it wasn’t about operating system, it was about encryption that was used in devices for sms and calls
right now the same or better encryption is achievable on free messengers like signal or wire
your regular voice call and sms isn’t really protected or secured by design, so blackberry used to add encryption to it
in the past “smartphones” very closed in their ecosystems, so having backdoor or RAT on your device wasn’t a real vector of attack
right now with android being fully functional OS and iOS and android having millions of third-party apps in store, that might act as a RAT, OS hardening became a problem
Latest Answers