Signal protocol is what you’re looking for. Started as text secure then changed name eventually. Whatsapp was never opensourced they just used the Text secure (now signal) protocol for their encrypted messages. They still allegedly use the signal protocol, I think, but with it being fully closed source and not implementing current signal feature for years afterwards.

In other words I have a high degree of faith our nuclear secrets will only be leaked to me when our DUI hires add me to the group, again using signal. But guarantee they’re also leaked to the Zuck using whatsapp.

More like WhatsApp is a Signal clone. Whatsapp uses the signal protocol for its encryption. Or at least it did until the Zuck took it closed source. It’s still at least, in part, signal though.

Course Signal is still very barebones. Great for a secure messenger, but not as super featured like telegram. Would’ve been nice if they kept SMS compatibility though.