eap Mailing List: Re: transport of EMSK

[Bernard Aboba (bernard_aboba]

Jari Arkko said:

(2) In the lower-layer based EMSK processing approach, the
EMSK is delivered to the authenticator along with the MSK.
KDF is selected either (a) by a default in EAP plus optional
negotiation in methods, choice is communicated to the
lower layer via AAA or (b) by lower layer negotiation alone.
The lower layer is responsible for all use of the AMSKs
in a local context. That is, no AAA key requests are needed
or possible."

RFC 3748 is fairly clear that transport of the EMSK is not allowed.  From 
Section 1.2:

     The EMSK is not shared with the authenticator or any other third 
party.

With respect to the handling of keys within EAP as well as AAA, RFC 4137 
defines separate variables for the key structure within the EAP peer & 
server (eapKeyData) and on the authenticator in the passthrough case 
(aaaEapKeyData).   So as far as I can tell, providing the EMSK to the lower 
layer on the EAP peer and server does not necessarily imply that it is 
transported via AAA to the authenticator.