Re: XACML's limitations in the access control for XML documents usecase - AW: AW: [xacml] CD-1 issue #11: strictness of xpath definition

[Erik Rissanen <erik@axiomatics.com>]

Hi Rich,

Some of the reasons why an XPath approach could be better are:

1. XPath contains many functions and other capabilities, which might not 
be as easily available in the URI based approach.

2. The TC would avoid the effort to define the URI approach. We would 
need to improve the xpath approach instead, but I suspect that the 
effort is smaller since we can reuse so much from xpath, compared with a 
wholly new URI based approach.

3. It is likely that an XML resource is already available in XML form, 
so an xpath implimentation can be applied to it directly, while the URI 
approach requires a transformation, which could degrade performance.

Note that it is not true that the whole XML has to be repeated for each 
resource since multiple <Attributes> elements are not required with the 
xpath approach, and with XACML 3.0 it is possible to reuse the same 
<Content> document for all the multiple queries.

Best regards,
Erik

Rich.Levinson wrote:
>
> Hi Jan, et al,
>
> I have had a busy week and not been able to respond until now, 
> however, looking over all the subsequent emails to the one to which 
> this is a response (
> http://lists.oasis-open.org/archives/xacml/200909/msg00081.html
> ), it appears to me that there are still unresolved issues, and from 
> my perspective, there are some assertions made, with which I disagree, 
> about AttributeDesignators, which I thought my suggested URI scheme 
> would address, but apparently it either needs further explanation or I 
> am missing something that I have not yet understood. In any event I 
> would very much like to determine whether these assertions are true or 
> false in order that the TC be of a single mind when comparing the 
> capabilities of AttributeSelectors and AttributeDesignators.
>
>    The assertion with which I disagree is that the AttributeDesignators
>    cannot do what the AttributeSelectors can do because the
>    AttributeDesignators lose the hierarchical structure. My response is
>    that if you don't throw away the hierarchical structure when
>    creating your AttributeDesignators then this perceived problem does
>    not exist.
>
> If I am wrong about this, I will accept that, however, I do not 
> believe that my approach to the AttributeDesignators has been 
> considered on its merits yet, and I will try to be totally explicit in 
> this email, and I will show how I think Jan's proposed solution can be 
> completely done using only AttributeDesignators and regexp string 
> matching.
>
> Having been thru some lengthy discussions earlier this year on the 
> hierarchical profile, I became quite sensitive to the node naming 
> issue, and one of the results of those earlier discussions was that if 
> hierarchical URIs are used to name nodes, that these names contain 
> within them the navigation necessary to locate the node, so that using 
> these names outside of an XML document does not lose the structural 
> relationships.
>
> Using James Clark's universal name syntax ("{namespace}elementname" 
> http://www.jclark.com/xml/xmlns.htm) combined with a transform to 
> replace the xml document with a list of name/value pairs (there are 
> several XML to JSON xslt transformers available free, which I expect 
> could readily be adapted to produce name/value pairs in the format 
> below), where each element and attribute is identfied by its full path 
> expressed as universal element names. For example, assuming the 
> document you gave as an example had a namespace = "foo":
>
> <objects xmlns:="foo">
>  <book>
>    <title>xxx</title>
>    <author>Bob</author>
>    <id>100</id>
>    <price>30</price>
>    <book-content>.....</book-content >
>  <book>
>  <book>
>    <title>yyy</title>
>    <author>Alice</author>
>    <id>200</id>
>    <price>80</price>
>    <book-content >...</book-content >
>  <book>
> </objects>
>
> The above document would first be transformed to the following set of 
> name value pairs (ignoring whitespace):
> /{foo}objects = ""
> /{foo}objects/{foo}book[1] = ""
> /{foo}objects/{foo}book[1]/{foo}title = "xxx"
> /{foo}objects/{foo}book[1]/{foo}author = "Bob"
> /{foo}objects/{foo}book[1]/{foo}id = "100"
> /{foo}objects/{foo}book[1]/{foo}price = "30"
> /{foo}objects/{foo}book[1]/{foo}content = "..."
> /{foo}objects/{foo}book[2] = ""
> /{foo}objects/{foo}book[2]/{foo}title = "yyy"
> /{foo}objects/{foo}book[2]/{foo}author = "Alice"
> /{foo}objects/{foo}book[2]/{foo}id = "200"
> /{foo}objects/{foo}book[2]/{foo}price = "80"
> /{foo}objects/{foo}book[2]/{foo}content = "..."
>
> The next step is to define resources, which for this use case would be 
> done based on multiple resource profile, where we would have 2 
> resources, using Erik's shorthand:
> <Resource>resource-id=/{foo}objects/{foo}book[1]</Resource>
> <Resource>resource-id=/{foo}objects/{foo}book[2]</Resource>
>
> The next step is to create xacml attributes for these resources using 
> the full universal names as AttributeIds (again w some shorthand), 
> resulting in the following 2 requests:
> (Note: since AttributeId requires anyURI datatype, the following 
> percent-encoding must be applied to the AttributeId values:
>
>    * { -> %7B
>    * } -> %7D
>    * [ -> %5B
>    * ] -> %5D )
>
>
> <Request>
> <Subject>subject-id="Bob"</Subject>
> <Resource>
> <Attribute>AttributeId="resource-id" 
> value="/{foo}objects/{foo}book[1]"</Attribute>
> <Attribute>AttributeId="/%7Bfoo%7Dobjects/%7Bfoo%7Dbook%5B1%5D/%7Bfoo%7Dtitle" 
> value = "xxx"</Attribute>
> <Attribute>AttributeId="/%7Bfoo%7Dobjects/%7Bfoo%7Dbook%5B1%5D/%7Bfoo%7Dauthor" 
> value = "Bob"</Attribute>
> <Attribute>AttributeId="/%7Bfoo%7Dobjects/%7Bfoo%7Dbook%5B1%5D/%7Bfoo%7Did" 
> value = "100"</Attribute>
> <Attribute>AttributeId="/%7Bfoo%7Dobjects/%7Bfoo%7Dbook%5B1%5D/%7Bfoo%7Dprice" 
> value = "30"</Attribute>
> <Attribute>AttributeId="/%7Bfoo%7Dobjects/%7Bfoo%7Dbook%5B1%5D/%7Bfoo%7Dcontent" 
> value = "..."</Attribute>
> </Resource>
> </Request>
>
> <Request>
> <Subject>subject-id="Bob"</Subject>
> <Resource>
> <Attribute>AttributeId="resource-id" 
> value="/{foo}objects/{foo}book[2]"</Attribute>
> <Attribute>AttributeId="/%7Bfoo%7Dobjects/%7Bfoo%7Dbook%5B2%5D/%7Bfoo%7Dtitle" 
> value = "yyy"</Attribute>
> <Attribute>AttributeId="/%7Bfoo%7Dobjects/%7Bfoo%7Dbook%5B2%5D/%7Bfoo%7Dauthor" 
> value = "Alice"</Attribute>
> <Attribute>AttributeId="/%7Bfoo%7Dobjects/%7Bfoo%7Dbook%5B2%5D/%7Bfoo%7Did" 
> value = "200"</Attribute>
> <Attribute>AttributeId="/%7Bfoo%7Dobjects/%7Bfoo%7Dbook%5B2%5D/%7Bfoo%7Dprice" 
> value = "80"</Attribute>
> <Attribute>AttributeId="/%7Bfoo%7Dobjects/%7Bfoo%7Dbook%5B2%5D/%7Bfoo%7Dcontent" 
> value = "..."</Attribute>
> </Resource>
> </Request>
>
> All the above processing to create the requests is done in the 
> ContextHandler, then the requests are submitted one at a time to the PDP.
> Now the rule that gets applied to each of these requests is the 
> following:
>
> <Rule effect=Deny>
>  Target:
>    reg-exp-string-match(resource-id, /{foo}objects/{foo}book\[\d+\]
>  Condition:
>    AttributeDesignator(AttributeId = 
> function:string-concatenate(resource-id, /%7Bfoo%7Dprice) > 50 and
>    AttributeDesignator(AttributeId = 
> function:string-concatenate(resource-id, /%7Bfoo%7Dauthor) = 
> AttributeDesignator(subject-id)
> </Rule>
>
> Unless I am mistaken, all the logic and structure is retained and it 
> has been done purely w AttributeDesignators and regexp.
>
> Assuming the above is correct, then the points I made about the 
> advantages over XPath (for an enterprise looking to use only URIs to 
> identify attributes):
>
>   1. The XML document does not need to be passed in with the request.
>      There is no node collection, only string operations.
>   2. For very large XML documents, say a catalog of 10,000 books, each
>      book is processed individually independent of the other books, as
>      compared to the XPath case, where one might expect the whole
>      document has to get parsed for each of the 10,000 individual 
> requests.
>   3. There is no paradigm shifting, or what I believe was referred to
>      in the discussion as "shifting semantics between XPath and XACML
>      in terms of representing the policies.
>
> Again, assuming the above is correct, I am not assuming this will be 
> desirable for everyone, however there may very well be organizations 
> for whom the advantages of this approach are decisive.
>
> A couple other points are that
>
>    * the "unsightliness" of the AttributeIds and the
>      AttributeDesignators can be "covered" up by policy tools that
>      facilitate defining policies based on XML Schemas, and can keep
>      all the encoding details transparent to the policy designers.
>    * the issue about basing policies on the structure of XML documents
>      is a legitimate concern, however, if structure of documents
>      change, then a legitimate case could probably made that the
>      namespace associated with that structure should also change, which
>      would mean the policy tools would need to be able to facilitate
>      upgrading of policies to new namespaces based on new revs of the
>      schemas.
>
> Comments and suggestions welcome.
>
>    Thanks,
>    Rich
>