kerberos error: PAC checksum type is not keyed

Andrew Bartlett abartlet at samba.org
Tue Oct 26 22:39:04 MDT 2010


On Wed, 2010-10-27 at 11:05 +1100, Andrew Bartlett wrote:
> On Thu, 2010-10-21 at 08:17 +1100, Andrew Bartlett wrote:
> > On Wed, 2010-10-20 at 10:22 -0400, Aaron Solochek wrote:
> > > I'm getting ever closer to having nfs4 working with the samba4 kdc.  Currently I
> > > seem to be blocking on the error "PAC checksum type is not keyed" which is
> > > generated by the kdc when nfs sends a PA-TGS-REQ for nfs/foo.bar.com.
> > > 
> > > >From googling, it seemed to be related to the des-cbc-crc enctype, so I set
> > > 
> > > default_tkt_enctypes = rc4-hmac des-cbc-md5
> > > 
> > > in krb5.conf on both client and server.
> > > 
> > > 
> > > Then the problem changes slightly.  With that option set, the client first
> > > requests nfs/foo.bar.com with enctypes "rc4-hmac des-cbc-md5", and that
> > > succeeds, but immediately following that the client sends the exact same
> > > request, only this time the enctypes are back to "des-cbc-crc des-cbc-md5
> > > des-cbc-md4" and it fails again with PAC checksum error.
> > > 
> > > 
> > > So it seems that I have 2 bugs.
> > > 
> > > 1) PAC checksum bug
> > > 
> > > 2) kerberos client (libraries or nfs4?) bug that causes the second request
> > > ignoring the enctypes specified in krb5.conf.
> > > 
> > > 
> > > What can I do about #1?
> > 
> > Someone needs to confirm what Windows does here.  The PAC security
> > relies on the checksum being keyed, so my gut feeling is to omit the
> > checksum in this case.  We need to determine if this is security issue
> > with Windows, or there is some other protection, or Windows omits it.
> > (This should not be relevant for NFSv4, which should never need to use
> > DES, but is important for AFS clients).
> 
> I've now looked into the windows behaviour, which is documented here:
> http://blogs.msdn.com/b/openspecification/archive/2010/01/01/verifying-the-server-signature-in-kerberos-privilege-account-certificate.aspx
> 
> The problem is that when different crypto-systems are used (Example 2),
> Heimdal objects because the keys involved are not the right type or
> length.  I'm working with upstream Heimdal to find out if we can get a
> better API for this, plus a way to handle the crazy windows behaviour
> exception noted there. 

Attached is my patch for testing.  

Andrew Bartlett

-- 
Andrew Bartlett                                http://samba.org/~abartlet/
Authentication Developer, Samba Team           http://samba.org
Samba Developer, Cisco Inc.

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