Hi Martin,
On Fri, 8 Jul 2022 at 01:11, Martin Bonner martingreybeard@gmail.com wrote:
On Thu, 7 Jul 2022 at 17:29, Martin Bonner martingreybeard@gmail.com wrote:
I have a 30MB FIT image as input, and I have added some debug to hash_calculate in rsa-checksum.c to print the amount of data being hashed. The answer is a rather scary "1106 bytes"! ...
Can anyone clarify what is happening?
Never mind. I have found fit_image_check_hash in image-fit.c (yay for gdb read watchpoints!) So the algorithm is basically "verify that the hashes of each image is correct", then calculate a hash which includes the hashes of the images (but not their data), and sign that. (I think it's overcomplicated, and complexity is the enemy of security - but it's much too late to change that.)
Some reasons: - it is faster to hash things only once (i.e. use the image hash we already have) - It is faster to hash smaller things (i.e. the meta data)
This of this as a tree of hashes...
Regards, Simon