Just encrypt it before sending it to their servers. How would you tell that apart from any other traffic it sends? (E.g. to check for new messages, to update who of your contacts is online, etc)
what does that have to do with anything? if you have to encrypt your messages manually yourself, that kind of proves the point that the service itself is not secure. And it’ll still show up on a network sniffer that they’re sending it to two places
Ok, let me break it down because clearly I didn’t explain it well.
What is supposed to happen, scenario 1: the client encrypts your messages with the public key of the recipient, sends it to the servers of WhatsApp (or whatever service) along with some encrypted metadata indicating the recipient, which then forward the message to the recipient.
What could happen, scenario 2: the client does the same, but also encrypts another copy of your message with a public key that belongs to WhatsApp, and send both versions to the WhatsApp servers. They decrypt and keep the second version while forwarding the first one to the recipient.
Or, scenario 3: they just never bother with end-to-end encryption, and always encrypt it with the WhatsApp key, still sending it to their servers which then reencrypt with the recipient’s key before forwarding.
In all cases, messages are sent only to the WhatsApp servers, not two places. The only visible difference is in scenario 2 where the communication is larger. You can’t inspect the metadata of the message with your network sniffer, because it is also encrypted, so there’s no way to rule out scenario 3.
If the protocol is designed to be transparent by not encrypting the entire payload sent to the servers, and you have access to the recipient’s private key (those are big ifs) then you could show that there is indeed an end-to-end encrypted message in there. But this is true for how many of these proprietary services? Maybe for WhatsApp.
