Re: [xacml] Minutes 7 March TC Meeting - action on ambiguity wrt set of returned Obligations, Advice

[Erik Rissanen <erik@axiomatics.com>]

Hi,

I agree with Hal in that the spec is currently clear on what may and may not happen in the PDP. It just happens to be that some of the combining algorithms are unordered, so you cannot use them to solve the use case you are presenting. But that does not mean that the spec is unclear on its meaning.

The problems are that there might be use cases which are not easily solved with the current spec. I'm thinking of resolving conflicts between obligations or more orthogonal specification of obligation collection, independently from effect combining for the sake of ease of policy authoring, for instance.

Best regards,
Erik


On 2013-03-13 17:28, rich levinson wrote:

Hi Hal,

I agree that a random number generator produces a result that is indeterminate
and there is nothing ambiguous about that.

However, to apply that paradigm to the present discussion, what we are then
discussing is a "random obligation generator", which is quite different, because
unlike "numbers" which may be regarded as equivalent in the functional context
of a random number generator, "obligations", in general, are not equivalent
in the same sense.

For example:

    if one rule had an obligation that said:
        "apply a $10 charge to the customer account"
    and a 2nd rule had an obligation that said:
        "apply a $500 charge to the customer account"

    then we'd have a situation where if the customer satisfied both rules
    that sometimes they would be charged $10 and other times they
    would be charged $500.

Therefore, I would assert that for any given customer transaction that
it is ambiguous how much the customer will be charged.

While this exact situation may not actually occur, it does demonstrate
the "nature of the ambiguity" that I have been addressing.

Furthermore, I am not saying it is right or wrong that the spec allows
for this situation, but that as the spec currently stands, this particular
situation, where one cannot accurately specify which obligations will
be produced for a given request, is an example of a situation where
the results may be considered ambiguous.

Finally, in terms of "optimization", my sense is that if the price of
optimization is such that one needs to introduce randomness to
the results, I would think that generally, for an authorization service,
this would be too high a functional price to pay for a performance
improvement.

    Thanks,
    Rich

On 3/12/2013 11:57 AM, Hal Lockhart wrote:

At the risk of pressing the good will of the TC, I still believe that a specification can describe behavior which is indeterminate without being ambiguous. For example, a spec can state than a random number, 128 bits in length must be included in a message. No one can predict what the number will be, but there is no ambiguity. I would argue that this case, like that of obligations meets both definitions you give.

 

Hal

 

From: rich levinson
Sent: Tuesday, March 12, 2013 11:46 AM
To: Hal Lockhart
Cc: XACML TC; William Parducci
Subject: Re: [xacml] Minutes 7 March TC Meeting - action on ambiguity wrt set of returned Obligations, Advice

 

Hi Hal,

Not to quibble (because, imo, it is important that discussions be conducted w a
common understanding of the terminology being used), but according to
Google/Webster online (google: defn ambiguous):

am·big·u·ous  

/amˈbigyo͞oəs/

Adjective

1. (of language) Open to more than one interpretation; having a double meaning.
2. Unclear or inexact because a choice between alternatives has not been made

I think #2 pretty much exactly describes the situation wrt to Obligations. i.e. the statement
in the spec:

"The following pseudo-code represents the normative specification of this combining algorithm.
The algorithm is presented here in a form where the input to it is an array with children
(the policies, policy  sets or rules) of the policy or policy set.
The children may be processed in any order, so the set of obligations or advice provided
by this algorithm is not deterministic."

Specifically, the "choice between alternatives" in terms of the order of processing is not
determined in advance, and so it is unclear which obligations will be produced, which
is precisely what's described in defn 2 of "ambiguous".

From an implementation point of view, this is more important than defn 1, which simply
has to do w unclear language, which can be identified and corrected. However, the 2nd
category of ambiguous, when correctly specified, as it appears to be in the spec really
makes the ambiguity a "property" of the system that it is up to developers to take
into consideration w their design and implementation.

One final note is that this "situation" was not introduced in 3.0, it existed in 2.0 as well.
What is different is that 3.0 goes into more detail defining the nature of the ambiguity.

  Thanks,
  Rich

On 3/12/2013 11:19 AM, Hal Lockhart wrote:

I am well aware of the fact that when the most common combining algorithms are used, different optimizing PDPs may produce different sets of Obligations from each other, given the same policies and inputs. If fact I was attempting to discuss exactly this issue in the context of reviewing Section 8 of the Healthcare Obligation s Profile.

 

In that context I made the following points. 1. This behavior has been understood and debated since XACML 1.0 it is not some kind of oversight. 2. A judicious choice of combining algorithms can force the evaluation of all policies containing Obligations and thus this is a question of whether to allow an optimization or not. 3. The Profile proposes a specific mechanism to force this evaluation, but since it alters PDP evaluation logic, it would be easier to get the Profile deployed if we could live without these new features. (The same goes for any PDP-based aspect of Obligation handling.)

 

An old argument of mine, which  I did not make last week and which I cannot prove is universally true is that generally policies can be easily structured so that Obligations are associated with the logic for the decision on the Effect and will generally produce the correct Obligations. For example, suppose one class of users can access the Resource under normal circumstances and another class of users can only access it under special conditions in which case the access should be recorded in the audit trail. By arranging to have the normal case checked first, the result will be that the audit will only happen under the special conditions.

 

During the call I was objecting to the fact that I thought you were describing the specification as “ambiguous”. The Merriam-Webster Collegiate Dictionary (the nearest one to hand) defines “ambiguous” as: “capable of being understood in two or more possible senses”. As the quote you cite makes clear, the specification is not ambiguous, more than one result is possible. The Obligations returned can depend on the PDP implementation. From the PEP’s point of view it is indeterminate, but the specification is not ambiguous.

 

IMO, Ambiguity in a Standard is Always a bug and must be fixed. However, allowing policy evaluation to produce different outcomes wrt Obligations may or may not be a good idea. Historically the view of the TC has been to allow optimized policy evaluation by default and let the policy author override it if he or she wishes to.

 

Hal

 

 

From: rich levinson
Sent: Friday, March 08, 2013 4:20 PM
To: XACML TC; Hal Lockhart
Cc: William Parducci
Subject: Re: [xacml] Minutes 7 March TC Meeting - action on ambiguity wrt set of returned Obligations, Advice

 

To Hal, TC,

At yesterday's mtg I made the point that was quoted in the minutes that:

"There seems to be a level of ambiguity in the evaluation (of policysets,
 policies, and rules) that has ramifications on Obligations. "

The comment was based on my understanding of the "ordered" combining
rules. The following comment is in the description of "deny-overrides":

"The following pseudo-code represents the normative specification of this
 combining algorithm. The algorithm is presented here in a form where the
 input to it is an array with children (the policies, policy sets or rules) of the
 policy or policy set.
The children may be processed in any order, so the set of obligations or
 advice provided by this algorithm is not deterministic."

The same comment appears in the "permit-overrides",  "deny-unless-permit",
and "permit-unless-deny" algorithms.

The same situation existed in XACML 2.0 although the effect on Obligations was
not as explicitly stated, although there was one general comment in
2.0 section 7.14 "Obligations".

    Thanks,
    Rich

On 3/7/2013 8:28 PM, William Parducci wrote:

Time: 15:00 ET (GMT-0500)

 Tel: 513-241-0892

I.  Roll Call

  Voting Members

   Richard Hill

   Mohammad Jafari

   Steven Legg

   Rich Levinson

   Hal Lockhart (Chair)

   Bill Parducci (Chair, minutes)

   Remon Sinnema

   John Tolbert

  Members

   Richard Skedd

  Quorum: YES (8 of 11 - 72%)

  Approve Minutes:

   21 February 2013 TC Meeting

   APPROVED UNANIMOUSLY

II. Adminstrivia

  Future TC meeting times

   Options (ET): 9:00am, 4:30pm, 5:00pm, 11:00pm

   TC has 24 hours to submit additional time proposals to Bill who will

   create a ballot on the Oasis site, duration one week. Format "vote

   against" poll. results will be used to update time of TC meetings

   going forward.

  Status EC-US/IPC Profiles

   Passed review for CS status. Ready for Attestations.

  Interop

   John: Demonstrated new technology, went well. Higher degree of

         interoperatbilty demonstrated over pervious years.

   Action Item: Hal will gather materials from interop, confirm

                approval to share and post demo materials.

   Oasis is asking for 2014 participation now. Any interested parties

   are encouraged to voice interest as soon as is feasible by posting

   to the list.

   John: A Profile for ISMAP would make for an interesting demo

  XACML v3.0 Issues and Errata

   Hal: There is an official process for errata. Main limitation is

        only releasable annually. The wiki is likely the best place to

        capture the errata. 

III. Issues

  REST Profile

   Stephen: Example in REST Profile in what response should be

            (non-normative). Remon explained that the text was

            speculative based upon assumptions of operation.

   Hal: suggest that there is a comment that highlights this

        understanding.

  Starter Document/Obligation Profile for Healthcare

   Mohammed: some of the Obligation material goes beyond the HC Profile.

             Those things should come out into a more general Profile

             and retain the HC specific content in a separate profile.

   Hal: suggested mechanism for ensuring consistency of version in the

        TC discussion

  Obligations

   Hal: It is important that our next foray into Obligations should

        drive semantics into a workable solution. TC members should

        start considering requirements. Hal offered that his preference

        is that the PDP remain unchanged in Obligation processing.

        Perhaps PDP changes could be considered later. Use cases that

        would not allow for this requested. Finally, the relationship

        between Policy processing and Obligation need to be revisited

        to address Obligations that are part of Policies dropped during

        evaluation.

   Mohammed: Combining algorithms seem to ignore Obligations.

   Bill: There are some old discussions re: Obligations on the wiki for

         those interested in looking at the historical discussions.

  XACML v3.0 - multiple category elements, normative ambiguity?

   Rich: There seems to be a level of ambiguity in the evaluation that

         has ramifications on Obligations.

   Hal: Please post any such finding so that we can explore it

meeting adjourned.

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