Neither Signal, WhatsApp, or Telegram are particularly secure. On Signal for example, your private key is protected only by a verification text message and short PIN. That's very low entropy compared to a full ED25519 keypair. It's better than SMS but don't be fooled into thinking you have Snowden-level opsec because you use one of these apps. Use PGP and email.
> your private key is protected only by a verification text message and short PIN. That's very low entropy compared to a full ED25519 keypair. Can you explain what you mean by this? In practice, I can see the argument that the only verified identity (phone number) for most users is the one protected by a flimsy SMS verification message, but I don't believe that that implies your private key is terribly vulnerable to…
Do you think so? The app provides a "Safety Number" and QR code with built-in scanner to verify that yup, your device and their device have nobody in the middle. It has a visible reminder that you checked this person's actual identity (if you did) and a message appears if the keys change. If you are "rigidly adhering" to protocol your response should be to arrange an in-person meet-up to reconfirm, not "New phone? Cool".