From a technical standpoint, it would seem impossible to completely prevent all users from making copies of such media as CDs, DVDs, videotapes, computer software discs, or video game discs. The basic technical fact is that all these types of media require a "player" -- a CD player, DVD player, videotape player, computer, or video game console, in these five examples. The player has to be able to read the media in order to display it to a human. In turn, then, logically, a player could be built that first reads the media, and then writes out an exact copy of what was read, whether to the same type of media that was read, or to some other format, such as a file on a hard disk.
Information goods which are downloaded (rather than embedded in physical media) can be "protected" more effectively. They can be encrypted in a fashion which is unique for each user's computer, and the decryption system can be made tamper resistant (see also traitor tracing).
At a minimum, digital copy protection is subject to the analog hole: regardless of the digital protections, if music can be played on speakers, it can also be recorded. Copying text in this way is more tedious, but if it can be printed or displayed, it can also be scanned and OCRed.
Since this basic technical fact exists, copy protection is not intended to stop professional piracy, in which well-funded teams work to create copies of media, but rather to stop casual copying in which one friend makes a copy of a disc for another friend and thus (arguably) decreases the possible market for that disc by 1 copy.
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2 Copy protection for computer software 3 Copy protection for audio CDs 4 Copy protection in recent digital media |
Copy protection has been attempted in many ways, long before computers and digital media entered the picture. For example, the ancient practice of watermarking is an attempt to, if not prevent a copy, at least prove the authenticity of the original. The music industry in particular has long sought a reliable copy prevention method - early attempts included adding a high frequency spoiler signal to an analogue recording so that tape recorders would generate an unpleasant whistle when the spoiler heterodyned with the bias oscillator. These attempts were largely unsuccessful since the spoiler was either audible to the listener, or else so high that it would not be reproduced reliably when played back. Videotape manufacturers had more success, with companies like Macrovision inventing clever schemes that would make copies unusable if they were created with a normal VCR, and licensing this technology to videotape manufacturers.
Some modern forms of copy protection are invisible to the end-user, such as CD subchannel data or other protection mechanisms such as SafeDisc which only become apparent once an attempt to copy is made.
Copy protection for early home computer software, especially for games, started a long cat-and-mouse struggle between publishers and crackers, programmers who as a hobby would defeat copy protection on software, often add their alias to the title screen, and then distribute the cracked product to the network of BBS warez sites that specialized in distributing pirated software.
Software copy protection schemes for early computers such as the Apple II and Commodore 64 computers depended on precise knowledge of what exactly would happen if that hardware were forced to do something unusual, such as to read a disk sector that was unformatted, or to take just a few microseconds longer than necessary when instructing the floppy disk drive arm motor to move. This sort of physical copy protection continues today on software shipped on CD-ROM, with companies like Macrovision and Sony providing copy protection schemes that work by writing data to places on the CD-ROM where a CD-R drive cannot normally write. Such a scheme has been used for the Sony Playstation and cannot be circumvented easily without the use of a modchip.
For software publishers, a less expensive method of copy protection is to write the software so that it requires some evidence from the user that they have actually purchased the software, usually by asking a question that only a user with a software manual could answer (for example, "what is the 4th word on the 6th line of page 37?"). This approach can be defeated by users who have the patience to copy the manual with a photocopier, and it also suffers from BTO vulnerability, so that once pirates circumvent the copy protection on a piece of software, the resulting pirated product is more convenient than the original software, creating a disincentive to buying an original. As a result, user-interactive copy protection of this kind has mostly disappeared.
Other software copy protection techniques include:
Starting in 2000, record publishers started to sell CDs with various copy protection schemes. Most of these are play protections that aim to make the CD unusable in devices that can also be conveniently used for duplicating (i.e. CD-ROM drives in computers), leaving only dedicated audio CD players for playback.
This is achieved by assuming certain feature levels in the drives: The CD Digital Audio is the oldest CD standard and forms the basic feature set beyond which dedicated audio players need no knowledge. CD-ROM drives additionally need to support mixed mode CDs (combined audio and data tracks) and multisession CDs (multiple data recordings each superseding and incorporating data of the previous session).
The play protections in use intentionally deviate from the standards and intentionally include malformed multisession data or similar with the purpose of confusing the CD-ROM drives to prevent correct function. Simple dedicated audio CD players would not be affected by the malformed data since these are for features they don't support (for example, an audio player will not even look for a second session).
In practice, results vary wildly. CD-ROM drives may be able to correct the malformed data and still play them to an extent that depends on the make and version of the drive. On the other hand, some audio players may be built around drives with more than the basic intelligence required for audio playback. Especially car radios with CD playback, portable CD players, CD players with additional support for data CDs containing MP3 files and DVD players are likely to be problematic.
The deviation from the Red Book standard that defines audio CDs required the publishers of CDs protected in such a manner to refrain from using the official CDDA logo on the discs or the cases. The logo is a trademark owned by Philips and licensed to identify compliant audio discs only. To prevent dissatisfied customers from returning CDs which were misrepresented as compliant audio CDs, such protected CDs also started to carry prominent notices on their covers.
More recently, publishers of music and movies in digital form have turned to encryption to make copying more difficult. (CSS is a particularly famous example of this.) Technically, CSS isn't a form of copy protection, since encryption doesn't do anything to make it harder to make playable copies. With this technique, the work is encrypted using a key only known to authorized players, which only allow "legitimate" uses of the work (usually restricted forms of playback, but no conversions or modification). The Digital Millennium Copyright Act provides a legal protection for this in the US, making it illegal to distribute unauthorized players -- which was supposed to eliminate the possibility of building a DVD copier. However, DeCSS and other such software-based solutions have been reverse engineered, providng access to the encryption keys and methods. The cat and mouse struggle continues...
See also: Digital rights management, broadcast flagCopy protection on various media
Copy protection for computer software
The two latter methods imply tying the software installation to a specific machine by noting some particular unique feature of the machine. Some machines have a serial number in ROM, others do not, and so some other metric, such as the date and time (to the second) of initialisation of the hard disk can be used. On machines with Ethernet cards, the MAC address, which is unique and factory-assigned, is a popular surrogate for a machine serial number (however, this address is programmable on modern cards). The problem with these sorts of schemes are that they can cause problems for a validly licensed user who upgrades to a new machine, or reinstalls the software having reinitialised the disk. Like other software, copy-protection software not infrequently contains bugs, whose effect may be to deny access to validly licensed users. As with all similar schemes, they are often easy to crack, and the resulting cracked software is perceived as being more valuable than the uncracked version.Copy protection for audio CDs
Copy protection in recent digital media