CC1110-RTR1 datasheet(126/203 Pages) TI

126 / 203 page

CC1110

CC1110 PRELIMINARY Datasheet (Rev. 1.01) SWRS033A

Page 126 of 202

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

only

downloaded

the

coprocessor in CBC-MAC mode except for

the last block, one block at a time. 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

22.

Name

Designation

First block for authentication in CCM mode

Byte

Name

Flag

NONCE

L_M

Figure 22: Message Authentication Phase Block 0

There is no restriction on the NONCE

value. L_M is the message length in bytes.

The content of the Authentication Flag

byte is described in Figure 23.

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 23: Authentication Flag Byte

(3)

If some Additional Authentication Data

(called a later) is needed (that is A_Data

=1), the software creates the A_Data

length field, called L(a) by :

•

(3a)

If l(a)=0, (that is A_Data =0),

then L(a) is the empty string. We

note l(a) the length of a in octets.

•

(3b)

If 0 < l(a) < 2

16 - 28 , then L(a)

is the 2-octets encoding of l(a).

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