The Project 25 standard covers all of the parts of a system for public safety land mobile radio communications. These systems include subscriber units, base stations and other fixed equipment. The subscriber units include portable radios for handheld operation and mobile radios for vehicular operation. The base stations are used for geographically fixed installations. Other fixed equipment is used for wide area operation and console operator positions. Computer equipment may be used to interface between each of these equipment items. A Common Air Interface (CAI) (Reference 2) allows these radios to send and receive digital information over a radio channel.

Many of the parts of a public safety land mobile radio communications system will use encryption to protect the information which is sent through the system. The encryption algorithms require keys in order to protect the confidentiality of this information. The process by which these encryption keys are generated, stored, protected, transferred, loaded, used and destroyed is known as key management. These keys must be protected from inadvertent disclosure and require updating or replacement in order to maintain system security. Key distribution is often accomplished manually. However, the most convenient way to distribute keys is to electronically send the keys from a key management facility to the destination equipment. This involves sending keys over the CAI and this procedure is referred to as Over-the Air-Rekeying (OTAR). OTAR is a method of encrypting and sending the encryption keys and other related key management messages through the CAI in such a way that they are protected from inadvertent disclosure and, in some cases, unauthorized modification.