Subject: [eap] A few comments on draft-puthenkulam-eap-binding-03.txt

From: "DongGook Park" <[email-masked]>
Date: Mon, 15 Sep 2003 17:56:18 +0900

Dear Jose Puthenkulam,

I have some comments and questions with regard to your recent IETF draft
entitled " The Compound Authentication Binding Problem"
(draft-puthenkulam-eap-binding-03.txt), which you can find below in this
mail. 

Best regards,
DongGook Park

=====================================
Information Security Research Division
Korea Telecom
17 WooMyeon-Dong SeoCho-Gu Seoul 137-792 
Korea, South
 
Telephone: +82 2 526 6173
Email: [email-masked]
Homepage: http://home.naver.com/dgpark6/
=====================================

######  A few comments on draft-puthenkulam-eap-binding-03.txt  #######

This draft describes MITM attacks against tunneled authentication
protocols, and proposes some countermeasures. I've found a bit
misleading descriptions from the draft:

1. [Comment]  On page 15, the draft describes the Binding Phase Exchange
with compound keyed MACs. The draft reads "... The validation of the
compound protects against the MITM attack, as the attacker is unable to
get any of the inner method keys. ..."  This description seems to be
rather misleading... As far as I understand, the attack cannot succeed
not because the attacker is not able to access the inner method keys,
but because the new additional message exchange of "Binding Phase" is
not expected message from the viewpoint of the victim entity. For the
victim entity has simply executed a legacy authentication protocol (not
as an inner protocol of the tunneled authentication protocol) which does
not include the additional Binding Phase exchange. 

2. [Comment]  On page 18, before the start of Section 3,4, the authors
argue that "Stage 2" is REQUIRED for additional protection on top of
"Stage 1" protection. Strangely, however, the draft does not explain why
it is the case, but rather they only emphasize the importance of "Stage
1" countermeasure and the insufficiency of "Stage 2" being used alone...
I think, rather, readers would expect why the additional countermeasure
"Stage 2" is used on top of the essential protection "Stage 1". By the
way, I don't see why Stage 2 is necessary in addition to Stage 1 as far
as the man-in-the-middle attack is concerned. IMHO, each stage can be
considered as a selective countermeasure; Stage 1 as a full-blown costly
fix (due to additional messages and hence some relevant
addition/modification to existing EAP protocols), and Stage 2 as a
lightweight solution which does not entail message
addition/modification, a penalty of which is some delayed detection of
whether a MITM attack has occurred or not.

3. [Question]  On page 17, Section 3.5 "Solution approaches", the first
proposed approach for accommodation of Stage 1 or 2 (the draft, in fact,
describes only the case with "Stage 1") is to "implement the binding
phase exchange as a new EAP method". Does this mean that we need to have
something like EAP-BindingPhaseExchange in addition to e.g. EAP-TTLS? 

4. [Question]  On page 13, Section 3.2 "Solution Concepts", the
description headed by "[S1]" argues that cryptographic binding solution
will not work for "non-key-deriving methods" without breaking at least
one of the solution criteria given in the draft. Most probably,
password-based authentication protocols such as CHAP will correspond to
the non-key-deriving methods. But why? Are we not allowed to use the
password instead of the inner method key in the case of the situation
where any inner key is not available from the inner protocol?

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