NRF52810-QFAA-R datasheet(110/483 Pages) NORDIC

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Peripherals

RFU

EPL

MIC

CRC

READY

TXEN

END

key-stream

generation

KSGEN

encryption

ENDCRYPT

READY

START

SHORTCUT

RADIO

AES CCM

RU: Ramp-up of RADIO

P: Preamble

A: Address

H: Header (S0)

L: Length

RFU: reserved for future use (S1)

EPL: encrypted payload

CRYPT

ENDKSGEN

SHORTCUT

PPI

Figure 30: On-the-fly encryption of short packets (MODE.LENGTH = Default) using a PPI connection

6.4.6 Decrypting packets on-the-fly in radio receive mode

When the AES CCM is decrypting a packet on-the-fly at the same time as the RADIO is receiving it, the AES

CCM must read the encrypted packet from the same memory location as the RADIO is writing to.

The INPTR on page 115 pointer in the AES CCM must therefore point to the same memory location as

the PACKETPTR pointer in the RADIO, see Configuration of on-the-fly decryption on page 110.

AES CCM

RFU

EPL

L+4 RFU

Unencrypted packet

Encrypted packet

MIC

Scratch area

CCM data

structure

MODE = DECRYPTION

RADIO

RXEN

From remote

transmitter

H: Header (S0)

L: Length

RFU: reserved for future use (S1)

PL: unencrypted payload

EPL: encrypted payload

OUTPTR

INPTR

PACKETPTR

SCRATCHPTR

CNFPTR

Figure 31: Configuration of on-the-fly decryption

In order to match the RADIO’s timing, the KSGEN task must be triggered early enough to allow the key-

stream generation to complete before the decryption of the packet shall start.

For short packets (MODE.LENGTH = Default) the KSGEN task must be triggered no later than when the

START task in the RADIO is triggered. In addition, the CRYPT task must be triggered no earlier than when

the ADDRESS event is generated by the RADIO.

If the CRYPT task is triggered exactly at the same time as the ADDRESS event is generated by the RADIO,

the AES CCM will guarantee that the decryption is completed no later than when the END event in the

RADIO is generated.

This use-case is illustrated in On-the-fly decryption of short packets (MODE.LENGTH = Default) using a

PPI connection on page 111 using a PPI connection between the ADDRESS event in the RADIO and the

CRYPT task in the AES CCM. The KSGEN task is triggered from the READY event in the RADIO through a PPI

connection.

For long packets (MODE.LENGTH = Extended) the key-stream generation will need to be started even

earlier, for example at the time when the RXEN task in the RADIO is triggered.

4430_161 v1.0

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