CC2430
Peripherals : AES Coprocessor
13.12 AES Coprocessor
The CC2430 data encryption is performed using
a dedicated coprocessor which supports the
Advanced Encryption Standard, AES. The
coprocessor allows encryption/decryption to be
performed with minimal CPU usage.
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•
Supports all security suites in IEEE
802.15.4
ECB, CBC, CFB, OFB, CTR and CBC-
MAC modes.
Hardware support for CCM mode
128-bits key and IV/Nonce
DMA transfer trigger capability
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•
•
The coprocessor has the following features:
13.12.1 AES Operation
To encrypt a message, the following procedure
must be followed (ECB, CBC):
The AES coprocessor works on blocks of 128
bits. A block of data is loaded into the
coprocessor, encryption is performed and the
result must be read out before the next block
can be processed. Before each block load, a
dedicated start command must be sent to the
coprocessor.
•
•
•
Load key
Load initialization vector (IV)
Download
encryption/decryption.
and
upload
data
for
13.12.2 Key and IV
Before a key or IV/nonce load starts, an
appropriate load key or IV/nonce command
must be issued to the coprocessor. When
loading the IV it is important to also set the
correct mode.
The key, once loaded, stays valid until a key
reload takes place.
The IV must be downloaded before the
beginning of each message (not block).
Both key and IV values are cleared by a reset
of the device.
A key load or IV load operation aborts any
processing that could be running.
13.12.3 Padding of input data
The AES coprocessor works on blocks of 128
bits. If the last block contains less than 128
bits, it must be padded with zeros when written
to the coprocessor.
13.12.4 Interface to CPU
The CPU communicates with the coprocessor
using three SFR registers:
When using DMA with AES coprosessor, two
DMA channels must be used, one for input
data and one for output data. The DMA
channels must be initialized before a start
command is written to the ENCCS. Writing a
start command generates a DMA trigger and
the transfer is started. After each block is
processed, an interrupt is generated. The
interrupt is used to issue a new start command
to the ENCCS.
• ENCCS, Encryption control and status
register
• ENCDI, Encryption input register
• ENCDO, Encryption output register
Read/write to the status register is done
directly by the CPU, while access to the
input/output registers should be performed
using direct memory access (DMA).
13.12.5 Modes of operation
When using CFB, OFB and CTR mode, the
128 bits blocks are divided into four 32 bit
blocks. 32 bits are loaded into the AES
coprocessor and the resulting 32 bits are read
out. This continues until all 128 bits have been
encrypted. The only time one has to consider
this is if data is loaded/read directly using the
CPU. When using DMA, this is handled
automatically by the DMA triggers generated
by the AES coprocessor, thus DMA is
preferred.
Both encryption and decryption are performed
similarly.
The CBC-MAC mode is a variant of the CBC
mode. When performing CBC-MAC, data is
downloaded to the coprocessor one 128 bits
block at a time, except for the last block.
Before the last block is loaded, the mode must
CC2430 Data Sheet (rev. 2.1) SWRS036F
Page 136 of 211
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