I can't wait for signal/session/simplex to be whatsapp compatible, but I'm not sure they can provided the e2ee gurantees since whatsapp is closed source.
I fear that Signal won't implement cross compatibility for WhatsApp since they already said that they are not a fan of potentially giving up E2EE to get it to work. And I can understand that but I still really would like to have the cross compatibility.
I hope Signal doesn't if it won't be E2EE. I like knowing that if it's in Signal, it's E2EE, and being able to tell less technically sophisticated people to whom I recommend Signal that everything in it is secure against eavesdropping.
Signal and also lots of other privacy focused messenger-services (threema e.g.) already said the will not implement this forced interoperability since it will lower their already high standards regarding their users privacy. Sad but i guess it makes sense :(
The ability to sell proprietary versions of Signal libraries is literally the reason for Signal's Contributor License Agreement: https://signal.org/cla/
signal may have given a fully vetted and correct implementation to whatsapp, but because its closed source we don't know if it has changed, or if its really implemented on all conversations.
It changes the trust model of conversation participants.
To answer your query, if signal was closed source, I wouldn't trust it either.
signal may have given a fully vetted and correct implementation to whatsapp
They were not "given" it. They are literally the contractor who worked on that: "Over the past year, we’ve been progressively rolling out Signal Protocol support for all WhatsApp communication across all WhatsApp clients." –https://signal.org/blog/whatsapp-complete/
but because its closed source we don’t know if it has changed, or if its really implemented on all conversations.
I'm not an encryption developer. I can't vet this for Signal's own app either.
...and because its closed source, community cryptography developers and researchers can't vet it for you either. That is the core issue, its not about trust.
It's about capabilities that inform the threat model, and the exposure model.