Method for over the air rekeying of multiple communication groups

In a secure communication system 100, communication units in different communication groups may be simultaneously rekeyed over the air by having a managing key unit 101 generate a key packet 121. This key packet 121, which includes a unit key 122, a key alias 123, and a key representation 125, is transported to at least one fleet key unit 102. Upon receiving the key packet 121, the fleet key unit determines, based on the key alias, which communication units of which communication groups are to be rekeyed. Once this is determined, the key representation 125, which represents the unit key (encryption key) 122, is transmitted to the communication units, thus rekeying multiple communication units in multiple communication groups simultaneously.