For open source messengers, you can check whether they actually encrypt your messages and whether the server has access to your encryption keys but what about WhatsApp? Since it’s not open source, you can’t be sure that the encryption keys aren’t sent to the server, right? Has there been a case where a government was able to access WhatsApp chats without reading them from the phone itself?
The code is not open source, so it’s hard to verify how good the encryption is or if it has backdoors.
I’m not an expert in cryptography, but from my limited knowledge, the cryptographic keys used are very important. If Meta or the government can somehow know the decryption key to your messages or predict it, then they can see your messages.
But they most likely don’t need to decrypt it in transit. One of the vulnerabilities in this system is Google firebase, which delivers notifications to your phone when WhatsApp messages arrive. Ever noticed how those notifications include the message content and the sender? Google has access to this information, despite the encryption.
That’s just an example. Google has access to a lot on your phone.
Another thing to consider is message metadata. The content of your message is encrypted, but what about information like the destination of your message, its recipients, time sent and received, and frequency? I’d even argue this is more important than content in many situations. Sometimes, linking person A to person B tells me a lot about person A.