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

[Hannes Tschofenig <[e-mail]>]

Hi Bernard,

thanks for your input. Please find a few more thoughts below:

Bernard Aboba wrote:
> It is not clear to me whether the requirements do in fact prohibit 
> server-side authentication.  As you note, without server-side 
> authentication man-in-the-middle attacks are possible;  however, even 
> with server-side authentication, additional requirements may need to 
> be imposed in order to provide the desired level of security.
>
I also believe that the requirements does not rule out server-side 
authentication.

> Requirement #1 is "No Pre-configured trust relationship".  This could 
> refer to pre-configuration of the server with respect to the expected 
> client credential (PSK or certificate), or it could refer to 
> pre-configuration of the client with respect to the server, (such 
> trust anchors).  The text seems focused on the former more than the 
> latter.  Assuming that clients can be pre-configured with trust 
> anchors, then TLS-based EAP methods could meet the requirement.
>
I agree that this aspect of no pre-configured trust relationship is 
quite likely to refer to pre-established shared secrets.

> 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.
>
I doubt that the performance argument counts a lot here given that the 
exchange is, as you indicated, local.

> 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.
Whereby "legitimate network" is already a quite difficult requirement.

>
> 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.
>
> 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.
>
I don't think it make a lot of sense to bind the certificate to the 
advertised SSID.


WHAT CAN YOU ACCOMPLISH WITH SERVER-SIDE AUTHENTICATION?

The important question here, I believe, is what you would do with 
server-side authentication in such a context given that it has entirely 
different semantic than the server-side authentication that is typically 
exercised in EAP exchanges between the peer and the EAP server in the 
user's home network.

I don't think that addressing man-in-the-middle attacks is the main 
objective. Instead, I could imagine that when something goes wrong then 
the end user might be able to indicate that he or she was interacting 
with a specific network.I see this more as a "debugging" tool.

When some verification steps fails, for example because the certificate 
of the server is expired, then in an emergency situation you will just 
continue rather than dropping the conversation. Additionally, everyone 
should be able to setup networks and hence the adversary can easily do 
the same. What would the network of an adversary differentiate from one 
that is from someone else?


WHAT ASSUMPTIONS DO WE MAKE WITH RESPECT TO THE SERVER'S CERTIFICATE AND 
THE TRUST ANCHORS?

Do we assume that persons that setup networks, such as my home network, 
a coffee shop, the IETF network, have to obtain a certificate for their 
network from
a) from any company that sells certs typically found in web servers
b) from a dedicated emergency services provider

(Note1)

May I re-use existing trust anchors, such as those available with my web 
browsers, or do end devices need to add new root certs into their 
certificate store?

In case (b) can I assume that the PSAP also uses the same trust anchor 
so that I could potentially use DTLS-SRTP for end-to-end media security 
to ensure that I am actually speaking to a real PSAP rather than to an 
adversary?


Ciao
Hannes

Note 1: Would it be allowed to just skip server-side authentication and 
to end-up with a unauthenticated Diffie-Helman exchange? One might argue 
that this does not provide a lot of advantages and we could also skip 
the EAP exchange entirely but that's not completely true that 
architectures, such as Wimax, assume that keying material is exported 
during network attachment and that these keys are used for other 
protocols. Hence, if one would change then the impact for the other work 
done before would be too large.

>
>
>
>
>
>> 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-unauthenticated-acces
>> 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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