Here's something that is bugging me recently: suppose that me and my friend establish an OTR session and - as a result of that - DH key exchange is performed. My friend verifies my key, but I cannot verify his fingerprint. Despite that, we have a secure channel over which he can send me one bit of information - whether my key was valid or not. Can I trust the OTR session if he successfully verified my key and sent me this confirmation or is there still a risk of a man-in-the-middle attack?

  • $\begingroup$ I guess the intuitive answer is that if you have nothing to see if you are communicating with your friend, then an MITM attack is always possible. Although the secure channel exists, you don't know who it has been established with. If you are indeed connected to your friend, then everything after that is indeed secured (assuming everything is implemented as it should be, of course). $\endgroup$
    – Maarten Bodewes
    Jan 24 '15 at 19:27

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