually it does "know" because the XPathCategory in an xPathExpression
value does not affect the processing by the AttributeDesignator. The
AttributeDesignator will fetch values of the nominated attribute and
data-type from the nominated Attributes element. If those values have
the xPathExpression data-type, then they will have an XPathCategory
XML attribute, but the AttributeDesignator will just provide these XML
attributes in the bag of XACML attribute values that it passes back.
It is the XPath-based functions that process the XPathCategory XML

The ContextSelectorId of an AttributeSelector will make the AttributeSelector
pay attention to the XPathCategory XML attribute, but section 5.30 of the
core specification says that the "XPathCategory attribute of the referenced
attribute MUST be equal to the Category attribute of the attribute selector"
ruling out the possibility of referencing into a different Attributes

Thus the indirection you are proposing for the AttributeDesignator does
not have a precedent in the processing of xPathExpression values by
attribute designators and selectors.

  * "The correlations between different attributes of the related entity
       are lost, just as in attribute flattening."

      o Ok, I'm not sure what you mean by "attribute flattening",
        but I do not believe anything is "lost" in my proposal.

Attribute flattening is described in the IPC profile.

        The only thing that changes here is that the access is
          "indirect", just as it is w the XPathExpression that uses
          XPathCategory and AttributeValue to navigate to the
          actual value.

        i.e. in this case the "navigation" is also using XPathCategory
          to identify the Attributes element, but instead of "XPath'ing"
          to the target attribute value, it is "AttributeId'ing" to the
          target AttributeValue.

        Bottom line is that nothing should be lost because all the
          info is retained either in the referencing or the referenced
          Attribute element, and the same metadata controls can
          be applied as in the direct case. It's just the presence
          of (the poorly named) XPathCategoryId triggers a
          slightly more complicated navigation mechanism.

Suppose there is a second boolean attribute of an organization that we want
to use in decisions. Let's call it US-organization. We can't reference it
using the same access-subject/organization attribute we used for
non-profit-organization because that would be ambiguous. We need two
access-subject attributes to do the job. Let's call them organization-np
and organization-us. Allow that the access subject can be associated with
zero, one or more organizations.

We can fetch the values the subject's organizations' non-profit-organization
attributes using an attribute designator like:


We can fetch the values of subject's organizations' US-organization
attributes using an attribute designator like:


We can ask a question like "is the subject associated with a non-profit
organization and associated with a US organization ?" using an expression

    <Apply FunctionId="and">
      <Apply FunctionId="boolean-is-in">
        <AttributeValue DataType="boolean">true</AttributeValue>
      <Apply FunctionId="boolean-is-in">
        <AttributeValue DataType="boolean">true</AttributeValue>

However, we have no way to ask the question "is the subject associated
with an organization that is both a non-profit organization and a US
organization ?" unless we know for certain that the user is associated
with exactly one organization. The above "and" expression will work
in that case because we know that any and all values returned for
organization-np and organization-us are properties of the same
organization. However, when the subject can be associated with one or
more organizations we don't know which organization-np values belong
with which organization-us values. These are the correlations that
get lost with attribute flattening and with your proposal.


Hopefully this makes the proposal a little more clear. I certainly
welcome any issues that you may raise, but I think the current
issues are just indicators that clarification of the mechanism
was needed as opposed to substantive functional issues, which
may still be present, but I don't see them yet.


On 7/16/2013 8:33 PM, Steven Legg wrote:

Hi Rich,

On 13/07/2013 3:48 PM, rich levinson wrote:
To Steven, Mohammad, & TC:

At yesterday's meeting, I mentioned that I thought it might be possible
to implement the "relationship based access control" reqts using the
current 3.0 spec, but also that I have not had time to fully analyze
the reqts and the applicability of the soln.

In any event, I will explain things as far as I have gotten looking
at this capability.

The first thing that brought this to my attention was when I was
looking at examples using XPathCategory in the 3.0 spec. I was
aware that this had something to do w AttributeSelectors, but
I was surprised to find one in an AttributeDesignator (in Rule 1
in sec

    1090 [f60] <AttributeDesignator
    1091 [f61]     MustBePresent="false"
    1092 [f62] Category="urn:oasis:names:tc:xacml:3.0:attribute-category:resource"
    1093 [f63] AttributeId="urn:oasis:names:tc:xacml:3.0:content-selector"
    1094 [f64] DataType="urn:oasis:names:tc:xacml:3.0:data-type:xpathExpression"/>
    1095 [f65]

The example is connected to the request in section 4.2.2, and, sure enough,
there is an Attribute there that will be resolved w this designator:

      963 [e43] <Attribute IncludeInResult="false"
      964 [e44] AttributeId="urn:oasis:names:tc:xacml:3.0:content-selector" >
      965 [e45]   <AttributeValue
      966 [e46] XPathCategory="urn:oasis:names:tc:xacml:3.0:attribute-category:resource"
      967 [e47]       DataType=" urn:oasis:names:tc:xacml:3.0:data-type:xpathExpression"
      968 [e48] >md:record/md:patient/md:patientDoB</AttributeValue>
      969 [e49] </Attribute>

There is nothing particularly remarkable about this particular example,
however, it is fairly obvious that the xpathExpression value could apply
to the <Content> of any <Attributes> element in the <Request>, simply
by changing the value of XPathCategory to point to the Category w
the desired value, such as category:action, or category:access-subject.

Therefore we have a potential starting point for referencing attributes
in some other category than the category that the associated
Attribute element is in. i.e. in the above example the Attribute
is in the category:resource collection, but its value can be in
the <Content> element of either the category:resource collection
OR any other <Content> element in some other category:xxx
collection, simply by setting the Value of XPathCategory

There is an additional benefit that the xpath selection mechanism
can also be associated w the metadata of the xpathExpression
Attribute, which I don't think is the case w the plain vanilla
AttributeSelector (but this is a secondary note, not the main
point of this discussion).

There are a couple of choices that I considered for making this
mechanism more general:

 1. We could allow XPathCategory to be used w any
    DataType, in which case the content of the AttributeValue
    would contain the AttributeId of the desired Attribute
    in the other Attributes collection-category.  I believe this
    is functionally equivalent to the way the xpathExpression
    is used, if one considers the path in the content to
    effectively be the "id" of the attribute. Also, it could
    be the presence of the XPathCategory attribute that
    would trigger the semantic interpretation of the value
    as a reference instead of a normal value.

So if a value fetched by an attribute designator has the XPathCategory
XML attribute (not the best name for it, obviously), then the attribute
designator instead fetches the attribute nominated by the original value's
content from the entity nominated by the XPathCategory XML attribute for
the data-type specified by the attribute designator ?

For example, if this is the attribute designator:


and this is one of the values of the organization attribute in the


then the attribute designator would fetch the boolean values of the
non-profit-organization attribute in the access subject's organization

If I've joined the dots correctly, then I see a number of drawbacks:

- The content of the attribute value in the access subject (a URI)
  doesn't conform to it's nominated syntax (boolean).

- It's not obvious from the attribute designator what attribute values
  are being fetched.

- The correlations between different attributes of the related entity
  are lost, just as in attribute flattening.


 2. I can't remember the other choices at the moment, but
    I think the above was the best one as I recall.

So, I think the above should get the basic idea across. I had
intended to go thru the emails on attributes of relations
in more detail to determine if the above had the potential
of meeting the reqts, but I have not had the time to do
that, so I am asking the interested parties to that discussion
if they think this approach would be viable, and also a path
of minimal impact on the existing xacml specs.


Thanks, Rich

Oracle <http://www.oracle.com>
Rich Levinson | Internet Standards Security Architect
Mobile: +1 978 5055017 <tel:+1%20978%205055017>
Oracle Identity Management
45 Network Drive | Burlington, Massachusetts 01803
Green Oracle <http://www.oracle.com/commitment> Oracle is committed to developing practices and products
that help protect the environment

Thanks, Rich

Oracle <http://www.oracle.com>
Rich Levinson | Internet Standards Security Architect
Mobile: +1 978 5055017 <tel:+1%20978%205055017>
Oracle Identity Management
45 Network Drive | Burlington, Massachusetts 01803
Green Oracle <http://www.oracle.com/commitment> Oracle is committed to developing practices and products
that help protect the environment

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