Re: [xacml-comment] Multiple decision result of type xpathExpression

[Steven Legg <steven.legg@viewds.com>]

Hi Clement,

On 6/09/2013 12:59 AM, Pellerin, Clement wrote:
> > On Sep 05, 2013 1:20 AM, Steven Legg wrote:
> > > On 31/08/2013 6:04 AM, Pellerin, Clement wrote:
> > > I'm still confused what to do when the element to test is a complex type.
> > > For example, how can I accept an element called public knowing
> > > it is a complex type and has no significant text value?
> > >
> > > Let's say I'm satisfied to write the condition in XPath,
> > > how can the boolean returned by XPath be used by the Match element?
> >
> > It can't. XACML requires that the result of the XPath expression is
> > a node-set. Anything else causes an AttributeSelector to generate an
> > error. Thus, there are limits to what it is possible to do with XPath
> > expressions in XACML.
>
> Yes, this is what I had concluded but I was hoping to be wrong.
>
> It would be an easy extension for XACML 4.
> Instead of doing the conversion from Node to String in the AttributeSelector,
> the XPath expression could return a String, which could be converted to
> the XACML primitive type by the new kind of AttributeSelector.

It is an extension that could also be done in a profile of XACML 3.0.
The existing conversion of nodes to strings could remain as it is and new
cases could be added for converting boolean, number and string results of
XPath expressions. An implementation that didn't support the profile would
return indeterminate errors if it encounters a boolean, number or string
instead of a node-set, which is a completely appropriate response.

> This still leaves unanswered my opening question.
> The XACML request contains a content-selector with an XPath expression
> that is known to return a single complexType Node with no text value.
> How can I write an efficient XACML expression to test some condition
> about that Node?

The Path argument of an AttributeSelector is relative to the node identified
by the ContextSelectorId attribute, so you can pull out any descendant nodes
of the single complexType Node as XACML attribute values and apply tests
to them, though it does require the PEP/PIP to provide the XPath expression
identifying the complexType Node as the value of the ContextSelectorId attribute.

For example:

    <Apply FunctionId="urn:oasis:names:tc:xacml:1.0:function:and">
      <Apply FunctionId="urn:oasis:names:tc:xacml:1.0:function:any-of">
        <Function FunctionId="urn:oasis:names:tc:xacml:1.0:function:integer-less-than"/>
        <AttributeSelector
          Category="urn:oasis:names:tc:xacml:3.0:attribute-category:resource"
          ContextSelectorId="urn:oasis:names:tc:xacml:3.0:content-selector"
          Path="Amount/child::text()"
          DataType="http://www.w3.org/2001/XMLSchema#integer";
          MustBePresent="false"/>
        <AttributeValue DataType="http://www.w3.org/2001/XMLSchema#integer";>200</AttributeValue>
      </Apply>
      <Apply FunctionId="urn:oasis:names:tc:xacml:1.0:function:any-of">
        <Function FunctionId="urn:oasis:names:tc:xacml:1.0:function:string-equal"/>
        <AttributeSelector
          Category="urn:oasis:names:tc:xacml:3.0:attribute-category:resource"
          ContextSelectorId="urn:oasis:names:tc:xacml:3.0:content-selector"
          Path="Colour/child::text()"
          DataType="http://www.w3.org/2001/XMLSchema#string";
          MustBePresent="false"/>
        <AttributeValue DataType="http://www.w3.org/2001/XMLSchema#string";>red</AttributeValue>
      </Apply>
    </Apply>

However, you are still limited to XPath expressions that identify nodes,
and there is no way to reliably apply non-trivial tests to more than
one complexType Node as part of the same individual decision request
(because ContextSelectorId attribute is explicitly required to identify one
node).

Regards,
Steven

> I believe Node is not a primitive type in XACML 3, and
> bag of Nodes are not supported either. This means a nodeset
> can only be the result of an intermediate result within the semantics
> of a built-in component like AttributeSelector or an intermediate result
> of a built-in function like xpath-node-equal.
>
> It seems hopeless to evaluate the content-selector to use the result
> as the context node of another XPath expression.
> Even writing a custom function seems difficult since it would require
> the built-in behavior of AttributeSelector up to the point where the
> conversion occurs.
>
> Hopefully, I have overlooked a simple solution,
> otherwise this would expose a serious lack of functionality
> for such a common use case.