Jump to content

Compact Disc and DVD copy protection

From Wikipedia, the free encyclopedia

This is an old revision of this page, as edited by 68.41.75.41 (talk) at 16:37, 5 August 2007 (→Illegal sub-channels: m (grammer)). The present address (URL) is a permanent link to this revision, which may differ significantly from the current revision.

CD/DVD copy protection is a set of copy protection mechanisms that prevent users from copying compact discs (CDs) or DVDs. These mechanisms vary widely and include DRM, CD-checks, Dummy Files, illegal table of contents, over-sizing or over-burning the CD, physical errors, bad sectors and more. Many of these protection schemes rely on breaking compliance with the CD and DVD standards, leading to playback problems on some devices.

All CD/DVD copy protection schemes rely on some kind of distinctive feature that

  • can be applied to a medium during the manufacturing process, making a protected medium distinguishable from an unprotected one,
    • The medium has to comply with industry-standards. Application of the protection must not harm the medium's ability of standard compliance.
  • whose presence can be checked for in the end-user environment
    • Hard- and software found in the end-user environment must be able to gather some kind of information from the medium that makes it possible to distinguish between an original/protected medium and a copied/unprotected one without impairing the medium's original purpose.
  • which can not be faked, copied and/or retroactively applied to an unprotected medium with the help of end-user soft/hardware.

Technology

Illegal filesystems / Dummy files

Most CD-ROMs use an ISO9660-filesystem to organize the available space into folders and files. Most times it is superposed by a more advanced filesystem like Joliet to circumvent some restrictions, but the ISO9660-filesystem is always present. The most basic approach for a distinctive feature is to purposely fake some information within the filesystem. Early generation of software copied every single file one by one from the original medium and re-created a new filesystem on the target medium, losing the faked/illegal information.

Illegal sectors

A sector is the top-level data structure of a CD-ROM and the only one that is accessible by software (including the OS). Each sector contains 2048 byte of user-data (content) and 304 bytes of structural information (for a MODE1 CD-ROM). Among other things the structural information consists of

Using the EDC and ECC-field, the drive can detect and repair read-errors.

Copy protections can use these fields as a distinctive feature by purposely crafting sectors with improper EDC/ECC fields during manufacturing. The protection's 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 supposed to be a copy.

Modern soft/hardware is able to reproduce raw sectors, therefore this approach does not give protection anymore.

A modification of this approach uses large regions of unreadable sectors with small islands of readable ones in between. 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's software now expects the sectors to be readable, supposing the medium to be a copy when read-errors occurs.

Illegal sub-channels

Beside the main-channel which holds all of the user-data, a CD-ROM contains a set of sub-channels where certain meta-information can be stored. One of these channels — the Q-channel — states the drive's current position relative to the beginning of the CD and the current track. This is a leftover from Audio-CDs, where this information is used to keep the drive on-track; nevertheless the Q-channel is filled even on Data-CDs.

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):

Standard CD-ROM
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:

Protected CD-ROM
Sector's logical address ... 6551 6552 6553 6553 6554 6555 6556 6557 ...
Sector's content ... Jack and Jill Mary went up the hill ...

As you can see, 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.

Note that 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.

Although protection-vendors using this technique say it's "physically impossible" to copy this kind of protection, specialized software can produce perfect clones of such CD-ROMs. However, this process is very time-consuming.

Current situation

The Red Book audio specification does not include any copy protection mechanism. Starting in early 2002, attempts were made by record companies to market "copy-protected" non-standard compact discs. Philips has stated that such discs are not permitted to bear the trademarked Compact Disc Digital Audio logo because they violate the Red Book specification. It also seems likely that Philips' new models of CD recorders will be designed to be able to record from these "protected" discs. However, there has been great public outcry over copy-protected discs because many see it as a threat to fair use. For example, audio tracks on such media cannot be easily added to a personal music collection on a computer's hard disk or a portable (non-CD) music player. Also, many ordinary CD audio players, e.g. in car radios, have problems playing copy-protected media, mostly because they use hardware and firmware components also used in CD-ROM drives. The reason for this reuse is cost efficiency.

In late 2005, Sony BMG Music sparked the Sony CD copy protection scandal when it included a form of copy protection called Extended Copy Protection ("XCP") on discs from 52 artists.[1] Upon inserting such a disc in the CD drive of a computer running Microsoft Windows, the XCP software would be installed. If CD ripper software were to subsequently access the music tracks on the CD, XCP would substitute white noise for the audio on the disc.

Technically inclined users found that XCP contains a root kit component. After installation, XCP went to great lengths to disguise its existence, and it even attempted to disable the computer's CD drive if XCP was forcibly removed. XCP's efforts to cloak itself unfortunately allowed writers of malware to amplify the damage done by their software, hiding the malware under XCP's cloak if XCP had been installed on the victim's machine. Several publishers of antivirus and anti-spyware software updated their products to detect and remove XCP if found, on the grounds that it is a trojan horse or other malware; and an assistant secretary for the United States' Department of Homeland Security chastised companies that would cause security holes on customers' computers.

Facing apparently unanimous 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]

Copy protection schemes currently in circulation

Commercial CD protections

Commercial DVD Protections

Commercial Audio CD/DVD Protections

Console CD Protections

Protected DVD Copiers

Retrieved from "https://en.wikipedia.org/enwiki/w/index.php?title=Compact_Disc_and_DVD_copy_protection&oldid=149366952"