Massey Omura multiple users key distribution

We address a simple protocol that can be used to distribute a shared key to three or more users over an insecure channel. We use the Elgamal protocol to set up the system and we define a generalization of Massey Omura protocol to allow one user to send a message or a shared key to a group of users. Moreover we analyse some problems about the decryption shared key of Massey Omura generalized protocol and the man-in-the-middle problem.