eap Mailing List: RE: Strawman -10/EMSK deletion requirement?

[Avi Lior (avi]

Hi Joe and Madjid,

The only reason for caching the EMSK is if you have to generate an AMSK
for another application associated with the current session.

So the question is do you have to generate all possible AMSKs and then
delete the EMSK or can you generate the AMSKs as needed?

Finally I have an slight issue with the statement that a KEY should be
deleted so that it wont be used for other purposes.  I find that
statements like that are an over specification.  What makes anyone think
that making a statement that "X should/must be deleted" is going to be
taken more seriously then "X should/must only be used for purpose X and
should/must not be transported out of Y"?

I think that whether it gets deleted or not is an implementation issue.

Finally if we insist on saying that X must be deleted we should really
say that X must be securely deleted.  There is a difference.  When a key
is securely deleted the software takes extra care that the memory
location containing the key is first erased before the memory location
is freed to the heap.  Perhaps we can state that somewhere. 

> -----Original Message-----
> From: Salowey, Joe [mailto:jsalowey [at] cisco.com] 
> Sent: Thursday, March 02, 2006 2:49 PM
> To: Nakhjiri Madjid-MNAKHJI1; Rafa Marin Lopez; Bernard Aboba
> Cc: eap [at] frascone.com
> Subject: RE: [eap] Strawman -10/EMSK deletion requirement?
> 
>  
> 
> > -----Original Message-----
> > From: Nakhjiri Madjid-MNAKHJI1 [mailto:Madjid.Nakhjiri [at] motorola.com]
> > Sent: Thursday, March 02, 2006 8:44 AM
> > To: Salowey, Joe; Rafa Marin Lopez; Bernard Aboba
> > Cc: eap [at] frascone.com
> > Subject: RE: [eap] Strawman -10/EMSK deletion requirement?
> > 
> > Hi Joe,
> > 
> > Thanks for the email. I think you responded to the old piece of the 
> > email from Rafa and I am to blame for causing that confusion, as I 
> > kept that part to provide context.
> > Again, my question was why an entity needs to delete EMSK after 
> > generating the first AMSK (or first set of AMSKs?)? This 
> seems to be 
> > the requirement regardless of two options:
> > 
> > 1) keep EMSK at EAP layer, create AMSK at EAP layer based 
> request from 
> > AAA layer, delete EMSK Immediately (this means EAP layer must have 
> > KDFs for AMSK=KDF(EMSK, etc)
> > 2) push EMSK down to AAA layer at backend server, create 
> AMSK at AAA 
> > layer and delete EMSK immediately (this means AAA layer must have 
> > KDFs)
> >
> [Joe] If the AAA layer contains the AAA client and AAA server 
> then the EMSK should not be available to this layer, if the 
> AAA layer means something else then I don't know about (1).  
> The AMSK should be generated in the EAP and exported, option (2).
>  
> > 
> > In both cases we require deletion of EMSK after generation of AMSK, 
> > why?
> > 
> [Joe] To minimize the chance of exposure of the EMSK.  Why do 
> you need to cache it? Could you generate and cache an AMSK instead?  
> 
> 
> > Thanks,
> > 
> > Madjid
> > 
> > -----Original Message-----
> > From: Salowey, Joe [mailto:jsalowey [at] cisco.com]
> > Sent: Wednesday, March 01, 2006 5:17 PM
> > To: Nakhjiri Madjid-MNAKHJI1; Rafa Marin Lopez; Bernard Aboba
> > Cc: eap [at] frascone.com
> > Subject: RE: [eap] Strawman -10
> > 
> >  
> > 
> > > -----Original Message-----
> > > From: Nakhjiri Madjid-MNAKHJI1 
> [mailto:Madjid.Nakhjiri [at] motorola.com]
> > > Sent: Wednesday, March 01, 2006 2:38 PM
> > > To: Rafa Marin Lopez; Bernard Aboba
> > > Cc: eap [at] frascone.com
> > > Subject: RE: [eap] Strawman -10
> > > 
> > > Madjid>>Again, why is deletion of EMSK after generation of
> > > one AMSK is a
> > > requirements. What if we need to create multiple AMSKs 
> and that at 
> > > multiple occassions?
> > > 
> > > 
> > > 
> > > Well, actually, lower layer authenticator implementation
> > should expect
> > > (MSK,EMSK) in the case EAP method is executed by the standalone 
> > > authenticator or (MSK,AMSK) in the case EAP method is executed by 
> > > backend authentication server. If it receives (MSK,EMSK)
> > should create
> > 
> > > AMSK and delete EMSK. If it receives (MSK,AMSK) , that's
> > all, correct?
> > 
> > [Joe] Not really, strictly speaking the lower layer 
> shouldn't expect 
> > to receive the EMSK as that would break mode independence.  An 
> > architectural description should not have the lower layer receiving 
> > the keys. From a supplicant perspective it must appear the same 
> > whether an external EAP-Server or a collocated EAP server is used.  
> > Now I don't know what is going on inside your box, it could all be 
> > monolithic when a internal EAP server is used but that shouldn't be 
> > visible to the external world.  If I was interested in 
> cryptographic 
> > module separation I might not be too happy if you shared 
> the EMSK with 
> > the lower layer.
> > 
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