CC2430-RTR1-F32 datasheet(136/234 Pages) TI

136 / 234 page

CC2430

CC2430 PRELIMINARY Data Sheet (rev. 1.03) SWRS036A

Page 136 of 232

the mode must be changed to CBC. The

last block is then downloaded and the

block uploaded will be the MAC value.

CCM is a combination of CBC-MAC and

CTR. Parts of the CCM must therefore be

done in software. The following section

gives a short explanation of the necessary

steps to be done.

13.9.5.1 CBC-MAC

When performing CBC-MAC encryption,

data is downloaded to the coprocessor in

CBC-MAC mode one block at a time,

except for the last block. Before the last

block is loaded, the mode is changed to

CBC. The last block is downloaded and

the block uploaded is the message MAC.

CBC-MAC

decryption

similar

encryption. The message MAC uploaded

must be compared with the MAC to be

verified.

13.9.5.2 CCM mode

To encrypt a message under CCM mode,

the following sequence can be conducted

(key is already loaded):

Message Authentication Phase

This phase takes place during steps 1-6

shown in the following.

(1)

The software loads the IV with zeros.

(2)

The software creates the block B0.

The layout of block B0 is shown in Figure

28.

Name

Designation

First block for authentication in CCM mode

Byte

Name

Flag

NONCE

L_M

Figure 28: Message Authentication Phase Block 0

There is no restriction on the NONCE

value. L_M is the message length in bytes.

For 802.15.4 the NONCE is 13 bytes and

L_M is 2 bytes.

The content of the Authentication Flag

byte is described in Figure 29.

L is set to 6 in this example. So, L-1 is set

to 5. M and A_Data can be set to any

value.

Name

FLAG/B0

Designation

Authentication Flag Field for CCM mode

Bit

Name

Reserved

A_Data

(M-2)/2

L-1

Value

Figure 29: Authentication Flag Byte

(3)

If some Additional Authentication Data

(denoted a below) is needed (that is

A_Data =1), the software creates the

A_Data length field, called L(a) by :

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