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CCM Mode
Name
Designation
A0
First CTR Value for CCM Mode
Byte
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
Name
Flag
Nonce
CTR
Figure 15-3. Message Encryption Phase Block
Name
Designation
FLAG/A0
Encryption Flag Field for CCM Mode
Bit
7
6
5
4
3
2
1
0
Name
Reserved
—
L
–
1
Value
0
0
0
0
0
1
0
1
Figure 15-4. Encryption Flag Byte
8. The software loads A0 by selecting a Load IV/nonce command. To do so, it sets the mode to CFB or
OFB at the same time it selects the Load IV/nonce command.
9. The software calls a CFB or an OFB encryption on the authenticated data T. The uploaded buffer
contents stay unchanged (M = 16), or only its first M bytes stay unchanged, the others being set to 0
(M
–
16). The result is U, which is used later.
10. The software calls a CTR-mode encryption immediately on the still-padded message blocks. It must
reload the IV when the CTR value is any value but zero.
11. The encrypted authentication data U is appended to the encrypted message. This gives the final
result, c.
Result C = encrypted message(m) + U
156
AES Coprocessor
SWRU191C
–
April 2009
–
Revised January 2012
Copyright
©
2009
–
2012, Texas Instruments Incorporated