Hiding data on 3.5" using "40 track mode"

[Peter Gutmann]

Ben Laurie <ben at algroup.co.uk> writes:
>Dave Howe wrote:
>>Oh - before I forget, I was thinking about covert channels and cds a few days
>>ago and realised there is already one - CDs support a special mode called "CD+G"
>>- this is used making "karaoke" cds to support the video data stream; the vast
>>majority of pc drives cannot read this data - there are exceptions of course.
>>however, karaoke players (and many low-end dvd players) CAN, and by design
>>display them on the screen of the playback device.
>>I used to write CD device drivers (ancient history, I wrote, with my brother,
>one of the first CD rippers ever: CD-GRAB). As I remember it, most drives
>_can_ read this extra data, but drivers often don't support it.

I recently had to check out the low-level capabilities of a range of CD/DVD
writers for reasons too tedious to mention, CD+G read support is at best about
50:50.  I used cdrinfo.com to get the data, they use Nero's InfoTool to ask
the drive for capabilities rather than relying on manufacturer specs, since
they can change between printing the data sheets and shipping the drive, and
across different firmware versions.  CD+G read support is treated as a drive
firmware/hardware characteristic since the drives that don't support it simply
won't send the streams to the host system even if they can read them.  Beyond
that, your software driver also has to support it.

You can get assorted software to write CD+G, but it's mostly targeted at the
DIY Karaoke market and not for use as subliminal channels (I believe Nero can
write CD+G, but haven't tried it yet - if it's like the other CD+G progs,
it'll only allow insertion of song text and graphics).  In addition you only
get 4 lots of 16 bytes per 2352-byte sector, and can then use 6 bits of each
byte for data.

>Note that CD data is actually unreliable when used for audio - I definitely
>saw the same CD show bitwise differences on successive reads, much to my
>surprise, so if you want to do this, remember to include error correction :-)

The cdrinfo folks really thrash the drives they test, including running
through a pile of special disks with artificial errors and other problems.
It's a good source of info on which drives handle errors best... of course
then you could end up creating CDs that are only readable on one drive type,
or even one drive.

Peter.

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