[TLS] Comments on draft-friel-tls-eap-dpp-01

["Scott Fluhrer (sfluhrer)" <[e-mail]>]

Again, last minute reviews...

It would appear that the exact computations that both the client and the server need to perform needs to be explicitly spelled out, as there are several possibilities.

Here is the one I could see that appear to have the security properties that you appear to be looking for:

Variable names:
                g - Well known group generator
                h - The secret generator that is private to the client and the server
                z - The secret value known to the client; g^z = h
                x - The client's ephemeral DH private value
                y - The server's ephemeral DH private value:

Client keyshare:
                This is the value g^x

When the server receives this, he selects y (and retrieves the value h); he then transmits (as his keyshare) the value:
                h^y
and stirs the value (g^x)^y into his KDF

When the client receives this (h^y), he computes:
                (h^y) ^ (x z^-1)
(where z^-1 is the modular inverse of z modulo the group order), and stirs that value into his KDF.

With this protocol, it appears that the client needs to know not only h, but also the value z.  However, this really needs to be spelled out (and run past the CFRG to check for subtle issues)