[zooko_LeastAuth]

Dear makers and backers of Telegram:

Perhaps in response to my requests (https://news.ycombinator.com/item?id=6933179 , https://twitter.com/zooko/status/413552420522708993 , https://twitter.com/zooko/status/413552466748133376 ), your FAQ (http://core.telegram.org/contestfaq) now says:

------- Q: Does Paul send the same message to Nick every day?

No, just as in real life, Paul‘s messages to Nick can be different each time. The only thing that doesn’t change is the secret email address in his daily messages.

Q: Could you provide an example of a Paul's message to Nick?

Sure. The message may look like “Hey Nick, so here is the secret email address for the bounty hunters – {here goes the email}”. -------

There are some things that I don't understand about the structure of this contest. Why is the target secret an email address rather than a magic word like "squeamish ossifrage"?

I asked for an “examples of the actual message”, and you posted an possible example, but what I meant to ask for was actually the exact text of one of the messages. Except, of course with the target string (the email address) replaced by X's.

For redditors following along, getting a (partial) copy of the exact message that was sent would be an example of what cryptographers call (partial) "known plaintext". If your cryptosystem is secure against Known Plaintext Attack, then it doesn't matter if an attacker (me) gets copies of some of the messages. If your cryptosystem is insecure in this model, then your users have to be careful with what they type into their messages. For example, they might need to be careful not to cut and paste long strings from other sources, or to otherwise insert strings into their messages that their attacker might guess.

All good, modern cryptosystems are secure in the Known Plaintext Attack model! (And, in fact, all good, modern cryptosystems are secure in much more rigorous models in which attackers get more powers beyond peeking at plaintext.)

So if the makers of Telegram are confident in the security of their protocol, they should have no problem posting the complete, verbatim text of the first message that Paul sent to Nick, with the target email address replaced by "XXX"'s.

[zooko_LeastAuth]

Taylor Hornby has written a good introductory explanation of the Known Plaintext Attack model and the more powerful attack models, in the context of the Telegram cracking contest:

http://www.cryptofails.com/post/70546720222/telegrams-crypta...

[djm_]

A simple way to understand the gravity of this: the Nazi's Enigma machine was broken with a known-plaintext attack a.k.a a Turing Bombe break. Furthermore, it was the known plain text of previously decrypted messages that was used in further attacks against new keys issued by the Nazis.

[zooko_LeastAuth]

Somebody pointed out to me that this isn't reddit, but hackernews. Oops, sorry.