CC2430 Datasheet, PDF (138/212 Pages) Texas Instruments – A True System-on-Chip solution for 2.4 GHz IEEE 802.15.4 / ZigBee-TM

CC2430
Peripherals : AES Coprocessor
unchanged, while setting the lower bits to 0 (M
!= 16).
The result is called T.
Message Encryption
(7) The software creates the key stream block
A0. Note that L=6, with the current example of
the CTR generation. The content is shown in
Figure 30.
must be zero. When encrypting message
blocks using CTR mode, CTR value must be
any value but zero.
The content of the Encryption Flag byte is
described in Figure 31.
Note that when encrypting authentication data
T to generate U in OFB mode, the CTR value
Name
A0
Designation
First CTR value for CCM mode
Byte 0
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Name Flag
NONCE
CTR
Figure 30: Message Encryption Phase Block
Name
Designation
FLAG/A0
Encryption Flag Field for CCM mode
Bit
7
6
5
4
3
2
1
0
Name
Reserved
-
L-1
Value
0
0
0
0
0
1
0
1
Figure 31: Encryption Flag Byte
Message Encryption (cont.)
(8) The software loads A0 by selecting a Load
IV/Nonce command. To do so, it sets Mode to
CFB or OFB at the same time it selects the
Load IV/Nonce command.
six-bit word. So, the value L is set to 6. To
decrypt a CCM mode processed message, the
following sequence can be conducted (key is
already loaded):
Message Parsing Phase
(9) The software calls a CFB or an OFB
encryption on the authenticated data T. The
uploaded buffer contents stay unchanged
(M=16), or only its first M bytes stay
unchanged, the others being set to 0 (M-16).
The result is U, which will be used later.
(10) The software calls a CTR mode
encryption right now on the still padded
message blocks. It has to reload the IV when
CTR value is any value but zero.
(11) The encrypted authentication data U is
appended to the encrypted message. This
gives the final result, c.
Result c = encrypted message(m) + U
Message Decryption
CCM Mode decryption
In the coprocessor, the automatic generation
of CTR works on 32 bits, therefore the
maximum length of a message is 128 x 2³²
bits, that is 2³⁶ bytes, which can be written in a
(1) The software parses the message by
separating the M rightmost octets, namely U,
and the other octets, namely string C.
(2) C is padded with zeros until it can fill an
integer number of 128-bit blocks;
(3) U is padded with zeros until it can fill a 128-
bit block.
(4) The software creates the key stream block
A0. It is done the same way as for CCM
encryption.
(5) The software loads A0 by selecting a Load
IV/Nonce command. To do so, it sets Mode to
CFB or OFB at the same time as it selects the
IV load.
(6) The software calls a CFB or an OFB
encryption on the encrypted authenticated
data U. The uploaded buffer contents stay
unchanged (M=16), or only its first M bytes
stay unchanged, the others being set to 0
(M!=16). The result is T.
CC2430 Data Sheet (rev. 2.1) SWRS036F
Page 138 of 211