Lots of FUD out there there about Telegram not being encrypted that's just not true. There's nothing either side can to do send a message in clear text / unencrypted.
This is proven by an extremely simple experiment: you log in on your new phone, enter password and instantly see all chats.
Another simple experiment points that chats are unlikely to be even encrypted at rest is that Telegram has an extremely fast server side message search. You log into a web client, half a second later you can type a search query and uncover chats from years ago.
How much data there are on your chats? 1 megabyte is around one thick book in plaintext.
AES-CBC as example method decrypts more than 2 gigabits per second with hardware opcodes (2012 processor), for example if we look this data https://www.bearssl.org/speed.html
It is impossible to say based on delay when searching plaintext on this level whether there is encryption.
What this means is that any conversations where you do select E2EE are the ones the "authorities" will take interest in, even if only to the extent of metadata.
That's the fundamental problem with E2EE-by-exception, rather than by default. It calls attention to specific data, even if its not cleartext, rather than obscuring everything.
also curious - how does telegram support encryption for chatrooms without the parties being known in advance? or are those chats not encrypted?
E2EE means that the users exchange encryption keys, and they encrypt the data at the client, so that only the other client can decrypt it. Meaning Telegram can never inspect the data if they wanted to.
I haven't dug enough to know what telegram does or claims to do.
E2EE: As long as it is correctly set up and no significant breakthroughs happens in math, nobody except the sender, the receiver can read the messages.
> Does the former mean that telegram itself can read non-private-chat messages if it so chooses?
Correct. They say they store messages encrypted and store keys and messages in different jurisdictions, effectively preventing themselves from abusing it or being coerced into giving it away, but this cannot be proven.
If your life depends on it, use Signal, otherwise use the one you prefer and can get your friends to use (preferably not WhatsApp though as it leaks all your connections to Facebook and uploads your data unencrypted to Google for indexing(!) if you enable backups.
Edited to remove ridiculously wrong statement, thanks kind SquishyPanda23 who pointed it out.
E2EE means the service provider cannot read the messages.
Only the sender and receiver can.