The present invention involves establishing a top-level key and optionally also a verification tag. The top-level key is used as the MDP key for encrypting a broadcast medium. Only the part of the key message that contains the encrypted top-level key is authenticated, e.g. using a signature or a Message Access Code (MAC). Any known group-key distribution protocol can be used that is based on the creation of a hierarchy of keys. Examples of such methods are the LKH and SD methods. The group-key distribution protocol output key H, traditionally used as the MDP key, or a derivative thereof is used to encrypt the top-level MDP-key. The invention, further, includes optimization of a group-key message by eliminating unnecessary message components relative a specified group or sub-group of users. The optimization can be made in dependence of contextual data such as user profile, network status, or operator policies.