Encryption

During packet encryption, CCM will read the unencrypted packet located in memory at the address specified in register IN.PTR, encrypt the packet and append an M byte long message authentication code (MAC) field to the packet.

The message to authenticate and encrypt (m) and additional authenticated data (a) are included in the MAC generation. The first byte in the packet header can be masked 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.

Encryption is started by triggering the START task with the MODE register set to Encryption. The END event will be generated when packet encryption is completed.

The AES CCM will modify the l(c) output field of the packet to adjust for the appended MAC field, that is, add MODE.MACLEN bytes to l(m), and store the resulting packet back into memory at the locations specified in the OUT.PTR list, as illustrated in the following figure. The maximum length of l(m) plus MODE.MACLEN cannot exceed 65535 bytes.

AES CCM packet encryption — Figure 2. Encryption

If the following occurs, the ERROR event is generated, the CCM stops, and the ERRORSTATUS register will report the type of error that triggered the ERROR event:

The IN.PTR job list ends before reading out the complete CCM data structure
The OUT.PTR job list ends before writing out the complete encrypted CCM data structure
The CCM is not able to operate fast enough to run concurrently with the RADIO as the RADIO transmits the encrypted packet.
The EasyDMA engine encounters an error, see EasyDMA and ERROR event

Any values of l(m) and l(a) are allowed. If encrypting empty packets, i.e. l(m) = l(a) = 0, no encryption will take place; the END event is generated, and CCM operation is stopped.

For Bluetooth (MODE.PROTOCOL=Ble), valid packets with 0 payload (l(a) is larger than 0 but l(m) is 0) will not be authenticated but instead moved unmodified through the AES CCM peripheral, and thus no MAC will be generated.

For IEEE 802.15.4 (MODE.PROTOCOL=Ieee802154), valid packets with 0 payload (l(a) is larger than 0 but l(m) is 0) will be authenticated, and thus a MAC will be generated as part of the output data.