7.2.4.9.2 SSL Padding

Auto Padding is enabled by writing a ‘1’ to AES_EMR.APEN and SSL padding mode is selected by writing a ‘1’ to AES_EMR.APM.

Figure 7-12. SSL Padding

Each byte of the padding area contains the padding length.

The padding length is configured in AES_EMR.PADLEN.

AES_BCNT.BCNT defines the length, in bytes, of the message to process. It must be configured before writing the first data in AES_IDATARx and the remaining bytes to process can be read at anytime (BCNT value is decremented after each AES_IDATARx access).

AES_BCNT.BCNT and AES_EMR.PADLEN must be configured so that the length of the message plus the length of the padding section is a multiple of the AES block size (128 bits).

To process a complete SSL message, the sequence is as follows:

  1. Set AES_MR.OPMOD to either CBC or CTR mode.
  2. Set AES_EMR.APEN to ‘1’, AES_EMR.APM to ‘1’, AES_EMR.PADLEN to the desired padding length in bytes.
  3. Set AES_BCNT.BCNT with the whole message length, without padding, in bytes.
  4. Write the key.
  5. Set AES_IVRx.IV if needed.
  6. Fill AES_IDATARx.IDATA with the message to process according to the SMOD configuration used. On the last data block write only what is necessary (e.g., write only AES_IDATAR0 if last block size is ≤ 32 bits).
  7. Wait for the DATRDY flag to be raised, meaning auto-padding completion and last block processing.