[Emu] RE: Draft liaison response for IEEE 802.11u EAP method for emergency calls

["Bernard Aboba" <[e-mail]>]

>[Joe] I agree, that if "No pre-configured trust relationship" refers to
>configuration of client on the server then we are in a better position.
>However it seems that in you discussion below that the peer does not
>typically have enough information to validate the server.

I think we need to distinguish between "validating the server" and 
"validating the authenticator".  If the EAP peer has trust anchors, it can 
validate the server.  The question is whether that should be sufficient to 
allow the peer to connect to the authenticator or not.

It may matter whether the peer has a pre-existing profile.   For example, if 
there is a profile corresponding to the "EXAMPLE" network, and the peer 
successfully validates the "example.com" server, then the question is 
whether the peer should apply the "EXAMPLE" profile based on validation of 
the "example.com" server certificate.  If the "EXAMPLE" profile includes a 
requirement for validation of a certificate from "example.com", then that 
should be sufficient.

However, if there is no pre-existing profile then it is not clear to me what 
security services can be required.  Should the peer be satisfied if the 
server certificate chains to any trust anchor, regardless of the SSID?  If 
the SSID in question represents a pre-existing profile requiring chaining to 
a particular trust anchor, I'd say "no".  However, if the SSID is unknown 
and there is an unknown SSID that advertises emergency services, then it is 
probably better to try that (and maybe even go ahead if the server cert 
fails validation) than to fail -- leaving the user without the ability to 
make an emergency call in a life and death situation.


> >
> > Requirement #2 is "Small" number of messages.  While this
> > requirement clearly excludes long exchanges, it is not clear
> > to me that TLS-based methods are excluded, particularly if
> > the method is to be implemented on the AP itself, which
> > potentially would result in lower round-trip times and
> > eliminate the possibility of AAA-based frame loss.
> >
>[Joe] the requirement is a bit unclear, however I agree that it is worth
>including text about this.
>
> > It may be possible that requirements #1 and #2 are compatible
> > with proposals involving unauthenticated client access
> > combined with server authentication.
> >    However, due to lack of a pre-configured network access
> > profile, this scenario presents additional threats that are
> > worthy of further discussion.
> >
> > The presentation refers to the desire to for confidentiality
> > (presumably at L2, rather than using SRTP).  Where
> > confidentiality is desirable, it will also presumably be
> > important for the client to determine that it has connected
> > to a legitimate network.
> >
> > Where a pre-configured network access profile exists, the
> > binding between a validated server certificate and an
> > advertised SSID is pre-configured.
> > However, where there is no pre-configured network access
> > profile, the binding may be difficult to establish without
> > imposition of additional requirements.
> >
> > For example, the server certificate/SSID binding cannot be
> > determined solely via verification of the server certificate.
> >  An attacker could obtain a valid server certificate for
> > "example.com"; does this entitle them to advertise an SSID of
> > "Emergency Network" or even "Example"?  Since SSIDs are not
> > globally unique, there is no verifiable mapping between a
> > Server-Id and an allowed set of SSIDs. In general, a CA has
> > no way of determining whether a server has the rights to a
> > particular SSID or not, so that a CA cannot in practice vouch
> > for an RFC 4334 SSID extension within a server certificate.
> >
>[Joe] If this is the case then I don't think there is a valid trust
>relationship for the peer to validate the server.
>
>
> > Therefore verification of a binding between the server
> > identity and the advertised network would only seem to be
> > possible by requiring the advertised network name to match
> > the Server-Id advertised in the server certificate.  This in
> > turn would require restricting the allowable SSIDs, or adding
> > another field to the IEEE 802.11 Beacon/Probe Response.
> >
>[Joe] It seems like there are at least two possibilities here:
>
>1. Have a CA that only issues emergency services certificates and an
>indication in the beacon/probe of which SSIDs are emergency services.
>
>2. Have a CA that issues different types of certificates and have
>specific SSIDs and names in the certificates to create the authorization
>linkage.
>
> >
> >
> >
> >
> >
> >
> > >From: "Joseph Salowey (jsalowey)" <[e-mail]>
> > >To: <[e-mail]>
> > >CC: <[e-mail]>, "Bernard Aboba"
> > ><[e-mail]>
> > >Subject: Draft liaison response for IEEE 802.11u EAP method for
> > >emergency calls
> > >Date: Sun, 16 Sep 2007 22:26:36 -0700
> > >
> > >The EMU working group has a liaison request from IEEE 802.11u on EAP
> > >methods for emergency calls.  The liaison request can be
> > found on the
> > >liaison statement page, https://datatracker.ietf.org/liaison/ (May
> > >2007).  We had a presentations and discussion of this topic at the
> > >Chicago EMU meeting.  Below is a draft response based on the
> > discussion
> > >in the meeting.  It would be good to have comments on or approval of
> > >the text by Monday, October 1, so a revised response can be
> > created to
> > >be sent as a response to the IEEE.
> > >
> > >==============================================================
> > >
> > >802.11u Liaison response for EAP Methods for Emergency Communications
> > >
> > >We have had discussion of EAP method for Emergency services
> > at the last
> > >IETF meeting in Chicago.  The following is a summary of
> > working group
> > >discussion on this topic.
> > >
> > >Currently there are no standards track EAP methods that meet the
> > >requirements as understood by the EMU working group.  There
> > are several
> > >possible candidates of existing EAP methods that may meet or be
> > >slightly modified to meet some of the 802.11u requirements for
> > >emergency services, especially if minimal latency is not the
> > strongest
> > >requirement.  TTLS (draft-funk-eap-ttls-v0-01.txt) and EAP-FAST
> > >(RFC4851) are TLS based methods that can support server only
> > >authentication.  It was also pointed out that EAP-TLS
> > >(draft-simon-emu-rfc2716bis-11.txt) could be modified to
> > create a new
> > >EAP method that only requires server side authentication.
> > In order to
> > >truly support emergency services these methods would need to forego
> > >server certificate validation which negates much of the security they
> > >provide by allowing man-in-the-middle attacks.   These TLS
> > based methods
> > >also require a significant number of round trips that may not be
> > >acceptable for emergency communication.
> > >
> > >There were also several questions raised in the working group during
> > >the discussion that might help in further determining the
> > best approach.
> > >These are summarized below:
> > >
> > >1) It is not clear how to make the tradeoff between security and
> > >low-latency.  If there is not existing trust relationship there are
> > >limits as to what security properties can be provided.  What
> > security
> > >properties are desirable and what is the tolerance for extra-round
> > >trips for the communication?
> > >
> > >2) PSK was described as having worse DOS resistance
> > properties that EAP.
> > >It seems that in many cases EAP would have worse DOS resistance that
> > >PSK, which cases is EAP better?
> > >
> > >3) It seems that most public access networks already provide an open
> > >access network, why couldn't this network be used for emergency
> > >communication?
> > >
> > >4) What regulatory requirements are driving the need for encryption?
> > >This creates some conflicts because encryption without
> > authentication
> > >does not satisfy most useful security requirements.
> > >
> > >As the 802.11u group is certainly aware, there are other
> > groups within
> > >the IETF that are looking at unauthenticated emergency services.  In
> > >particular, the ECRIT group within the IETF has ongoing work in this
> > >area:
> > >
> > >http://tools.ietf.org/html/draft-schulzrinne-ecrit-unauthenti
> > cated-acce
> > >s
> > >s-00
> > >
> > >We encourage IEEE working group members to continue the
> > discussion with
> > >the IETF in the EMU and the ECRIT working groups.
> >




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