Re: Miscellaneous CAMMAC issues

On Wed, 2012-02-15 at 13:42 -0500, Greg Hudson wrote:
> On 02/15/2012 01:10 PM, Jeffrey Hutzelman wrote:
> > You keep saying that, but it is not true.  A service can print a ticket
> > for itself whenever it wants, not only when using S4U2Proxy.  A "normal"
> > TGS request is authenticated by a TGT, but a request to modify a ticket
> > is authenticated only by the ticket to be modified, which means that any
> > data signed by either the ticket session key or the service's long-term
> > key cannot be trusted by the KDC.
> 
> I hadn't considered renewal/validation of service tickets.  (Which is
> rare, but probably not so rare that we can throw it away.)
> 
> If the KDC needs to re-sign CAMMACs during renewal/validation, then it
> does need a way to verify that it originated them.  Otherwise the target
> service could get the KDC to help it forge a trusted-svc-signature on
> arbitrary authdata elements.
> 
> I'm not sure whether the current drafts require the CAMMAC to be
> re-signed on renewal/validation.  Certainly, if the svc-signature uses
> the session key and the trusted-svc-signature encompasses the
> svc-signature (as Sam suggested) then re-signing would be necessary.

If AD-Id-ANCHOR is not modified you will need to chnge the expiration
time it stores and I think it would be reasonable to allow KDCs to
recompute AD-PAD-DATA too if they prefer. So yes, contents, changes, so
new signatures are needed, so verification is needed.
That, I think, makes the KDC checksum not optional.

Simo.

-- 
Simo Sorce * Red Hat, Inc * New York

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