I am writing a small chat program to explore some methods and concepts in cryptography and currently I am using a shared AES key tunneled with RSA which upon further reading revealed many flaws both in the mode choice and the concept itself. So after some more videos, articles and posts I decided to implement 2 ECDH pairs which will be used to derive 2 different AES-GCM keys which in terms will be used for message encryption in both directions. On top of that I would like to implement a key derivation per request so that every message is encrypted with its own key derived from the previous. (Synchronicity problems will be handled by the protocol)
Since the initial AES key is not a human password but a high entropy string (which eliminates bruteforcing) is it enough to just hash it with SHA-256 and get the first 128 bits for the seed of the new AES key for efficiency and ease of use? This would prevent decryption of previous messages if the key is compromised which in combination of re-establishing the ECDH pair every so often will limit the threat of a compromised key to minimum in both time directions.
Just to make it clear because i added too much context around the question: I am asking if a simple hash function as SHA-256 would play a sufficient role of a KDF in a double ratchet?
This is the proof of concept that I am going to use: https://netnix.org/2015/04/19/aes-encryption-with-hmac-integrity-in-java