#12 [**] Subsession keys (section 5 and 8)

I also don't think it is necessary to forbid using sub session keys.
but Michael pointed that there were some discussion about this topic
in the mailing list long time before.  It might be necessary to consider
something.

If we allow using sub session key, then we have to add a text to
the document.  we have to describe precisely where a session key or
a sub session key is used.
and if my understanding is correct, the responder can change the
subsession key by the responder's policy.  So if the responder
changes the sub session key, the exchange needs the 3 way handshake.
because the initiator will have to recalculate the KEYMAT from
the subsession key from the responder.