Compact Disc and DVD copy protection
CD/DVD copy protection is a blanket term for various methods of copy protection for CDs and DVDs. Such methods include DRM, CD-checks, Dummy Files, illegal tables of contents, over-sizing or over-burning the CD, physical errors and bad sectors. Many protection schemes rely on breaking compliance with CD and DVD standards, leading to playback problems on some devices.
Protection schemes rely on distinctive features that:
- can be applied to a medium during the manufacturing process, so that a protected medium is distinguishable from an unprotected one.
- cannot be faked, copied, or retroactively applied to an unprotected medium using typical hardware and software.
Technology
Filesystems / Dummy files
Most CD-ROMs use the
Sectors
A sector is the primary data structure on a CD-ROM accessible to external software (including the OS). On a Mode-1 CD-ROM, each sector contains 2048 bytes of user-data (content) and 304 bytes of structural information. Among other things, the structural information consists of
- the sector number, the sector's relative and absolute logical position
- an error detection code (EDC), which is an advanced checksum used to detect (if possible) read-errors
- an error correction code (ECC), an advanced method of detecting and correcting errors
Using the EDC and ECC information, the drive can detect and repair many (but not all) types of read-error.
Copy protections can use these fields as a distinctive feature by purposely crafting sectors with improper EDC/ECC fields during manufacture. The protection software tries to read those sectors, awaiting read-errors. As early generations of end-user soft/hardware were not able to generate sectors with illegal structural information, this feature could not be re-generated with such soft/hardware. If the sectors forming the distinctive feature have become readable, the medium is presumed to be a copy.
A modification of this approach uses large regions of unreadable sectors with small islands of readable ones interspersed. Most software trying to copy protected media will skip intervals of sectors when confronted with unreadable ones, expecting them all to be bad. In contrast to the original approach, the protection scheme expects the sectors to be readable, supposing the medium to be a copy when read-errors occur.
Sub-channels
Beside the main-channel which holds all of the user-data, a CD-ROM contains a set of eight sub-channels where certain meta-information can be stored. (For an audio CD, the user-data is the audio itself; for a data CD, it is the filesystem and file data.) One of the sub-channels — the Q-channel — states the drive's current position relative to the beginning of the CD and the current track. This was designed for Audio-CDs (which for a few years were the only CDs), where this information is used to keep the drive on track; nevertheless the Q-channel is filled even on Data-CDs. Another sub-channel, the P-channel (which is the first of the subchannels) carries even more primitive information—a sort of semaphore—indicating the points where each track starts.
As every Q-channel field contains a 16-bit checksum over its content, copy protection can yet again use this field to distinguish between an original medium and a copy. Early generations of end-user soft/hardware calculated the Q-channel by themselves, not expecting them to carry any valuable information.
Modern software and hardware are able to write any information given into the subchannels Q and P.
Twin sectors
This technique exploits the way the sectors on a CD-ROM are addressed and how the drive seeks from one sector to another. On every CD-ROM the sectors state their logical absolute and relative position in the corresponding sector-headers. The drive can use this information when it is told to retrieve or seek to a certain sector. Note that such information is not physically "hard-wired" into the CD-ROM itself but part of user-controlled data.
A part of an unprotected CD-ROM may look like this (simplified):
Sector's logical address | ... | 6551 | 6552 | 6553 | 6554 | 6555 | 6556 | 6557 | ... |
---|---|---|---|---|---|---|---|---|---|
Sector's content | ... | Jack | and | Jill | went | up | the | hill | ... |
When the drive is told to read from or seek to sector 6553, it calculates the physical distance, moves the laser-diode and starts reading from the (spinning) disc, waiting for sector 6553 to come by.
A protected CD-ROM may look like this:
Sector's logical address | ... | 6551 | 6552 | 6553 | 6553 | 6554 | 6555 | 6556 | 6557 | ... |
---|---|---|---|---|---|---|---|---|---|---|
Sector's content | ... | Jack | and | Jill | Mary | went | up | the | hill | ... |
In this example, a sector was inserted ("Mary") with a sector-address identical to the one right before the insertion-point (6553). When the drive is told to read from or seek to sector 6553 on such a disc, the resulting sector-content depends on the position the drive starts seeking from.
- If the drive has to seek forwards, the sector's original content "Jill" is returned.
- If the drive has to seek backwards, the sector's twin "Mary" is returned.
A protected program can check whether the CD-ROM is original by positioning the drive behind sector 6553 and then reading from it — expecting the Mary version to appear. When a program tries to copy such a CD-ROM, it will miss the twin-sector as the drive skips the second 6553-sector, seeking for sector 6554.
There are more details about this technique (e.g. the twin-sectors need to be recorded in large extents, the SubQ-channel has to be modified etc.) that were omitted. If the twin sectors are right next to each other as shown, the reader would always read the first one, Jill; the twin sectors need to be farther apart on the disc.
Data position measurement
Stamped CDs are perfect clones and have the data always at the same position, whereas writable media differ from each other.
Changes that followed
The Red Book
In late 2005,
Technically inclined users and computer security professionals found that XCP contains a
Facing resentment and class action lawsuits[2] Sony BMG issued a product recall for all discs including XCP, and announced it was suspending use of XCP on future discs. On November 21, 2005 the Texas Attorney General Greg Abbott sued Sony BMG for XCP[3] and on December 21, 2005 sued Sony BMG for MediaMax copy protection.[4]
United Kingdom position
The provisions of law allow for redress to buyers of Audio CDs with Copyright-Protection. The Copyright, Designs and Patents Act 1988 contains provisions in section 296ZE part VII that allow for "[a] remedy where effective technological measures prevent permitted acts".
In practice, the consumer would make a complaint to the
Schedule 5A of the Copyright, Designs and Patent Act 1988 lists the permitted acts, to which the provisions of section 296ZE apply (i.e. lists the cases in which the consumer can use the remedy, if the copy protection prevents the user doing a permitted act).
See also
References
- ^ "Archived copy". Archived from the original on 2008-12-24. Retrieved 2008-12-24.
{{cite web}}
: CS1 maint: archived copy as title (link) - ^ BBC NEWS | Technology | Sony sued over copy-protected CDs
- ^ "Texas Attorney General". Archived from the original on 2010-07-25. Retrieved 2006-07-23.
- ^ Texas Attorney General