This section takes the integrity verification of firmware data during OTA upgrades as an example to introduce how data integrity verification is designed. Figure 13.5 shows that during firmware updates, integrity verification is performed before data transmission and updated firmware loading.
In the process of OTA upgrades, if HTTPS protocol is used to transmit data, the sender generates a CRC checksum for the data before transmission, and the receiver recalculates a CRC checksum from the received data, followed by verification similar to the process shown in Figure 13.4. It is worth noting that when using the HTTPS protocol to transmit data, there is no need to worry about CRC verification, as the HTTPS protocol automatically performs this verification internally.
In addition, when the device uses the firmware stored in flash, it also checks the integrity of the firmware. Every time the device is restarted to load the app firmware, it will perform integrity verification to ensure that the app firmware for loading is not damaged. This process occurs automatically and does not need manual intervention.
However, it is far from enough to rely only on integrity verification for ensuring data security. Since the mechanisms and implementations of the integrity verification algorithms are usually open-source, malicious attackers can use the same CRC verification algorithm to add a CRC checksum to a custom firmware and flash it into the device's flash, thereby passing the CRC check. To prevent such attacks, it is necessary to identify the source of the data, which involves the data legitimacy protection scheme – Secure Boot, which will be covered in detail in Section 13.4.2.