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SubjectRe: [PATCH] nvmem: sunxi-sid: SID content is not a valid source of randomness
On Mon, Oct 24, 2016 at 10:10:20PM +0200, Maxime Ripard wrote:
> On Sat, Oct 22, 2016 at 03:53:28PM +0200, Corentin Labbe wrote:
> > Since SID's content is constant over reboot,
>
> That's not true, at least not across all the Allwinner SoCs, and
> especially not on the A10 and A20 that this driver supports.
>

On my cubieboard2 (A20)
hexdump -C /sys/devices/platform/soc\@01c00000/1c23800.eeprom/sunxi-sid0/nvmem
00000000 16 51 66 83 80 48 50 72 56 54 48 48 03 c2 75 72 |.Qf..HPrVTHH..ur|
00000010 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
*
00000100 16 51 66 83 80 48 50 72 56 54 48 48 03 c2 75 72 |.Qf..HPrVTHH..ur|
00000110 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
*
00000200
cubiedev ~ # reboot
cubiedev ~ # hexdump -C /sys/devices/platform/soc\@01c00000/1c23800.eeprom/sunxi-sid0/nvmem
00000000 16 51 66 83 80 48 50 72 56 54 48 48 03 c2 75 72 |.Qf..HPrVTHH..ur|
00000010 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
*
00000100 16 51 66 83 80 48 50 72 56 54 48 48 03 c2 75 72 |.Qf..HPrVTHH..ur|
00000110 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
*
00000200

So clearly for me its constant.

> > it must not be used as source of randomness.
>
> And I don't think that's true either. A constant entropy provider will
> not add any entropy, but will not remove any, would it?

I cced linux-crypto at the begining for confirmation on that.
But the problem is increased as a part of the content is predicatable over same type of device (at least the thirst bytes and all the zeros).

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