Re: [kitten] CAMMAC open issues

[Jeffrey Hutzelman <[e-mail]>]

On Mon, 2013-11-11 at 17:46 -0500, Greg Hudson wrote:
> On 11/11/2013 04:51 PM, Jeffrey Hutzelman wrote:
> > I fail to see how these are alternatives.  As a wrapper, AD-IF-RELEVANT
> > has special semantics which nullify the criticality of the element it
> > wraps.  AD-KDCIssued does not have this property.
> 
> My reading of RFC 4120 section 5.2.6.2 is that AD-KDCIssued authdata are
> implicitly non-critical.  The text isn't as precise as I would like, but:
> 
> * "The KDC-issued ad-data field is intended to provide a means for...
> positive authorization...".

Not relevant.


> * "Elements encapsulated with in the KDC-issued element MUST be ignored
> by the application server if this 'signature' is not present."

I'm not even sure what this means, since the 'signature' is contained in
a non-optional field; if it's not present, the AD won't parse.

> * "This element and the elements it encapsulates MAY safely be ignored
> by applications, application servers, and KDCs that do not implement
> this element."

I somehow missed this.  In which case, yes, wrapping in AD-KDCIssued
would indeed obviate the need to wrap in AD-IF-RELEVANT.  However, that
means that wrapping in AD-KDCissued is not a substitute for using the
svc-verifier, since the former carries semantics affecting criticality.
If we drop svc-verifier, then there is no way to issue credentials
containing AD that is both authenticated and critical!

IMHO, we should simply note that most already-deployed services will not
understand AD-CAMMAC, and specify that it SHOULD be enclosed in
AD-IF-RELEVANT unless it contains authorization data which is critical
with respect to the service or some principal named in other-verifiers.

Probably we should also indicate that AD-CAMMAC-BINDING SHOULD be
enclosed in AD-IF-RELEVANT unless the containing AD-CAMMAC is already so
wrapped, and modify the text so that an AD-CAMMAC whose first element is
an AD-IF-RELEVANT wrapping AD-CAMMAC-BINDING is permissible.


Incidentally, it's not clear that wrapping AD-CAMMAC in AD-IF-RELEVANT
automatically confers full non-criticality on all of the wrapped
elements!  The definition of AD-IF-RELEVANT in RFC4120 5.2.6.1 is
ambiguous in this respect.  It may be desirable to clarify that if a
particular AD-CAMMAC is wrapped in AD-IF-RELEVANT, the non-criticality
applies to all of its elements.

-- Jeff