Subject: Comments on draft-ietf-pppext-eap-ttls-00.txt (Section 2)

From: "Glen Zorn" <[email-masked]>
Date: Wed, 30 Jan 2002 02:07:59 -0800

Section 2
---------
Paragraph 3 says: "Users have also been willing to entrust their passwords
to their service providers, or at least to allow their service providers to
view challenges and hashed responses which are then forwarded to their home
authentication servers using, for example, proxy RADIUS, without fear that
the service provider will mount dictionary attacks on the observed
credentials."  This statement is highly questiaonable: at least some people
have been very leery of that trust, one of the reasons that RADIUS doesn't
meet the criteria in RFC 2477.
"Because a user typically has a relationship with a single service provider,
such trust is entirely manageable."  This passage leads me to believe that
the authors are ignoring the general case of Internet roaming in favor of a
very limited corporate dial outsource model.  In fact, they seem to think
that brokers don't exist (or maybe it's just more convenient to ignore their
existence, since EAP-TTLS is basically useless in that enviroment).

Paragraph 5 says: "Wireless connections...may enable channel hijacking, in
which an attacker gains fraudulent access by seizing control of the
communications channel after authentication is complete."  True, but
irrelevant, since EAP-TTLS by itself does nothing to ameliorate this threat.

Paragraph 8 says: "In a wireless network, however, the user does not get to
choose an access domain, and must connect with whichever access point is
nearby;..."  It is not clear what is meant here by "wireless network", but
both 802.11 WLANs (through the SSID) and GSM cellular networks allow the
user to choose a provider in a roaming situation. Although the SSID doesn't
provide authentication of the SPs, it does at least allow the user to
distinguish between them; the user is not quite the weed blowing in the wind
that this passage suggests.  The passage contiues: "...providing for the
routing of the authentication from an arbitrary access point to the user's
home domain may pose a challenge."  This is certainly true, but the authors
neglect to mention that it is a challenge that has been overcome, both by
cellular operators and Internet roaming brokers such as iPass.

Paragraph 14 says: "The authentication mechanism must support roaming among
small access domains with which the user has no relationship and which will
have limited capabilities for routing authentication requests."  Yet another
red herring: while the user may not have a pre-established relationship with
the "small access domains" (what does size have to do with it, anyway?), the
"access domain" must have a relationship (either direct or brokered) with
the user's home domain or roaming is impossible anyway.  Therefore, any
limitations on the routing capabilities of the local domain are irrelevant:
either thelocal domain can route to the user's home or not.

~gwz

Glen Zorn
CTO Consulting Engineer
Security and Integrity Group
Consulting Engineering
Cisco Systems

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