Shortcuts with AES CCM mode encryption

[Elisa Bld]

I'm using the peripheral AES CCM to encrypt frames on the nrf52832 board but I'm not using the on-the-fly encryption (at the same time as the radio).

I managed to correctly encrypt and decrypt frames but a have an event error (NRF_CCM_EVENT_ERROR) when I'm using the shortcut between the ENDKSGEN event and the CRYPT task (only with the shortcut, not by triggering the task CRYPT in the event ENDKSGEN).

Also, this error happens only with the encryption and the frames are well encrypted even with this error returned.

I'm using the encryption with a data rate of 2 Mbits and I noticed that if I configure the peripheral to 1 Mbit which is the same as the RADIO peripheral, I do not have the event error.
I assume there is a verification for the on-the-fly encryption. Is there a way to use the shortcut correctly with a different data rate configured ?


Thank you in advance

[Einar Thorsrud]

Hi,

There is no documented link between the configured data rate and the error event, and I do not see why it should matter in practice in your case when the data is not comming from the radio but is awailable in the RAM buffer before you start the crypto operation. That said, the CCM peripheral is designed specifically to work syncrhoneously with the radio and this is the primary use case.

As this is not documented I will need to check with the digital designers to know more about this behaviour. Do you have a (prefrably minimal) project that reproduce this that you can share here?