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Let me respectfully disagree with you here. Secret Chats in Telegram provide users with a way to detect a server-side MITM attack. http://core.telegram.org/tech

• nwh 13y ago
  That only protects against a MITM between peers who have communicated with a "secret chat" previously, not two fresh peers. As "secret charts" are disabled by default it's not really a defence against infiltration; users will presumably only enabled the "secret chat" mode when they have something sensitive to talk about. When they do enabled it for the first time, we can instantly MITM them using the attack against the "image verification" I mentioned lower down (https://news.ycombinator.com/item?id=6932053 https://news.ycombinator.com/item?id=6932053), and we can assume that the conversation is worth our while listening in on. The user will hopefully expose themselves in the belief that they are safe, and the game is over.
• h0cked 13y ago
  It's simple unauthenticated Diffie-Hellman key agreement, which is known for MITM attack. Yes, you ask A to accept B's identity upon key exchange, but to what extend A would know B is really B not the server playing along? A plausible method would have A and B exchange certificates separate from the Diffie-Hellman key exchange process, and use those as the identity verification mechanism.