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Subject: Re: [xacml-users] Clarification of Hierarchical Resource Profile

Hi Steve,

I find your explanation much easier to understand now. It sounds like:
  • there is one underlying "physical" resource, "c", that can be obtained either
    • by application "A" by navigating via /a/b/c
    • or by application "X" by navigating via /x/y/c
  • basically "A" presents user with /a/b/c,
    and "X" presents user with /x/y/c
    where /a/b and /x/y are "steps" the user takes thru the appls
I think the following Policy elements placed in a Target would make the Policy
applicable to any resource prefixed with "file:///a/b"
      DataType="http://www.w3.org/2001/XMLSchema#string" /> 
A similar match statement could be prepared with "file:///x/y/.*" to match any request
prefixed with "file:///x/y".

These match statements can be combined within a Resource statement to create an "AND",
or combined within 2 separate Resource statements to create an "OR".

Also, I have added a prefix "file://" in order to make these proper URIs, but will not
use it below. The point is while the example does not use full URI syntax, there is no
reason why it can't, and we will just assume it is shortened for convenience.

I think the rest of my comments can now go in line.


Steve Bayliss wrote:
003c01cbb88f$8f8c1eb0$0301010a@asusp4t533" type="cite"> Message
Hi Rich
Many thanks for your response.  I realise my questions are not clear, I was struggling myself with how to phrase them, and my terminology is rather loose in places.  Perhaps the best approach is for me to step back and outline two distinct scenarios; particularly to address what I was trying to express with "resources-in-path-context".
In both scenarios there is a resource c that is present in two hierarchies:
And in both scenarios, there is a user application that allows a user to browse through the hierarchies to locate the resource.  The user could browse by starting at the top of either hierarchy (a or x) to locate the resource c.
Scenario 1:
The user browses starting at "x", and locates the resource under the hierarchy /x/y/c
Is it possible to write a policy that applies based on which hierarchy the user navigated to locate the resource, ie a policy that applies to their browsing path of /x/y/c, so that the policy would apply if they had browsed through x->y->c but would not apply if they had browed to the resource through a->b->c?
The first (and only) ResourceMatch element above I think is what such a Policy would need
to contain to match the "/a/b/c" path.
003c01cbb88f$8f8c1eb0$0301010a@asusp4t533" type="cite">
(This is what I was attempting to express with "resource-path-in-context", the context being the route a user took to locate the resource.)
That is much more clear now, thanks.
003c01cbb88f$8f8c1eb0$0301010a@asusp4t533" type="cite">
The application's PEP would be able to supply the browsing path in the request context.
Agree, the general assumption is that the PEP is able to collect information about the
request and transform that information to an appropriate set of attributes.
(If additional attributes are required they can be obtained via the ContextHandler or
by using the MissingAttributesDetail technique.)
003c01cbb88f$8f8c1eb0$0301010a@asusp4t533" type="cite">
Is this something that the Hierarchical Resource Profile is suited to?
I believe so. It appears to me to be a perfect fit.
003c01cbb88f$8f8c1eb0$0301010a@asusp4t533" type="cite">
Or should this "navigation path" should be a separate attribute outside of those specified in the Hierarchical Resource Profile?
I don't think it is necessary for an additional "navigation path" attribute. I think the navigation
path makes for a perfectly good resource-id.
003c01cbb88f$8f8c1eb0$0301010a@asusp4t533" type="cite">
I can see how a policy could be written using a "navigation path" attribute, my question is whether the scenario is relevant to/better implemented with the Hierarchical Resource Profile, using the resource-id attribute to represent the navigational path used by the user.
The only distinction I can think of right now is that a "navigation path" in the sense of tracing everything the user
has been doing may have redundant info. However if the "navigation path" removes backsteps, then I think it
is effectively equivalent to the URI path, which is a legitimate resource-id value.

Am I missing something here?
003c01cbb88f$8f8c1eb0$0301010a@asusp4t533" type="cite">
Scenario 2:
The user browses to the resource c.  It is irrelevant from a policy perspective of how they navigated to the resource.
Presumably, within this particular example there are only the two paths available. In any event, even if there
were more paths, the profile should work just as well.
003c01cbb88f$8f8c1eb0$0301010a@asusp4t533" type="cite">
Two separate policies are to be written, one for all resources that are descendents of "a", and another for all resources that are descendents of "x".  Both policies should be applied when the user requests resource "c".
This sounds like it could be done with an "AND" of the two ResourceMatch elements (one for "/a.*",
and the other for "/x.*") within a single Resource element.
003c01cbb88f$8f8c1eb0$0301010a@asusp4t533" type="cite">
It seems that this is suited to the Hierarchical Resource Profile.
Agree, but also think scenario 1 is just as applicable.
003c01cbb88f$8f8c1eb0$0301010a@asusp4t533" type="cite">
It would be the responsibility of the Context Handler to provide two hierarchical resource-id attributes.
The algorithm in HRP XACML 3.0 section 3.3.1 shows how resource-ids can be constructed given a list
of unique resource identifiers (can be a simple integer starting from zero and incremented for each resource
that is added to the collection). This "list" can be regarded as "column 0" of the "resource map".

For each hierarchy applied to the collection an additional column is created for the map. Within the column,
the resource identifier of the parent of the current resource is assigned to create the hierarchy.

The context handler can use this map to create all the resource-ids needed for the resource.
003c01cbb88f$8f8c1eb0$0301010a@asusp4t533" type="cite">
Policies could be then written using (URI regex match) /a/.* and /b/.* (or indeed separate rules as you suggest).  Alternatively the "ancestor scheme" could be used as you suggest, in which case the Context Handler would provide the resource-parent, resource-ancestor, resource-ancestor-or-self attributes (and the policy would be written in terms of these rather than a resource-id regex match.
I agree the "ancestor scheme" can be used here. Personally, I think the URI scheme is sufficient for
the use cases I have considered to date. I think it is just a matter of how the information is viewed
as to which approach is preferable.

With the DAGs, I think the URI would not work very well, if at all since there are no definitive
unambiguous URIs that can be constructed. So, if resources are viewed as DAG, then should
use ancestor approach. Same algorithm in 3.3.1 is applicable.
003c01cbb88f$8f8c1eb0$0301010a@asusp4t533" type="cite">
Further comments:
To help my understanding of the difference between the URI scheme and the "ancestor scheme":
  • If the URI scheme is used, the resource-id attribute is a URI specifying the hierarchical path to the resource.  Multiple resource-id attributes would be present if the resource is to be found under more than one path.
Yes, that is correct, and it is allowed as specified in XACML 2.0 section 6.3 lines 2947-2954,
and  XACML 3.0 generally Attributes can contain multiple AttributeValues. It probably would
be more correct to say something like "the Attribute with AttributeId = "...resource-id" generally
can contain multiple AttributeValue elements with different values".
003c01cbb88f$8f8c1eb0$0301010a@asusp4t533" type="cite">
  • If the "ancestor scheme" is used, the resource-id does not have to be a URI, and does not have to contain any information about which hierarchies the resource belongs to.
003c01cbb88f$8f8c1eb0$0301010a@asusp4t533" type="cite">
  • Instead resource-parent, resource-ancestor and resource-ancestor-or-self attributes are used to represent this information
Yes, although, this probably is only relevant to DAGs, and is superfluous for any number of single parent hierarchies.
003c01cbb88f$8f8c1eb0$0301010a@asusp4t533" type="cite">
The "relaxation" I thought I had identified for the resource-id in the URI scheme was due to the 2.0 document stating (section 2.2) that "The identity of a node in a hierarchcial resource ... SHALL be represented as a URI ...", whereas the 3.0 document (again section 2.2) has "The identity of a node in a hierarchical resource ... MAY be represented as a URI...".  However on re-reading the subsequent text in section 2.2 of the 3.0 spec, it now seems clear to me that the resource-id must indeed be a URI.
I hope this goes someway to clarify the questions I was attempting to pose in my original post.
Yes, I think we are in agreement, assuming my comments are consistent with what your
statements are intended to mean.

003c01cbb88f$8f8c1eb0$0301010a@asusp4t533" type="cite">
-----Original Message-----
From: Rich.Levinson [mailto:rich.levinson@oracle.com]
Sent: 20 January 2011 04:23
To: Steve Bayliss
Cc: xacml-users@lists.oasis-open.org
Subject: Re: [xacml-users] Clarification of Hierarchical Resource Profile

Hi Steve,

Not sure if I fully understand your questions, but will try to respond inline. Note:
my first few responses are attempting to understand your question, the later
responses are probably closer to being in the answer space of your questions.

I do not expect that this will fully answer your questions but hopefully it will
move the ball down the field a bit.


Steve Bayliss wrote:
001e01cbb7f7$85a884d0$0301010a@asusp4t533" type="cite">
I'm seeking to clarify my understanding / the intent of the Hierarchical Resource Profile.  Primarily XACML 2.0, but I understand that 3.0 mainly refines and adds clarity, so anything from 3.0 that's relevant in this respect I am interested in.
I'm iinterested in the hierarchical path representations of non-XML nodes, ie 2.0 spec lines 190 et seq.
I am having trouble parsing the following paragraph. I will deal with each segment in sequence:
001e01cbb7f7$85a884d0$0301010a@asusp4t533" type="cite">
My question is if the Hierarchical Resource Profile deals (is intented to deal with) with resources-in-path-context. 
The sentence above appears to be asking the question:

    Does the HRP deal with "resources-in-path-context"?

If that is the question, then what does the term "resources-in-path-context" mean? For example,
XACML Policy deals with information in the RequestContext. Is this what you are referring to?
001e01cbb7f7$85a884d0$0301010a@asusp4t533" type="cite">
That is, if a resource belongs to multiple hierarchies/has multiple parents/ancestors, can a policy be written for this resource-in-path-context. 
The answer to the first part of the question is "Yes, a policy can be written for it.".
As above the term "resource-in-path-context" is undefined, at least to me, so I do not understand the last part of the question.
001e01cbb7f7$85a884d0$0301010a@asusp4t533" type="cite">
(to put it another way, policies dealing with resources when being accessed as part of a particular collection having a specific policy, rather than a resource being accessed, and because it is a member of - rather than being accessed in the context of being a member of - having a specific policy).
I do not understand the above statement at all. The first phrase appears to use the term policy redundantly:
"policies (dealing with resources when being accessed as part of a particular collection) having a specific policy"
The remainder I find in more difficult to parse. My point here is not to criticize your sentence structure, but to
try to understand what the point is you are trying to make, and possibly by explaining why I find it confusing,
will help to establish a terminology where the issues can be addressed. That being said, the last phrase appears
to me to be saying:
There is some kind of distinction between:
  • a resource being accessed because it is a member of (a specific collection?)
  • a resource being accessed in the context of being a member of (a specific collection?)
Is this the point, that the above two bullets are distinct in some manner?
001e01cbb7f7$85a884d0$0301010a@asusp4t533" type="cite">
Example paths for a resource "c" (ignoring the fact these aren't URIs, I gather this restriction was relaxed in 3.0)
Ok, assuming the above URI pathname portions plus leading "/" are provided as resource-ids (where multiple resource-ids
for the same resource are allowed as specified in section 6.3), the two provided above seem like a reasonable pair of example paths.
Note: I do not believe there was intentional "relaxation" in 3.0. Note there are 2 non-XML node identification schemes in the HRP:
  • The URI scheme: section 2.2 (of both XACML 2.0 and 3.0)
  • The "ancestor scheme": section 3.2 of XACML 2.0, and section 2.3 of XACML 3.0.
If the URI scheme is used the resource-id must be a URI. If the ancestor scheme is used, then that is not a necessary condition.
001e01cbb7f7$85a884d0$0301010a@asusp4t533" type="cite">
According to the spec, a request for resource c should result in two separate resources in the request context, a resource-id with value /a/b/c and a resource-id with value /x/y/z - is that correct?
There is only one resource, but it will have 2 resource-id's, so the answer to above is "no" to the first part and "yes" to the 2nd part.
001e01cbb7f7$85a884d0$0301010a@asusp4t533" type="cite">
And two different policies, one specifying (as URI regex match) /a/b/.* and the other /x/y/.* would both match?
There is no requirement for two Policies, as opposed to say two Rules, but yes, it is likely that both the regex's you
propose would appear.
001e01cbb7f7$85a884d0$0301010a@asusp4t533" type="cite">
Is it also permissible for the incoming request to the PEP to specify the full path, for instance to state that resource c is being accessed in the context of collection b which is a member of collection a?  Thus only the first policy would match?
It is ok to specify only one path in the request, but it depends on the Policy structure whether that would
be accepted or not. i.e. does the Policy state:
  • ( /a/b/.* AND /x/y/.* ), or
  • ( /a/b/.* OR /x/y/.* )
001e01cbb7f7$85a884d0$0301010a@asusp4t533" type="cite">
That is, because a full resource-id is already present then the engine shouldn't go away and build the paths.
Sorry, I am not sure what you are trying to say here. There is no requirement on the engine to use any
specific method for processing the above scenario.
001e01cbb7f7$85a884d0$0301010a@asusp4t533" type="cite">
And is it conceivable that both forms could exist together?
I assume so, however, it sounds like you may have a concern that for some reason both forms should not
co-exist. Is that a concern?
001e01cbb7f7$85a884d0$0301010a@asusp4t533" type="cite">
For instance, if there was a resource attribute of object-ID, and it is defined that resource-id is formed of the full path to object-ID then we could have
Request one:
specifies resource object-id = "c"
result: engine builds all applicable paths to "c" - ie two separate resources with their own resource-id (so both above policies match)
I'm afraid you are going to have to be more specific here about what information is being processed.
My assumption is that all required info is in the request context. If all that is there is "c" then how does one
derive full paths?
001e01cbb7f7$85a884d0$0301010a@asusp4t533" type="cite">
Request two:
specifies resource-in-path-context, specifying resource-id directly as "/a/b/c"
result: engine does not build resource-id and only the first policy matches
Correct, but see comment above. It depends if Policy requires resource to be member
of both hierarchies or only one hierarchy.
001e01cbb7f7$85a884d0$0301010a@asusp4t533" type="cite">
Any guidance on this appreciated.  My reading so far of the specs suggests that the Hierarchical Resource Profile isn't intended for resource-in-path-context (or resource-accessed-as-member-of-collection).
Again, I am not sure what the term "resource-in-path-context" precisely means or the alternate phrase:
"resource-accessed-as-member-of-collection", at least in the context of the HRP. The HRP is intended
to represent single nodes in a hierarchy, and to also consider using the Multiple Resource Profile
for defined "scopes" of nodes within a hierarchy.
001e01cbb7f7$85a884d0$0301010a@asusp4t533" type="cite">

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