Subject: [Sipping] extensible authentication

From: Jari Arkko <[email-masked]>
Date: Wed, 05 Dec 2001 14:17:04 +0200

Hello,

I have been informed that I should motivate why 'extensible
authentication' is a requirement for multimedia in 3G
networks. This requirement is described in 6.23.1 of the
Garcia draft, and also behind our work on HTTP EAP.

As a background, all security systems require some
amount of infrastructure as well as some authentication
tokens that are delivered to the users off-line.
The tasks of building the infrastructure and token
delivery (e.g. password agreement) are typically
the most expensive, and maybe the most security
problematic tasks in any network. In particular,
when building a long-lasting very large network
these are extremely complicated tasks.

The effect of this towards protocols is felt
on many areas. For instance, we've never really
gotten the kind of PKI deployment in the world
that was envisioned in the IETF some time ago.
As an example, the IPsec/IKE protocol set has been
forced to be enhanced with so called "Remote Access"
functionality which means legacy authentication
using the current RADIUS (or whatever) databases
companies have on their employees, because the
companies didn't want to build an additional PKI
infrastructure. Unfortunately, standardization of
IPsec Remote Access was taken up quite late, so
there now exists not just one standard solution but
also a number of proprietary schemes, and interoperability
is quite hard.

The situation with the extensible authentication
requirement from 3GPP is similar to the above. The 3G
networks already use an authentication infrastructure
that includes (a) a specific algorithm, (b) databases,
(c) tamper-proof smart cards, and most importantly,
(d) all the personnel, procedures, and tools at
operator sites for handling the process. And a
previous version of the infrastructure has proven
to be very scalable: the process has worked reasonably
well up to a 0.5 *billion* subscribers globally.
Now, the 3G operators are in a similar situation
to what the companies were when they wanted to
use IPsec/IKE but didn't want to switch over
to a PKI. In theory it might be feasible to do
something else, but economically it looks like
it is crucial to be able to reuse the same
infrastructure and smart cards (one smart
card slot per phone etc) for multimedia
authentication tasks as well. More seriously,
there's really no proof that anything else can
scale to the size needed. Certainly no one
would claim PKIs could scale to this. The
situation that 3g networks have is, however,
not unique and could well exist in other
networks deploying SIP in the future.

In addition, there are a few related issues, such
as the need for tamper-proof protection of the keys
which happens to be easily solvable with the particular
authentication scheme that 3G networks use due to a
particular split of functionality to the card, available
APIs, and so on. Of course, nobody wants a specific
authentication scheme to be added to SIP. However,
what we could do is to allow a family of extensible
authentication protocols, in a way similar to EAP,
GSS_API or the like. 

Unfortunately, we really have no good way to today
in IETF to deal with legacy / extensible authentication
for SIP. (IPsec/IKE/IPSRA would work, but is limited to
h2h and in most cases - like in the 3G networks - we
really need an end-to-end or end-to-middle solution).

So, this was the background for the extensible
authentication requirement. How do people feel about
it, does the above make sense? Perhaps we could try
to reach a consensus on the requirement -- and then move
on to discuss the various ways of providing this feature
(there are many).

Jari

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