There are some serious issues with Telegram's cryptography it seems: https://security.stackexchange.com/questions/49782/is-telegr... and http://unhandledexpression.com/2013/12/17/telegram-stand-bac...
See also http://thoughtcrime.org/blog/telegram-crypto-challenge/ (comments: https://news.ycombinator.com/item?id=6936539 )