For those who don't know: Telegram is a partially open source Whatsapp alternative (Server is closed source) which offers secret chats and normal chats. Secret chats are encrypted with Diffie-Hellman key exchange and are end-to-end encrypted. One can verify his peer's signature using a barcode. Normal chats are not end-to-end encrypted, but have the benefit of being synced between one's devices.
Telegram has been widely criticized for using a brand new protocol, MTProto. Whether MTproto is actually secure or not is out of the scope of this question, let us assume it is insecure.
Since DH is used in secret chats, will a compromise of MTProto compromise secret chats? Are DH and MTproto coupled in such a way that if MTProto fails, DH fails? Or is it layered so that the two must fail for secret chats to become vulnerable?
In short, if one does not trust MTProto, can one still trust secret chats thanks to DH?
Note: MTProto also uses DH for device registeration, this is unrelated.
Useful official documents:
Anton Garcia Dosil stated that DH is just a way to distribute keys and is not an encryption method itself. This is definitely true, and I apologize for being a bit vague here. A clearer formulation of my question would be: Once the two peers exchange DH keys and begin end-to-end encryption, does MTProto use an encryption method which is known to be secure? or does it use yet another home brewed encryption scheme? If it does use a known encryption method X for secret chats, Are X and MTproto coupled in such a way that if MTProto fails, X fails? Or is it layered so that the two must fail for secret chats to become vulnerable?