DTC T-2350, Руководство пользователя

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21
Digital Encryption
In the “Digital Encrypt” mode of opera-
tion, this unit uses a 56-bit DES-OFB
Encryption Key per the APCO Project 25
specification. Each Encryption Key is
identified/labeled using a shorter Key
Identifier (16 bits), known as the KeyID.
If the unit uses the Digital Encrypt mode
of operation, one or more KeyID/key
pairs need to be entered.
0000
0001
0010
0011
0100
0101
0110
0111
1000
1001
1010
1011
1100
1101
1110
1111
(Table 1)
Encryption
Figure A
Hexa
decimal
Binary
Encryption
1. To start entering KeyID/Key pairs, select the “Tx Options” tab , and click the Edit
button under “Encryption_Keys”.
2. To enter a new pair, click “Add Item”. In the dialog box (see Figure A), enter a
four-digit hexadecimal number for the KeyID and click “OK”.
3. The KeyID will be displayed at the top of the dialog box, and the program will
then ask to enter the Encryption Key (16 hexadecimal digits). When done
entering the Key, click OK, and the KeyID/Key pair will be entered into the avail-
able list. If the parity of any of the bytes is not odd, the user will be warned
“Wrong parity entered” and the KeyID/Key will have to be re-entered. To protect
the Key, it will not be shown or displayed once it is entered.
Associating a Key/KeyID pair with a
Channel
For each channel with the “Digital
Encrypt” mode selected, a Key/KeyID
pair must be selected. Under the “Tx
Channels” tab, select the channel and
click “Edit Chan”. The Key/KeyID pair
is selected under “key_IDs”. Note that
you only see the KeyID, not the Key
itself. Note that multiple channels
may use the same KeyID/Key pair if
desired. If the mode is not “Digital
Encrypt”, the key_ID setting is ignored
and has no effect.
Entering Key/KeyID pairs
The KeyID is essentially a label for the
DES-OFB Encryption Key. The KeyID is
a 16-bit value, represented by four
hexadecimal digits, with each hexadeci-
mal digit representing 4-bits (see Table
1 below). The Encryption Key is a 64-bit
value (56-bit key + 8 bits parity), repre-
sented by eight hexadecimal digits. Per
the APCO Project 25 specification, each
byte (i.e. pair of hexadecimal digits) in
the Encryption Key must have odd
parity. Odd parity means that the
number of “one’s” in the binary (0/1)
representation of the 8-bit byte must be
odd. For example, the following is a
valid Encryption Key: 2F AB 08 E3 B5
9D 4C 16 .
1
2
3
4
5
6
7
8
9
A
B
C
D
E
F