Re: New ID of potential interest

On Aug 10, 2010, at 4:12 PM, Jim Schaad wrote:
>> 
>> On Apr 10, 2010, at 9:07 PM, Jim Schaad wrote:
>> 
>>> A couple of questions and comments on this document
>>> 
>>> 1.  Is there a reason not to make the sequence require at least two
> elements
>>> for a union and an intersection?
>> 
>> No, there is no reason. And the ASN.1 module can be easily changed to
> enforce the
>> presence of at least two elements in the sequence. But what are the pros
> and cons of
>> doing so? Will anything be gained by requiring that unions and
> intersections have at
>> least two elements?
> 
> I see no down side to make this requirement - it avoids the need to parse
> and evaluate a node in the tree that contains a union or an intersection of
> one node.  This means that the tree is smaller as is the encoded attribute.

Makes sense. I'll change it.


>>> 3.  Given that we are discussion SETs.  Are there any reasons not to
> impose
>>> some restrictions on those who create SetKetParticipentSet values so
> that
>>> they are not every empty?  Thus an intersection SHOULD NOT result in an
>>> empty result
>> 
>> I fear that might turn out to be an onerous run-time requirement. These
> sets might be
>> represented by community-identifiers, remember. If we wish to enforce that
>> intersections be non-empty (for example) then we are requiring that
> someone go chase
>> down the membership-lists of all communities. That may be an undesirable
> run-time
>> requirement for some settings.
>> 
>> Also, that requirement may not be well-defined, if these communities
> change. A set may
>> be non-empty when generated, but become empty later as come component
> communities
>> change their membership.
>> 
>> But let me ask: what is gained by requiring that generated SETs be
> non-empty?
> 
> So yes there is a good reason not to require this is the leaf nodes can be
> dynamic.  If they are static then doing a pre-evaluation would make things
> smaller and faster for all clients to evaluate.

I'll make that clear in the document.


>> 
>>> 4.  Are there any locations (such as SetMember) where you one should
>>> explicitly state that this field may be expanded at a later date with
>>> additional values that may need to be recognized as being present, even
> if
>>> they are not evaluated.
>> 
>> I'm not sure what you mean here. Can you rephrase?
> 
> Are you ever going to want to add additional, currently unknown, items to
> the structure SetMember?  If so do you want to require that they be handled
> gracefully by people written to the current specification?  Similarly are
> you ever going to want to add new operations to the SetKeyParticapantSet in
> the future and how should existing clients deal with such items?

Ah, I see what you are asking. Good question. We'd like keep this as extensible as possible, and so what you say makes sense. But I don't immediately see what it means to 'handle gracefully' the case where one doesn't know how to interpret a set. For example, what would it mean to 'handle gracefully' the case where the set is 

  Intersection of (set A) and (X)

when you don't know how to interpret X? I suspect the answer depends on the intended use of this attribute, which can differ from setting to setting.

Given that we can't define what it means to 'handle gracefully' the case where current implementations encounter future extensions, I don't think we should require it in the spec. But at the same time, we don't want to rule out future extensions. Here's an alternate idea: what if we were to include a version number in this attribute? Then we could simply increment this version-number when we extend the attribute. Also, implementations would be able to tell right away whether or not they would understand the set-representation of the attribute. They would then either process the attribute to get the so-represented set, or gracefully handle the attribute as they would a completely unfamiliar attribute. 



>> 
>>> 5.  When doing an intersection, does none need to understand the value
> of
>>> SetMember, or can the evaluation be done on the ASN.1 encoding of the
>>> SetMember.   Specifically would one need to understand that a
> certificate
>>> containing a specific public key be identified EITHER by the pub key
> info or
>>> by the issuer/serial?
>> 
>> That's a very good question. I hadn't thought about it. Do you have any
> suggestions?
> 
> So the first question that comes up is whether you are expecting this to be
> evaluated in the context of a specific certificate (or set of certificates)
> or is this to be evaluated and this later applied to a set of certificates.

I'm not sure what you mean, but I don't envision that a public key would be included in a set-definition between creation and certification. That is, by the time a public key gets to the point where it will be used in a set definition, then one of the two following statements is true:

1) It has already been put in a certificate, and therefore can be referenced by issuer/serial number, or
2) It is an ephemeral/anonymous key, and will *never* be embedded in a certificate.

If this is a reasonable assumption, then we might simply wave the problem away by specifying that it is invalid to refer to a key using the SubjectPublicKeyInfo form when it could also be referenced using the IssuerAndSerialNumber form. But I'm not sure that the underlying assumption is valid...

I'm trying to think of how this might come up. Suppose that the same key were referred to in two different ways-- once using IssuerAndSerialNumber (the 'I' form), and once as SubjectPublicKeyInfo (the 'S' form). Lets further suppose that implementations believed that the two forms referred to two different keys. Then how could things go wrong?

* They could mis-count the number of elements in a set. They could count the key twice, for example, believing there to be one more person/key in the set than there really is.

* They can also over-count the number of elements in the set if they encounter the form "{I} SetDiff {S}". This set should be empty, but the implementation may believe that it is actually equivalent to the set {I}. 

* Then again, they might undercount the number of elements in a set if they encounter the form "{I} Intersection {S}". This set actually contains the key in question, but might be mistaken for empty.

I think what I will propose is that when creating a SetKey attribute, implementations that wish to name a certified key SHOULD use the issuer and serial number form. However, implementations that receive and process a SetKey attribute SHOULD NOT regard different names as always indicating different keys.  And that I add a section to the document explaining these pitfalls.


> In the second case one cannot say that an issuer serial number and a public
> key info refer to the same certificate.  Part of the question can be
> addressed as a security consideration and part by expected usage.  That is a
> single community might only use a single way of identifying certificates and
> thus the question is moot.

For our intended usage, it is the case that a given community would use only one way to refer to a certificate.


> You might also have some problems from Alexy if you refer to things as
> names, but you don't use a name form to store them in.  You might be better
> using groupId rather than groupName  (same with participantName).  Note that
> you don't currently seem to expect that you can get from a participantName
> to a certificate to check for matches so the best rule might be that
> different name forms are treated as different identifiers even if they can
> later be mapped to the same individual.


Will do.


While you chew on this, I'll go ahead and make the changes described above and send the new version to you.


Thanks for your comments.
-- 
Jonathan Herzog							voice:  (781) 981-2356
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