Altering file inside firmware requires new checksum

Question

I have a firmware-file for a picture frame that I want to alter. The purpose of this altering is to make the device display a custom image during boot.

I've managed to switch the existing JPEG-file with my own custom file, but it is not possible to flash the device with this new firmware-file. The device complains about an invalid checksum, which seems correct since I haven't recalculated it.

Any tips about how to go forward with this? I've tried looking for the checksum in the file, but so far I haven't had any success.

Original unaltered and custom altered firmware in one zip-file

Answer 1

There isn't a general answer to this question, the answer depends on the device, and what checksum it uses to verify the firmware.

In general a checksum is used to verify the file hasn't got damaged during transfer.

It could be that the checksum is simply a CRC or MD5 or other hash, appended to the end of the firmware, or it could be in a header somewhere.

If you are unlucky then the checksum is combined with a digital signature which would prevent you from 'correcting' the checksum for your changes.

If you are lucky then someone will be able to tell you for format for the firmware and what the checksum algorithm is.