Telegram reached 500 million users last year, many of them in Russia and Ukraine. Since Russia invaded its neighbour, Telegram has topped app store charts worldwide, as people try to get to the heart of what's happening and as Ukrainian civilians maintain vital lines of communication. Telegram likes to tout its privacy credentials, but cybersecurity researchers say all is not what it seems. Notably, end-to-end encryption is an opt-in feature, not a default. Peter O'Brien explains in this week's Tech 24.