Hmmm non-disclosed? If you’re talking about the fact that the backed code is not public, then you are right. They didn’t release it yet. Though, MTProto is fully open and documented and anyone can check its implementation in the clients by checking their source code, which is public. The whole point of e2e encryption is that you don’t need to trust the server to know your chats are actually encrypted (as long as you verified your chat partner’s key fingerprint through another trusted channel), so being able to audit the implementation of the protocol from the clients public code is enough to know the server cannot snoop your conversations