So, what you're suggesting is some sort of end to end encryption sort of how Whatsapp does it? I'm not sure if I'm missing something, but how would that work on a public forum?

I personally don't get the obsession with changing everything to decentralisation, since it isn't something that needs to be implemented into everything, in fact I tend to believe in certain situations, decentralisation is a horrible idea.

Also, the forum doesn't need to be decentralised, and quite frankly at least from how I envision it would be absolutely horrible, and no one would use it. There's a reason why there's no true decentralised forum out there, since it would just become unusable, and way too much effort to implement in the first place without any real upsides to it.