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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 out 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).

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 10.1).

Algorithm ID (ALGID) - Identifies the algorithm (DES or AES) used to encrypt a 
message. This ID and the Key ID are transmitted with each non-analog 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. 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 in some circumstances by addressing the KMM to a 
group RSI. Refer to “KEK” for more information.

Common Key Reference (CKR) Group - Same as Storage Location Number (SLN).

Crypto Group - A group of up to 16 keysets containing the same type of keys (either TEK 
or KEK). Although a crypto group can contain up to 16 keysets, only two are normally 
used. Only one keyset in a crypto group is active at a time. EFJohnson radios currently 
support only one crypto group: for TEKs, crypto group 0.

Cryptographic Variable - The variable used by a cryptographic algorithm to encrypt a 
message. Also called a “key”.

Key - A variable used by a cryptographic algorithm to encrypt voice or data. Also called 
“Cryptographic Variable”. Keys are identified by their Algorithm ID and Key ID values.