potential for harm in DES AD/MIT trust

[David Botsch]

Running Win 2003 SP1 and Win2000 latest SP (forget the num), we were forced to
add in the des-cbc-md5 encryption type for all users. The reason seemed to have
to do w. the session key being set up for the user.

So, we've seen the following behavior:

AS-REP has the TGT encrypted with des3-cbc-sha1, the reply itself encrtyped
with arcfour, and a session key of des-cbc-crc. 

The TGS-REP for the cross-realm Active Directory tgt has a reply encrypted with
des-cbc-crc, ticket encrypted with des-cbc-md5, and session key of des-cbc-crc

Using the arcfour encrypted type for the cross realm tgt principal did not work
(in fact, MS's documentation mentions this). So, we had to set up the cross
realm principal with the des-cbc-md5 encryption type.

When we did not add the des-cbc-md5 type to the individual principals, the
server would choose to use des3-sha1 which, of course, Windows does not parse
:(

We're running MIT Kerbeors 1.3.5 with the latest security patches.

On Sat, Jun 04, 2005 at 03:27:27PM +0000, Jeffrey Altman wrote:
> David Ressman wrote:
> > As it's been pointed out to me, many of our peer institutions have
> > accepted the risk and have set up trusts in their production domains
> > using des-cbc keys. What do they know that I don't?
> 
> David:
> 
> The MIT Kerberos team worked with the Microsoft Windows Security team
> to make sure that RC4-HMAC could be used for cross-realm authentication
> by Windows Server specificly because of the concerns you raise.   DES
> keys are very weak and if they must be used because that is all that is
> supported, then they keys must be replaced on a very regular basis
> until such time as they no longer need to be used.
> 
> With 2003 Server SP1 there should no longer be a reason to use DES keys
> for anything but compatibility with Java 1.5 and earlier.
> 
> Jeffrey Altman
> 
> 
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-- 
********************************
David William Botsch
Consultant/Advisor II
CCMR Computing Facility
dwb7 at ccmr.cornell.edu
********************************