- Messages by default are encrypted in transit. Client to server. Yes Telegram does have access to those messages. (I don't believe we had any e2e encrypted chat service before the likes of signal, matrix etc. Whatsapp added it after Telegram too if my memory is right.)
- The library IS used for all encryption including the above client to server encryption. As far as I can tell from casual use the other end does not need to be online for secret chats per se. There's a key exchange with picture verification that requires the party on the other end to accept the chat request.
- The phone bits in your and the other commenters response sound a little bit handwavy to me.
- Telegram client(s) are also open source. The comment was about the server and interoperability with other clients.
After all it doesn't seem to me that I am more misinformed than yourself.
- The library IS used for all encryption including the above client to server encryption. As far as I can tell from casual use the other end does not need to be online for secret chats per se. There's a key exchange with picture verification that requires the party on the other end to accept the chat request.
- The phone bits in your and the other commenters response sound a little bit handwavy to me.
- Telegram client(s) are also open source. The comment was about the server and interoperability with other clients.
After all it doesn't seem to me that I am more misinformed than yourself.