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Subject: Re: [xacml] Combining algorithm combining orders

Hi Rich,

I misunderstood you on some points, so thanks for the clarifications. 
With those, yes, I think we are in agreement on the nature of the problem.

Some comments:

1. The calculatedEffect and extended Indeterminate are exactly the same 
thing, we just call them different names and I used a different 
notation. (I would perhaps prefer "potentialEffect" instead.)

2. If I understand you correctly, you would like to have a global 
setting in the PDP to decide whether combining algorithms would be 
biased or not. I think this is not the correct way to do it, since I 
think you would probably want to use unbiased combining at lower levels 
in the policies and propagate errors upwards because there might be 
other low level policies which do not have errors and could take over 
and produce a correct result. The biased algorithm would be typically be 
used at a higher level in the policies to filter out errors if the 
happen at lower levels and the lower levels cannot sort them out 
themselves. So I think separate biased/unbiased algorithms are a better 

3. I would not like to change the PolicySet/Policy/Rule structure, 
although PolicySet/Rule would be enough. This would be a change which 
probably makes migration from 2.0 to 3.0 more difficult because of 
compatibility issues which I guess are likely to result from this. (But 
I haven't thought much about this yet, so I don't know for sure.)

4. Yes, currently returning Indeterminate means that information is 
lost. It would be possible to include the calculatedEffect with each 
Indeterminate as they are propagated upwards. But I don't think this is 
a big concern in practice, and it is probably not worth making the 
standard more complex like this.

Best regards,

Rich.Levinson wrote:
> Hi Erik,
> Thanks for the detailed response. I think we are pretty much on the 
> same wavelength and it is just a matter of identifying the key details 
> and how to represent them in 2.0 and 3.0 (w no intent to change 
> anything in 2.0 except possibly combining algorithm details if they 
> are found to be inconsistent with what we perceive to be intended).
> Responses inline: (Note: I am still not 100% sure of the logic I am 
> suggesting (i.e. I may be missing something), but am offering it as 
> analysis, which appears to me at least to yield a possible 
> simplification, which appears to remove the problem of Permits being 
> converted into Denys, plus fairly simply also removes the need for 
> separate rule and policy combining algorithms.)
> Erik Rissanen wrote:
>> Rich, All,
>> It's a very interesting discussion you have here with some good 
>> thoughts, but there are some things that I don't agree with.
>> It's a long thread, so it's a bit hard to make responses, so I will 
>> just write up my comments here, in the hope that everybody can figure 
>> out what I mean. :-)
>> 1. You say that we don't need all of the levels PolicySet, Policy and 
>> Rule. I kind of agree. I have thought the same thing before. I don't 
>> see the need to differentiate between PolicySet and Policy. A Rule on 
>> the other hand is something entirely different, and should be 
>> distinct. (When you say that a Rule and Policy need not be separated, 
>> you are at the same time modifying the Policy to contain an Effect, 
>> essentially making it what a Rule is today.)
> Actually, all I am saying is that conceptually, based on the Effects 
> of the Rules that a Policy contains that a Policy may or may not have 
> a defined Effect. If all the Rules within a Policy are Deny, then the 
> Policy cannot possibly produce a Permit, and vice versa. I am also 
> saying that when the Policy processes the Rules it can keep track of 
> the Effects and determine if it effectively has a Permit or Deny or no 
> Effect. Then a PolicySet, with its policy combining algorithm can 
> apply the same type of logic: i.e. if a Policy evaluates to 
> Indeterminate, it can easily within the rule combining algorithm have 
> retained the fact whether all the rules had the same Effect or not, 
> and, if so, which value the Effect would have.
> What happens with the current combining algorithms is that in the rule 
> combining deny overrides there is a built-in unbiased choice being 
> made which passes up ambiguity which results in the PolicySet that is 
> delivered this rule combining result is unable to determine whether or 
> not the Policy could have possibly returned a Deny or not.
> That is an observation, not intended to say whether the algorithm is 
> right or wrong, just that it appears to be unnecessarily removing 
> information that might have been useful.
> The other obvious choice is to return a Deny, which is what I 
> suggested, which is what I would call a "deny-biased deny overrides" 
> rule combining algorithm. i.e. it would return deny if it found a deny 
> could possibly be returned. This might be slightly less ambiguous than 
> unbiased version, but also has the disadvantage of introducing 
> ambiguity into whether the Deny was real or potential.
> Actually, given the asserted objective of the algorithm, I think both 
> choices are bad and that a better choice is to calculate whether there 
> is a game changer in the Indeterminates. This way the rule combining 
> algorithm would return one of the following:
>     * Permit
>     * Deny
>     * Indeterminate + (calculatedEffect = Permit, Deny, or None)
>     * NotApplicable
> where the "calculatedEffect" is just an indicator as to whether all 
> the applicable Rules were Permit or all Deny or a mix in which case 
> the Policy has no implicit Effect.
> I believe this slight change would have two results:
>    1. It would remove the distinction between rule combining and
>       policy combining because both types of algorithm would now
>       process definitive Deny's and Permits plus have a possible
>       modifier indicating the character of an Indeterminate result
>       that might have been obtained.
>    2. With that distinction removed, the PolicySet and Policy no
>       longer need to be distinct either.
> I think the net effect would be that the Indeterminates could be cut 
> off at whatever level desired by simply inserting a "biased" combining 
> algorithm wherever it was deemed to be needed.You could put it in the 
> top level policy set and have a flag as to whether you had a biased 
> PDP or not, and if not, pass it on up and leave the choice to the PEPs.
> But I think the net effect is that we could have a uniform structure 
> of PolicySets containing Rules and PolicySets, just like a file system 
> has directories that contains directories and files. Each PolicySet 
> would have a "combining algorithm", which could be either a biased or 
> un-biased flavor.
> For 2.0 we simply could define rule combining algorithms that return a 
> calculatedEffect along with the result. For a biased algorithm, the 
> calculatedEffect would be irrelevant since it is already embodied in 
> the result, and for an unbiased algorithm it passes up the potentially 
> useful information that the Policy could have produced only one 
> possible Effect.
>> However, I don't think this is important enough to motivate a change 
>> which complicates compatibility between 2.0 and 3.0.
>> 2. In your first email you suggest that the rule combining algorithm 
>> should be changed (around the test for a "potential deny"). I don't 
>> agree. It's the policy combining algorithm which is wrong, the rule 
>> combining algorithm is right and by changing it you would introduce 
>> the same kind of error in the rule combining algorithm as well.
> I believe this was answered above - basically the point I was making 
> there was that by making the rule combining biased, that the policy 
> combining would no longer be in a position to turn Permits into Deny 
> because potential Denys would be pre-resolved and that if the policy 
> combining had the calculatedPolicyEffect then it would be able to make 
> its own decision in the unbiased case. Basically, at that point I was 
> concerned with the fact that information was being screened out of the 
> policy combining which caused it to be in the position you originally 
> characterized of one where it reversed a Permit rule into a Deny. As 
> indicated above, I think there are no "good" choices if you don't 
> return the calculatedEffect. Either choice removes information, 
> causing policy combining to be dealing with ambiguous results.
>> 3. You say that policies should be constrained by an Effect. I don't 
>> agree with this. Policies/PolicySets are not the leaf nodes in the 
>> policy tree, where the effect should be. Policy/PolicySet is for 
>> combining results from lower levels. This is completely right.
> I didn't say the policies "should be constrained by an Effect". I said 
> that policies could calculate an Effect, which policy combining could 
> use or not depending on the algorithm. Again the only "Effect" a 
> Policy would have is "Permit" if all the applicable Rules were 
> Permits, i.e. similar to a single Rule with Effect of Permit in a 
> Policy should not be able to produce a Deny, I think one can argue 
> that a collection of Rules, all whose Effects are Permit, should not 
> be able to produce a Deny either. That is the only meaning to this 
> calculatedPolicyEffect that I am considering.
>> Instead, what you are kind of approaching here is something similar 
>> to what Olav Bandmann did at SICS when we did some formal 
>> analysis/modeling of XACML.
>> He suggested to me that the concept of Indeterminate could be 
>> extended to include a set of "potential results". Currently an 
>> Inderminate means "There was an error, and it could have been a Deny, 
>> Permit or NA".
>> Instead, we could extend Indeterminate so it also includes a more 
>> fine grained set of potential results. For instance, if a rule with 
>> Effect=Permit is evaluated to Indeterminate, we know that it could 
>> never have been a Deny, so we could say that the result is 
>> Indet{P/NA}, meaning that "there was an error, and it could have been 
>> eitther a Permit or a NA". There are two other possible versions of 
>> Indeterminate in this model: Indet{D/NA} and Indet{P/D/NA}. Under the 
>> old rules, every Indeterminate is equivalent to an Indet{P/D/NA}. 
>> (It's this model Olav used to analyze the current algorithms, and 
>> that was when he found the errors we are currently debating.)
> I believe that the "calculatedEffect" I describe above serves this 
> same function. It is simply a byproduct of rule processing that can be 
> factored into policy combining algorithmically rather than defining 
> different classes of "Indeterminate".
>> Now, if a deny overrides combining algorithm would have to combine a 
>> Permit and a Indet{P/NA}, it could see that the indeterminate could 
>> never be Deny, so it can safely return a Permit, despite the error.
>> I think Rich you were looking for something similar. But it's the 
>> Indeterminate result which needs to be extended with the effect, not 
>> the Policy.
>> Under this extended model, the extended set of the Indeterminate 
>> could be something entirely internal to the PDP, just for the 
>> combining algorithms to use, or it could also be exposed to the 
>> outside the response.
>> Anyway, Olav made the suggestion a couple of years ago, but I never 
>> posted it to the list since we both thought it would be too 
>> controversial to be accepted by the TC.
>> It is interesting to note that the rule combining algorithms already 
>> work like this. That's what the "potential-deny/permit" tests do in 
>> effect. If this analysis would have been followed also on the policy 
>> combining algorithms, they would not have been made the way they are 
>> today.
> I pretty much agree with all this. What I think happens with the 
> current structure is that by removing the notion of "Effect" at the 
> policy combining level, that we are introducing unnecessary complexity 
> conceptually which results in all kinds of strange behavior such as 
> the Permit becoming Deny that you originally described. By simply 
> retaining the "notion" of "Effect" we are able to have uniform logic 
> in the combining algorithms, which preserves information of a 
> consistent character that is currently being thrown away plus removes 
> the need to have distinct policy or rule combining algorithms.
>> I guess that the reason the policy combining algorithms are wrong is 
>> the use case you are talking about Rich: sometimes policy authors 
>> want to specify in their policy that indeterminates should not be 
>> propagated upwards. To handle this, the end result in the design 
>> became the current algorithms. (I am just speculating here, I wasn't 
>> a member of the TC back then, so I don't know.)
>> Sure, I agree that sometimes it is useful to not propagate errors, 
>> but in many (most?) cases it is not. We need to separate these two 
>> use cases into two different algorithms.
> I agree. I think this is the notion of biased (consumes errors) vs 
> unbiased (propagates errors).
>> So I propose that:
>> A. For 2.0 we change nothing. This is is too big to be errata.
> I agree 2.0 should be left alone.
>> B. For 3.0 we change the behavior of the currently defined 
>> permit/deny-overrides algorithms so that they always propagate 
>> Indeterminates upwards. (We do not change the identifiers.)
> This I would consider the "unbiased" set of algorithms, which, if they 
> do not include a calculatedPolicyEffect, remove the fact of 
> potentialDeny or potentialPermit that might have been encountered.
>> C. For 3.0 we introduce new combining algorithms 
>> "permit-overrides-indeterminate" and "deny-overrides-indeterminate", 
>> which never propagate an Indeterminate upwards, rather they will 
>> default to Permit/Deny in case of an error.
> This I would consider to be the "biased" set of algorithms, which, 
> regardless of whether they retain a calculatedPolicyEffect remove the 
> information as to whether the Deny or Permit being reported was 
> potential or real.
>> (I think the concept of extended Indeterminate is a bit complex, and 
>> I am not sure if it's really worth so much, so I do not propose that 
>> we introduce it.)
> I believe the "calculatedPolicyEffect" very cheaply produces the same 
> information as the "extended Indeterminate" and would remove the need 
> to have separate rule and policy combining algorithms which I believe 
> have the effect of removing information causing unnecessary ambiguity 
> at the policy combining level.
>> Best regards,
>> Erik
>> Rich.Levinson wrote:
>>> Hi Daniel,
>>> I do not understand your objection. My suggestion removes nothing 
>>> from the existing logic in terms distinguishing between effect and 
>>> decision. In fact, it adds the ability to determine effect at the 
>>> policy level as well, which the current algorithms do not include 
>>> which results in the ambiguous and contrary results as described by 
>>> Erik in his original email:
>>> http://lists.oasis-open.org/archives/xacml/200701/msg00020.html
>>> where a Rule with Effect="Permit" results in a parent Policy with 
>>> decision = "Deny".
>>> If you follow the logic I described in detail, especially in the 3rd 
>>> email (although it also exists in the first two as well):
>>> http://lists.oasis-open.org/archives/xacml/200810/msg00044.html
>>> I think you will find it to be the case that current effect logic is 
>>> preserved, as well as all distinctions between errors and results.
>>> The main point is to define what "Effect" really means, which is 
>>> that the contained logic can produce one and only one of the 
>>> decision types: i.e. either Permit or Deny, but not both (obviously 
>>> not "both" at the same time, but "both" in the sense that until 
>>> evaluation it is not determisitic which can be produced, which 
>>> becomes the case when multiple Rules are present within a Policy 
>>> that have different "Effects".). Once characterized in this manner, 
>>> it is straight-forward to carry it up along with rest of the 
>>> combining information.
>>>     Thanks,
>>>     Rich
>>> Daniel Engovatov wrote:
>>>> I disagree with this assessment. I think it was important to 
>>>> separate the ability to explicitly handle errors from results. So 
>>>> effect and decision are distinct on purpose.
>>>> What we really need is a normative and portable way to define and 
>>>> exchange combining policies.
>>>> Daniel. -----Original Message-----
>>>> From: "Rich.Levinson" <rich.levinson@oracle.com>
>>>> Date: Sun, 26 Oct 2008 04:25:31 To: Erik Rissanen<erik@axiomatics.com>
>>>> Cc: XACML TC<xacml@lists.oasis-open.org>
>>>> Subject: Re: [xacml] Combining algorithm combining orders
>>>> Hi All,
>>>> While there are probably some details in the previous email that 
>>>> need to be cleaned up, I think that it identifies a basic problem 
>>>> that needs to be addressed.
>>>> As indicated, Erik came up with a single rule policy where the rule 
>>>> had an effect of permit, and yet the policy that wrapped it 
>>>> effectively produced a deny at a higher level.
>>>> I believe the root cause of this problem is that the concept of 
>>>> "Effect", and its implementation in the current  XACML specs is 
>>>> fundamentally flawed and needs to be addressed.
>>>> Again, with Erik's use case, the flaw is obvious: a Rule with an 
>>>> Effect of Permit results in a Policy that causes a Deny. The flaw 
>>>> is that the concept of Effect is not handled properly. "Effect" 
>>>> effectively means that the block of logic contained within this 
>>>> element can ONLY produce either a Deny or a Permit, but not both. 
>>>> On the other hand, because a Policy does not have an Effect, it 
>>>> implicitly can produce either a Deny or a Permit. Therefore, to 
>>>> preserve the notion of "Effect", we have to include it also at the 
>>>> Policy level, with the following constraints: The Effect of a 
>>>> Policy must equal the "AND" of the Effects of all of its Rules. 
>>>> Therefore if a Policy has only Rules that are Permits, then the 
>>>> Effect of the Policy is Permit. If the Policy has Rules with both a 
>>>> Permit and Deny, then the Policy effectively has no "Effect" 
>>>> because the meaning of "Effect" is that only one type of decision 
>>>> is possible, and now the Policy can produce two types of decisions.
>>>> Therefore, I think my basic conclusions in prev email are correct 
>>>> about converging Policy and Rule into a single element - i.e. the 
>>>> only thing that distinguishes the Rule and single Rule Policy is 
>>>> the fact that the Rule has an Effect and the Policy doesn't, which 
>>>> has been shown above to be wrong and correctable by giving the 
>>>> Policy an Effect attribute, which effectively removes the 
>>>> distinction between Policy and Rule.
>>>> Comments and suggestions will be much appreciated.
>>>>     Thanks,
>>>>     Rich
>>>> Rich.Levinson wrote:
>>>>> Hi Erik and TC,
>>>>> As indicated at today's meeting, I have concerns about the 
>>>>> proposed changes. I have read this email from Erik fairly 
>>>>> carefully, as well as the first part of the cited reference:
>>>>> http://lists.oasis-open.org/archives/xacml/200701/msg00020.html
>>>>> and I also started to follow the thread initiated by Daniel:
>>>>> http://lists.oasis-open.org/archives/xacml/200810/msg00027.html
>>>>> In the first cited reference, Erik states:
>>>>>    "P1 has a singe rule with a Permit effect."
>>>>> then goes on about how this Rule gets propagated up and 
>>>>> effectively turns into a Deny under the deny-overrides algorithm.
>>>>> I agree that this appears incorrect, and went to look for the 
>>>>> source of that problem.What I found was that it appears this 
>>>>> problem results from what appears to me to be an error in the 
>>>>> deny-overrides *rule *combining algorithm, which has what I 
>>>>> believe is an incorrect piece of logic on lines 5181-5184 of the 
>>>>> XACML 2.0 core spec, p 133:
>>>>>    5181     if (potentialDeny)
>>>>>    5182     {
>>>>>    5183         return Indeterminate;
>>>>>    5184     }
>>>>> It is my opinion the line 5183 should say "return Deny", because 
>>>>> it is at this point that the "potentialDeny" condition that the 
>>>>> algorithm goes to the trouble to find, now throws away, which ends 
>>>>> up creating the situation where a rule with effect = Permit can 
>>>>> produce a Deny at the policy combining level.
>>>>> I also read Daniel's comment up to the point where he said:
>>>>>    "Just throwing an exception all the way to the client is what 
>>>>> we have tried to avoid.",
>>>>> with which I agree. I expect that many policy administrators will 
>>>>> want definitive results to come out of the PDP, and not any 
>>>>> "Indeterminates".
>>>>> So, my first suggestion would be to fix what I consider to be an 
>>>>> error on line 5183 described above. The practical reason why I 
>>>>> consider this an error is that if there is insufficient 
>>>>> information to evaluate a rule that has an effect = Deny, then, I 
>>>>> believe it is legitimate to define a rule combining algorithm that 
>>>>> says that rule should return the effect = Deny so that one cannot 
>>>>> remove input attributes in the hopes of reversing a decision. One 
>>>>> could also define a companion combining algorithm the evaluated to 
>>>>> Indeterminate in order to provide that option for those who want 
>>>>> it. This way a policy administrator could choose on a case by case 
>>>>> basis which behavior is preferable based on specific situations.
>>>>> My second suggestion is more broad-based, and, in fact, I think we 
>>>>> should consider for 3.0 if there is some agreement. I am not 
>>>>> expecting what follows to be bought hook line and sinker at first 
>>>>> glance, but I do think it is worth consideration, especially since 
>>>>> we are already making a major paradigm shift with the 
>>>>> category-based attributes that are defined in 3.0, that 
>>>>> effectively adds a new "dimension" to the domain of attributes, 
>>>>> which I believe is a desirable step forward from the 
>>>>> Subject,Resource,Action,Environment-based attributes of 2.0, which 
>>>>> based on previous discussions about the what appear to me as 
>>>>> ambiguous and unnecessary semantic restrictions on the Action 
>>>>> attribute "category". Personally, I would recommend in section B.6 
>>>>> of 3.0 that the action attributes be defined within a given 
>>>>> "resource" category, as I believe that actions are attributes of 
>>>>> resources and do not have any meaningful definition outside of a 
>>>>> resource context. However, that is just a minor aspect of the 
>>>>> overall suggestion I have related to the combining algorithms.
>>>>> The main suggestion about combining-algorithms is that I believe, 
>>>>> but have not definitively proven yet, that there is an unnecessary 
>>>>> layer of combining with two types of rule and policy combining 
>>>>> that exists in 2.0. One easy way to see that this might be true is 
>>>>> to do the following thought experiment:
>>>>>   1. Wrap every "Rule" within a Policy element, which effectively 
>>>>> turns
>>>>>      every policy into a single rule policy. However, if we do this,
>>>>>      then we lose the "Effect" behavior of the Rule, therefore
>>>>>   2. Move the Effect attribute up to the Policy level. Therefore, 
>>>>> each
>>>>>      single Rule Policy now also preserves the Effect.
>>>>>   3. Clearly all multi-rule Policies must now become PolicySets, and
>>>>>      the policy-combining algorithm must be replaced by the logic of
>>>>>      the rule combining algorithm to take into account the Effect of
>>>>>      the Policies.
>>>>>   4. We are now left with a collection of single rule Policies 
>>>>> (with an
>>>>>      effect) and PolicySets that have no effect since it was factored
>>>>>      out in step 3. This is where it gets "fuzzy", but I suspect that
>>>>>      we might be able to show that we can still use the rule 
>>>>> combining
>>>>>      algorithms on a mix of entities some of which have an effect and
>>>>>      some which don't. I suspect that for example on lines 5174-5177
>>>>>      that we can just skip this block if the entity being tested is a
>>>>>      PolicySet, and execute it if the entity is a Policy. If so, 
>>>>> then I
>>>>>      think we are done and have factored the distinction between
>>>>>      rule-combining and policy-combining out of the structure.
>>>>> If the above logic is correct, and we can reduce the structure to 
>>>>> be simply Policies with an effect and PolicySets with no effect, 
>>>>> and allow the combining algorithms to only use the "effect" 
>>>>> segments on the objects that contain an effect, then I think we 
>>>>> reduce the complexity of the syntax (either no longer have "Rule"s 
>>>>> or no longer have "Policy"s) at the cost of only what appears to 
>>>>> me to be a tiny increase in the complexity of the rule-combining 
>>>>> syntax (which is to effectively allow for rules that have no 
>>>>> effect and simply bypass the effect logic).
>>>>> Bottom line is that I am offering this as a potential suggestion. 
>>>>> Because the logic to analyze it is fairly complex, I allow for the 
>>>>> fact that maybe I have missed something essential. However, since 
>>>>> we are making some fairly broad changes in 3.0, I am offering this 
>>>>> as a potential "simplification". At the very least, I am looking 
>>>>> to find what I might be missing that would prevent this 
>>>>> "simplification".
>>>>>    Thanks,
>>>>>    Rich
>>>>> Erik Rissanen wrote:
>>>>>> All,
>>>>>> In my opinion the both the deny overrides and permit overrides 
>>>>>> policy combining algorithms are wrong. They treat errors in an 
>>>>>> incorrect and unsecure manner. The corresponding rule combining 
>>>>>> algorithms are correct, and the policy combining algorithms 
>>>>>> should work in a similar manner.
>>>>>> The Indeterminate result in XACML is used in those places where 
>>>>>> the result of some part of the policy cannot be determined 
>>>>>> because of some form of error. It is important that if such an 
>>>>>> indeterminate result affects the final decision of the PDP, that 
>>>>>> the indeterminate is returned to the PEP so that the PEP knows 
>>>>>> that there is an error. If so is not done, errors in policies 
>>>>>> might not be noticed or an attacker might be able to force the 
>>>>>> PEP into some bad action by causing errors in the PDP (for 
>>>>>> instance by interrupting attribute provisioning services).
>>>>>> If we want to determine whether an Indeterminate should affect a 
>>>>>> result, we can consider the indeterminate to be a result which 
>>>>>> potentially could be either Permit, Deny or NotApplicable and for 
>>>>>> each of them see what the overall result would be. If the overall 
>>>>>> result depends on which choice we make for the Indeterminate, 
>>>>>> then the Indeterminate should be propagated "further up" in the 
>>>>>> policy evaluation. (If you don't understand what I mean, just 
>>>>>> read on, I will provide examples.)
>>>>>> Currently the permit overrides policy combining algorithm works 
>>>>>> like this (using the updated textual description):
>>>>>> 1.    If any policy evaluates to "Permit", the result is "Permit".
>>>>>> 2.    Otherwise, if any policy evaluates to "Deny", the result is 
>>>>>> "Deny".
>>>>>> 3.    Otherwise, if any policy evaluates to "Indeterminate", the 
>>>>>> result is "Indeterminate".
>>>>>> 4.    Otherwise, the result is "NotApplicable".
>>>>>> Notice how a deny goes before indeterminate. That is a bad 
>>>>>> priority. Consider a policy set with two policies A and B. Assume 
>>>>>> that A is deny and B is indeterminate. Consider what the overall 
>>>>>> result could be if policy B would not have an error in it:
>>>>>> A: Deny
>>>>>> B: could be Deny
>>>>>> -> overall would be Deny
>>>>>> A: Deny
>>>>>> B: could be NotApplicable
>>>>>> -> overall would be Deny
>>>>>> A: Deny
>>>>>> B: could be Permit
>>>>>> -> overall would be Permit
>>>>>> Notice how the result depends on what the indeterminate could 
>>>>>> potentially be. However the current definition gives a definite 
>>>>>> Deny in all cases. This breaks the error propagation safety of 
>>>>>> the combining algorithm. In some applications availability could 
>>>>>> be critical, and a PEP might have a bias that indeterminate means 
>>>>>> that access is allowed. In such a case harm could be caused by 
>>>>>> this behavior.
>>>>>> Now consider the deny overrides instead. Currently the deny 
>>>>>> overrides policy combining algorithm works like this (using the 
>>>>>> updated textual description):
>>>>>> 1.    If any policy evaluates to "Deny", the result is "Deny".
>>>>>> 2.    Otherwise, if any policy evaluates to "Indeterminate", the 
>>>>>> result is "Deny".
>>>>>> 3.    Otherwise, if any policy evaluates to "Permit", the result 
>>>>>> is "Permit".
>>>>>> 4.    Otherwise, the result is "NotApplicable".
>>>>>> Note how an indeterminate is converted to a Deny. This gives the 
>>>>>> following behavior.
>>>>>> A: Permit
>>>>>> B: could be Deny
>>>>>> -> overall would be Deny
>>>>>> A: Permit
>>>>>> B: could be NotApplicable
>>>>>> -> overall would be Permit
>>>>>> A: Permit
>>>>>> B: could be Permit
>>>>>> -> overall would be Permit
>>>>>> Again, there is a definite result from the combining algorithm 
>>>>>> although the underlaying logic is ambiguous.
>>>>>> As I said we have discussed this previously. The threads are here:
>>>>>> http://lists.oasis-open.org/archives/xacml/200701/msg00020.html
>>>>>> http://lists.oasis-open.org/archives/xacml/200702/msg00019.html
>>>>>> I propose that we define new identifiers for new combining 
>>>>>> algorithms for 3.0 which work like this:
>>>>>> Deny overrides:
>>>>>> 1.    If any policy evaluates to "Deny", the result is "Deny".
>>>>>> 2.    Otherwise, if any policy evaluates to "Indeterminate", the 
>>>>>> result is "Indeterminate".
>>>>>> 3.    Otherwise, if any policy evaluates to "Permit", the result 
>>>>>> is "Permit".
>>>>>> 4.    Otherwise, the result is "NotApplicable".
>>>>>> Permit overrides:
>>>>>> 1.    If any policy evaluates to "Permit", the result is "Permit".
>>>>>> 2.    Otherwise, if any policy evaluates to "Indeterminate", the 
>>>>>> result is "Indeterminate".
>>>>>> 3.    Otherwise, if any policy evaluates to "Deny", the result is 
>>>>>> "Deny".
>>>>>> 4.    Otherwise, the result is "NotApplicable".
>>>>>> Best regards,
>>>>>> Erik
>>>>>> --------------------------------------------------------------------- 
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