Re: [kitten] draft-ietf-kitten-cammac kdc-verifier omission

Tom Yu <tlyu@mit.edu> Thu, 26 February 2015 20:40 UTC

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From: Tom Yu <tlyu@mit.edu>
To: Benjamin Kaduk <kaduk@mit.edu>
References: <x7d7fvo404h.fsf@equal-rites.mit.edu> <alpine.GSO.1.10.1502121341360.3953@multics.mit.edu> <54DD194D.2010201@mit.edu> <ldv1tluzpt6.fsf@sarnath.mit.edu> <alpine.GSO.1.10.1502181348460.3953@multics.mit.edu> <1424287116.6980.35.camel@willson.usersys.redhat.com> <alpine.GSO.1.10.1502201601450.3953@multics.mit.edu>
Date: Thu, 26 Feb 2015 15:40:01 -0500
In-Reply-To: <alpine.GSO.1.10.1502201601450.3953@multics.mit.edu> (Benjamin Kaduk's message of "Fri, 20 Feb 2015 16:04:44 -0500")
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Cc: kitten@ietf.org, Simo Sorce <simo@redhat.com>
Subject: Re: [kitten] draft-ietf-kitten-cammac kdc-verifier omission
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Benjamin Kaduk <kaduk@MIT.EDU> writes:

> On Wed, 18 Feb 2015, Simo Sorce wrote:
>
>> On Wed, 2015-02-18 at 13:55 -0500, Benjamin Kaduk wrote:
>>
>> Would it make more sense to say something like:
>>
>>         The KDC MUST NOT make verbatim copies of CAMMAC contents into a
>>         new CAMMAC unless the original CAMMAC is authenticated by an
>>         originator that is fully trusted by the KDC.
>>         The KDC MAY choose to filter, transform, or propagate elements
>>         from the original CAMMAC according to local policy.
>
> It might.  Is this the first time we are using the word "trusted" in a
> Kerberos RFC, though?  And being concrete does have something going for
> it (we just have to get it right).

I prefer to avoid using the word "trusted" here.

The KDC makes a policy decision about which CAMMACs have contents that
are safe to copy verbatim into a new CAMMAC.  We can recommend that the
KDC limit verbatim CAMMAC copying to cases where only the local realm
KDCs know the key that authenticates the CAMMAC.

We could define a term "authenticated as originating from a local realm
KDC" to mean:

1. kdc-verifier is present and validates;

2. svc-verifier is present, validates, and uses a key known only to
   local realm KDCs; or

3. no verifiers are present, the ticket-encrypting key is known only to
   local realm KDCs, and all local realm KDCs properly filter out
   client-submitted CAMMACs

(I think I didn't include case 2 in my earlier message, and think it's
somewhat unlikely to get used in practice, but I think mentioning it
makes the definition more complete.)

We can say that the KDC SHOULD only make verbatim copies of CAMMAC
contents to a new CAMMAC when it has authenticated the CAMMAC as
originating from a local realm KDC.  We might also say that when the KDC
has not authenticated a CAMMAC as originating from a local realm KDC, it
SHOULD NOT apply a kdc-verifier to a new CAMMAC whose contents are a
verbatim copy from that first CAMMAC.  (The cross-realm case is somewhat
of an exception, though there generally should be some filtering.)

I guess "verbatim copying" is really shorthand for "verbatim copying
without a complete policy check on the copied stuff".