libsodium mess

[Kurt Roeckx]

On Thu, Jan 19, 2017 at 01:00:50PM -0800, Gary E. Miller wrote:
> Yo Kurt!
> 
> On Thu, 19 Jan 2017 21:20:23 +0100
> Kurt Roeckx <kurt at roeckx.be> wrote:
> 
> > On Thu, Jan 19, 2017 at 02:30:35PM -0500, Eric S. Raymond wrote:
> > > Gary E. Miller <gem at rellim.com>:  
> > > > > - to fuzz the low-order bits of the clock.  
> > > > 
> > > > Hmm, can you expand on this a bit?  Which clock?  How much fuzz?
> > > > Does this degrade anything?  
> > > 
> > > Whenever ntpd polls the system clock, it fuzzes the lowest-order
> > > digits of the result. The amount of fuzz to apply is bounded by
> > > half the measured interval between system clock ticks.
> > > 
> > > That shouldn't degrade anything. I presume it's a measure to foil
> > > timing attacks of some sort.  Daniel might be able to say more.  
> > 
> > Adding random (white) noise to a measurement is done to improve the
> > resolution after averaging, it's ussually in combination with
> > oversampling. Adding this white noise is done in the analog signal,
> > before you convert it to digital.
> 
> Yeah, I have seen that before.  It can be good, or it can be bad.

You of course need to know what you're actually doing for it to be
useful.

> In my GR-601W experiments I can show it would be bad.

I really have no idea what kind of experiment that was, but I
doubt that it somehow has an ADC.

> > And I guess improving the resolution was important
> > when on some systems you have a resolution in the order of 1 to 50
> > ms.
> 
> I'm worried about 1 micro Second or less.  And one should not
> confuse accuracy with resolution.  A PPS signal only has a resolution
> of one Second, but can eaaily have an accuracy of 10 nano seconds.

Please don't confuse accuracy with precision. I'm not sure your 10
nano seconds is the accuracy or precision, both are possible, but
I think you're talking about precision. The accuracy is probably
not that easy to measure.

> A signal on a USB 2.0 bus can only have a resolution of about 1 micro
> Second, but that can be locked to a PPS to 100 micro Seconds jitter.

I'm not sure what you're trying to say.

But if you're talking about jitter, you're really talking about
the precision.

> > But I'm currently not really sure that it either improves
> > things, make things worse, or has no effect at all.
> 
> Hal does not think it is in the measurement path at all.  If it is
> then it will be eauy to do some testing.

As far as I understood, the packets that go out have their
timestamp adjusted to add a random value.


Kurt