CCM — AES CCM mode encryption

A new keystream needs to be generated before a new packet encryption or packet decryption operation can start.

A keystream is generated by triggering the KSGEN task. An ENDKSGEN event is generated after the keystream has been generated.

Keystream generation, packet encryption, and packet decryption operations utilize the configuration specified in the data structure pointed to by CNFPTR. It is necessary to configure this pointer and its underlying data structure, and register MODE before the KSGEN task is triggered.

The keystream will be stored in the AES CCM peripheral’s temporary memory area, specified by the SCRATCHPTR, where it will be used in subsequent encryption and decryption operations.

For default length packets (MODE.LENGTH = Default), the size of the generated keystream is 27 bytes. When using extended length packets (MODE.LENGTH = Extended), register MAXPACKETSIZE specifies the length of the keystream to be generated. The length of the generated keystream must be greater or equal to the length of the subsequent packet payload to be encrypted or decrypted. The maximum length of the keystream in extended mode is 251 bytes, which means that the maximum packet payload size is 251.

If a shortcut is used between the ENDKSGEN event and CRYPT task, pointer INPTR and the pointers OUTPTR must also be configured before the KSGEN task is triggered.

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

Encryption is started by triggering the CRYPT task with register MODE 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, see Encryption.

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.

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

The packet header (S0) and payload are included in the MIC authentication. Bits in the packet header can be masked away by configuring register HEADERMASK.

Decryption is started by triggering the CRYPT task with register MODE 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.

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.

Empty packets (length field is set to 0) will not be decrypted but instead moved unmodified through the AES CCM peripheral. These packets will always pass the MIC check.

CCM supports different widths of the LENGTH field in the data structure for decrypted packets. This is configured in register MODE.

The CCM peripheral is able to encrypt/decrypt data synchronously to data being transmitted or received on the radio.

In order for CCM to run synchronously with the radio, the data rate setting in register MODE needs to match the radio data rate. The settings in this register apply whenever either the KSGEN or CRYPT tasks are triggered.

The data rate setting of register MODE can also be overridden on-the-fly during an ongoing encrypt/decrypt operation by the contents of register RATEOVERRIDE. The data rate setting in this register applies whenever the RATEOVERRIDE task is triggered. This feature can be useful in cases where the radio data rate is changed during an ongoing packet transaction.

When the AES CCM peripheral encrypts a packet on-the-fly while RADIO is transmitting it, RADIO must read the encrypted packet from the same memory location that the AES CCM peripheral is writing to.

The OUTPTR pointer in the AES CCM must point to the same memory location as the PACKETPTR pointer in the radio, see Configuration of on-the-fly encryption.

In order to match RADIO’s timing, the KSGEN task must be triggered early enough to allow the keystream generation to complete before packet encryption begins.

For short packets (MODE.LENGTH = Default), the KSGEN task must be triggered before or at the same time as the START task in RADIO is triggered. In addition, the shortcut between the ENDKSGEN event and the CRYPT task must be enabled. This use-case is illustrated in On-the-fly encryption of short packets (MODE.LENGTH = Default) using a PPI connection. It uses a PPI connection between the READY event in RADIO and the KSGEN task in the AES CCM peripheral.

For long packets (MODE.LENGTH = Extended), the keystream generation needs to start earlier, such as when the TXEN task in RADIO is triggered.

Refer to Timing specification for information about the time needed for generating a keystream.

When the AES CCM peripheral decrypts a packet on-the-fly while RADIO is receiving it, the AES CCM peripheral must read the encrypted packet from the same memory location that RADIO is writing to.

The INPTR pointer in the AES CCM must point to the same memory location as the PACKETPTR pointer in RADIO, see Configuration of on-the-fly decryption.

In order to match RADIO’s timing, the KSGEN task must be triggered early enough to allow the keystream 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 RADIO is triggered. In addition, the CRYPT task must be triggered no earlier than when the ADDRESS event is generated by RADIO.

If the CRYPT task is triggered exactly at the same time as the ADDRESS event is generated by RADIO, the AES CCM peripheral will guarantee that the decryption is completed no later than when the END event in RADIO is generated.

This use-case is illustrated in On-the-fly decryption of short packets (MODE.LENGTH = Default) using a PPI connection using a PPI connection between the ADDRESS event in RADIO and the CRYPT task in the AES CCM peripheral. The KSGEN task is triggered from the READY event in RADIO through a PPI connection.

For long packets (MODE.LENGTH = Extended) the keystream generation will need to start even earlier, such as when the RXEN task in RADIO is triggered.

Refer to Timing specification for information about the time needed for generating a keystream.

CCM implements an EasyDMA mechanism for reading and writing to RAM.

When the CPU and EasyDMA enabled peripherals access the same RAM block at the same time, increased bus collisions might disrupt on-the-fly encryption. This will generate an ERROR event.

EasyDMA stops accessing RAM when the ENDKSGEN and ENDCRYPT events are generated.

If the CNFPTR, SCRATCHPTR, INPTR, and the OUTPTR are not pointing to the Data RAM region, an EasyDMA transfer may result in a HardFault or RAM corruption. See Memory for more information about the different memory regions.