fit_check_sig not hashing everything.

[Simon Glass]

Hi Martin,

On Fri, 8 Jul 2022 at 01:11, Martin Bonner <martingreybeard at gmail.com> wrote:
>
> On Thu, 7 Jul 2022 at 17:29, Martin Bonner <martingreybeard at 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