Blog post by crypto professor Matthew Green, discussing what Telegram does (I wasn’t familiar with it) and criticizing its cryptography. He says Telegram by default is not end-to-end encrypted. It does have an end-to-end “secret chat” feature, but it’s a nuisance to activate and only works for two-person chats (not groups) where both people are online when the chat starts.
It still isn’t clear to me why Telegram’s founder was arrested. Green expresses some concern over that but doesn’t give any details that weren’t in the headlines.
And I’m saying that despite people constantly throwing shade on the protocol, nobody has actually showed any flaws in it over many years. The whole dogma that non-standard protocols and algorithms should be shunned out of principle is just that. Meanwhile, plenty of exploits have been found in standard protocols. Not only that there are known cases where US security agencies introduced exploits into popular protocols. https://www.theverge.com/2013/12/20/5231006/nsa-paid-10-million-for-a-back-door-into-rsa-encryption-according-to