eap Mailing List: RE: Issue: AAA Key Caching effectively prohibited?

[Salowey, Joe (jsalowey]

> -----Original Message-----
> From: Nakhjiri Madjid-MNAKHJI1 [mailto:Madjid.Nakhjiri [at] motorola.com] 
> Sent: Wednesday, November 02, 2005 9:44 AM
> To: Salowey, Joe; Bernard Aboba; aboba [at] internaut.com
> Cc: jari.arkko [at] piuha.net; eap [at] frascone.com
> Subject: RE: [eap] Issue: AAA Key Caching effectively prohibited?
> 
>  > 1. We've talked about deletion of transported keys from the AAA 
> > server.  So if an MSK (or AMSK) is calculated within the 
> AAA layer and
> 
> > subsequently transported, that key is destroyed and cannot 
> be cached.
> > 
> [Joe] I don't think we have said that the AMSK must be 
> deleted or cannot be cached.  It should be the contrary, the 
> AMSK is a quantity that an application such as AAA can use 
> for the purpose it has defined. 
> 
> Madjid>> AAA can _use_? How about create and keep?
> 
[Joe] The AAA can keep the AMSK if that is what the application
specifies to be done.  Generating an AMSK from an EMSK is a more
specialized function that needs to exist outside of AAA since EAP does
not require AAA.

> 
> [Joe] This depends on how you define your key hierarchy.  If 
> you derive your keys directly from the EMSK you have a 
> problem.  If you define your keys from an AMSK derive for the 
> purpose of pre-emptive key distribution then things work out 
> a bit better. 
> 
> Madjid>>Are you saying we create our application specific keys out of
> AMSK that is already called application master key? Not sure 
> what "rainy day" we are saving EMSK for? Why have so many key 
> levels, does anybody care about the CPU and battery 
> consumptions down at a tiny mobile device that may have to do 
> all this? 
> 
[Joe] The EMSK is the root of a key hierarchy used only to generate
AMSKs.  This is to ensure that application keys are cryptographically
separate from one another so that one application does not cause a
compromise in another.  If the EMSK is used by multiple applications
this opens you up to a catastrophic compromise if one application has a
problem.