CCM Mode
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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
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