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Re: Please add lzip support in the repository




Henrique de Moraes Holschuh writes ("Re: Please add lzip support in the repository"):
> On Fri, 16 Jun 2017, Adrian Bunk wrote:
> > On Thu, Jun 15, 2017 at 08:36:48PM -0300, Henrique de Moraes Holschuh wrote:
> > >...
> > > We pretty much need Debian packages to be 100% correct in the first
> > > place, they are not going to be subject to lossy recovery from
> > > corruption (which is where lzip is supposed to be much better than xz):
> > > we need to replace any that is even slightly corrupt with a fully
> > > correct copy.
> > > 
> > > So, it would make more sense to have a par2 (or create a modern version
> > > of it, actually) ECC layer on top of the compression layer, at which
> > > point we can use one of the already supported compression formats.
> > >...
> > 
> > A digital signature is an ECC layer.
> 
> ECC as in eliptic-curve crypto?  That's useless for repair.
> 
> It should have been obvious by context, especially since I even
> mentioned "par2", but it was ECC as in Error-Correcting Code.
> 
> https://en.wikipedia.org/wiki/Error-correcting_code

Adrian's point is correct.  He meant ECC as in error-correcting code.
Of course most digital signature schemes, including the ones we use,
are not able to correct errors.  But they are very good at detecting
them.

I think corrupted downloads are rare enough (due to ECC applied at
lower layers in networks and storage media) that the additional
complication of ECC would not be valuable.

And, our current practice is to sign the compressed streams.  That
means that a corrupted download would be rejected by our digital
signature mechanism, before any compression-layer ECC would see it.

We don't want to reverse the order, and do the compression after
signature, for a number of reasons.  One very useful benefit of the
digital signature scheme is that it means the decompressor used during
package download (which typically runs with rather too much
privilege) is not exposed to potentially hostile input.  Another is
that the signatures can be verified with standard tools.

For Debian binary and source packages, there is no benefit in ECC
in the compression layer.

I'm not sure why all of this isn't obvious.

As an aside: I am sceptical of the value of ECC as part of a general
purpose compression scheme.  That proponents of lzip are pushing this
as an advantage merely adds to the scepticism which their marketing
campaign is generating in my mind.

Ian.