Actually, it's the private key that needs to be kept secret... I'm not sure how that's guaranteed or how we can actually verify that the private key is secure.

Actually, it's the private key that needs to be kept secret... I'm not sure how that's guaranteed or how we can actually verify that the private key is secure. Knowing that whatsapp is built by spying and uploading all your contacts and phone numbers to their servers, well, do you trust them to not upload the private key also?