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

Greg Rose wrote:

>> Has anyone determined whether or not a 128-bit RES is backward 
>> compatible?
> 
> Definitely not backward compatible, although that might not yet be a big 
> deal. In the meantime, however, the vote in SA3 has been to go with 
> IPsec for integrity protection, so the problem is now moot; 32-bit RES 
> is sufficient in that context.

As far as I understand, whether IPsec or James' enhanced digest was adopted
as the solution for first hop integrity protection does not affect the RES
discussion at all. Both use the generated session key, IK, and not RES.

The only discussion has been on whether Digest AKA -- the home authentication
scheme for 3G networks -- uses RES in the clear or whether it ties the RES to the
rest of the message using MD5. The former approach is kind of like basic using
one time passwords, the latter is regular digest usage. The former is always
vulnerable to someone replacing the message contents but keeping RES; the
latter is not vulnerable to this except when RES is short.

The input from the IETF meeting was that the Digest approach is favored.

Jari

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