I have a PSK. I don't want to use it directly. I want to pass a "salt"/password into a KDF and get a new key for authenticated encryption with ChaCha20-Poly1305 .
I need to give the other side the nonce used.
I don't have enough space to fit all of this in a packet.
My plan was to generate a random nonce. But then thought...
Is it bad to use my pass/salt (16 bytes) as the nonce (12 bytes) as well? I could save the space of sending the nonce by just lopping 4 bytes off the salt.
I know all about the dangers of reusing a nonce. But I'm not sure it applies here because my "old PSK" is thrown into the KDF with the salt/pass so does it matter if that also makes up the nonce?