@feld thanks for the source
last time I checked the mtproto had such an obvious flaw to diffie-hellman it would have been a beginner question in crypto 1 at my uni (and my uni is worst in the country for c.s.)
we're only talking about the secure chat feature here. Every other chat (nearly 100 %) is still unencrypted (and they're open about that).
They sometimes communicate this premise in a way very confusing to the uninitiiated