Operating Manual Part 15

October 2008 5300 ES Series Mobile Radio Operating Manual 10-7
Secure Communication (Encryption)
10.3.3 Talk Group Encryption Override
On conventional digital (P25) channels, the encryption strapping mode and PID are
programmed on a per talk group basis similar to the trunked modes (SMARTNET/
SmartZone/P25 Trunked). However, with this mode only, there is the option to override
the talk group encryption programming and program it on a per-channel basis. Therefore,
if desired on conventional digital channels, encryption can be programmed differently for
each channel. Conventional analog channel encryption is always programmed on a per-
channel basis.
10.4 Over-The-Air Rekeying (OTAR)
OTAR stands for “Over-The Air-Rekeying”. This is the process of sending encryption
keys and related key management messages over-the-air to specific radios. The advantage
of OTAR is that it allows these keys to be quickly and conveniently updated when
necessary. It is no longer necessary to periodically travel to the radio location or bring the
radio into a maintenance facility to load new keys.
The actual OTAR rekeying functions are performed by a Key Management Facility
(KMF) that sends Key Management Messages (KMM) to the RSI (Radio Set Identifier)
assigned to a specific radio or radios. These messages are themselves encrypted using a
unique key called the UKUK (Unique Key Encryption Key). Radios must be OTAR-
compatible, programmed for OTAR, and the UKEK loaded for OTAR for this type of
rekeying to occur.
Note The RSI is enabled in the KMF and must be assigned to the radio under OTAR in PC
Configure.
OTAR is available only on P25 conventional and trunked channels, and only to program
DES-OFB and AES keys. It is not used on SMARTNET/SmartZone channels or to load
DES keys.
10.4.1 Encryption Key Types
There are two types of keys used with OTAR:
Traffic Encryption Key (TEK) - The key used to encrypt voice and data traffic. All
radios using encryption must have at least one of these keys.
Key Encryption Key (KEK) - The key used to encrypt keys contained in OTAR Key
Management Messages (KMMs). All radios which use OTAR must contain at least one
of these keys. The KEK used to decrypt/encrypt keys in an OTAR message is defined
by the algorithm and key IDs transmitted in the decryption instructions field. A KEK