Subject: [Ietf-krb-wg] PA-ETYPE-INFO(2) a MUST in AS-REP when using FAST?

From: "Srinivas Cheruku" <[email-masked]>
Date: Wed, 22 Jul 2009 14:10:59 +0530

Sam,

When using FAST, the AS-REP is encrypted using the reply key which is
generated using the client long-term key and the strengthenkey. So, the
client should know the etype of the client long-term key used so that it can
generate the correct reply key to decrypt the AS-REP. 

As of now, some KDCs (Windows2003 and 2008 - I know FAST is not supported by
these) doesn't return PA-ETYPE-INFO(2) in AS-REP and also RFC 4120 says that
the PA-ETYPE-INFO(2) MAY be used in AS-REP.

Also, we cannot use the etype of the enc-part of the KDC-REP for generating
the client long-term key, as it is the etype of the reply key and not the
etype of the client long-term key and  the etype of the reply key maynot be
same as the etype of the client long-term key e.g. the armor ticket might
use etype=18 and the client might be trying to get the ticket using
etype=17.

So, when FAST is supported, I think the PA-ETYPE-INFO(2) must be used in
AS-REP. Do you agree?

Thanks,
Srini

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From: Sam Hartman <[email-masked]>
Date: Wed, 29 Jul 2009 18:00:47 -0400

Hi.  Sorry for taking so long to respond to this.  I wanted to delve
into RFC 4120 for a bit.

Briefly, I don't think a change is necessary, because I believe the
practice of always including etype-info2 to indicate the key is
already implemented where needed.

As I read RFC 4120, the KDC is required to include etype-info2
whenever pre-authentication data is used (3.1.3, I think) both in a
kdc-error and an as-reply.  Pedantically a KDC may omit etype-info2 if
pre-authentication is not used.

The preauth framework assumes explicitly that the KDC will use
etype-info2 to indicate the parameters of the long-term key.  It does
mandate that the etype-info2 be included.

I think that from a pedantic correctness of spec standpoint that
anytime FAST is being used, pre-authentication is being used and thus
RFC 4120 already requires etype-info2 be included.

If you had brought this point up during the WG LC, I think that a
clarification would be reasonable.  However, since even though it is
kind of indirect, the spec does seem to say the right thing now, my
preference is to make no change at this point.

I completely agree with you that the client cannot function if the KDC
fails to include etype-info2 and the long-term key is used.
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