8.2.1 Bluetooth Low Energy OTA DFU Image File Definition

The OTAU binary export file is distributed as part of the regular firmware release package. The OTAU file is segmented as an OTAU header and Flash image. The Bluetooth Low Energy OTAU header contains the OTAU file information necessary for the DFU client (mobile application) to perform the OTAU DFU procedure, and it is not transferred OTA. Whereas, the Flash image is divided as metadata and executable firmware. The metadata contains information about the manufacturer, firmware version, firmware size, firmware signature and so on.

Unlike the OTAU header, the Flash image is encrypted to avoid the risk of exposing the plain text to mobile applications and the Cloud. The Flash image is encrypted using the AES CBC engine using a private key and AES key and is, later, decrypted at the device side when the image is received OTA.
Figure 8-2. OTA DFU Image File Definition
  • Flash image – Meta-data Header + Executable Firmware. This is full image content that is programmed in the device Flash.
  • Meta-data header – Flash image that has a metadata header, metadata payload and metadata footer that gives the Bootloader firmware information about the location of the firmware image, security decryption information, signature, sequence number and more. Digital signatures ensure the authenticity of the image and integrity of the data in the image. A digital signature also ensures that the data within the image was not modified (preserving integrity) and is intact as it was generated at the source.
  • OTAU file encryption – The executable firmware can be encrypted. Encrypting the image ensures the confidentiality of the data. This makes it so no unauthorized parties can peek at the contents of the image. Only the end device can decrypt the image. The method in use is the AES128-CBC encryption. The OTAU header is not encrypted.
  • Bluetooth Low Energy OTAU header – OTAU file information for Bluetooth Low Energy OTA DFU Client (for example, mobile application) to perform OTA DFU procedures.