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Subject: Walkthrough of multiple profile (related to public review issue #11)


All,

(I am mailing this in HTML since some of the lines are long, and I don't want them to be cut off. I hope this works.)

One issue which was brought up several times during the TC call yesterday was that some TC members said that the multiple resource profile does not define in any normative manner how the XPath based multiple request is divided into individual requests. I don't agree, so here is a walkthrough of an example with references to the specification.

The relevant section is 2.2 of the multiple resource profile. I am using the PDF version of the CD-1, so line numbers refer to it.

Section 2.2.2 describes the original request context. This says that the resource-id must be an XPath expression over the <Content> element of the resource category. The scope attribute must be "XPath-expression". Here is an example of such a request:

<Request
      xmlns="urn:oasis:names:tc:xacml:3.0:core:schema:cd-1"
      xmlns:md="http://www.medico.com/schemas/record">
    <Attributes Category="urn:oasis:names:tc:xacml:1.0:subject-category:access-subject">
        <Attribute
              AttributeId="urn:oasis:names:tc:xacml:1.0:subject:subject-id">
            <AttributeValue
              DataType="http://www.w3.org/2001/XMLSchema#string"
              >Julius Hibbert</AttributeValue>
        </Attribute>
    </Attributes>
    <Attributes Category="urn:oasis:names:tc:xacml:3.0:attribute-category:resource">
        <Content>
            <md:record>
                <md:patient_info>
                    <md:name>Bart Simpson</md:name>
                </md:patient_info>
                <md:diagnosis_info>
                    <md:diagnosis>
                        <md:item type="primary">Gastric Cancer</md:item>
                    </md:diagnosis>
                </md:diagnosis_info>                
            </md:record>
        </Content>
        <Attribute
              IncludeInResult="true"
              AttributeId="urn:oasis:names:tc:xacml:1.0:resource:resource-id">
            <AttributeValue
              XPathCategory="urn:oasis:names:tc:xacml:3.0:attribute-category:resource"
              DataType="urn:oasis:names:tc:xacml:3.0:data-type:xpathExpression"
              >md:record/descendant-or-self::node()</AttributeValue>
        </Attribute>
        <Attribute
              AttributeId="urn:oasis:names:tc:xacml:2.0:resource:scope">
            <AttributeValue
              DataType="http://www.w3.org/2001/XMLSchema#string">XPath-expression</AttributeValue>
        </Attribute>
    </Attributes>
</Request>

Section 2.2.3 of the spec, lines 137-139, say that each node selected by the XPath expression shall be considered as an individual request. The rest of section 2.2.3 specifies how the individual requests are produced. It simply states that each individual request is identitical to the above request, except that the scope attribute is not present and that the resource-id is replaced with an xpath which points to the individual resource (each in turn).

There is no normative statement about the exact form of this XPath expression, except that it must identify an individual node. Issue #11 from the public review concerns this XPath expression. As will be clear, its exact form does not matter, which is the reason the specification does not specify a particular form.

The Axiomatics implementation constructs the expression by referencing the nodes by their order in the tree, so in our implementation the individual requests look like this:


<Request
      xmlns="urn:oasis:names:tc:xacml:3.0:core:schema:cd-1"
      xmlns:md="http://www.medico.com/schemas/record">
    <Attributes Category="urn:oasis:names:tc:xacml:1.0:subject-category:access-subject">
        <Attribute
              AttributeId="urn:oasis:names:tc:xacml:1.0:subject:subject-id">
            <AttributeValue
              DataType="http://www.w3.org/2001/XMLSchema#string"
              >Julius Hibbert</AttributeValue>
        </Attribute>
    </Attributes>
    <Attributes Category="urn:oasis:names:tc:xacml:3.0:attribute-category:resource">
        <Content>
            <md:record>
                <md:patient_info>
                    <md:name>Bart Simpson</md:name>
                </md:patient_info>
                <md:diagnosis_info>
                    <md:diagnosis>
                        <md:item type="primary">Gastric Cancer</md:item>
                    </md:diagnosis>
                </md:diagnosis_info>                
            </md:record>
        </Content>
        <Attribute
              IncludeInResult="true"
              AttributeId="urn:oasis:names:tc:xacml:1.0:resource:resource-id">
            <AttributeValue
              XPathCategory="urn:oasis:names:tc:xacml:3.0:attribute-category:resource"
              DataType="urn:oasis:names:tc:xacml:3.0:data-type:xpathExpression"
              >./*[1]/*[1]/*[1]</AttributeValue>
        </Attribute>
    </Attributes>
</Request>

For each individual request, the XPath expression "./*[1]/*[1]/*[1]" will be different, like "./*[1]/*[1]/*[1]/text()[1]", "./*[1]/*[2]" and so on, covering all nodes selected by the original XPath expression. Other implementations may use another form, but this form is easy to produce since we don't need to track element names, namespaces, etc, just their order.

Somebody mentioned XML overhead before, so I will note that there is no reason to construct each individual XML document in their entirety, rather one can point to pieces of the orignial XML and keep a small chache on the side for the resource-id value. So there is not really any overhead to it. (In XACML 2.0 it is necessary to construct the full <Request> XML since an XPath could be used to refer to any part of it. That is not allowed in XACML 3.0, so the implementation can be efficient.)

A policy could look like this:

<Policy
      xmlns="urn:oasis:names:tc:xacml:3.0:core:schema:cd-1"
      xmlns:md="http://www.medico.com/schemas/record"
      PolicyId="urn:FIXME:policy1"
      RuleCombiningAlgId="urn:oasis:names:tc:xacml:1.0:rule-combining-algorithm:deny-overrides">
    <Target/>
    <Rule
          RuleId="urn:FIXME:rule1"
          Effect="Permit">
        <Target>
            <AnyOf>
                <AllOf>
                    <Match
                          MatchId="urn:oasis:names:tc:xacml:1.0:function:string-equal">
                        <AttributeValue
                              DataType="http://www.w3.org/2001/XMLSchema#string">Julius Hibbert</AttributeValue>
                        <AttributeDesignator
                              Category="urn:oasis:names:tc:xacml:1.0:subject-category:access-subject"
                              AttributeId="urn:oasis:names:tc:xacml:1.0:subject:subject-id"
                              DataType="http://www.w3.org/2001/XMLSchema#string"/>
                    </Match>
                </AllOf>
            </AnyOf>
            <AnyOf>
                <AllOf>
                    <Match
                          MatchId="urn:oasis:names:tc:xacml:3.0:function:xpath-node-match">
                        <AttributeValue
                              XPathCategory="urn:oasis:names:tc:xacml:3.0:attribute-category:resource"
                              DataType="urn:oasis:names:tc:xacml:3.0:data-type:xpathExpression">
                              md:record/md:patient_info
                        </AttributeValue>
                        <AttributeDesignator
                              Category="urn:oasis:names:tc:xacml:3.0:attribute-category:resource"
                              AttributeId="urn:oasis:names:tc:xacml:1.0:resource:resource-id"
                              DataType="urn:oasis:names:tc:xacml:3.0:data-type:xpathExpression"/>
                    </Match>
                </AllOf>
            </AnyOf>
        </Target>
    </Rule>
</Policy>

This policy will allow Julius Hibbert access to the patient_info section of the medical record, but not the rest. Note how the policy uses an xpath match function, so the exact form of the xpath expression is not significant.

So, there will be results like this:

The md:record/md:diagnosis element:

  <xacml-ctx:Result>
    <xacml-ctx:Decision>NotApplicable</xacml-ctx:Decision>
    <xacml-ctx:Status>
      <xacml-ctx:StatusCode Value="urn:oasis:names:tc:xacml:1.0:status:ok"/>
    </xacml-ctx:Status>
    <xacml-ctx:Attributes Category="urn:oasis:names:tc:xacml:3.0:attribute-category:resource">
      <xacml-ctx:Attribute AttributeId="urn:oasis:names:tc:xacml:1.0:resource:resource-id" IncludeInResult="true">
        <xacml-ctx:AttributeValue XPathCategory="urn:oasis:names:tc:xacml:3.0:attribute-category:resource" DataType="urn:oasis:names:tc:xacml:3.0:data-type:xpathExpression">./*[1]/*[2]</xacml-ctx:AttributeValue>
      </xacml-ctx:Attribute>
    </xacml-ctx:Attributes>
  </xacml-ctx:Result>

The string "Bart Simpson":

  <xacml-ctx:Result>
    <xacml-ctx:Decision>Permit</xacml-ctx:Decision>
    <xacml-ctx:Status>
      <xacml-ctx:StatusCode Value="urn:oasis:names:tc:xacml:1.0:status:ok"/>
    </xacml-ctx:Status>
    <xacml-ctx:Attributes Category="urn:oasis:names:tc:xacml:3.0:attribute-category:resource">
      <xacml-ctx:Attribute AttributeId="urn:oasis:names:tc:xacml:1.0:resource:resource-id" IncludeInResult="true">
        <xacml-ctx:AttributeValue XPathCategory="urn:oasis:names:tc:xacml:3.0:attribute-category:resource" DataType="urn:oasis:names:tc:xacml:3.0:data-type:xpathExpression">./*[1]/*[1]/*[1]/text()[1]</xacml-ctx:AttributeValue>
      </xacml-ctx:Attribute>
    </xacml-ctx:Attributes>
  </xacml-ctx:Result>

The resource-id is part of the <Result>s since the original resource-id has IncludeInResult="true", see the lines 144-147 in the spec.

So, it is not true that the specification does not have any normative statements of how the XPath "variant" works. Personally I also think the text is quite clear. I can agree that the spec is not good learning material, but I don't think that is the role of standard specification since good learning material needs to be verbose and redundant because one needs examples and different ways to explain the same thing to help learning. But those are qualities which are undesirable in a technical specification, so I would like to keep the text as it is.

Best regards,
Erik



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