Re: Evaluation: draft-ietf-pkix-wlan-extns-04.txt

[hardie@qualcomm.com]

At 10:26 AM -0400 08/19/2003, Russ Housley wrote:
> Ted:
>
> The Security Considerations section does address this concern.  It says:
>
>    SSID values are unmanaged; therefore SSIDs may not be unique.  Hence,
>    multiple client certificates may contain the same SSID.  In this
>    case, automatic selection of the certificate SHOULD fail, and the
>    implementation will require help from the user to choose the correct
>    certificate.  If the implementation chooses to automatically select
>    the client certificate and choose the incorrect one, then information
>    such as the list of SSIDs may be disclosed.
>
> If your concern is implementations that do not have human operators, 
> then sequentially trying the certificates may be necessary.
>
> Whether the implementation has a human user associated with it or 
> not, the sequential attempt with each certificate only needs to be 
> done once if a cache of AP MAC addresses with which the certificate 
> has successfully authenticated.  I think this is a good addition to 
> the document.  I propose the following text:
>
>    The Wireless LAN (WLAN) System Service identifiers (SSIDs) public key
>    certificate extension is always non-critical.  It contains a list of
>    SSIDs.  When more than one certificate includes an extended key usage
>    extension indicating that the certified public key is appropriate for
>    use with the EAP in the LAN environment, the list of SSIDs MAY be
>    used to select the correct certificate for authentication in a
>    particular WLAN.
>
>    Since SSID values are unmanaged, the same SSID can appear in
>    different certificates that are intended to be used with different
>    WLANs.  When this occurs, automatic selection of the certificate
>    SHOULD fail, and the implementation will require help from the user
>    to choose the correct certificate.  However, by maintaining a cache
>    of Access Point (AP) MAC addresses with which the certificate has
>    successfully authenticated, user involvement can be minimized.

I believe that the text on maintaining a cache of AP MAC addresses with
which the certificate has successfully authenticated is useful.  I'm concerned,
though, about saying that the automatic selection SHOULD fail and that the
implementation will require user intervention.  First, there will be cases in
which there is no human user, the user is not attending this process, or the
user interface is not available at the time this error would be 
raised.  Second,
I'm not sure what the implementation would present to the user to allow
them to distinguish among certificates which contain the same SSIDs; is
there a particular set of certificate attributes which feel would be valuable
to present to the user in this case?

I'd suggest having some default method of dealing with it, but warning
of the consequences of using it.  How does this look as a re-write:

   Since SSID values are unmanaged, the same SSID can appear in
   different certificates that are intended to be used with different
   WLANs.  When this occurs, automatic selection of the certificate
   will fail, and the implementation will either require help from a user
   in selecting the correct certificate or MAY choose to present multiple
   certificates.  The second option discloses information, such as the
    list of SSIDs, which would otherwise remain confidential, and it is
    therefore recommended that it be used only when user selection is
    unavailable or impractical.

Thanks for your consideration of these points,
				regards,
					Ted Hardie