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