DFU firmware encryption

[Jonathan]

Hi,

I would like to implement DFU with firmware encryption. I'm aware this isn't present in SDK 12 and would like to modify the secured bootloader to implement it.

As far as I understand, a DFU package is a zip file containing a manifest, and sets of 2 files, a .dat and a .bin . Is it right that the .bin is pure image, and .dat is the header/init packet?

My plan was to encrypt the .bin file, then, in the DFU bootloader, decrypt just before writing to flash. Does this sound like a good approach?

Thanks for your help!

[Hung Bui]

I don't see any problem with this.

And you are correct, .bin file is the pure image, and .data is the init packet.

If you going to encrypt the .bin file and decrypt it after you receive it on the bootloader, you need to modify the bootloader so that hash and signature is only validated after the image is decrypted.

[Jonathan]

Ok there's just one thing I think I need to clear... When checking the image hash, what does the bootloader use? Does it compute new firmware hash by reading it from the flash? That's how I understand it. And then it seems to me I could leave .dat file intact, encrypt .bin after generating .dat, and decrypt data just before writing to the flash memory. CRC is always computed on encrypted data, SHA2 is always computed on the decrypted data. Am I wrong? I'm gonna flash again my firmware and put a breakpoint in nrf_dfu_postvalidate. Thank you guys!