ECB mode decryption of N blocks of ciphertext into N blocks of plaintext without CPU interaction is achieved through the use of 2 DMA channels (referred to as DMA_A and DMA_B). To implement ECB decryption follow these steps:
- Configure the AESACTL0 register to pregenerate the decryption key (last round key):
- Clear CMEN to disable block cipher mode
- Set OPx to 0x2 (decryption key generation)
- Write key into AESAKEY register as described in Section 23.2.2
- Wait for the BUSY status in the AESSTAT register to clear, indicating key generation has completed
- Configure the AESACTL0 register for block cipher decryption mode using ECB:
- Set CMEN to enable block cipher mode
- Set CMx to ECB mode
- Set OPx to 0x3 (decryption mode)
- Set AESKEYWR bit in AESSTAT (to use pregenerated key from step 1)
- Configure DMA_A channel for saving plaintext:
- Set DMA channel trigger selection to AES0 trigger
- Set DMA channel source address to AESADOUT
- Set DMA channel destination address to location where plaintext is to be stored (for example, SRAM)
- Set DMA channel transfer size to N∗4
- Set DMA channel mode to single transfer mode
- In the AES event registers, unmask DMA0 in the IMASK register of DMA_TRIG
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- Configure DMA_B channel for loading ciphertext:
- Set DMA channel trigger selection to AES1 trigger
- Set DMA channel source address to location where ciphertext is stored (for example, SRAM)
- Set DMA channel destination address to AESADIN
- Set DMA channel transfer size to N∗4
- Set DMA channel mode to single transfer mode
- In the AES event registers, unmask DMA1 in the IMASK register of DMA_TRIG
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- Configure and enable the DMA interrupt for the DMA_A channel in the DMA controller
- Start decryption by writing the block count N to BLKCNTx in the AESACTL1 register
- Wait for the DMA_A channel interrupt which indicates completion of the operation. The plaintext output will be stored in the location configured in step 3c.