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Subject: Re: [xacml] Minutes of 12/02/02 comments subcommittee call

> You don't have to "guess".  If the AttributeSelector occurs in a Target Subject,
> then pre-pend "Request/Subject.  If the AttributeSelector occurs in a Target
> Resource, then pre-pend Request/Resource, etc.  If the AttributeSelector
> occurs in a Condition, then prepend Request/.

Well, you do have to "guess" since, as the original comments pointed out, the
expression may already have been absolute, or may contain back pointers or
other XPath wierdness. You can parse the expression, figure out what it has,
and then do some pre-pending, but that's slower, and means you can't as
easily use existing libraries (see next comment). If you require a valid
XPath expression from the start, you don't need to worry about this, though
you should probably check that you're pointing into the right part of the
request (which _can_ be done with most libraries).

> Such pre-pending is just for an implementation that somehow is able to use an
> XPath library even though, as far as I know, no such library would be able
> to deal with the "notional" Request Context and be helpful in resolving
> attributes not supplied in the physical request.  Since a conforming implementation
> must deal with such attributes, a conforming implementation will probably have
> to parse XPath expressions itself (possible with some help), and can validate
> the expression according to the XACML-specified root.

Not true. A conforming implementation that uses AttributeSelectors must be
able to look in the physical request document, and if nothing is found, it
should pass the request on to the Context Handler. In this case, the first
step can be done with an off-the-shelf XPath library, and if that fails, the
expression can be passed to the Context Handler, which can choose what to
do next. I'm not saying that this is the way it must be done, but it's
certainly reasonable to expect that implementors will use existing libraries
for this step (and probably use them in part for the next step).


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