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Subject: Re: [xacml] BTG comment

Hal, Erik

is some sort of consensus arising that we need an entity in the XACML 
model (call it a StateDB for now) that holds the state of the system in 
the form of one or more attributes, whose values can be changed. When an 
attribute value changes this represents a state change of the system.

The PDP is and remains stateless.

The request context carries the current state of the system to the PDP.

A component of the XACML system (might be a PIP, or it might be 
something new) picks up the current state of the system from the StateDB 
and inserts these attributes into the request context for passing to the 

When certain authz decision requests are granted (or denied) this might 
cause a change in the state of the system. A new component is needed 
which updates the StateDB when this occurs.

This type of state based system is more generic than BTG, and can 
incorporate BTG as one of its use cases. You can for example use this 
type of state based system to
- support withdrawals from a credit card machine up to a daily limit,
- block a user's account after 3 concurrent denies,
- implement BTG by setting the glass to broken when a user is granted 
permission to break the glass

Does this also answer Mike's suggestion that BTG is an example of 
something more generic?



On 24/02/2011 17:48, Erik Rissanen wrote:
> Hal,
> Yes, I agree that a special entity with the explicit purpose of
> maintaining authorization state would be something good to add. It's not
> unusual for real world access control needs to depend on state, which
> may be itself affected by an access decision. XACML should support this,
> but not in the PDP itself.
> Erik
> On 02/24/2011 06:04 PM, Hal Lockhart wrote:
>> When we say that the PDP is stateless, what is meant is that no state
>> is carried by the server from one decision to the next. Policy
>> decisions are purely a function of policies in force and the content
>> of the request context.
>> I agree with the general view that this is a valuable property which
>> not should be lightly changed across the board. However, I am keeping
>> an open mind on the desirability of explicitly defining entities
>> within the access control architecture which do maintain state.
>> For example, the RBAC profile as it currently exists implies some kind
>> of stateful mechanism to keep track which dynamic roles have been
>> enabled. The exact mechanism was deliberately not specified as it was
>> recognized that many designs were possible.
>> Hal
>>     -----Original Message-----
>>     *From:* Rich.Levinson
>>     *Sent:* Wednesday, February 23, 2011 11:57 AM
>>     *To:* Davis, John M.
>>     *Cc:* erik@axiomatics.com; xacml@lists.oasis-open.org
>>     *Subject:* Re: [xacml] BTG comment [WAS: [xacml] Proposed Agenda
>>     10 February 2011 TC Meeting]
>>     Hi Mike, Erik, Paul, and David,
>>     Here's my two cents on what I've taken from the emails on this
>>     thread, David's BTG profile and protocol description.
>>     I agree w Erik's statement that the XACML model does not have
>>     state built in, and that there is not yet a compelling reason to
>>     add it.
>>     However, my interpretation of "state" is wrt the Policy definitions.
>>     i.e. the Policy defns themselves are "static".
>>     However, for "Policy evaluation", the "state" is the RequestContext,
>>     namely whatever values are in the Attributes in the RequestContext
>>     at Policy evaluation time may be regarded as the "state".
>>     So, for "BTG", what I see as the essential ingredient is the defn of
>>     a BTG-State Attribute, that can be supplied by the PEP, or some
>>     PIP that supplies it to an attribute finder during policy evaluation.
>>     Testing the attribute would be triggered by its presence in a Policy
>>     with it MustBePresent xml attribute set to true, and the Policy
>>     should be defined, so that the BTG-State Attribute is not tested
>>     unless the request is not permitted without it.
>>     This is equivalent to what we did in the RSA-2008 Interop, where
>>     the "BTG-State" attribute was "hl7:pea-001", which would allow an
>>     external user access, where they would otherwise be denied.
>>     i.e. the presence of hl7:pea-001 effectively changed the "state"
>>     of Policy evaluation, which enabled BTG logic to be applied.
>>     With respect to David's documents, I basically agree with the
>>     approach, except for statements like in the protocol descr step 8,
>>     where it says: "The PDP sets the relevant BTG-state variable
>>     to true ...".
>>     I think it is fine to return an Obligation as the Profile describes,
>>     and that this be used as the basis of the User obtaining a
>>     "BTG-State" Attribute, which can be submitted in the
>>     follow-up request. I think the presence of this attribute
>>     is sufficient "state" for the Policy to evaluate properly.
>>         Thanks,
>>         Rich
>>     Davis, John M. wrote:
>>>     I still think BTG is a  subset of a more general use case.
>>>     Following along with the obligation, my impression was that this must be followed with an ageement to abide by the obligation constraints, e.g. a "promise".  The promise  could act to tell the PDP to change the policy set (in this case to BTG).  With the "state" change the decision is now correct.
>>>     Mike Davis
>>>     ----- Original Message -----
>>>     From: Erik Rissanen<erik@axiomatics.com>
>>>     To:xacml@lists.oasis-open.org  <xacml@lists.oasis-open.org>
>>>     Sent: Wed Feb 23 04:42:41 2011
>>>     Subject: Re: [xacml] BTG comment [WAS: [xacml] Proposed Agenda 10 February 2011 TC Meeting]
>>>     Hi Paul and Mike,
>>>     I agree, but isn't this exactly what David is proposing? That is how I
>>>     understand it, at least for the "PEP state" mode. The other alternative
>>>     with the PDP maintaining state is something I don't think is a good idea.
>>>     To make David's proposal better, it needs:
>>>     - Drop the PDP state approach since this goes against the capability of
>>>     the XACML model which does not have a state built in.
>>>     - Define identifiers for at least the BTG obligation/advice (advice is
>>>     better, but for XACML 2.0 an obligation needs to be used), the action-id
>>>     for breaking glass (as well as giving some kind of direction of what the
>>>     BTG request should look like in relation to resources being accessed).
>>>     - It would be nice with a full, worked through example.
>>>     Best regards,
>>>     Erik
>>>     On 2011-02-23 05:06, Davis, John M. wrote:
>>>>     Concur with Paul's analysis.  We also see BTG as a state change.
>>>>     Regards, Mike Davis, CISSP
>>>>     Department of Veterans Affairs
>>>>     Office of Health Information
>>>>     Security Architect
>>>>     760-632-0294
>>>>     -----Original Message-----
>>>>     From: Tyson, Paul H [mailto:PTyson@bellhelicopter.textron.com]
>>>>     Sent: Tuesday, February 22, 2011 7:48 AM
>>>>     To: David Chadwick;xacml@lists.oasis-open.org
>>>>     Subject: [xacml] BTG comment [WAS: [xacml] Proposed Agenda 10 February
>>>>     2011 TC Meeting]
>>>>     I have some reservations about David's BTG proposal, primarily because
>>>>     the semantics are not well specified, and because it muddies the
>>>>     distinction between what should happen within the XACML system, and what
>>>>     should happen outside of XACML.
>>>>     Obligations, as currently defined, require the PEP to take some action
>>>>     prior to executing or acting on the PDP's decision for a particular
>>>>     request.  The proposed BTG obligation is entirely different: it says
>>>>     "you can't have access, but you can break the glass if you choose".  We
>>>>     added Advice in XACML 3 to accommodate this sort of
>>>>     message-passing--messages which may or may not have anything to do with
>>>>     the original request or the decision.  I don't think the TC should
>>>>     encourage the use of Obligations for anything other than what they are:
>>>>     an obligation to discharge certain actions regarding the PDP's decision.
>>>>     This is the only way to assure predictable system behavior--or to detect
>>>>     a faulty system.
>>>>     As we discussed on the last call, the scope and definition of "glass"
>>>>     should be specified.  If not, we are just standardizing some features of
>>>>     the request/response protocol without knowing what effect these features
>>>>     have, either on the PDP or the PEP.  Does the glass cover a specific
>>>>     request, a specific class of subjects, a class of resources, or what?
>>>>     Or is glass-condition just another attribute in the request context,
>>>>     whose effective meaning is given by the policy rules?
>>>>     However you define glass-condition, it will have to be implemented as a
>>>>     XACML attribute in the request context, or as some extra-xacml
>>>>     functionality.  Either way, you are now using the XACML request
>>>>     mechanism to change the policy evaluation state directly.  While this
>>>>     isn't prohibited by the standard, it makes it harder to reason about the
>>>>     behavior of the system.
>>>>     David's "BreakTheGlass" action-id is egregious in that it is not just a
>>>>     request for permission to do something (outside the XACML system)--it is
>>>>     a directive to change the state of the policy evaluation with regard to
>>>>     a particular (non-XACML) resource, subject, or action.  Now, if state is
>>>>     maintained in PEP, you could say this is a normal request, but
>>>>     nevertheless it has muddied the distinction between requests for
>>>>     permission to do something (outside of XACML) and directives to do
>>>>     something "inside" of XACML.
>>>>     I would favor one of two alternative approaches (or some combination):
>>>>     1. Simply return an Obligation (along with a Permit decision) if the
>>>>     requestor is authorized to "break the glass".  The PEP would be obliged
>>>>     to display the list of consequences associated with accessing the
>>>>     resource, and the user could choose to accept the consequences and see
>>>>     the resource, or cancel the request.  The consequences could include
>>>>     changing the state of the system to allow access to other resources, or
>>>>     allow other people to see the same resource, or whatever the business
>>>>     meaning of "break the glass" carries in a specific application.  The
>>>>     obligations would be discharged outside the XACML system.  (I'm not sure
>>>>     that this approach really warrants a standard profile, unless we just
>>>>     want to provide a distinguished obligation id value for BTG.)
>>>>     2. Write policies specifically to allow "break-the-glass" actions to
>>>>     certain subjects in certain conditions.  The PEP would request
>>>>     permission to break the glass, and if allowed would use some non-XACML
>>>>     mechanism to change the policy evaluation state.  Obligations could be
>>>>     returned with the decision to advise user of the consequences.  Then the
>>>>     application would request access to the desired resource.  (Again I
>>>>     question whether this is worthy of standardization, unless we want to
>>>>     name a distinguished "break-the-glass" action id.)
>>>>     Either of these approaches would meet the BTG use case without altering
>>>>     the semantics or conventional usage of XACML.
>>>>     Regards,
>>>>     --Paul
>>>>>     -----Original Message-----
>>>>>     From: David Chadwick [mailto:d.w.chadwick@kent.ac.uk]
>>>>>     Sent: Thursday, February 10, 2011 05:22
>>>>>     To:xacml@lists.oasis-open.org
>>>>>     Subject: Re: [xacml] Proposed Agenda 10 February 2011 TC Meeting
>>>>>     Dear All,
>>>>>     in preparation for this evening's call, I attach a revised version of
>>>>>     the BTG profile for you consideration
>>>>>     regards
>>>>>     David
>>>>     <snip>
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David W. Chadwick, BSc PhD
Professor of Information Systems Security
School of Computing, University of Kent, Canterbury, CT2 7NF
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