Diffie-Hellman is only a key exchange protocol and does not provide forward secrecy. Imagine that my and your client figure out our shared secred (key) with DH, then encrypt our chat with that. If someone breaks only a bit of our traffic, then they can read newer messages as well.
https://en.wikipedia.org/wiki/Forward_secrecy
Forward secrecy means that at any point messages are encrypted with keys that aren’t reused forever, which means if an attacker can intercept traffic and read a bit, or crack the encryption, that they don’t automatically get access to our future conversation as well.
Since Ig we all know this message which appears when the “keys” ig, changed
I don’t know if I fully understand you here.
To my understanding whatsapp does it like this too. The key changing must be the high level key, since the message means that it’s an unknown key (thus not yet trusted). From new install or new device.
Double Ratchet Messaging Encryption - Computerphile
https://en.wikipedia.org/wiki/Double_Ratchet_Algorithm
This is the encryption that they developed. Whatsapp and OMEMO (XMPP) use it as well. It has forward secrecy because if an attacker has the key for a message they can’t calculate the key for the key for the next message etc.
Wdym?