[libdvdcss-devel] cache: use libxdg-basedir and save cache into XDG-compliant location.

[Reimar Döffinger]

On Wed, Feb 27, 2013 at 01:21:42PM +0100, Bernard Lang wrote:
> Using my drive, with no region set, I found that about 25 to 30% of
> the DVDs have a title that cannot be found by libdvdcss (though I am
> not sure whether the region setting of the drive has a responsibility
> in this).

When the region is not set, libdvdcss can't just ask the drive to
give it the keys.
Though I believe some drives cause even more trouble when the region
is not set, breaking even the fallback methods of cracking the keys -
though maybe I misremember.

> * Jean-Baptiste Kempf <jb at videolan.org>, on 27-02-13, replied:
> > Some people actually use that in dvdcss?
> 
> I do.
> 
> Actually, I often managed to get the correct keys by other means, and
> added them by hand in the cache. I confirm it is easier in text format.

In the two cases I had that the keys were identical or simply increasing
by one between the titles.

> My first question is the following. Given a title and a CSS key, is
> there a reliable way (other than viewing, which is not always
> adequate) to check that it is the proper key to decode that title.  My
> experience is that some titles will be 'decoded' (?) without fuss by
> many keys, and the bad keys will produce garbage without any
> noticeable protest from the decoding software (libdvdcss, not always
> within VLC).

If the video is long enough you should get decoding errors quite
reliably with the wrong keys. However I believe there is no really
certain way to detect if they key is correct (if there is, we definitely
should be using it in DVDCSS, I had issues where it came up with the
wrong key, though I think they were never reproducible enough to debug,
possibly related to drive read errors).

> Another question is the following. Given a coded title and a decoded
> one. Is it possible to check that the first is an encoded version of
> the second. Is it possible to identify more easily the key ? Can that
> be somewhat easier than finding the key with only the encoded version
> of the title.  I have met that situation in practice.

As far as I can see the most problematic step of the cracking is to
figure out what the decoded data looks like, see the function
AttackPattern in the code.
Since they key is only 40 bit cracking it is fairly trivial if you
know what some encrypted data should decrypt to.

> My last question concerns my DVD player.  I do not wish to change
> the (absent) region setting, as I have DVDs from several different
> regions (some films are just not published in region 2).

Setting the region should not make anything worse when it comes
to playing DVDs from a different region as far as I know.
For most drives there are also ways to either disable RPC or
at least allowing to change the region as often as you want.

> Would
> libdvdcss be able to discover more keys if the player were set to the
> region corresponding to the DVD.

In that case it should be able to easily get all keys, just like
a proper DVD player would.