The Project 25 standard covers all of the parts of a system for public-safety Land Mobile Radio communications. These systems include portable radios for hand held operation, mobile radios for vehicular operation, base stations for fixed installations, and other fixed equipment for wide area operation and console operator positions, as well as computer equipment for data mmmunications. The standard defines the means for this equipment to send and receive digital information, in the form of either voice or data (¡.e. non-voice) messages.

The reader of this document should be familiar with Appendix C, STATEMENT OF REQUIREMENTS, of reference 1. One of the requirements is to be able to protect digital communications with encryption. The implication is that a means for encryption and decryption must be included in those system elements (e.g. portable and mobile radios) that ntend to protect the messages which they exchange in conformance to the other parts of Project 25 standard. It is the scope of this document to describe how to use encryption to protect messages for the Project 25 standard.

Reference 1 also contains a General System Model to describe a Project 25 standard system. Such a system is decomposed into functional groups with simple designations like apos;MRapos; for mobile radio, and apos;BRapos; for base station (radio). These functional groups are loosely correlated to real products and devices, but they need not be. Each functional group has one or more interfaces to other functional groups that allow information to be transferred through the system. The interface for communications over a radio channel is called the Common Air Interface (CAI). The formats for transmission of information over the Common Air Interface are described in reference 3.

The functions of encryption and decryption generally take place near the end points of a message path in a system, in order to maintain the confidentiality of the information through as much of the system as possible. This means that the encryption and decryption functions can be provided at points where voice information is coded with IMBE, such as an MR (mobile or portable radio) or a CON (console). The functions may also be provided at points where data (non voice) information enters the system such as an RFG (RF system Gateway). This is diagrammed in figure 1-1 for clarity.

This DES Encryption Protocol defines the operation of encryption and decryption in a way that is compatible with information transfer through an APCO Project 25 standard system, and especially, through the CAI of such a system. It is important to note that the DES Encryption Protocol is not actually part of the CAI because encryption and decryption may take place in system devices which are not directly connected to the CAI.