You don't have to trust a 3rd party. Just compare your security codes IRL or through another trusted channel. Without doing that, you can never be sure if ther

You don't have to trust a 3rd party. Just compare your security codes IRL or through another trusted channel. Without doing that, you can never be sure if there is a MITM attack. Not with Signal, not with Matrix, not with WhatsApp. EDIT: MITM may not be the best description, let's say: "you cannot be sure, that you talk to the correct person."