Subject: EAP/SIM roundtrips

From: [email-masked]
Date: Mon, 7 May 2001 16:21:45 +0300

Hi all,

I have been asked why the EAP/SIM draft 
(draft-haverinen-pppext-eap-sim-01.txt) uses so many roundtrips 
and if it would be possible to save them. Here is a 
clarification.

Yes, we could save a roundtrip by sending the challenge already 
in the first EAP/SIM Request like this:

Client       Authenticator
<- EAP-Req/Identity 
EAP-Resp/Identity ->
<- EAP-Req/SIM (RAND, MAC_RAND)
-> EAP-Resp/SIM (NONCE_MT, MAC_SRES)
<- EAP Success

A problem here is that in this case NONCE_MT wouldn't
be included in the calculation of the MAC_RAND authenticator,
and the terminal couldn't recognize if the EAP-Req/SIM is a
replay. This would mean weaker network authentication.
NONCE_MT could still be included in MAC_SRES and the key.

As this is a tradeoff between security and roundtrips,
I think it is better to prefer security.

Regards,
Henry