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Comment by cprecioso | Hacker News Reader

Can anyone familiar with the matter say how secure is the 4-emoji verification code?

> Thanks to this modification, it becomes possible to prevent eavesdropping (MitM attacks on DH) with a probability of more than 0.9999999999 by using just over 33 bits of entropy in the visualization. These bits are presented to the users in the form of four emoticons. We have selected a pool of 333 emoji that all look quite different from one another and can be easily described in simple words in any language.

33 bits of entropy seems quite low to me.

https://core.telegram.org/techfaq#q-how-are-voice-calls-auth...

That would be low if we were considering a long-term key. It seems fine for a one-shot authentication token? Eve is going to have to MitM a lot of calls before she guesses this token correctly.

I wonder about the possibility to use "collisions" between similar-looking emojis in order to fake a match more easily.

I think a while ago someone on HN suggested emoji as a joke as a more information-dense and internationalized alternative to numeric or alphanumeric verification codes. I don't know about security, but if this uses all 1,088 emoji from Unicode 9.0, then four emoji have about 40 bits of information.

They say they are using 333 emoji to represent ~34 bits.

33.52 bits to be precise, which is where the many-nines claim comes from: 1-(1/(333^4)) = ~0.999999999918

Emojis in this case are just sort of visual representation of the keys two users exchange before communication.

In theory this makes MitM attack impractical.

https://core.telegram.org/techfaq#q-how-are-voice-calls-auth...

https://core.telegram.org/api/end-to-end/voice-calls#key-ver...

Based on their tech docs[1], it seems pretty good.

1: https://core.telegram.org/api/end-to-end/voice-calls#key-ver...

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