[OGSA-AUTHZ] Latest profile specs

[David Chadwick]

Hi Tom

thanks for your input

Tom Scavo wrote:
> On Nov 28, 2007 1:38 PM, David Chadwick <d.w.chadwick at kent.ac.uk> wrote:
>> i) why did you delete Identity Provider from being synonymous with
>> Attribute Authority? If you think they are not technically equivalent
>> can you say why.
> 
> An identity provider manages identity information for principals
> (users). 

But identity is simply a set of attributes isnt it?

  An attribute authority asserts attributes about a subject.

And one such attribute could be the user's DN, another could be his 
address or his mothers maiden name. So as a minimum there is a clear 
overlap between the IDP and AA, even if they are not synonymous.


> The latter is what you want, I think.  In any event, the term
> "identity provider" is not used in this document, so it need not be
> defined.

I could go along with that, except that Shibboleth used to have AAs in 
its documentation, then it changed them into IdPs, so if we only mention 
AAs, it will make it more difficult for readers to relate our document 
to other ones that mention IdPs. This is one reason why I wanted to 
include both and show the relationship between them.


> 
>> ii) I suggest changing Credential to Authorisation Credential, because
>> as you point out, Credentials are a superset of signed attribute assertions.
> 
> A credential is information that is transferred from one entity to
> another entity to establish a claimed identity.  See:
> 
> http://www.itu.int/rec/T-REC-X.800-199103-I/en
> 
> So when I think of "credential," I think of authentication.  Rather
> than overload the word "credential," I believe it's better to use the
> term "signed attribute assertion," but it's your call.

Since an identity attribute is just one type of attribute, then it 
logically follows that a signed identity attribute, which you are 
presumably happy to call a credential, is just one type of credential. 
Thus we can have authentication credentials and authorisation 
credentials, and both are signed attribute assertions, are they not?

> 
>> iv) you ask how the CIS is different from an AA. They are clearly
>> related. An AA is the authority behind the attribute assertions that are
>> released, and it does not have to sign the attribute assertions that are
>> issued. A CIS is a service of an AA, and it does have to sign the
>> assertions.
> 
> That's not enough distinction to warrant a new term, I believe.

However, Microsoft have coined the term Security Token Service for the 
service that provides both the CIS function and the CVS function. I 
think this is too general, since it does not say what type of security 
token service it provides. Not all STSs will be CISs, and not all will 
be CVSs. This is why I think we need to separate the STS down into its 
component functionalities.


> 
>> In the grid we are only interested with digitally signed
>> tokens (not symmetrically encrypted ones, MACed ones, or unsigned ones).
> 
> I disagree.  Our implementation, for example, does not require signed
> assertions.  It requires mutual authentication, yes, but message-level
> security is but one way to achieve that.

So do you implement zero proof symmetric encryption methods for mutual 
authentication?

> 
>> So we introduce the CIS to show that it is signed attribute assertions
>> that we are concerned with, and the CIS is the service of the AA that
>> does this. We also need to have the converse validation service to the
>> issuing service, hence the CVS. If we replace CIS by AA, then we should
>> also replace CVS, perhaps by AVS.
> 
> I'm afraid I don't understand your point.  In any event, the use of
> the word "credential" is misleading, I think.  On the other hand, the
> word "attribute" is well understood, so why not use that?

Because we need to differentiate between an entity having an attribute, 
a random (unprovable) assertion that an entity possesses an attribute, 
and a signed (believable) assertion that an entity possesses an 
attribute. It is the job of the authorisation infrastructure to 
disentangle these, and to be able to tell the PDP that this entity has 
these attributes, give a set of unsigned and signed attribute 
assertions. Do you appreciate the difference between these three 
elements? Giving each of these elements different names, and giving 
names to the functions that provide and validate these elements, seems 
to be a useful thing to do in my opinion. This is why we have introduced 
(authorisation) credential, CIS and CVS to be more precise in our 
terminology and to indicate what elements we are actually talking about. 
Personally I think many people are confused about the differences 
between these three elements (heck many people are confused between 
identification, authentication and authorisation!)


> 
>> v) I think its useful to keep the MS STS terminology in the document
>> since some readers may already be familiar with this concept, and it
>> gives them a handle on our terminology. Its also good to relate
>> different terms together when they are talking about the same conceptual
>> entities. This helps people figure out how all these disparate terms fit
>> together. (which is related to point i) above)
> 
> There already is a section on WS-Trust and the STS, which is fine.  I
> don't think you need to add confusing parenthetical remarks in the
> definitions, however.  Indeed, the phrase "synonymous with the
> validation service of Microsoft's Security Token Service" is false,
> since a CVS/CIS is not an STS.

Well I think it is, since an STS has three functions, token issuing, 
token validation and token exchange, where the latter is a combination 
of the two former functions.

regards

David

> 
> Tom
> 

-- 

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David W. Chadwick, BSc PhD
Professor of Information Systems Security
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