CC2520_11 Datasheet, PDF (93/133 Pages) Texas Instruments – 2.4 GHZ IEEE 802.15.4/ZIGBEE RF TRANSCEIVER

CC2520 DATASHEET
2.4 GHZ IEEE 802.15.4/ZIGBEE® RF TRANSCEIVER
SWRS068 – DECEMBER 2007
26 Security Instructions
CC2520 has extensive support for security operations defined in IEEE 802.15.4. The latest specification
version [2] describes only the CCM* mode of operation. CCM* uses CTR-mode encryption for confidentiality
and CBC-MAC for authentication. In CC2520 these operations are available as separate instructions. In
addition the basic ECB instruction is available and an instruction for manipulation of the counter used in
CTR-mode encryption/decryption.
Note that all the different security operations in IEEE802.15.4 only use AES 128bit encryption. Decryption is
never used and thus CC2520 only supports encryption. ECB decryption is not supported.
Note that for all the security instructions, the key and counter should reside in RAM in reversed byte order
compared to the data. This can either be done by reversing the byte order of the key/counter before it is
written to the RAM, or the MEMCPR instructions can be used to reverse the byte order of keys/counters that
are already in the RAM.
26.1 Decoding of the Flags Field in CC2520
This section defines the security flags used during counter mode encryption and CBC-MAC mode
authentication (also includes CCM*) and how these are represented in CC2520 RAM.
Flags byte written to RAM
7
6
5
4
3
2
1
0
-
CTR Flag
bits 7:6
CBC Flag
bits 7:6
L
m(1:0)
LUT(m(1:0))
7
6
5
4
3
2
1
0
Res Res 0
0
0
L
Flags byte used for CTR operation
7
6
5
4
3
2
1
0
Res Adata
M
L
Flags byte used for CBCMAC operation
Figure 35: Security flags
Figure 36 shows how the most significant byte of the counter in CC2520 RAM represents both the CTR and
CBC-MAC security flags.
For the CBC-MAC flags, a lookup procedure is used to translate the two least significant bits of the m
parameter to the CBCMAC instruction in the M value that is used in the flag byte. The same translation is
used for the CBC-MAC part of the CCM instruction.
Table 19: Lookup table for M value
m(1:0)
M
00
000
01
001
10
011
11
111
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