CC2430 Datasheet, PDF (138/234 Pages) Texas Instruments – A True System-on-Chip solution for 2.4 GHz IEEE 802.15.4 / ZigBee-TM

CC2430
(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 does not have to
reload the IV/CTR.
(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 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
(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.
(7) The software calls a CTR mode
decryption right now on the encrypted
message blocks C. It does not have to
reload the IV/CTR.
Reference
Authentication
tag
generation
This phase is identical to the
Authentication Phase of CCM encryption.
The only difference is that the result is
named MACTag (instead of T).
Message Authentication checking
Phase
The software compares T with MACTag.
13.9.6 Sharing the AES coprocessor
between layers
The AES coprocessor is a common
resource shared by all layers. The AES
coprocessor can only be used by one
instance one at a time. It is therefore
necessary to implement some kind of
software semaphore to allocate and de-
allocate the resource.
13.9.7 AES Interrupts
The AES interrupt, ENC, is produced
when encryption or decryption of a block is
completed. The interrupt enable bit is
IEN0.ENCIE and the interrupt flag is
S0CON.ENCIF.
13.9.8 AES DMA Triggers
There are two DMA triggers associated
with the AES coprocessor. These are
ENC_DW which is active when input data
needs to be downloaded to the ENCDI
register, and ENC_UP which is active
when output data needs to be uploaded
from the ENCDO register.
The ENCDI and ENCDO registers should be
set as destination and source locations for
DMA channels used to transfer data to or
from the AES coprocessor.
13.9.9 AES Registers
The AES coprocessor registers have the
layout shown in this section.
CC2430 PRELIMINARY Data Sheet (rev. 1.03) SWRS036A
Page 138 of 232