RM0440 Rev 4 1503/2126
RM0440 AES hardware accelerator (AES)
1538
In CBC decrypt mode, like in ECB decrypt mode, the secret key must be prepared to
perform an AES decryption.
After the key preparation process, the decryption goes as follows: the first 128-bit ciphertext
block (after the swap operation) is used directly as the AES core input block I1 for decrypt
operation, using the 128-bit or 256-bit key. Its output O1 is XOR-ed with the 128-bit IVI field
(that must be identical to that used during encryption) to produce the first plaintext block P1.
The second ciphertext block is processed in the same way as the first block, except that the
I1 data from the first block is used in place of the initialization vector.
The decryption continues in this way until the last complete ciphertext block is decrypted.
If the message size is not a multiple of 128 bits, the final partial data block is decrypted in
the way explained in Section 34.4.6: AES ciphertext stealing and data padding.
For more information on data swapping, refer to Section 34.4.13: AES data registers and
data swapping.
ECB/CBC encryption sequence
The sequence of events to perform an ECB/CBC encryption (more detail in Section 34.4.4):
1. Disable the AES peripheral by clearing the EN bit of the AES_CR register.
2. Select the Mode 1 by setting to 00 the MODE[1:0] bitfield of the AES_CR register and
select ECB or CBC chaining mode by setting the CHMOD[2:0] bitfield of the AES_CR
register to 000 or 001, respectively. Data type can also be defined, using
DATATYPE[1:0] bitfield.
3. Select 128- or 256-bit key length through the KEYSIZE bit of the AES_CR register.
4. Write the AES_KEYRx registers (128 or 256 bits) with encryption key. Fill the
AES_IVRx registers with the initialization vector data if CBC mode has been selected.
5. Enable the AES peripheral by setting the EN bit of the AES_CR register.
6. Write the AES_DINR register four times to input the plaintext (MSB first), as shown in
Figure 514.
7. Wait until the CCF flag is set in the AES_SR register.
8. Read the AES_DOUTR register four times to get the ciphertext (MSB first) as shown in
Figure 514. Then clear the CCF flag by setting the CCFC bit of the AES_CR register.
9. Repeat steps 6-7-8 to process all the blocks with the same encryption key.
Figure 514. ECB/CBC encryption (Mode 1)
MS18936V3
WR
PT3
WR
PT2
WR
PT1
WR
PT0
Wait until flag CCF = 1
RD
CT3
RD
CT2
RD
CT1
RD
CT0
Input phase
4 write operations into
AES_DINR[31:0]
Computation phase
Output phase
4 read operations of
AES_DOUTR[31:0]
PT = plaintext = 4 words (PT3, … , PT0)
CT = ciphertext = 4 words (CT3, … , CT0)
MSB LSB MSB LSB
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