Encrypting packets on-the-fly when MODE.LENGTH = Extended

[Hiihtaja]

Hello !

In PS1.7 chapter 6.4.5 Encrypting packets on-the-fly in radio transmit mode is explained encryption of a short packets (MODE.LENGTH = Default).

But when MODE.LENGTH = Extended is only clear "the keystream generation needs to start earlier, such as when the TXEN task in RADIO is triggered".

So it should/can be generated beforehand.  What kind of radio event should start CRYPT Task by using PPI ? 

RADIO->EVENTS_ADDRESS -> CCM->TASKS_CRYPT ?

I think SHORTS are also no needed.

Please clarify PPIs and SHORTs usage in case encryption of the Extended packets.

Regards,

Eugene

[Sigurd]

Hi,

RADIO->EVENTS_ADDRESS -> CCM->TASKS_CRYPT ?

Correct, PPI channel 25 is already pre-programmed with this connection. https://infocenter.nordicsemi.com/topic/ps_nrf52840/ppi.html?cp=4_0_0_5_15_0#preprogrammed_channels

[Hiihtaja]

Hi !

Looks like in case of decryption, the same channel should be used and no need nothing else

if NRF_CCM->TASKS_KSGEN = 1; is enabled beforehand

NRF_PPI->CHEN |= (1UL << NRF_PPI_CHANNEL25);

Is this so ?

Can KSGEN be  triggered even before than TASKS_TXEN/RXEN tasks ?

Or better to start it synchronously with TXEN/RXEN ? Or attach to other radio events ?

I have done KSGEN beforehand but looks like something wrong and encrypted data are not decrypted properly on RX side.

What about RFU byte ( S1) ? Looks like  if on-fly mode is enabled, it must be present.

S1LEN and S1INC in PCFN0 should be > 0

Is this so ?

Regards,

Eugene

[Sigurd]

Hi,

Maybe it would be helpful to take a look at how it's done in the Zephyr Bluetooth Controller.

https://github.com/zephyrproject-rtos/zephyr/blob/zephyr-v2.7.1/subsys/bluetooth/controller/ll_sw/nordic/hal/nrf5/radio/radio.c#L1335

https://github.com/zephyrproject-rtos/zephyr/blob/zephyr-v2.7.1/subsys/bluetooth/controller/ll_sw/nordic/hal/nrf5/radio/radio.c#L1380

[Hiihtaja]

Hi !

There I found some exact settings of MAXPACKET size :

Should MAXPACKET size be so exact  in case of RX/TX ?

Can I set to max value 251 always ?

Or it have some effect for encrypt/decrypt process ?

I think only LENGTH of packet is in use.

Regards,

Eugene