[Sip-security] Re: [Sipping] SIP authentication problem when using RES in Digest-AKA

At 08:16 AM 3/15/2002 +0200, Jari Arkko wrote:
>Thanks for an interesting describing this interesting attack! I believe
>while making draft-niemi the authors have been assuming that we do not
>use the GSM compatibility mode (which I believe is the reason why the RES
>could be only 32 bits). That is, when full AKA is used this isn't a problem.

Regrettably, this is not correct. RES could be as little as 32 bits *even 
in full AKA*.

>So, we could either
>
>(1) Require the full use of AKA
>(2) Switch to using IK and not RES as input in the Digest process

IK is the obvious (to me) candidate.

>Greg, is the IK free of similar limitations when GSM compatibility
>is used?

If I understand your question correctly -- yes. IK is always 128 bits 
coming out of the USIM, even if it is subsequently "dumbed down" for GSM 
compatibility (which should never happen in anything capable of packet data 
and IMS). When a *GSM SIM* is used, you will only get out a 64-bit K_c, but 
even that is a lot better than a 32-bit RES.

regards,
Greg.

Greg Rose                                       INTERNET: ggr@qualcomm.com
Qualcomm Australia          VOICE:  +61-2-9817 4188   FAX: +61-2-9817 5199
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