I'd like to add that MTProto is impossible to MITM because there are no trusted third parties. The handshake that happens before the user logs into their account includes a step with RSA. The public keys for that step are hardcoded into clients.

To anyone who wants to complain about Telegram not encrypting chats by default and using "homebrew crypto" I'd like to say that it's XMPP we're discussing here. Telegram offers marginally better security than XMPP over TLS. The "homebrew crypto" is still not broken in 10 years and not for the lack of trying.