eap Mailing List: RE: Strawman -10

[Salowey, Joe (jsalowey]

> -----Original Message-----
> From: Nakhjiri Madjid-MNAKHJI1 [mailto:Madjid.Nakhjiri [at] motorola.com] 
> Sent: Wednesday, March 01, 2006 2:44 PM
> To: Salowey, Joe; Bernard Aboba
> Cc: eap [at] frascone.com
> Subject: RE: [eap] Strawman -10
> 
>  > The difference is subtle, but potentially important for issues such
> as 
> > crypto-agility.  Allowing the lower layer to obtain the 
> EMSK enables 
> > the lower layer to negotiate the PRFs used in AMSK 
> generation, whereas
> 
> > this is not possible if AMSK generation is handled in the 
> EAP layer.  
> > It also maintains backward compatibility so that a lower 
> layer using 
> > the AMSK can be introduced without requiring changes to 
> existing EAP 
> > implementations.
> > 
> > As long as the EAP peer does not need to be aware of whether the 
> > authenticator is configured in standalone or pass-through mode, I 
> > think that the requirements of mode independence have been met.
> 
> [Joe] This worries me.  It ties the key derivation to the lower layer,
> which could be problematic.  A goal of the EMSK to AMSK 
> derivation is to
> contain the problem of a misbehaving lower layer to the lower layer
> itself.  A lower layer that determines the key derivation algorithm
> conflicts with this goal.  What happens if there are multiple AMSKs
> being derived for different purposes? Who decides the KDF?   
> 
> 
> I think I would prefer to see a default KDF specified with the
> capability of an EAP method to override it with a KDF of its own.  
> 
> Madjid>> Are you saying all EMSK-AMSK generations will have to follow
> the same KDF defined based on EAP method? And ruling out use of
> different KDFs for different AMSKs?
> 
[Joe] Yes. A single coordinated KDF definition is a robust and
deterministic way to avoid one key derivation scheme compromising other
keys derived from another scheme.

> 
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