Operating Manual

March 2008 5100 ES Models II/III Portable Radio Operating Manual 10-9
Secure Communication (Encryption)
The KMF performs OTAR functions by exchanging Key Management Messages (KMMs)
with the radios. Both the KMF and radio can originate messages. Some functions
performed by the KMF are as follows:
• Loading new keys
• Modifying keys
• Initiating active keyset switchovers
• Modifying keyset names
• Deleting keys
• Deleting all keys (zeroize) when a radio may be lost or compromised
• Determining if a radio is on the air and reading key information
• Changing individual and group RSIs
• Manage Subscriber Groups (groups of radios using the same keys).
10.4.4 Message Number Period (MNP)
One of the parameters that is programmed in a radio using OTAR is Message Number
Period (MNP). This parameter is used to minimize the possibility of someone sending
messages to “spoof” the system or radio. The MNP is used as follows.
Every message sent out has a message number. The message number increments by one
with each message sent. The MNP is the maximum difference allowed between
messages. For example, if the MNP is set to 1000 and the last message number received
by the radio is older by more than 1000 from the current message number, the current
message is ignored, and a message number resynchronization process will be
automatically attempted. If the MNP is set to 65535, message numbers are ignored.
When determining this number, consider the likelihood of someone trying to spoof the
system by retransmitting recorded messages and then adjust the MNP accordingly. The
higher the risk, the smaller the MNP should be. A disadvantage of setting a low MNP is
that the chance of rejecting valid messages becomes higher. In addition, traffic level
and terrain contribute to lost messages and should also be considered when selecting
this number. If the message numbers become out-of-sync between the radio and the
KMF, an automatic resynchronization process will take place, which requires
additional bandwidth. Note that this process is not possible if the radio does not share a
TEK with the KMF.
A setting of 1000 will block very old messages but is unlikely to affect anything
intentionally sent out by the system (1000 messages sent to a single radio is not likely to
occur in a system in less than a year with normal usage).