MRF24XA_15 Datasheet, PDF (106/258 Pages) Microchip Technology – Low-Power, 2.4 GHz ISM-Band IEEE 802.15.4™ RF

MRF24XA
4.7 Security Suites
MRF24XA provides extensive hardware support for
security suites defined in 802.15.4-2003/2006 stan-
dard. The security suites are based on the AES-128
block cipher transformation. Block ciphers are ciphers
that work on a plaintext block of a fixed length to pro-
duce a ciphertext block of the same length. Given a
particular Key (K), there is a 1-to-1 correspondence
between the Plaintext Block (P) and the Ciphertext
Block (C).
• Encryption operation: C_i = EK(P_i)
Ciphertext block (i) is produced by Encrypting
Plaintext block (i) using key K.
• Decryption operation: P_i = DK(C_i)
Plaintext block (i) is produced by Decrypting
Ciphertext block (i) using key K.
4.7.1
ELECTRONIC CODE BOOK MODE
(ECB)
The simple usage of the block cipher is known as ECB
mode. There are several issues in using a block cipher
in ECB mode to encrypt data. Each plaintext block is
encrypted to the same ciphertext block and it is possi-
ble to associate the ciphertext block with an event with-
out knowing that the plaintext block itself. To trigger the
event, user can resend the ciphertext block, a process
known as Replay Attack. In addition, most block cipher
algorithms in ECB mode do nothing to scramble repet-
itive data, making the plaintext block reversible from
the ciphertext block.
4.7.2 COUNTER MODE (CTR)
In CTR mode, each chipertext block is produced by
XOR'ing, the plaintext block with the encrypted version
of a counter input. The initial value of the counter
serves as the Initialization Vector (IV) for the message
block, and may be changed for each message block.
Although the term “counter” is used, this does not man-
date the use of a true counter. An easy-to-compute
function, which is practically non-repeating (at least for
a long time) may be used. CTR mode may be 100%
parallelized for both encryption and decryption.
802.15.4-2003 defines CTR mode as follows:
CTR₀ = 0
Loop on each 16 byte block of plaintext
C_i = P_i xor EK({ENCFLAGS, SECNONCE, CTR_i})
CTR_i+1 = CTR_i+1
End
Where, ENCFLAGS is defined by the standard (0x82),
SECNONCE is defined in the standards and CTRi is a 2
byte block counter. If the last block of the plaintext is not
16 byte, it is zero-padded and only the required number
of MSB bytes is used in the XOR operation. The host
can override ENCFLAGS field when the SECFLAGOVR
bit in MACCON1 register is enabled, and the new flags
through SECENCFLAG register is set. The host can
overwrite the SECNONCE register anytime through
SECNONCE1..13 registers.
802.15.4-2006 does not define CTR mode.
4.7.3
CIPHER BLOCK CHAINING MODE
(CBC)
In CBC mode, each plaintext block is XOR’ed with the
result of the previous block encryption operation before
being encrypted. In this way all plaintext blocks depend
on the previous block, making it difficult to remove, add
or change individual blocks without detection. In addi-
tion, the encryption for the first block is performed using
XOR of the plaintext block and an Initial Value (IV). This
initial value changes for each message, making it more
resistant to Replay Attacks.
MRF24XA does not support CBC mode.
4.7.4
CIPHER BLOCK CHAINING
MESSAGE AUTHENTICATION
CODE MODE (CBC-MAC)
A CBC-MAC protects the authenticity of a message,
and therefore implicitly its integrity. The algorithm takes
the variable length input message and a secret key,
and produces a MIC TAG (MIC and MAC terms are
often used interchangeably). Any change to the content
of the message results in a change of the MIC tag,
which guarantees the integrity of the message. As the
same message with a different secret key also pro-
duces a different MIC tag, a MAC mode also provides
protection of authenticity.
802.15.4-2003 defines CBC-MAC mode as follows:
P = {LENGTH, MACHDR, MACPAYLOAD}
O₁ = EK(P₁)
Loop on each 16 byte block of plaintext
O_i = EK(P_i xor O_{i -1})
End
MICTAG is the leftmost M bit of O_end
Where LENGTH is the number of bytes to be authenti-
cated (MAC header and payload).
802.15.4-2006 does not define CBC-MAC mode.
DS70005023C-page 106
Preliminary
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