Subject: [Sipping] questions on SIP and AKA

From: Marco Aime <[email-masked]>
Date: Wed, 04 Jun 2003 16:01:40 +0200

Hi all,

I'm wondering about implications of selecting AKA as the preferred (or 
unique?) strong authentication method for SIP messages

AKA is defined with a precise authentication model in mind, the one 
currently used in mobile telephony networks:
- the "Home Domain" delegates the "Visited Domain" for User 
authentication (this require the network domain to be completely 
trusted). AKA add network authentication...
- no data privacy nor integrity is achieved between User and the "Home 
Domain". AKA could change this...

Is this the current model for SIP authentication?

AKA can derive session keys (CK and IK): between whom?
-if between the User and the "Home Domain" (like in TS 24.228) we could 
get in troubles with scenarios with hierarchical registration (how 
authenticate local REGISTERs?)
- if between User and "Visited Domain" we are in the traditional 
authentication model and we loose User-to-"Home Domain" privacy/integrity

AKA does not seem particularly fit for multi-layer authentication models 
(User-to-"Home Domain", "Visited Domain"-"Home Domain", User-"Visted 
Domain"), at least I was not able to find some references...

(as it sounds rigth) the recent draft-ietf-sipping-aaa-req-03.txt does 
not define any trust model among User "Home Domain" and "Visited 
Domain": selecting AKA is going to do it? Am I missing something?

thanks in advance
bye
Marco

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Marco AIME
Dipartimento di Automatica e Informatica
Politecnico di Torino
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Mail: [email-masked] ([email-masked])
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From: Jari Arkko <[email-masked]>
Date: Wed, 04 Jun 2003 17:29:46 +0300

> unique?) strong authentication method for SIP messages

I don't think any IETF document has claimed specific preference
for the use of Digest AKA against other potential mechanisms. AKA is
most useful when you have existing SIM cards in your device and when
your security model matches what can be arranged with it.
And quite clearly, the amount of protection offered solely
by Digest is very minimal, so typically Digest AKA would
be used in connection with other mechanisms.

So, depending on what you want to do, you may wish to
employ AKA with say TLS, or you might want to employ
TLS only in a SIP trapezoid model, or you might do TLS
and S/MIME.

--Jari

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From: Marco Aime <[email-masked]>
Date: Wed, 04 Jun 2003 18:19:13 +0200

> your security model matches what can be arranged with it.

right, what I meant was wether the WG has investigated (or is
invetigating) how the currently available authentication mechanisms for
SIP impact authentication/authorization models among Users, Home
Domains, etc...

> TLS only in a SIP trapezoid model, or you might do TLS

right, but TLS, as well as ipsec, can provide for hop-by-hop security
features: (not assuming a PKI) you need, at least, to integrate them
with AAA to get an authentication/authorization model (as with the
ongoing work in other IETF WGs): is it correct?

for what concern S/MIME, I was just pointing at
authentication/authorization schema based on S/MIME-like mechanisms: is
there (or the was) some investigation on this topic?

thanks
bye
marco