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Subject: Re: [xacml] Attributes of Relations

Hi Steven,

Yes, that is my proposal. See inline for responses to your concerns.

On 2012-12-19 00:44, Steven Legg wrote:


Hi Erik,

On 18/12/2012 2:18 AM, Erik Rissanen wrote:

Hi Steven,
My point in my email was that you cannot just flatten attributes like that, as you describe yourself, but 
also that I don't think it is worth doing in XACML.
One option is to extend XACML. There are multiple ways to do that. One is to define a tuple/relation datatype and operators on it, which I think is a relatively nice and clean solution. Another one is what you propose is to iterate over categories, which I don't quite like as much since it appears more kludgy than 
well established relational algebra on tuples. We could debate this...
However, you can also handle a requirement like this in a PIP, in which case you do not need any changes to 
XACML.
Given that, we could define these extensions into XACML, but I have doubts about what the value of this would be since it complicates the XACML language, making XACML harder to learn and implement. And since these operators/iterators are already available in commonly used PIP backends such as SQL and LDAP, so the XACML industry would essentially be re-inventing these features into the XACML language, although every organization already has easy access to solve these use cases using the existing XACML spec and PIPs. 

You really need to describe your thinking in more detail.
Since the PIP only supplies attributes, my best guess is that you expect to use XACML attribute values, typically of the boolean data-type, as surrogates for the result of SQL statements or LDAP queries evaluated internally by the PIP. For example, an attribute called subject-employed-by-ip-owner that indicates the boolean result of an internal PIP query to evaluate whether the subject is an employee of the IP owner. I see a number of problems with an approach like this.
(1) It assumes that the PIP has all the necessary information to query. Some attributes (like ip-owner) may only come from the PEP in the authorization request. It is only the context handler and PDP that are guaranteed to have the complete 
request context for evaluation.
Typically in an implementation a PIP can ask the context handler for more "key values" which it would need, so this is not a problem in practice given that the implementation is done right.
(2) It is necessary to define a new attribute and configure the PIP with a new SQL statement, LDAP query, or whatever, for each new expression the policy writer wants to use that is beyond the current capabilities for XACML to express. 



That is correct. I don't see this as a huge problem.
(3) It splits the policy evaluation between the PDP and the PIP. It is no longer sufficient to examine the XACML policies to understand the effect of access control policy. It is necessary to also examine the SQL statements, LDAP queries, etc. to understand what the complete authorization policy is. For interworking, it is also necessary to export the XACML policies from the PAP and the SQL statements, LDAP 
queries, etc. from the PIP.
Yes, this is the main drawback in my opinion. But it's something one can live with.
If that isn't the approach you had in mind, then you need to describe what you are thinking. An example of how you would tackle a condition like "the subject is 
an employee of the IP owner" would be useful for comparison.



This is the approach I do have in mind.
The reason I prefer this is because the amount of "new stuff" we would have to add to XACML to handle all such cases of attributes of relations is a big deal. We will basically end up re-implementing a SQL like language within XACML, and I doubt what the value of that would be since SQL implementation are ubiquitous and can already do all this for us.
Adding something SQL like within XACML is going to be costly in terms of TC effort, implementation effort and customer learning curve for XACML products. And in the end, it won't do anything which cannot be done today, with the exception of a more fully visible authorization logic in the XACML policy. 

Best regards,
Erik


Regards,
Steven



Best regards,
Erik

On 2012-12-17 07:43, Steven Legg wrote:


Hi Richard,

On 30/11/2012 8:13 AM, Hill, Richard C wrote:
The topic of "attributes of relations" came up during our work on the IPC profile that I would like to bring 
to the list for discussion.
For example, an intellectual property agreement (Copyright, Patent, Proprietary, etc.) is essentially a contract between parties (subjects) regarding the use of resources. The "agreement" is a relationship between subject and resource. The question of how to best model relationships like this with XACML attributes is what I would like bring up for discussion. So far two approaches to this problem have been 
proposed:
1.) Creating new attribute categories that would represent the relationships. Below is an excerpt from Hal 
regarding this approach.
2.) Determine the relationship at the PIP. This is one approach that IPC profile suggests regarding the use of the Agreement-Id, Valid-Agreement-Exists and Number-Of-Valid-Agreements attributes. Below is an excerpt 
from Erik on some of his thoughts on this topic.


I think Hal is on the right track but hasn't taken it far enough.
I've encountered use cases where I need to reference attributes that are not part of the subject, but rather are attributes of some entity/object that is related to subject. For example, attributes of the organization that employs the subject, or attributes of another person, being the parent/guardian of the subject. The IPC and EC-US profiles have other examples of a need to reference attributes of things related to the subject or resource, but not really part 
of the subject or resource.
The IPC and EC-US profiles try to shoehorn these attributes of related objects into the subject and resource categories by "flattening", but this is woefully 
inadequate when the relationships are one-to-many or many-to-many.
Consider this simple example as an illustration. I want to indicate that the subject is an employee of Ajax Inc, a commercial organization, which is a customer of Widget Inc., a non-profit organization and also the IP owner. The subject is also a contractor to Widget Inc. The IPC profile would have me create a request context something like this (from here on I've shortened all the URIs to just the last part and removed the IncludeInResult XML attributes 
in the interest of readability):

<Attributes Category="access-subject">
  <Attribute AttributeId="subject-to-organization-relationship">
    <AttributeValue DataType="anyURI">employee</AttributeValue>
    <AttributeValue DataType="anyURI">contractor</AttributeValue>
  </Attribute>
  <Attribute AttributeId="organization">
    <AttributeValue DataType="string">Ajax Inc.</AttributeValue>
    <AttributeValue DataType="string">Widget Inc.</AttributeValue>
  </Attribute>
  <Attribute AttributeId="organization-type">
    <AttributeValue DataType="anyURI">commercial</AttributeValue>
    <AttributeValue DataType="anyURI">non-profit</AttributeValue>
  </Attribute>
  <Attribute AttributeId="business-context">
    <AttributeValue DataType="anyURI">customer</AttributeValue>
  </Attribute>
  <!-- Other subject attributes here. -->
</Attributes>
<Attributes Category="resource">
  <Attribute AttributeId="ip-owner">
    <AttributeValue DataType="string">Widget Inc.</AttributeValue>
  </Attribute>
  <!-- Other attributes of the resource. -->
</Attributes>
The "flattening" process makes the request context ambiguous and leaves policy evaluation open to false-positive and false-negative results. Which organization is the subject an employee of ? Is it both ? Which organization is commercial and which is non-profit ? There's no way to tell from the request context.
The real world allows objects of many kinds that can be related to each other in many ways. To capture that reality, XACML needs the ability to represent a 
request context that is a graph of objects, rather than a small list of
predefined categories. If I reinterpret a category as an object (i.e., a list of attributes) and use attribute values of the anyURI data-type as references between objects, then I can turn the request context into a graph without changing the syntax at all. Having a predetermined category URI isn't adequate when there is a need to represent multiple instances of the same kind of object, as Erik points out. The answer is to decouple the kind of a object from the category URI by regarding the category URI as simply a unique identifier for a distinct object. These URIs could be anything, e.g., UUIDs, LDAP URLs, OID URNs, or whatever. The predefined category URIs become well known aliases for singling out the particular objects that are significant to the access attempt, e.g., the subject and resource. These objects are the entry points 
into the graph.

If I break out the different entities and relationships into separate
<Attributes> elements and use anyURI values to link them, then I get a
request context that looks like this:

<Attributes Category="access-subject">
  <Attribute AttributeId="relationship-ref">
    <AttributeValue DataType="anyURI">relationship-1</AttributeValue>
    <AttributeValue DataType="anyURI">relationship-2</AttributeValue>
  </Attribute>
  <!-- Other subject attributes. -->
</Attributes>
<Attributes Category="relationship-1">
  <Attribute AttributeId="subject-to-organization-relationship">
    <AttributeValue DataType="anyURI">employee</AttributeValue>
  </Attribute>
  <Attribute AttributeId="organization-ref">
    <AttributeValue DataType="anyURI">organization-1</AttributeValue>
  </Attribute>
</Attributes>
<Attributes Category="relationship-2">
  <Attribute AttributeId="subject-to-organization-relationship">
    <AttributeValue DataType="anyURI">contractor</AttributeValue>
  </Attribute>
  <Attribute AttributeId="organization-ref">
    <AttributeValue DataType="anyURI">organization-2</AttributeValue>
  </Attribute>
</Attributes>
<Attributes Category="organization-1">
  <Attribute AttributeId="organization"
    <AttributeValue DataType="string">Ajax Inc.</AttributeValue>
  </Attribute>
  <Attribute AttributeId="organization-type">
    <AttributeValue DataType="anyURI">commercial</AttributeValue>
  </Attribute>
  <Attribute AttributeId="business-context-ref">
<AttributeValue DataType="anyURI">business-context-1</AttributeValue> 
  </Attribute>
  <!-- Other attributes of organization "Ajax Inc." here. -->
</Attributes>
<Attributes Category="business-context-1">
  <Attribute AttributeId="business-context">
    <AttributeValue DataType="anyURI">customer</AttributeValue>
  </Attribute>
  <Attribute AttributeId="organization-ref">
    <AttributeValue DataType="anyURI">organization-2</AttributeValue>
  </Attribute>
</Attributes>
<Attributes Category="organization-2">
  <Attribute AttributeId="organization"
    <AttributeValue DataType="string">Widget Inc.</AttributeValue>
  </Attribute>
  <!-- Other attributes of organization "Widget Inc." here. -->
</Attributes>
<Attributes Category="resource">
  <Attribute AttributeId="ip-owner">
    <AttributeValue DataType="anyURI">organization-2</AttributeValue>
  </Attribute>
  <!-- Other attributes of the resource. -->
</Attributes>
The affiliation of the subject to an organization is represented by an object (<Attributes> element) containing the subject-to-organization-relationship attribute. The relationship of one organization to another is represented by an object containing the business-context attribute. Each of the organizations is represented by an object containing the organization attribute. Note that 
relationship-1, relationship-2, organization-1, organization-2 and
business-context-1 are not predefined URIs. I'm using them as placeholders 
for UUIDS, or something similar, that uniquely identify each object.
With the above request context I am able to faithfully represent the situation I originally expressed in English. I don't expect that the PEP would actually provide a request context like this. More likely, the context handler would construct the request context on demand by querying a PIP, given a bare-bones 
subject and resource provided by the PEP as a starting point.
The next thing I need to address is how a graph-like request context can be 
evaluated by policies.
The first thing to note is that attribute designators use predetermined URIs 
for the category, but most of the graph objects have URIs that won't be
known at the time a policy is written, and may vary over time. To overcome that I need an attribute designator in the form of a function so that I can 
feed into it the URI values fetched by other attribute designators.

So an attribute designator like this:

    <AttributeDesignator
      Category="access-subject"
      AttributeId="relationship-ref"
      DataType="anyURI"
      MustBePresent="false"/>

is equivalent to an attribute-designator function:

    <Apply FunctionId="attribute-designator">
      <AttributeValue DataType="anyURI">access-subject</AttributeValue>
<AttributeValue DataType="anyURI">relationship-ref</AttributeValue> 
      <AttributeValue DataType="anyURI">anyURI</AttributeValue>
      <AttributeValue DataType="boolean">false</AttributeValue>
    </Apply>
Each of the arguments, especially the first one, is able to be an arbitrary 
expression instead of a constant.

The attribute-designator function isn't enough by itself. To do really
interesting things with the graph I find I also need to invoke the iterator 
expressions I described on the comment list some time ago:
https://lists.oasis-open.org/archives/xacml-comment/201101/msg00007.html
Armed with the attribute designator function and the ForAny expression I can apply conditions to any part of the graph. For example, if I want to test whether the subject is an employee of the IP owner I could use this 
expression:

<ForAny VariableId="$a">
  <!-- $a is bound to each relationship-ref URI in turn -->
  <AttributeDesignator
    Category="access-subject"
    AttributeId="relationship-ref"
    DataType="anyURI"
    MustBePresent="false"/>
  <Apply FunctionId="and">
<!-- Return true if and only if the subject-to-organization-relationship of the relationship object referenced by $a contains the value "employee". --> 
    <Apply FunctionId="anyURI-is-in">
      <AttributeValue DataType="anyURI">employee</AttributeValue>
      <Apply FunctionId="attribute-designator">
        <VariableReference VariableId="$a"/>
<AttributeValue DataType="anyURI">subject-to-organization-relationship</AttributeValue> 
        <AttributeValue DataType="anyURI">anyURI</AttributeValue>
        <AttributeValue DataType="boolean">false</AttributeValue>
      </Apply>
    </Apply>
<!-- Return true if and only if the organization referenced by the relationship 
         object referenced by $a is the IP owner. -->
    <Apply FunctionId="anyURI-at-least-one-member-of">
      <Apply FunctionId="attribute-designator">
        <VariableReference VariableId="$a"/>
<AttributeValue DataType="anyURI">organization-ref</AttributeValue> 
        <AttributeValue DataType="anyURI">anyURI</AttributeValue>
        <AttributeValue DataType="boolean">false</AttributeValue>
      </Apply>
      <AttributeDesignator
        Category="resource"
        AttributeId="ip-owner"
        DataType="anyURI"
        MustBePresent="false"/>
    </Apply>
  </Apply>
</ForAny>
For the record, the answer is false for the graph request context I provided above. The current profile with "flattening" is incapable of such precision.
I would expect to write up the graph-like request context, the attribute designator function and iterator expressions as a separate profile since they 
are generally useful beyond just the IPC profile.

Regards,
Steven



- Richard
-------------------------------------------------------------------------------- 

On 2012-10-10 04:04, Hal Lockhart wrote (excerpt):
--------------------------------------------------------------------------------
...I have been aware for some time that the Categories of Subject, Resource, Action & Environment are insufficient as policies become more complex. The first use case I encountered of this kind was when trying to label certain operations as "Administrative". There is a need to associate the attribute not with the Resource or Action, but with the combination. For example, all operations on Usage Reports might be Administrative, whereas only creating an Account might be Administrative, other Account operations would 
not be.
Some Access Control Models consider Action to be a part of Resource, on the theory that to, for example, write a file and to write a database record are not really the same in important ways. I think the XACML treatment of Action as a first class Category has certain advantages, for example in the case where you want 
to write a policy about ALL reads or writes.
I have opposed using selectors to solve this problem for several reasons. First, it tends to hide what is intended in a complicated expression. Second, I think it is undesirable for references to attributes be dependent on the means used to obtain them. The foo attribute should be the foo attribute whether it came from LDAP, SAML or SQL. Also it makes the system fragile. If the attribute location changes, the policies have to change. I objected to Jan Hermann's proposal that we have Input Message and Output Message Categories for the same reason. Finally, I consider Selectors to be a mechanism to access attributes in the request context when a suitable designator has not (yet) been defined. Using a selector to access a 
repository seems like a kludge to me.
My preferred alternative in such cases is to define a new category type, say, Resource-Action. A possible objection is that we could end up with an explosion of Categories, but while this could be true, I think it comes from the real world nature of the problem and thus will be a feature of any solution. The selector will be different for every attribute and repository combination, possibly an even larger set.
Notwithstanding all that, I am not sure that is the problem in your case below. I don't think the patent, or copyright or license are attributes of the relationship, they are attributes of the Resource (document), The relationship is what you are trying to capture in the policy. For example, somebody has licensed Boeing to use this document. The policy says that since John Tolbert works for Boeing, he can use the document.
I think the problem in this case is that your attributes are not flat scalars, but contain multiple fields. I am not prepared to propose a data model, but it seems to me you problem is that the copyright attribute needs to be qualified by country and perhaps other fields. XACML requires new functions to deal with new attribute types as a whole, e.g. GeoXACML, but the existing functions can deal with one field at a time just 
fine.

Hal
-------------------------------------------------------------------------------- 

On 2012-10-09, 10:50 PM, Erik Rissanen wrote (excerpt):
--------------------------------------------------------------------------------
...There are different ways to deal with this issue. Defining a new category like you suggest Hal will not solve the problem because even if you have a new category, you cannot separate multiple instances of the 
same category, thus you cannot represent multiple relations this way.
My recommendation is to resolve the relations in a PIP. It puts some of the "logic" outside the XACML policy but it is the most pragmatic approach, since it does not need any extensions to XACML and leads to simple 
XACML policies.
Using complex data types, either in the form of xml <Content> and selectors, tuples represented as strings, or XACML data type extensions also work, but mean unwieldy expressions in policies or XACML extensions which 
need to be implemented in code in the PDP.
One idea I have been toying with in my mind for a long time has been to define a sort of generic tuple data type for XACML, but I have not been able to design a nice and clean set of operators on it, so I have not posted anything on the list. There is a suggestion like this on the XACML comments list, btw.
One could extend on this tuple idea, and when one thinks about the various operations which would be needed, one ends up re-inventing SQL, so I figured there is little practical value for XACML to do so. This can be done on the database side in a PIP. I guess there would be some value in terms of explicit
visibility, but I am not convinced, so I never pursued this thread of thought on the TC list. 

Best regards,

Erik




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