Figure 184. Mode 3: Decryption With 128-Bit Key Length - ST STM32L4x6 Reference Manual

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25.9.3 Mode 3: decryption
1. Disable the AES by resetting the EN bit in the AES_CR register.
2. Configure mode 3 by programming MODE[1:0] = 10 in the AES_CR register and select
which type of chaining mode needs to be performed by programming the CHMOD[2:0]
bits.
3. Select Key length 128-bit or 256-bit via KEYSIZE bits configuration in AES_CR
register.
4. Write the AES_KEYRx registers with the decryption key (this step can be bypassed if
the derivation key is already stored in the AES_KEYRx registers using mode 2: key
derivation). Write the AES_IVRx registers if CTR, CBC or GCM mode is selected. For
ECB mode, the AES_IVRx registers are not used.
5. Enable the AES by setting the EN bit in the AES_CR register.
6. Write the AES_DINR register 4 times to input the cipher text (MSB first) as shown in
Figure 184: Mode 3: decryption with 128-bit key
7. Wait until the CCF flag is set in the AES_SR register.
8. Read the AES_DOUTR register 4 times to get the plain text (MSB first) as shown in
Figure 184: Mode 3: decryption with 128-bit key
9. Repeat steps 6, 7, 8 to process all the blocks using the same derivation key stored in
the AES_KEYRx registers.
25.9.4 Mode 4: key derivation and decryption
1. Disable the AES by resetting the EN bit in the AES_CR register.
2. Configure mode 4 by programming MODE[1:0] = 11 in the AES_CR register. This mode
is forbidden when AES is configured in CTR, GCM, GMAC or CMAC mode. It will be
Advanced encryption standard hardware accelerator (AES)

Figure 184. Mode 3: decryption with 128-bit key length

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