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Subject: RE: [xacml] Proposal to address issue 11, and thoughts on whether it is advisable or not to separate out sections of hier, mult to a new profile

Rich, I think I am closer to understanding your use case, but I still believe there are architectural solutions that can be implemented using current features of the 3.0 specs.
See attached diagrams, which I put together to show the process of implementing a PEP in a multi-tiered application.  You have a requirement:
(1) "Authorization is determined prior to accessing any resources at all."
Therefore the only choice for a PEP is at the access control point (ACP) labeled "1" in the diagram, "base-system-no-authz.png".  At this point, all the system has is search parameters from the user.  With your URI-reference scheme, you are proposing that the PEP compose a series of URIs representing an XML document that might be created at a future point in the process (perhaps, after ACP 2).  But how could it compose these URIs without knowledge of what was in the database?  Why couldn't it issue a request to the PDP, "Can Jones query the database", and get the decision (with obligation) like "yes, but only for books not by Jones, or costing less than 50 dollars"?
I'm probably still missing something about your use case and architectural constraints.

From: Rich.Levinson [mailto:rich.levinson@oracle.com]
Sent: Tuesday, October 13, 2009 18:33
To: Tyson, Paul H
Cc: xacml
Subject: Re: [xacml] Proposal to address issue 11, and thoughts on whether it is advisable or not to separate out sections of hier, mult to a new profile

Hi Paul,

Thanks for the comments. I don't significantly disagree with any of the specific points you mentioned, however, in your point 3, which, if I read it correctly was addressing my point 1, I don't think your example directly addresses the point I was making, which is the following:
  • The URI scheme does not require access to either the document or the database. Authorization is determined prior to accessing any resources at all.
    The XPath scheme requires access to the actual document, or issuing the query to the database, as you indicated in your alternative suggestion: "Query book database for candidate records".
    The URI scheme would base the decision on the query alone, before it is executed. The policies would scope based on policy-specified URI w scope. The request URI would be evaluated based on whether it fell within the hierarchical scope of the policy-specified URIs. (all URIs in policy evaluation would be in the form described in the proposal)
To me, this is a significant distinction. The reason it is required in XPath is that the namespaces need to be resolved. With the URI scheme, the namespaces can be resolved with only the schema of the target resource, the actual resource does not need to be accessed. i.e. the risk that is avoided is having an unnecessary access of the resource prior to determination of authorization.


Tyson, Paul H wrote:
3898C40CCD069D4F91FCD69C9EFBF09603E1143B@txamashur004.ent.textron.com type="cite">
I still can find no compelling reason to introduce this into the standard.  Any enterprise could certainly adopt this system of resource identification without violating the XACML specification.  I cannot see any advantage in doing this, but I believe Rich when he says there may be highly constrained situations that force one to this type of solution.  That is not a good enough reason to put it in the standard, though.
To address some specific points:
1. XPath performance overhead.  I don't know what others have experienced, but I have used saxon xslt engine for years, and simply cannot slow it down no matter how complex my xpath expressions are, or how large the input documents are.  I don't know what xpath engines are commonly used in XACML implementations, but poor performance is not an inherent feature of xpath processing.  In the case of multiple authorization decisions on a single document, it would be a very naive implementation that actually re-parsed the document for every decision.
2. Paradigm shifting: Attribute(Designator|Selector), xpath vs. regex.  Really, who *likes* regex? ;-) XACML is an XML application, therefore the first and favored approach should be xpath.  It's nice to be able to use regex when necessary, but I have never seen a case where it is the best first choice in XML processing (except for datatyping and value checking, but those areas are outside the domain of xpath).  A developer who knows regex better than xpath would want to use regex first, but xpath is a much more natural fit with XML.
3. Not wanting to expose entire XML document to PDP.  If this is a hard constraint, there are other architectural alternatives.  Take the book catalog use case.  For my money, I would do something like this before using the alternative URI representation:
    a. Query book database for candidate records
    b. Send multi-decision request with attributes like ((user=Jones), (type=book, author=Jones, price=50), (type=book,author=Smith,price=30), ...)
    c. Receive results
    d. Form XML for delivery to user, based on results.
This model would work for all cases where the XML is manufactured from database or other backend sources.  Just move the authorization step ahead of the XML creation.
The book use case does not actually depend on any hierarchical relations for the decision, so it is not the best example.

From: Rich.Levinson [mailto:rich.levinson@oracle.com]
Sent: Monday, October 12, 2009 22:02
To: xacml
Subject: [xacml] Proposal to address issue 11, and thoughts on whether it is advisable or not to separate out sections of hier, mult to a new profile

Based on Oct 8 TC meeting, proposals were solicited to address both issue 11, and the broader issue of whether or not we should consider separating out the XML document parts of Hier, Mult to another profile.

The attached document represents a proposed addition to Hier profile to address issue 11 (it is the same as attachment to http://lists.oasis-open.org/archives/xacml/200909/msg00076.html, except w highlight changes turned off to make Hier sections 2.2, 2.2.1 easier to read). (It is also included as attachment to emphasize it is a proposal, as opposed to a draft of an agreed change, which would be rev'd in the repository)

The following comments state why I think the proposed addition to Hier is needed (#1, #2) and why I think the hierarchical properties of XML documents should remain in the Hier/Mult profiles (#3), and that if other profiles are developed for XML docs then those profiles should refer to Hier/Mult for their hierarchical access properties.
  1. The proposed addition to Hier is needed because it represents functionality that is currently missing from the Hier profile that enables identifying resources within an XML document without having to provide the XML document itself.

    • The problem introduced by requiring the presence of the XML document is that, for example, it requires actually accessing and exposing the protected resources in order to determine if access is allowed to those same resources. While this may be an acceptable increase in risk of exposure in some application environments, it may not be acceptable in others where very sensitive data is involved, and an alternative should be provided for those cases.

    • For more specific example, XML-frontended datastores contain resources in relational or other legacy storage mechanisms and primarily use XML as vehicle for containing and carrying those resources. Requiring construction of XML documents containing those resources, which could potentially contain very sensitive data, in order to construct a request to determine whether access to those resources is allowed should not be required if alternative mechanisms which do not require this exposure are readily available.

  2. The proposed addition is also needed to provide a unique uniform naming mechanism and policy reference mechanism for all hierarchical resources whether they are contained in an XML document structure or some other hierarchical structure. i.e. XML documents have an inherently simple hierarchical structure that has an implicit resolved name structure in the underlying XPath data model that should be able to be used for resource identification and policy definition despite the fact that the XPath language, itself, does not expose this capability of the underlying reference model.

    • The attached proposal uses a commonly used mechanism (Clark notation: curly braces around resolved namespace prefix) that addresses the omission from the XPath language of the ability to enable single string display representation of explicit full hierarchical path to each node. This path is also percent-encoded where required in order that it can be used as a URI fragment as described in section 2.2.1 of attachment, which seamlessly augments the existing Hier URI scheme in section 2.2.

  3. It is recommended to leave the XML document sections in Hier/Mult for the following basic reason: The introduction to the Hier profile (section 1, lines 41-54) makes it clear that XML documents are regarded as generally only one possible "representation" of the actual target hierarchical resources. Therefore there seems to be little to be gained by separating out one representation of the general hierarchical resources covered by the profile into a separate profile. What would seem to make more sense is that a more general XML/WebServices profile could reference the Hier profile when necessary for matters concerning the "hierarchical" access control aspects of the  more general XML/WebServices problem space addressed by that new profile.
Additional context for this proposal has already been discussed in tc emails and will not be repeated here, but may be found in the following references to those emails:
Comments and suggestions welcome.




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