User's Manual

Draft
December 2006 5300 ES Series Mobile Radio Operating Manual -11
11.4.5 Message Number Period (MNP)
One of the parameters that is programmed in a radio utilizing OTAR is Message Number
Period (MNP). This parameter is programmed as described in
Section 11.5.1. This
parameter is used to minimize the possibility of someone sending messages to “spoof” the
system. 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 different by more than 1000 from the current message number, the current message is
ignored. 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 blocking out intended messages becomes higher. In addition, traffic
level and terrain contribute to lost messages and should also be considered when selecting
this number.
A setting of 1000 is a good compromise because it blocks out real 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).
11.4.6 Definitions
Algorithm - Refers to the specific encryption standard (DES or AES) that is used to
encrypt a message. Each standard uses different calculations to perform the encryption
(see
Section 11.1).
Algorithm ID (ALGID) - Identifies the algorithm (DES or AES) used to encrypt a
message. This ID and the Key ID are transmitted with each message to uniquely identify
the key being used.
Black - Refers to information that is encrypted. The opposite is “Red” which refers to
unencrypted information.
Common Key Encryption Key (CKEK) - This is a KEK common to a group of
subscriber units which share the same encryption keys (are part of same crypto group).
These keys can be the DES or AES type. The use of a common key allows the subscriber
units to be rekeyed by the KMF using one Key Management Message. Refer to “KEK” for
more information.
Common Key Reference (CKR) Group - Same as Storage Location Number (SLN).