NRF52820-QDAA-R datasheet(98/437 Pages) NORDIC

98 / 437 page

Peripherals

If a shortcut is used between the ENDKSGEN event and CRYPT task, pointer INPTR on page 107 and the

pointers OUTPTR on page 107 must also be configured before the KSGEN task is triggered.

6.4.2 Encryption

The AES CCM periheral is able to read an unencrypted packet, encrypt it, and append a four byte MIC field

to the packet.

During packet encryption, the AES CCM peripheral performs the following:

• Reads the unencrypted packet located in RAM address specified in the INPTR pointer

• Encrypts the packet

• Appends a four byte long Message Integrity Check (MIC) field to the packet

Encryption is started by triggering the CRYPT task with register MODE on page 106 set to ENCRYPTION.

An ENDCRYPT event is generated when packet encryption is completed.

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

field. It adds four bytes to the length and stores the resulting packet in RAM at the address specified in

pointer OUTPTR on page 107, see Encryption on page 98.

Empty packets (length field is set to

0) will not be encrypted but instead moved unmodified through the

AES CCM peripheral.

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

configured in register MODE on page 106.

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 30: Encryption

6.4.3 Decryption

The AES CCM peripheral is able to read an encrypted packet, decrypt it, authenticate the MIC field, and

generate an appropriate MIC status.

During packet decryption, the AES CCM peripheral performs the following:

• Reads the encrypted packet located in RAM at the address specified in the INPTR pointer

• Decrypts the packet

• Authenticates the packet's MIC field

• Generates the appropriate MIC status

Decryption is started by triggering the CRYPT task with register MODE on page 106 set to DECRYPTION.

An ENDCRYPT event is generated when packet decryption is completed.

The AES CCM peripheral modifies the length field of the packet to adjust for the MIC field. It subtracts

four bytes from the length and stores the decrypted packet in RAM at the address specified in the pointer

OUTPTR, see Decryption on page 99.

CCM is only able to decrypt packet payloads that are at least five bytes long (one byte or more encrypted

payload (EPL) and four bytes of MIC). CCM will therefore generate a MIC error for packets where the

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

4463_156 v1.0

NRF52820-QDAA-R Datasheet(PDF) 98 Page - Nordic Semiconductor

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