How do they do it?
The messages are encrypted using the Signal protocol, which is by far the most robust and secure protocol ever developed. It amalgamates the Extended Triple Diffie-Hellman (X3DH) key agreement protocol, Double Ratchet algorithm, pre-keys, and uses Curve25519, AES-256, and HMAC-SHA256 as cryptographic primitives.
In simple words, what this means is that even if someone were to intercept messages sent on the signal app, all they will see is gibberish. Add to this, the requirement of passcodes, and what you have is a system that is both robust and reliable, not to mention – Secure.
What might interest you to know is that other chat applications also use the Signal protocol to execute the encryption feature in their respective platforms, but none come even close to the absolute privacy that Signal offers.
Overall, the Signal Messaging app is a great choice for your primary messenger as it includes high-grade security with all the features that are currently employed in other competitor messaging apps. If you want to try out the Signal Messaging app, you can download your copy from the links given below.