Just thinking out loud....

The administrative complexity of a cryptosystem is overwhelmingly in key 
management and identity management and all the rest of that stuff.  So imagine 
that we have a widely-used inner-level protocol that can use strong crypto, but 
also requires no external key management.  The purpose of the inner protocol is 
to provide a fallback layer of security, so that even an attack on the outer 
protocol (which is allowed to use more complicated key management) is unlikely 
to be able to cause an actual security problem.  On the other hand, in case of 
a problem with the inner protocol, the outer protocol should also provide 
protection against everything.

Without doing any key management or requiring some kind of reliable identity or 
memory of previous sessions, the best we can do in the inner protocol is an 
ephemeral Diffie-Hellman, so suppose we do this:  

a.  Generate random a and send aG on curve P256

b.  Generate random b and send bG on curve P256

c.  Both sides derive the shared key abG, and then use SHAKE512(abG) to 
generate an AES key for messages in each direction.

d.  Each side keeps a sequence number to use as a nonce.  Both sides use 
AES-CCM with their sequence number and their sending key, and keep track of the 
sequence number of the most recent message received from the other side.  

The point is, this is a protocol that happens *inside* the main security 
protocol.  This happens inside TLS or whatever.  An attack on TLS then leads to 
an attack on the whole application only if the TLS attack also lets you do 
man-in-the-middle attacks on the inner protocol, or if it exploits something 
about certificate/identity management done in the higher-level protocol.  
(Ideally, within the inner protcol, you do some checking of the identity using 
a password or shared secret or something, but that's application-level stuff 
the inner and outer protocols don't know about.  


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