8 years ago.

first find AN11782 and read the code and pdf's. It is not what you need, but it is a good starting place. OTA updates always have the ability to brick the device because of partial updates due to connectivity or power cycle. it is best not to update flash in place. If the process screws up, there is no fall back. Best practices would suggest that you save your update data stream in an external SPI or I2C EEPROM or your NAND flash. Then You can checksum and/or do decryption on the image.

Then boot the LPC824. The boot loader reads the image and sees a valid update signature, checsum, whatever.... If no signature, it just runs old firmware. If signature, then it reads the image blocks and writes LPC824 flash blocks. Bootloader Code should verify the flash is correct, and it then marks the signature as downloaded. you can have several images in the EEPROM/NAND, so you can go back a version if the current update is not good.

just my $0.02 after having done OTA updates before.