[dhcwg] Comments on draft-ietf-dhc-pktc-kerb-tckt-00.txt

[Sam Hartman <[e-mail]>]

Hi.  I'm not subscribed to the list, so please copy me on responses.
I'm sending these comments because a review of the option from a
Kerberos viewpoint was requested.

First, I do not believe this document is an appropriate work item of
the IETF  for the following reasons.  I do not think it likely there
will ever be multiple implementations of the standard, so I do not
think it likely that the  the standard will meet criteria for
advancement.  The standard seems focused on a specific deployment
situation and in fact on a specific category of tightly specified
devices and has insufficient generic applicability  to devices outside
that specific environment to be  an Internet standard.  I understand
these devices are being deployed on the open Internet.  However it is
not necessary for the IETF to specify all protocols that are used over
the Internet.  I realize this concern applies equally  to the base
packetcable configuration option and as such will probably be
discarded.

The second reason this document seems poorly scoped for the IETF is
that while based on Kerberos, the authentication technology discussed
is not in fact Kerberos.  The Packetcable group needed a standard and
could not wait for the Kerberos working group to conclude and publish
a document.  As such they adopted a draft that is incompatible both
with RFC 1510 and with current drafts within the Kerberos working
group.  Fields are added to protocol messages in a manner that does
cause implementations of RFC 1510 to reject the messages and will
interfere with future expansion of the protocol.  The mandatory to use
encryption systems for Packetcable Kerberos are not even specified in
the IETF drafts or published RFCs.  The Kerberos working group has
decided not to standardize the encryption systems that Packetcable
uses  in part because the encryption systems that are being
standardized are significantly stronger.  It may be possible to
implement  a system that supports both Packetcable Kerberos and IETF
Kerberos--I know one vendor who is trying to do that.

However it seems that the IETF should not spend time standardizing a
DHCP sub option for controlling an authentication system outside of the
scope of the IETF for use in a limited  environment.

If the IETF decides that this work is within our scope, a note needs
to be added to the draft clarifying the Kerberos compatibility issue.
A pointer to the Packetcable Kerberos specification should be added
along with a note that this specification is incompatible with current
RFC 1510 implementations and with ongoing working in the IETF.

The rationale for this draft claims that it will be used among other
reasons to cancel subscriber service.  The presence of a Kerberos
ticket should not be used to imply authorization to use a service;
this is incompatible both with RFC 1510 and with current Kerberos
drafts.  Even if the authorization data field is used to convey
authorization along with authentication, service tickets with should
not be issued with lifetimes longer than are acceptable.  I.E. you
should only issue a ticket with a lifetime as long as you are willing
to have authorization cached.

I believe that this problem should be fixed by removing the text
proposing that using this option is appropriate to notify a device
that service has been canceled and adding a requirement that the
security of the deployment MUST NOT depend on this option being
honored.  The option if it exists should serve to invalidate caches to
deal with operational problems such as emergency rekeying of services
or testing, and the only effect of failing to honor this option should
be a temporary loss of service.

The format of this option is insufficiently extensible.  Many of the
sixteen code points for available services have already been used and
experiences shows new Kerberos services tend to be deployed over time.
Why would you want to selectively invalidate some but not all tickets?
If this option exists, it seems like providing a mechanism to
completely invalidate the cache should be sufficient.

Finally security considerations need to discuss the disadvantages of
storing persistent Kerberos tickets.  Other deployments of Kerberos
are moving away from doing this and instead storing tickets in
volatile memory.  It would be excellent if the write up of this issue
in the security considerations section gave numbers to justify the
need for persistent tickets, although requiring this would be
excessive.


--Sam



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