NRF52811-QFAA-R7 datasheet(86/444 Pages) NORDIC

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Peripherals

The key-stream will be stored in the AES CCM’s temporary memory area, specified by the SCRATCHPTR on

page 95, where it will be used in subsequent encryption and decryption operations.

For default length packets (MODE.LENGTH = Default) the size of the generated key-stream is 27 bytes.

When using extended length packets (MODE.LENGTH = Extended) the MAXPACKETSIZE on page 96

must be greater or equal to the length of the subsequent packet payload to be encrypted or decrypted.

The maximum length of the key-stream in extended mode is 251 bytes, which means that the maximum

packet payload size is 251.

If a shortcut is used between ENDKSGEN event and CRYPT task, the INPTR on page 95 pointer and the

OUTPTR on page 95 pointers must also be configured before the KSGEN task is triggered.

6.4.2 Encryption

During packet encryption, the AES CCM will read the unencrypted packet located in RAM at the address

specified in the INPTR pointer, encrypt the packet and append a four byte long Message Integrity Check

(MIC) field to the packet.

Encryption is started by triggering the CRYPT task with the MODE on page 94 register set to

ENCRYPTION. An ENDCRYPT event will be generated when packet encryption is completed

The AES CCM will also modify the length field of the packet to adjust for the appended MIC field, that is,

add four bytes to the length, and store the resulting packet back into RAM at the address specified in the

OUTPTR on page 95 pointer, see Encryption on page 86.

Empty packets (length field is set to 0) will not be encrypted but instead moved unmodified through the

AES CCM.

The CCM supports different widths of the LENGTH field in the data structure for encrypted packets. This is

configured in the MODE on page 94 register.

AES CCM

RFU

EPL

L+4 RFU

Unencrypted packet

Encrypted packet

MIC

Scratch area

CCM data

structure

MODE = ENCRYPTION

H: Header (S0)

L: Length

RFU: reserved for future use (S1)

PL: unencrypted payload

EPL: encrypted payload

INPTR

OUTPTR

SCRATCHPTR

CNFPTR

Figure 27: Encryption

6.4.3 Decryption

During packet decryption, the AES CCM will read the encrypted packet located in RAM at the address

specified in the INPTR pointer, decrypt the packet, authenticate the packet’s MIC field and generate the

appropriate MIC status.

Decryption is started by triggering the CRYPT task with the MODE on page 94 register set to

DECRYPTION. An ENDCRYPT event will be generated when packet decryption is completed

The AES CCM will also modify the length field of the packet to adjust for the MIC field, that is, subtract

four bytes from the length, and then store the decrypted packet into RAM at the address pointed to by the

OUTPTR pointer, see Decryption on page 87.

The CCM is only able to decrypt packet payloads that are at least 5 bytes long, that is, 1 byte or more

encrypted payload (EPL) and 4 bytes of MIC. The CCM will therefore generate a MIC error for packets

where the length field is set to 1, 2, 3 or 4.

Empty packets (length field is set to 0) will not be decrypted but instead moved unmodified through the

AES CCM, these packets will always pass the MIC check.

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