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Disclaimer: I am not a cryptographer, and have only gotten to #40 in Matasano's challenges.

They appear to be trying to use encrypt and mac, and not as far as I can tell relying on IGE mode to provide authentication. See the "Message Key" and "Important Tests" sections here: https://core.telegram.org/mtproto/description

There is also not, as far as I can tell from reading the protocol description above and the source code here: https://github.com/ex3ndr/telegram-mt/blob/master/src/main/j... (in particular the EncryptedMessage method, lines 447 to 460) any sort of key in the MAC, it's just SHA-1 of the plaintext. It's also not encrypted as far as I can tell, it's listed in the unencrypted "external header" in the protocol description.

I can't think of an attack from this, but the fact that an adversary can generate a valid MAC for _any message_ by simply SHA-1'ing it seems really scary.

Kindly take at look at this technical FAQ that we've assebmled based on the conversations on Hacker News and elsewhere: http://core.telegram.org/techfaq

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