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.
Group chat is a tricky problem and the modern crypto group (moderncrypto.org) talked about it at great length a few years back. I don’t know whether any software exists that incorporates all those ideas, but that’s mostly because I haven’t really been looking for it.
There is the MLS standard now that was explicitly developed with e2ee group chat applications in mind. From what I have read so far, this new standard seems well regarded by cryptography experts.
Thanks, I’ll ask my crypto homies about it. I remember they were trying to handle some subtle problems.
MLS info: https://en.m.wikipedia.org/wiki/Messaging_Layer_Security