On Thu, 25 Jul 2013, Sergio Gelato wrote:
I've been poking a bit into this. First of all, let's make sure I don't
misunderstand your expectation here: do you want the KDC to be willing to
issue a ticket with a des-cbc-crc session key (as requested by old aklog)
even though the afs service principal does not have that enctype? Or are
we Heimdal users expected to add that enctype to afs/cell whenever we
rekey? The latter works with the Heimdal KDCs I've tried (the pre-1.4.0
from Debian squeeze and the pre-1.6 from Debian wheezy), the former doesn't.
If the KDC is in a state where it must choose a session key enctype in the
intersection of the service principal's keys and the client's list, then
the latter should always work. The DES key for the afs/cell principal
will need to be entered into the KeyFile or removed from the rxkad.keytab
in order for server-to-server authentication to work, though.
The relevant code seems to be in kdc/kerberos5.c:_kdc_find_etype(). It was
reworked in 2011, largely by Nico Williams, to use a new session key enctype
selection algorithm (controlled by {tgt,svc,preauth}-use-strongest-session-key
in krb5.conf, and now on by default) as an alternative to the old one. The
old code, which the comments claim conforms to RFC4120, still cannot select
an enctype that isn't in the intersection of the principal's and the client's
lists. The new code looks like it should (provided that allow_weak_crypto=true
for the KDC; the _kdc_is_weak_exception() mechanism won't help here) as a
last resort, except it forgets to set ret=0 in the relevant code path
(after "enctype = clientbest;"). It looks like the bug might still be there
at the tip of the master branch as of this writing. I'll try to test my
putative fix later tonight.
Thanks for looking into this.
-Ben
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