Decryption

During packet decryption, CCM will read the encrypted packet located in memory at the address specified in the IN.PTR pointer, decrypt the packet, authenticate the packet's MAC field and generate the appropriate MAC status.

The encrypted message in (c), is decrypted and authenticated together with additional authenticated data (a) and then matched against the decrypted MAC value. The decrypted MAC value is part of (c). Bits in the first byte of the data can be masked away before calculating the MAC value by configuring the ADATAMASK register. This is useful for Bluetooth header masking. For protocols other than Bluetooth, the ADATAMASK register must be set to 0xFF for correct CCM operation; the reset value is configured to support Bluetooth.

Decryption is started by triggering the START task with the MODE register set to FastDecryption.

CCM will write the l(m) value of the decrypted packet to the location provided in OUT.PTR, and then store the decrypted packet into memory at the locations given by the OUT.PTR list as illustrated in the following figure.

For Bluetooth (MODE.PROTOCOL=Ble), CCM is only able to authenticate messages where l(c) is at least MACLEN+1 bytes long. If l(c) is less than MACLEN+1, CCM will generate an END event and clear the MACSTATUS (indicating MAC check failure). Furthermore, empty packets (l(c)=0) will be moved unmodified through the AES CCM peripheral even though ERROR event shall be generated. In any other case that leads to a failed MACSTATUS or an ERROR event, the contents of the job addresses given in OUT.PTR are undefined.

For IEEE 802.15.4 (MODE.PROTOCOL=Ieee802154), CCM will also perform authentication on messages where only ADATA is present (i.e. l(m)=0 and l(a)>0). In this case MACSTATUS reflects the result of the authentication. If l(c)<MACLEN, then the ERROR event is generated, and the contents of the locations given in OUT.PTR are undefined.