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

[Bernard Aboba (bernard_aboba]

I would like to point out one other consequence of preventing lower layers 
(including AAA) from accessing the EMSK.  It appears to me that this 
severely restricts implementation of a key cache on the AAA server.

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.

2. We've talked about how the EMSK cannot be passed down to the lower layer 
(including AAA).  Therefore the EMSK also cannot be cached by the AAA layer.

3. Given 1) and 2) what keys *can* be cached in the AAA layer?  Limited 
answers appear to be available.  Caching of MSKs does not make much sense, 
even if they are not transported in a particular application, because some 
applications *do* transport MSKs.

It would be possible for the AAA layer to request more than one AMSK, then 
cache the AMSKs that are not transported.

However, this approach does not appear to enable implementation of schemes 
such as "Key Request" and "Pre-emptive Key Distribution", both of which 
appear to require the AAA layer to calculate AMSKs *on demand*.

To be able to do this, it would seem like the AAA server would need to have 
access to the EMSK -- since the EAP layer doesn't cache EMSKs, how else 
could AMSKs be calculated once EAP authentication completes?  In the case of 
roaming systems it is not feasible to ask the EAP layer to calculate every 
conceivable AMSK that could be useful -- there could be thousands of them.