Product Specs

29 CCM — AES CCM mode encryption
Page
270
key-stream
generation
encryption
MODE = DECRYP TION
AES CCM
SHORTCUT
ENDKSGEN CRYPT
AES CCM
KSGEN ENDCRYPT
RADIO
TXEN
PPI
READY
RU
P
A
H
L
RFU
EPL
MIC
CRC
END
READY START
RU: Ramp-up of RADIO
P: Preamble
A: Address
SHORTCUT
H: Header (S0)
L: Length
RFU: reserved for future use (S1)
EPL: encrypted payload
Figure 63: On-the-fly encryption using a PPI connection
29.6 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 pointer in the AES CCM must therefore point to the same memory location as the PACKETPTR
pointer in the RADIO, see Figure 64: Configuration of on-the-fly decryption on page 270.
OUTPTR
SCRATCH PTR
INPTR
&
PACKETPTR
Unencrypted packet
Encrypted packet
CCM data
structure
CNFPTR
H: Header (S0)
L: Length
RFU: reserved for future use (S1)
PL: unencrypted payload
EPL: encrypted payload
From remote
transmitter
RXEN
Figure 64: Configuration of on-the-fly decryption
In order to match the RADIO’s timing, 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 Figure 65: On-the-fly decryption using a PPI connection between the READY
event in the RADIO and the KSGEN task in the AES CCM on page 271 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.
Scratch area
H
L
RFU
PL
H
L+4
RFU
EPL
MIC
RADIO