> If you look at the history of TLS, you'll see how tricky is to get crypto right. There has been lots of attacks on the protocol, that no one person could've think of.

You'd hope so, but contrary to popular belief, actual cryptographers relatively rarely manage to prevent errors in the standards. For instance, Phil Rogaway warned against MAC-then-Encrypt used in TLS which led to the Padding Oracle attacks many many years before they were exploited, but the suggestions were ignored by the "practitioners" designing the protocols. In fact, the TLS example was one that is relatively easy to get right if the designers wouldn't have gone crazy and listened to actual cryptographers. Telegram will someday fall into the exact same bucket.