What is it?
The application key (AppKey) is an Advanced Encryption Standard (AES) 128-bit root key unique to the LoRaWAN® network’s end device.
How does it work?
Not only is the application key used by the end device but also by the join server. It helps to derive two other keys during over-the-air activation (OTAA) of devices that participate in the network.
First, this is about the Network Session Key (NwkSKey), which verifies the integrity of all data frames. Second, it supports the generation of the Application Session Key (AppSKey) used to encrypt and decrypt payload, actual data sent to or received from the server.
Another use for the application key consists in validating the integrity of join requests and join accept messages. These are exchanged during the OTAA process as part of the message integrity code (MIC), which serves to ensure whether data is not changed in transit.