I think the following is not possible, but I have no proof and would like to find out wether it is possible or not.
Let's say in a network, every participant cannot connect to each other participant directly. Every message is sent via a person M. Every participant can connect through M to every other participant.
Is there any way to know if the other participants are real and not fake participants created by M?
I am assuming, that you cannot swap information "offline", e.g. there is no key signing party and no way to exchange certficates offline. Also, you do not know the other participants.
The problem is: If the other participants are simulated by M, M has the data I wanted to share with another participant. Even if encrypted, it could be a key of M.
As you can see, M is already the man in the middle in the architecure.
Is there a proof, you cannot be sure, that the participants are real? Or am I missing some solution to my problem? The architecture has to stay the same for my question.
Thank you in advance!