[Replicant] backup stock ROM without root

[Denis 'GNUtoo' Carikli]

On Tue, 11 Sep 2018 14:40:39 +0000
Fil Lupin <fillupin at protonmail.com> wrote:

> Hi Denis,
Hi,

> Yes, of course. It seems I do not have access to modify the wiki
> pages. Waiting for those rights, those informations should probably
> go into a section into
> https://redmine.replicant.us/projects/replicant/wiki#Other
I've added wiki Access to 'Fil Lupin', so it should work now.

> Indeed, I am aware of that, in fact, this comes from 1st Clarke's law
> (https://en.wikipedia.org/wiki/Clarke%27s_three_laws). ;)
Thanks for the reference, I didn't know it.

> Actually, I did not enter into details but I precisely talked about
> that: an asymetric cryptographic signature to assure the partition is
> what is pretends to be.
This is what GPG signatures are.

MD5 was supposed to do something else, but then it has been broken, so
it cannot even be used for that something else.

> In fact, thinking about it, I conclude the same: this is essentially
> useful for research. If you want to get back to the device's original
> state, for documentation or testing, this is needed.
I do not have useful example that would require a recovery matching
exactly the installed OS. There might exist though, but I fear that the
probability of requiring the exact matching recovery and to need to know
it's exactly matching is very low.

> > If the stock OS is able to somehow check the integrity of the
> > recovery, there might be a way to find how to do it ourselves.
> 
> Galaxy S3 GT-I9300 does not seem to be able to do that.
> This is why it is so easy to change stock ROM with a custom one if I
> understood well:

> 0. recovery check that stock ROM is well signed so one has to get rid
> of recovery to change stock ROM.
As I understand this is not a strict requirement. A recovery enables
you to easily install an Android distribution. You could probably
manage to install one with only Heimdall but it would be more risky
and less convenient.
> 1. change recovery (through Heimdall/Odin) 2. then change stock ROM
> through custom recovery

Also note that if the device is not unlocked, the bootloader probably
checks the boot and recovery partitions.

Denis.
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