Updating Traffic Secrets

[tireddy2]

The derivation of application_traffic_secret_N+1 looks complicated to me. I don't see a need to use SealBase and OpenBase.

We could use HPKE encap and decap functions to solve the problem. The secret from the encap function can be used as input to the HKDF-Expand-Label to derive application_traffic_secret_N+1.

a) The server generates pkR and skR
b) The client will receive pkR from the server.
c) Client uses Encap
shared_secret, enc = Encap(pkR)
d) Client sends enc to server
e) The server uses Decap
shared_secret = Decap(enc, skR)

application_traffic_secret_N+1 =
HKDF-Expand-Label(shared_secret,
"traffic up2", application_traffic_secret_N, Hash.length)

The proposed approach in future will also be compatible with PQC KEM.