[OTR-dev] One-way OTR MITM?

Jacek Wielemborek d33tah at gmail.com
Wed Apr 8 08:51:14 EDT 2015


Here's something that keeps bugging me for a moment. It's a hypothetical
situation, so please refrain from question like "how did you send the FP
to your friend?".

1. My friend receives my OTR fingerprint over a secure channel (e.g.
meeting in person), but he doesn't give the fingerprint to me and
destroys the channel,

2. Me and my friend establish a perfectly secure channel that has two
limitations: my friend can only send messages to me (not the other way)
and only one bit of information can be transferred.

3. We agree that over this channel, he will tell me during a future OTR
session whether my fingerprint matched what he received from me during
step 1,

4. We go back home, establish an OTR session over the internet, which
isn't secure yet. My friend verifies my fingerprint based on what we got
during step 1 and tells me whether it matched over channel from step 2,

If the fingerprint he sees on the screen matches the thing he got from
step 1, can I assume that there can be no man in the middle? In other
words, is it possible to perform a one-way OTR MITM where my friend
actually sees my real fingerprint, but when he responds, I can't see
his, but one from the MITM?

Hopefully I explained this clearly. Cheers,
Jacek Wielemborek

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