[honestjohn]

I'm not sure if they can add a participant to an existing conversation, and if they did, your client would at least know. Also don't remember if the client will send message history, but I think Signal doesn't.

The simple bad scenario I have in mind is when you're initiating a new chat and the mitm it from the start. Or they could do it halfway through, which would notify you that the other end's key changed, but that message is non-threatening enough and happens enough for random other reasons that most people would probably ignore it.

[bsaul]

not a participant. Whatsapp is multi device ( a single user can have multiple devices, such as a web client, an iphone and a mac app). They can simply register another device as belonging to one of the participant, and everything should be forwarded to them, invisibly.

[honestjohn]

Multi-device mode is new enough that I might be wrong about this, but afaik the web client still needs to get the priv key from your phone, so they can't authorize a new client unilaterally. Or it'd be a really silly hole if they could.

Edit: Meant to say, the web client needs to somehow be authorized by the phone, not that it takes the privkey exactly. Probably gets a new key that the phone stores, so the phone is still the "master" device. I wouldn't expect the phone client to happily send the chat history to a new device it didn't authorize locally.