NRF52820-QDAA-R datasheet(97/437 Pages) NORDIC

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Peripherals

6.4 CCM — AES CCM mode encryption

Cipher block chaining - message authentication code (CCM) mode is an authenticated encryption

algorithm designed to provide both authentication and confidentiality during data transfer. CCM combines

counter mode encryption and CBC-MAC authentication. The CCM terminology "Message authentication

code (MAC)" is called the "Message integrity check (MIC)" in Bluetooth terminology and also in this

document.

The CCM block generates an encrypted keystream that is applied to input data using the XOR operation

and generates the four byte MIC field in one operation. CCM and RADIO can be configured to work

synchronously. CCM will encrypt in time for transmission and decrypt after receiving bytes into memory

from the radio. All operations can complete within the packet RX or TX time. CCM on this device is

implemented according to Bluetooth requirements and the algorithm as defined in IETF RFC3610, and

depends on the AES-128 block cipher. A description of the CCM algorithm can also be found in NIST

Special Publication 800-38C. The Bluetooth specification describes the configuration of counter mode

blocks and encryption blocks to implement compliant encryption for Bluetooth Low Energy.

The CCM block uses EasyDMA to load key counter mode blocks (including the nonce required), and to

read/write plain text and cipher text.

The AES CCM peripheral supports three operations: keystream generation, packet encryption, and packet

decryption. These operations are performed in compliance with the Bluetooth AES CCM 128 bit block

encryption, see Bluetooth Core specification Version 4.0.

The following figure illustrates keystream generation followed by encryption or decryption. The shortcut is

optional.

keystream

generation

KSGEN

encryption / decryption

CRYPT

ENDKSGEN

ENDCRYPT

SHORTCUT

Figure 29: Keystream generation

6.4.1 Keystream generation

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 on page 107. It is necessary to configure this

pointer and its underlying data structure, and register MODE on page 106 before the KSGEN task is

triggered.

The keystream will be stored in the AES CCM peripheral’s temporary memory area, specified by the

SCRATCHPTR on page 107, 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 on page

108 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.

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