Hi, I wanna know what is the most secure and best messaging app/platform...
Need an app that is crossplatform and has a very good numbers of features and security.
(And it has to be FLOSS)
I thought about XMPP clients, Signal, Session, IRC clients.. Propose and explain me your choice
You need to understand your threat model. Some apps are very secure but extremely inconvenient and hard to use. Others are more convenient but may not be able to hide the fact of a conversation between certain users for example
There is always a cost to security. How much you and your recipent ready to pay?
And, what you mean by "secure"? E2EE is basic. How about meta data? Or resilient to DPI? How about correlation attacks? Then the directory server. And the operator of the server. Where they located can be a factor too.
There can be a milion factors that can contribute to security. You can have it all but I don't know if such thing exist or not. For each factor, you gain some security but loss some in other places. You need to pick and choose what you need.
So you want self hosted? And what you mean by changing the encryption? Changing the encryption protocol and encryption algorithm are two different thing. OMEMO is kind of de facto for XMPP. Last I checked it seems doesn't let you choose the cipher suite like TLS 1.2 does. You can spin another if you have the capability to do so but not recommend.
That would be cool to have a messaging app that let me change almost what I want like the encryption protocol, changing the encryption keys... And so on, don't know if it's possible
Key? Oh, welcome to the land of GPG/OTP where platform and protocol doesn't matter. Good luck finding a friend willing to deal with this just to talk to you though. And loose some important security features like forward secrecy and double ratchet.