Lessons from the Sony CD DRM Episode
J. Alex Halderman and Edward W. Felten
Center for Information Technology Policy
Department of Computer Science
Princeton University
Extended Version
February 14, 2006
Abstract
In the fall of 2005, problems discovered in two Sony-BMG compact disc copy protection systems,
XCP and MediaMax, triggered a public uproar that ultimately led to class-action litigation and the recall
of millions of discs. We present an in-depth analysis of these technologies, including their design, im-
plementation, and deployment. The systems are surprisingly complex and suffer from a diverse array of
flaws that weaken their content protection and expose users to serious security and privacy risks. Their
complexity, and their failure, makes them an interesting case study of digital rights management that
carries valuable lessons for content companies, DRM vendors, policymakers, end users, and the security
community.
1
Introduction
This paper is a case study of the design, implementation, and deployment of anti-copying technologies.
We present a detailed technical analysis of the security and privacy implications of two systems, XCP and
MediaMax, which were developed by separate companies (First4Internet and SunnComm, respectively) and
shipped on millions of music compact discs by Sony-BMG, the world's second largest record company. We
consider the design choices the companies faced, examine the choices they made, and weigh the conse-
quences of those choices. The lessons that emerge are valuable not only for compact disc copy protection,
but for copy protection systems in general.
Before describing the technology in detail, we will first recap the public events that brought the issue
to prominence in the fall of 2005. This is necessarily a brief account that leaves out many details, some of
which will appear later in the paper as we discuss each part of the technology. For a fuller account, see [10].
The security and privacy implications of Sony-BMG's CD digital rights management (DRM) technolo-
gies first reached the public eye on October 31, 2005, in a blog post by Mark Russinovich [29]. While
testing a rootkit detector he had co-written, Russinovich was surprised to find an apparent rootkit (software
designed to hide an intruder's presence [16]) on one of his systems. Investigating, he found that the rootkit
was part of a CD DRM system called XCP that had been installed when he inserted a Sony-BMG music CD
into his computer's CD drive.
{jhalderm, felten}@cs.princeton.edu
Revision 1, February 15, 2006
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News of Russinovich's discovery circulated rapidly on the Internet, and further revelations soon fol-
lowed, from us1, from Russinovich, and from others. It was discovered that the XCP rootkit makes users'
systems more vulnerable to attacks, that both CD DRM schemes install risky software components without
obtaining informed consent from users, that both systems covertly transmit usage information back to the
vendor or the music label, and that none of the protected discs include tools for uninstalling the software.
(For these reasons, both XCP and MediaMax seem to meet the consensus definition of spyware.) These and
other findings outraged many users.
As the story was picked up by the popular press and public pressure built, Sony-BMG agreed to recall
XCP discs from stores and to issue uninstallers for both XCP and MediaMax, but we discovered that both
uninstallers created serious security holes on users' systems. Class action lawsuits were filed soon after, and
government investigations were launched, as Sony-BMG worked to repair relations with its customers.
While Sony-BMG and its DRM vendors were at the center of this incident, its implications go beyond
Sony-BMG and beyond compact discs. Viewed in context, it is a case study in the deployment of DRM into
a mature market for recorded media. Many of the lessons of CD DRM apply to other DRM markets as well.
Several themes emerge from this case study: similarities between DRM and malicious software such as
spyware, the temptation of DRM vendors to adopt malware tactics, the tendency of DRM to erode privacy,
the strategic use of access control to control markets, the failure of ad hoc designs, and the force of differing
incentives in shaping behavior and causing conflict.
Outline
The remainder of the paper is structured as follows. Section 2 discusses the business incentives
of record labels and DRM vendors, which drive their technology decisions. Section 3 gives a high-level
technical summary of the systems' design. Sections 4­9 each cover one aspect of the design in more detail,
discussing the design choices made in XCP and MediaMax and considering alternative designs. We discuss
weaknesses in the copy protection schemes themselves, as well as vulnerabilities they introduce in users'
systems. We cover installation issues in Section 4, recognition of protected discs in Section 5, player soft-
ware in Section 6, deactivation attacks in Section 7, uninstallation issues in Section 8, and compatibility and
upgrading issues in Section 9. Section 10 concludes and draws lessons for other systems.
2
Goals and Incentives
The goals of a CD DRM system are purely economic: the system is designed to protect and enable the busi-
ness models of the record label and the DRM vendor. Accordingly, any discussion of goals and incentives
must begin and end by talking about business models. The record label and the DRM vendor are separate
actors whose interests are not always aligned. We will see that incentive gaps between the label and the
DRM vendor can be important in explaining the design and deployment of CD DRM systems.
2.1
Record Label Goals
We first examine the record label's goals. Though the label would like to keep the music from the CD from
being made available on peer-to-peer (P2P) file sharing networks, this goal is not feasible [4]. If even one
user can rip an unprotected copy of the music and put it on a P2P network, it will be available to the whole
world. In practice, every commercially valuable song appears on P2P networks immediately upon release,
1As news of the rootkit spread, we added to the public discussion with a series of 27 blog posts analyzing XCP and MediaMax.
This paper provides a more systematic analysis, along with much new information. Our original blog entries can be read at
http://www.freedom-to-tinker.com/?cat=30&m=2005.
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if not sooner. No CD DRM system can hope to stop this. Real systems do not appear designed to stop P2P
sharing, but seem aimed at other goals.2
The record label's goal must therefore be to retard disc-to-disc copying and other local copying and use
of the music. Stopping local copying might increase sales of the music--if Alice cannot copy a CD to give
to Bob, Bob might buy the CD himself.
Control over local uses can translate into more revenue for the record label. For example, if the label can
control Alice's ability to download music from a CD into her iPod, the label might be able to charge Alice
an extra fee for iPod downloads. Charging for iPod downloads creates new revenue, but it also reduces the
value to users of the original CD and therefore reduces revenue from CD sales. Whether the new revenue
will outweigh the loss of CD revenue is a complex economic question whose answer depends on detailed
assumptions about users' preferences; generally, increasing the label's control over uses of the music will
tend to increase the label's profit.
Whether the label would find it more profitable to control a use, as opposed to granting it for free to
CD purchasers, is a separate question from whether copyright law gives the label the right to file lawsuits
relating to that use. Using DRM to enforce copyright law exactly as written is almost certainly not the record
label's profit-maximizing strategy.
Besides controlling use of the music, CD DRM can make money for the record label because it puts
software onto users' computers, and the label can monetize this installed platform. For example, each CD
DRM album includes a special application for listening to the protected music. This application can show
advertisements or create other promotional value for the label; or the platform can gather information about
the user's activities, which can be exploited for some business purpose. If taken too far, these become
spyware tactics; but they may be pursued more moderately, even over user objections, if the label believes
the benefits outweigh the costs.
2.2
DRM Vendor Goals
The CD DRM vendor's primary goal is to create value for the record label in order to maximize the price
the label will pay for the DRM technology. In this respect, the vendor's and label's incentives are aligned.
However, the vendor's incentives diverge from the label's in at least two ways. First, the vendor has a
higher risk tolerance than the label, because the label is a large, established business with a valuable brand
name, while the vendor (at least in the cases at issue here) is a start-up company with few assets and not
much brand equity. Start-ups face many risks already and are therefore less averse to taking on one more
risk. The record label, on the other hand, has much more capital and brand equity to lose if something goes
horribly wrong. Accordingly, we can expect the vendor to be much more willing to accept security risks
than the label.
The second incentive difference is that the vendor can monetize the installed platform in ways the record
label cannot. For example, once the vendor's DRM software is installed on a user's system, the software can
control use of other labels' CDs, so a larger installed base makes the vendor's technology more attractive
to other labels. This extra incentive to build the installed base will make the vendor more aggressive about
pushing the software onto users' computers than the label would be.
In short, incentive differences make the vendor more likely than the label to (a) cut corners and accept
security and reliability risks, and (b) push DRM software onto more user's computers. If the label had
perfect knowledge about the vendor's technology, this incentive gap would not be an issue--the label would
simply insist that the vendor protect the label's interests. But if, as seems likely in practice, the label has
imperfect knowledge of the technology, then the vendor will sometimes act against the label's interests.
2Music industry rhetoric about DRM often focuses on P2P, and some in the industry probably still think that DRM can stop P2P
sharing. We believe that industry decision makers know otherwise. The design of the systems we studied supports this view.
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2.3
DRM and Market Power
DRM affects more than just the relationships among the label, the vendor, and the user. It also impacts the
label's and vendor's positions in their industries, in ways that will shape the companies' DRM strategies.
For example, DRM vendors are in a kind of standards war--a company that controls DRM standards
has power to shape the online music business. DRM vendors fight this battle by spreading their platforms
widely. Record labels want to play DRM vendors off against each other and prevent any one vendor from
achieving dominance.
Major record companies such as Sony-BMG are parts of larger, diversified companies, and can be ex-
pected to help bolster the competitive position of their corporate siblings. For example, parts of Sony sell
portable music players in competition with Apple, so Sony-BMG has an incentive to take steps to undermine
Apple's market power.
Having examined the goals and motivations of the record labels and DRM vendors, we now turn to a
description of the technologies they deployed.
3
CD DRM Systems
CD DRM systems must meet difficult requirements. Copy protected discs must be reasonably compliant
with the CD Digital Audio standard so that they can play in ordinary CD players. They must be unreadable
by almost all computer programs in order to prevent copying, yet the DRM vendor's own software must be
able to read them in order to give the user controlled access to the music.
Most CD DRM systems use both passive and active anti-copying measures. Passive measures change
the disc's contents in the hope of confusing most computer drives and software, without confusing most
audio CD players. Active measures, in contrast, rely on software on the computer that actively intervenes to
block access to the music by programs other than the DRM vendor's own software.
Active protection software must be installed on the computer somehow. XCP and MediaMax use Win-
dows autorun, which (when enabled) automatically loads and runs software from a disc when the disc is
inserted into the computer's drive. Autorun lets the DRM vendor's software run or install immediately.
Once the DRM software is installed, every time a new CD is inserted the software runs a recognition
algorithm to determine whether the disc is associated with the DRM scheme. If the disc is associated, the
active protection software will interfere with accesses to the disc, except those originating from the vendor's
own music player application. This proprietary player application, which is shipped on the disc, gives the
user controlled access to the music.
As we will discuss further, all parts of this design are subject to attack by a user who wants to copy
the music illegally or who wants to make uses allowed by copyright law but blocked by the DRM. The
user can defeat the passive protection, stop the DRM software from installing itself, trick the recognition
algorithm, defeat the active protection software's blocking, capture the music from the DRM vendor's player,
or uninstall the protection software.
The complexity of today's CD DRM software offers many avenues of attack. On the whole, today's sys-
tems are no more resistant to attack than were simpler early CD DRM systems. When there are fundamental
limits to security, extra complexity does not mean extra security.
Discs Studied
Sony deployed XCP on 52 titles (representing more than 4.7 million CDs) [1]. We exam-
ined three of them in detail: Acceptance, Phantoms (2005); Susie Suh, Susie Suh (2005); and Switchfoot,
Nothing is Sound (2005). MediaMax was deployed on 37 Sony titles (over 20 million CDs) as well as dozens
of titles from other labels [1]. We studied three albums that used MediaMax version 3--Velvet Revolver,
Contraband (BMG, 2004); Dave Matthews Band, Stand Up (Sony, 2005); and Anthony Hamilton, Comin'
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from Where I'm From (Arista/Sony 2005)--and three albums that used MediaMax version 5--Peter Cetera,
You Just Gotta Love Christmas (Viastar, 2004); Babyface, Grown and Sexy (Arista/Sony, 2005); and My
Morning Jacket, Z (ATO/Sony, 2005). Unless otherwise noted, statements about MediaMax apply to both
version 3 and version 5.
4
Installation
Active protection measures cannot begin to operate until the DRM software is installed on the user's system.
In this section we consider attacks that either prevent installation of the DRM software, or capture music
files from the disc in the interval after the disc has been inserted but before the DRM software is installed
on the computer.
4.1
Autorun
Both XCP and MediaMax rely on the autorun feature of Windows. Whenever removable media, such as
a floppy disc or CD, is inserted into a Windows PC (and autorun is enabled), Windows looks on the disc
for a file called autorun.inf; if a file with that name is found, Windows executes commands found in
it. Autorun is commonly used to pop up a splash screen or simple menu, for example, to offer to install
software found on the disc. However, the autorun mechanism will run any program that the disc specifies.
Other popular operating systems, including MacOS X and Linux, do not have an autorun feature, so this
mechanism does not work on these systems. XCP ships only Windows code and so has no effect on other
operating systems. MediaMax ships with both Windows and MacOS code on the CD, but only the Windows
code can autorun. The MacOS code relies on the user to double-click an installer on the CD, which few
users will do. For this reason, we will not discuss the MacOS version of MediaMax further.
Current versions of Windows ship with autorun enabled by default, but the user can choose to disable it.
Many security experts advise users to disable autorun to protect against disc-borne malware [6]. If autorun is
disabled, the XCP or MediaMax active protection software will not load or run. Even if autorun is enabled,
the user can block autorun for a particular disc by holding down the Shift key while inserting the disc [13].
This will prevent the active protection software from running.
Even without disabling autorun, a user can prevent the active protection software from loading by cov-
ering up the portion of the disc on which it is stored. Both XCP and MediaMax discs contain two sessions,
with the first session containing the music files and the second session containing DRM content, including
the active protection software and the autorun command file. The first session begins at the center of the
disc and extends outward; the second session is near the outer edge of the disc. By covering the outer edge
of the disc, the user can prevent the drive from reading the second session's files, effectively converting the
disc back to an ordinary single-session audio CD. The edge of the disc can be covered with nontransparent
material such as masking tape, or by writing over it with a felt-tip marker [27]. Exactly how much of the
disc to cover can be determined by iteratively covering more and more until the disc's behavior changes,
or by visually inspecting the disc to look for a difference in appearance of the disc's surface which is often
visible at the boundary between the two sessions.
4.2
Temporary Protection
Even if the copy protection software is allowed to autorun, there is a period of time, between when a
protected disc is inserted and when the active protection software is installed, when the music is vulnerable
to copying. It would be possible to have the discs immediately and automatically install the active protection
software, minimizing this window of vulnerability, but legal and ethical requirements should preclude this
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option. Installing software without first obtaining the user's consent appears to be illegal in the U.S. under
the Computer Fraud and Abuse Act (CFAA) as well as various state anti-spyware laws [2, 3].
Software vendors conventionally obtain the user's consent to the installation of their software by dis-
playing an End User License Agreement (EULA) and asking the user to accept it. Only after the user agrees
to the EULA is the software installed. The EULA informs the user, in theory at least, of the general scope
and purpose of the software being installed, and the user has the option to withhold consent by declining the
EULA, in which case no software is installed. As we will see below, the DRM vendors do not always follow
this procedure.
If the discs didn't use any other protection measures, the music would be vulnerable to copying while the
installer waited for the user to accept or reject the EULA. Users could just ignore the installer's EULA win-
dow and switch tasks to a CD ripping or copying application. Both XCP and MediaMax employ temporary
protection mechanisms to protect the music during this time.
4.2.1
XCP Temporary Protection
The first time an XCP-protected disc is inserted into a Windows machine, the Windows autorun feature
launches the XCP installer, the file go.exe located in the contents folder on the CD. The installer
displays a license agreement and prompts the user to accept or decline it. If the user accepts the agreement,
the installer installs the XCP active protection software onto the machine; if the user declines, the installer
exits after ejecting the CD, preventing other applications from ripping or copying it.
While the EULA is being displayed, the
XCP installer continuously monitors the list of
processes running on the system. It compares
the image name of each process to a blacklist
of nearly 200 ripping and copying applications
hard coded into the go.exe program. If one
or more blacklisted applications are running,
the installer replaces the EULA display with
a warning (shown at right) indicating that the
applications need to be closed in order for the
installation to continue. It also initiates a 30-
second countdown timer; if any of the appli-
cations are still running when the countdown
reaches zero, the installer ejects the CD and
quits.3
This technique might prevent some unso-
phisticated users from copying the disc while
the installer is running, but it can be bypassed with a number of widely known techniques. For instance,
users might kill the installer process (using the Windows Task Manager) before it can eject the CD, or they
might use a ripping or copying application that locks the CD tray, preventing the installer from ejecting the
disc.
The greatest limitation of the XCP temporary protection system is the blacklist. Users might find ripping
or copying applications that are not on the list, or they might use a blacklisted application but rename its
executable file to prevent the installer from recognizing it. Since there is no mechanism for updating the
blacklist on existing CDs, they will gradually become easier to rip and copy as new applications not on the
3Similar application blacklisting techniques have been used in other security contexts. The client software for World of Warcraft,
a massively multiplayer online role playing game, checks running applications against a regularly updated blacklist of programs
used to cheat in the game [15].
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blacklist come into widespread use. Application developers may also adapt their software to the blacklisting
technique by randomizing their process image names or taking other measures to avoid detection.4
4.2.2
MediaMax Temporary Protection
MediaMax employs a different--and highly controversial--temporary protection measure. It defends the
music while the installer is running by installing, and at least temporarily activating, the active protection
software before displaying the EULA. The software is installed without obtaining consent, and it remains
installed (and in some cases, permanently active) even if the user explicitly denies consent by declining the
license agreement.
MediaMax discs install the active protection driver by copying a file called sbcphid.sys to the Win-
dows drivers directory, configuring it as a service in the registry, and launching it. Initially, the driver's
startup type is set to "Manual," so it will not re-launch the next time the computer boots; however, it remains
running until the computer is shut down, and it remains installed permanently [13]. Albums that use Media-
Max version 5 additionally install components of the MediaMax player software before displaying a license
agreement. These files are not removed if the EULA is declined.
Even more troublingly, under some common circumstances the MediaMax installer will permanently
activate the active protection software (by setting its startup type to "Auto," which causes it to be launched
every time the computer boots). This behavior is related to a mechanism in the installer apparently intended
to upgrade the active protection software if an older version is already installed. Under the following scenar-
ios, it is triggered even if the user previously declined the EULA:
· The user inserted a MediaMax version 3 album, then sometime later inserts an MediaMax version 5
album.
· The user inserted a version 5 album, then sometime later inserts a version 3 album.
· The user inserted an version 5 album, reboots, then sometime later inserts the same album or another
version 5 album.
These steps do not have to take place in a single session. They can happen over a period of weeks or months,
as users purchase new albums.
We can think of two possible explanations for this behavior. Perhaps the vendor, SunnComm, did not
test these scenarios to determine what their software did, and so did not realize that they were activating
the software without consent. Or perhaps they did know what would happen in these cases and deliber-
ately chose these behaviors. Either possibility is troubling, indicating either a deficient design and testing
procedure or a deliberate decision to install software after the user denied permission to do so.
Even if poor testing is the explanation for activating the software without consent, it is clear that Sunn-
Comm deliberately chose to install the MediaMax software on the user's system even if the user did not
consent. These decisions are difficult to reconcile with the ethical and legal requirements on software com-
panies. But they are easy to reconcile with the vendor's platform building strategy, which rewards the vendor
for placing its software on as many computers as possible.
Even if no software is installed without consent, the temporary activation of DRM software, by both
XCP and MediaMax, before the user consents to anything raises troubling ethical questions. It is hard to
argue that the user has consented to loading running software merely by the act of inserting the disc. Most
users do not expect the insertion of a music CD to load software, and although many (but not all) of the
affected discs did contain a statement about protection software being on the discs, the statements generally
4An extreme extension of this would be to adopt rootkit-like techniques to conceal the copying application's presence, just as
XCP hides its active protection software.
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were confusingly worded, were written in tiny print, and did not say explicitly that software would install or
run immediately upon insertion of the disc. Some in the record industry argue that the industry's desire to
block potential infringement justifies the short-term execution of the temporary protection software on every
user's computer. We think this issue deserves more ethical and legal debate.
4.3
Passive Protection
Another way to prevent copying before active protection software is installed is to use passive protection
measures. Passive protection exploits subtle differences between the way computers read CDs and the
way ordinary CD players do. By changing the layout of data on the CD, it is sometimes possible to confuse
computers without affecting ordinary players. In practice, the distinction between computers and CD players
is imprecise. Older generations of CD copy protection, which relied entirely on passive protection, proved
easy to copy in some computers and impossible to play on some CD players [12]. Furthermore, computer
hardware and software has tended to get better at reading the passive protected CDs over time as it became
more robust to all manner of damaged or poorly formatted discs. For these reasons, more recent CD DRM
schemes rely mainly on active protection.
XCP uses a mild variety of passive protection as an added layer of security against ripping and copying.
This form of passive protection exploits a quirk in the way Windows handles multisession CDs. When CD
burners came to market in the early 1990s, the multisession CD format was introduced to allow data to be
appended to partially recorded discs. (This was especially desirable at a time when recordable CD media
cost tens of dollars per disc.) Each time data is added to the disc, it is written as an independent series of
tracks called a session. Multi-session compatible CD drives see all the sessions, but ordinary CD players,
which generally do not support the multisession format, recognize only the first session.
Some commercial discs use a variant of the multisession format to combine CD audio and computer
accessible files on a single CD. These discs adhere to the Blue Book or "stamped multisession" format.
According to the Blue Book specification, stamped multisession discs must contain two sessions: a first
session with 1­99 CD audio tracks, and a second session with one data track. The Windows CD audio
driver contains special support for Blue Book discs. It presents the CD to playing and ripping applications
as if it was a normal audio CD. Windows treats other multisession discs as data-only CDs.
XCP discs deviate from the Blue Book format by adding a second data track in the second session. This
causes Windows to treat the disc as a regular multisession data CD, so the primary data track is mounted as
a file system, but the audio tracks are invisible to player and ripper applications that use the Windows audio
CD driver. This includes Windows Media Player, iTunes, and most other widely used ripper applications.
We developed a procedure for creating discs with this flavor of passive protection using only standard CD
burning hardware and software.
This variety of passive protection provides only limited resistance to ripping and copying. There are a
number of well-known methods for defeating it:
· Advanced ripping and copying applications avoid the Windows CD audio driver altogether and issue
commands directly to the drive. This allows programs such as Nero [24] and Exact Audio Copy [33]
to recognize and read all the audio tracks.
· Non-Windows platforms, including MacOS and Linux, read multisession CDs more robustly and do
not suffer from the limitation that causes ripping problems on Windows.
· The felt-tip marker trick, described above, can also defeat this kind of passive protection. When the
second session is obscured by the marker, CD drives see only the first session and treat the disc as a
regular audio CD, which can be ripped or copied with ease.
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5
Disc Recognition
The active protection mechanisms employed by XCP and MediaMax regulate access to raw CD audio,
blocking access to the audio tracks on albums protected with a particular scheme while allowing access to
all other titles.
To accomplish this, the schemes install a background process that interposes itself between applications
and the original CD driver. In MediaMax, this process is a kernel-mode driver called sbcphid.sys.
XCP uses a pair of filter drivers called crater.sys and cor.sys that attach to the CD-ROM and IDE
devices [29]. In both schemes, the active protection drivers examine each disc that is inserted into the
computer to see whether access to it should be restricted. If the disc is recognized as copy protected, the
drivers monitor for attempts to read the audio tracks, as would occur during a playback, rip, or disc copy
operation, and corrupt the audio returned by the drive to degrade the listening experience. MediaMax
introduces a large amount of random jitter, making the ripped audio sound like it has come from a badly
scratched or damaged CD; XCP replaces the audio with random noise.
Each scheme's active protection software interferes with attempts to rip or copy any disc that is protected
by the same scheme, not merely the disc from which the software was installed. This requires some mecha-
nism for identifying discs that are to be protected. This section discusses the security requirements for such
a recognition system, describes the design and limitations of the actual recognition mechanism employed by
the MediaMax scheme, and presents an improved design that better satisfies the requirements.
5.1
Recognition Requirements
Any disc recognition system detects some distinctive feature on discs protected by a particular copy protec-
tion scheme. Ideally, such a feature would satisfy these requirements:
1. Uniqueness. The feature should identify protected discs without accidentally triggering the copy
protection on unprotected titles.
2. Detectability. It should be possible for the active protection drivers running on client systems to
reliably and quickly detect the feature in protected discs. In practice, this limits the amount of audio
that can be read from the disc before deciding whether to apply protection.
3. Indelibility. The feature should be hard to remove without substantially degrading the quality of the
audio; that is, it should be difficult to make a copy of the copy protected disc that does not itself trigger
the protection.
4. Unforgeability. It should be difficult to apply the feature to an unprotected album without the cooper-
ation of the protection vendor, even if the adversary has access to protected discs.
This last requirement stems from the DRM vendor's platform building strategy, which tries to put the
DRM software on as many computers as possible and to have the software control access to all marked discs.
If the vendor's identifying mark is forgeable, then a record label could mark its discs without the vendor's
permission, thereby taking advantage of the vendor's platform without paying.5
There are advantages and disadvantages for an entity placing unauthorized marks. Copyright would
prohibit rogue publishers from distributing an installer for the active protection software, though they might
depend on the existing installed base if the software was included on many widely sold titles. They would
5Forging a mark is probably not copyright infringement. Unlike the musical work in which it is embedded, the mark itself is
functional and contains little or no expression, and therefore seems unlikely to qualify for copyright protection. In principle, the
mark recognition process could be covered by a patent, but we are unaware of any such patent relating to XCP or MediaMax. Even
if the vendor does have a legal remedy, it seems worthwhile to design the mark to prevent forgery if the cost of doing so is low.
9

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also be prevented from employing the components of the protection software that allow users to access
restricted copies of the music; however, they could create their own software to provide this capability if
they desired. On the other hand, free riding publishers would not be restricted to marking their disc for
only one scheme. By identifying their discs as copy protected with multiple schemes (e.g., both XCP and
MediaMax), they could invoke multiple layers of security and provide stronger protection than is available
with any single technique, all without paying. Preventing free riding by publishers requires some kind of disc
authentication mechanism to control access to installed active protection software--a meta-copy protection
technique.
5.2
MediaMax Disc Recognition
To find out how well the disc recognition mechanisms employed by CD DRM systems meet the ideal require-
ments, we examined the recognition system built into MediaMax. This system drew our attention because
MediaMax's creators have touted their advanced disc identification capabilities, including the ability to iden-
tify individual tracks within a compilation as protected [22]. (XCP appears to use a less sophisticated disc
recognition system based on a marker stored in the data track of protected discs; we did not include it in this
study.)
We determined how MediaMax identifies protected albums by tracing the commands sent to the CD
drive with and without the active protection software running. These experiments took place on a Windows
XP VMWare virtual machine running on top of a Fedora Linux host system, which we modified by patching
the kernel IDE-SCSI driver to log all CD drive activity.
With this setup we observed that the MediaMax software executes a disc recognition procedure imme-
diately upon the insertion of a CD. The MediaMax driver reads two sectors of audio at a specific offset
from the beginning of audio tracks--approximately 365 and 366 frames in (a CD frame stores 1 second
75
of sound). On unprotected discs, the software scans through every track in this way, but on MediaMax-
protected albums, it stops after the first three tracks, apparently having detected an identifying feature. The
software decides whether or not to block read access to the audio solely on the basis of information in this
region, so we inferred that the identifying mechanism takes the form of an inaudible watermark embedded
in this part of the audio stream.6
Locating the watermark amid megabytes of audio might have been difficult, but we had the advantage
of a virtual Rosetta Stone. The actual Rosetta Stone--a 1500 lb. granite slab, unearthed in Rosetta, Egypt,
in 1799--is inscribed with the same text written in three languages: ancient hieroglyphics, demotic (sim-
plified) hieroglyphics, and Greek. Comparing these inscriptions provided the key to deciphering Egyptian
hieroglyphic texts. Our Rosetta Stone was a single album, Velvet Revolver's Contraband, released in three
different versions: a U.S. release protected by MediaMax, a European release protected by a passive scheme
developed by Macrovision, and a Japanese release with no copy protection. We decoded the MediaMax
watermark by examining the differences between the audio on these three discs. Binary comparison re-
vealed no differences between the releases from Europe and Japan; however, the MediaMax-protected U.S.
release differed slightly from the other two in certain parts of the recording. By carefully analyzing these
differences--and repeatedly attempting to create new watermarked discs using the MediaMax active protec-
tion software as an oracle--we were able to deduce the structure of the watermark.
The MediaMax watermark is embedded into the audio of each track in 30 clusters of modified audio
samples. Each cluster is made up of 288 marked 16-bit audio samples followed by 104 unaltered samples.
Three mark clusters exactly fit into one 2352-byte CD audio frame. The watermark is centered at approxi-
mately frame 365 of the track; though the detection routine in the software only reads two frames, the mark
extends several frames to either side of the designated read target to allow for imprecise seeking in the audio
6By locating the watermark nearly five seconds after the start of the track rather than at the very beginning, MediaMax reduces
the likelihood that it will occur in a very quiet passage (where it might be more audible) and makes cropping it out more destructive.
10

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portion of the disc (a typical shortcoming of inexpensive CD drives). The MediaMax driver detects the
watermark if at least one mark cluster is present in the region read by the detector.
A sequence of 288 bits that we call the raw watermark is embedded into the 288 marked audio samples
of each mark cluster. A single bit of the raw watermark is embedded into an unmarked audio sample by
setting one of the three least significant bits to the new bit value (as shown in bold below) and then setting
the two other bits according to this table:7
Original bits
Marked bits
0
0
0
1
1
1
111
011
101
110
111
111
111
110
011
101
110
110
110
111
101
011
101
100
101
110
101
100
011
100
100
100
110
101
011
011
001
010
100
011
011
010
010
001
010
100
010
011
001
001
001
000
100
010
001
000
000
000
000
100
010
001
The position of the embedded bit in each sample follows a fixed sequence for every mark cluster. Each of
the 288 bits is embedded in the first-, second-, or third-least-significant bit position of the sample according
to this sequence:
2,3,1,1,2,2,3,3,2,3,3,3,1,3,2,3,2,1,3,2,2,3,2,2,2,1,3,3,2,1,2,3,3,1,2,2,3,1,2,3,3,1,1,2,2,1,1,3,
3,1,2,3,1,2,3,3,1,3,3,2,1,1,2,3,2,2,3,3,3,1,1,3,1,2,1,2,3,3,2,2,3,2,1,2,2,1,3,1,3,2,1,1,2,1,1,1,
2,3,2,1,1,2,3,2,1,3,2,2,2,3,1,2,1,3,3,3,3,1,1,1,2,1,1,2,2,2,2,3,1,2,3,2,1,3,1,2,2,3,1,1,3,1,1,1,
1,2,2,3,2,3,2,3,2,1,2,3,1,3,1,3,3,3,1,1,2,1,1,2,1,3,3,2,3,3,2,2,1,1,1,2,2,1,3,3,3,3,3,1,3,1,1,3,
2,2,3,1,2,1,2,3,3,2,1,1,3,2,1,1,2,2,1,3,3,2,2,3,1,3,2,2,2,3,1,1,1,1,3,2,1,3,1,1,2,2,3,2,3,1,1,2,
1,3,2,3,3,1,1,3,2,1,3,1,2,2,3,1,1,3,2,1,2,2,2,1,3,3,1,2,3,3,3,1,2,2,3,1,2,3,1,1,3,2,2,1,3,2,1,3
The active protection software reads the raw watermark by reading the first, second, or third bit from
each sample according to the sequence above. It determines whether the resulting 288-bit sequence is a valid
watermark by checking certain properties of the sequence (represented below). It requires 96 positions in
the sequence to have a fixed value, either 0 or 1. Another 192 positions are divided into 32 groups of linked
values (denoted a­z and ­ below). In each group, three positions share the same value and three share
the complement value. This allows the scheme to encode a 32-bit value (value A), though in the discs we
studied it appears to take a different random value in each mark cluster of each protected title. The final 32
bits of the raw watermark may have arbitrary values (denoted by below) and encode a second 32-bit value
(value B). MediaMax version 5 uses this value to distinguish between original discs and backup copies
burned through it proprietary player application.
0, a, b, c, d, e, 0, 0, f, 0, g, 0, h, 0, i, d, j, ¯
j, k, 0, l, m, 0, n, o, p, ¯
e, q, ¯
e, r, 0, ¯
p, s, d, ¯
m, t, u, v, w, t, ¯
l, a, x, c, u, 0, ¯
r, l,
f, ¯
d, v, 0, m, 0, ¯
q, 0, y, c, z, 0, j,¯i, ¯
g, , ¯
s, ¯
w, ¯
h, v, y, n, 0, 0, ¯
h, ¯
j, ¯
u, a, , 0, ¯
v, g, j, 0, 0, ¯
, ¯i, e, ¯
z, 0, r, , ¯
a, , ¯
d, ¯
z, 0, ¯
v,
, 0, x, s, ¯
g, ¯
r, 0, ¯
b, o, b, r, 0, y, ¯
, ¯
m, h, 0, ¯
a, n, ¯
f , ¯
t, 0, ¯
o, 0, ¯
, ¯, ¯
e, 0, 0, ¯
k, ¯
c, ¯
x, 0, ¯
f , p, z, ¯
x, i, 0, 0, , ¯
g, 0, 1, w, ¯
t, ¯
n, ¯
w,
i, 0, 0, ¯
j, m, x, , ¯
y, ¯
p, ¯
q, 0, 0, 0, e, ¯
, 0, 0, 1, g, 0, p, l, 0, ¯
, t, h, ¯
d, ¯, ¯
w, , ¯
, 0, ¯
p, q, ¯
f , 0, 1, , 0, ¯
c, , ¯
, ¯
s, ¯
b, ¯
, , 0, o,
0, q,¯i, 0, 0, ¯
, s, , ¯, ¯
h, 0, ¯
k, ¯
n, ¯
, , ¯
s, ¯
z, ¯
n, ¯
c, ¯
o, ¯
b, 0, ¯
t, 0, ¯
y, ¯
v, 0, , ¯
o, 0, ¯
, 0, u, , 0, ¯
y, k, ¯
u, z, ¯
, ¯
q, k, ¯
r, ¯
u, ¯
, ¯
, ¯
l, ¯
l,
w, ¯
k, ¯
a, 0, ¯
, 0, , ¯
m, b, f, 0, 0, ¯
x, , , 0, , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , ,
7This design seems to be intended to lessen the audible distortion caused by setting one of the bits to the watermark value. The
change in the other two bits reduces the magnitude of the difference from the original audio sample, but it also introduces a highly
uneven distribution in the three least significant bits that makes the watermark easier to detect or remove.
11

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5.3
Attacks on the MediaMax Watermark
The MediaMax watermark fails to satisfy the indelibility and unforgeability requirements of an ideal disc
recognition system. Far from being indelible, the mark is surprisingly brittle. Most advanced designs
for robust audio watermarks [8, 7] manipulate the audio in the frequency domain and attempt to resist
removal attempts that use lossy compression, multiple conversions between digital and analog formats, and
other common transformations. In contrast, the MediaMax watermark is applied in the time domain and
is rendered undetectable by even minor changes to the file. An adversary without any knowledge of the
watermark's design could remove it by converting the tracks to a lossy format like MP3 and then burning
them back to a CD, which can be accomplished easily with standard consumer applications. This would
result in some minor loss of fidelity, but a more sophisticated adversary could prevent the mark from being
detected with almost no degradation by flipping the least significant bit of one carefully chosen sample from
each of the 30 watermark clusters, thereby preventing the mark from exhibiting the pattern required by the
detector.
The watermark also fails to satisfy the unforgeability requirement. The mark's only defense against
forgery is its complicated, unpublished design, but as is often the case this security by obscurity has proved
tedious rather than impossible to defeat. As it turns out, an adversary needs only limited knowledge of
the watermark--its location within a protected track and its confinement to the three least significant bits
of each sample--to forge it with minimal loss of fidelity. Such an attacker could transplant the three least
significant bits of each sample within the watermarked region of a protected track to the corresponding
sample from an unprotected one. Transplanting these bits would cause distortion more audible that that
caused by embedding the watermark since the copied bits are likely to differ by a greater amount from the
original sample values; however, the damage to the audio quality would be limited since the marked region
is only 0.4 seconds in duration. A more sophisticated adversary could apply a watermark to an unprotected
track by deducing the full details of the structure of the watermark, as we did; she could then embed the
mark in an arbitrary audio file just as well a licensed disc producer.
As a proof-of-concept, we created a utility called scmark that can detect, embed, or remove the Media-
Max watermark. The program is invoked on one or more WAVE audio files as follows:
usage: scmark --detect [-p <pos>] [-c <count>] <file.wav> [files]
--embed [[a] <b>] [-p <pos>] [-c <count>] <file.wav> [files]
--remove [-p <pos>] [-c <count>] <file.wav> [files]
When scmark is executed with only the --detect parameter on a track ripped from a MediaMax-
protected album, the watermark is detected almost instantaneously. The -p <pos> parameter gives a hint
as to the position of the watermark within the file; and -c <count> indicates how many mark clusters
to read or write. For embedding, the a and b parameters specify the two 32-bit values A and B encoded
in the watermark, as described in Section 5.2. The --remove switch searches for an existing watermark
and removes it by overwriting it with a random raw watermark that lacks the properties required by the
MediaMax detector.
5.4
Secure Disc Recognition
Having shown that the MediaMax watermark fails to provide either strong resistance to removal or strong
resistance to forgery, we ask whether it is possible to securely accomplish either or both of these goals.
As far as indelibility is concerned, watermarking schemes have a poor history of resisting removal
[8, 20, 26]. This is especially true against an adversary who has oracle access to the watermark detector, as
was the case with a previous application of watermarks to audio copy protection, SDMI [8], and with CD
DRM systems. Making marks that are both indelible and unforgeable is likely much more difficult. There
12

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seems to be tension between marks that are difficult to remove and ones that are hard to forge. Enforcing
both requirements creates two ways to fool the detector--by rendering the mark invisible and by making
it appear forged. If, as in CD DRM systems, either situation leads to the same result (no protection), the
attacker's power is magnified.
In contrast, a mark strongly robust to forgery is simple to create based on digital signatures if we aren't
concerned with its being easy to remove. A very simple scheme works as follows:
1. To sign an audio track, the licensed publisher reads a fixed portion L1 of the audio data (say, the
first ten seconds), then computes a cryptographic hash of L1 and signs it using a public key signature
algorithm to derive the signature SL := Sign
(Hash(L
is then stored at a second location
1
KS
1)). SL1
in the track by setting the LSB of each sample in the region to the corresponding bit in the signature.
A 320-bit DSA signature could be embedded in this way using approximately the same space as one
mark cluster of the MediaMax watermark.
2. The publisher keeps the signing key KS secret, and builds the corresponding verification key KV into
the client software. When presented with a CD, the software checks for a valid signature. First it
reads the audio from the signed area of the track and hashes it, and it locates and extracts the signature
stored in the LSBs in the second mark location. Next, it verifies the signature on the hash using KV .
If the signature is correct, the watermark is valid and genuine; otherwise, forgery or data corruption is
indicated.
The scheme could be strengthened against natural errors by applying the mark to several regions of the
disc, as is the MediaMax watermark.
Forging such a mark would require defeating the digital signature scheme or splicing both L1 and SL1
from a legitimately marked album. We set L1 to be several seconds of audio to make such splicing less
appealing.
Clearly this watermark is highly vulnerable to removal. If even a single bit of the hashed region is
changed, the mark will not be recognized as valid. Yet the watermark MediaMax actually uses is also vul-
nerable to corruption by a single bit (in each mark cluster) while being far less resistant to forgery. Though
robustness to removal could be improved by using error correction methods, we believe that robustness,
while desirable in principle, is of limited value in real CD DRM applications, and should not be traded off
against forgeability. Removal of the watermark is unlikely to be the weakest link protecting the audio, and
while the gains from creating a more indelible watermark are slight, the loss to free riders from an easily
forgeable mark is potentially much greater.
6
CD DRM Players
Increasingly, personal computers--and portable playback devices that attach to them--are users' primary
means of organizing, transporting, and enjoying their music collections. Sony-BMG and its DRM vendors
recognized this trend when they designed their copy protection technologies. Rather than inhibit all use
with PCs, as some earlier anti-copying schemes did [12], XCP and MediaMax provide their own proprietary
media players, shipped on each protected CD, that allow certain limited uses subject to restrictions imposed
by DRM software.8
The XCP and MediaMax players launch automatically using autorun when a protected disc is inserted
into a PC. Both players have similar feature sets. They provide a rudimentary playback interface, allowing
8The restrictions imposed by the DRM players only loosely track the contours of copyright law. Some uses that could be
prohibited under copyright--such as burning three copies to give to friends--are allowed by the software, while some perfectly
legal uses--like transferring the music to one's iPod--are prevented.
13

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users to listen to protected albums, and they allow access to "bonus content," such as album art, liner notes,
song lyrics, and links to artist web sites. The players access music on the disc, despite the active protection,
by using a special back door interface provided by the active protection software.
XCP and MediaMax version 5 both permit users to burn copies of the entire album a limited number of
times (typically three). These copies are created using a proprietary burning application integrated into the
player. The copies include the player applications and the same active (and passive, for XCP) protection as
the original album, but they do not allow any subsequent generations of copying.
Another feature of the player applications allows users to rip the tracks from the CD to their hard disks,
but only in DRM-protected audio formats. Both schemes support the Windows Media Audio format by
using a Microsoft product, the Windows Media Data Session Toolkit [23], to deliver DRM licenses that are
bound to the PC where the files were ripped. The licenses allow the music to be transferred to portable
devices that support Windows Media DRM or burned onto CDs, but the Windows Media files will not be
usable if they are copied to another PC. Because XCP and MediaMax create Windows Media files, they are
vulnerable to any attack that can defeat Windows Media DRM. Often, DRM interoperation allows attacks
on one system to defeat other systems as well, because the attacker can transfer protected content into the
system of her choice in order to extract it.
The XCP and MediaMax version 5 players both exhibit similar spyware-like behavior: phoning home to
the vendor or record label with information about users' listening habits despite statements to the contrary
from the vendors. Whenever a protected disc is inserted, the players contact web servers to retrieve images
or banner ads to display. Part of the request is a code that identifies the album. XCP discs contact a Sony web
site, connected.sonymusic.com [28]; MediaMax albums contact license.sunncomm2.com, a
site operated by MediaMax's creator, SunnComm. These connections allow the servers to log the user's IP
address, the date and time, and the identity of the album. This undisclosed data collection, in combination
with other practices--installation without informed consent and the lack of an uninstaller--make XCP and
MediaMax fit the consensus definition of spyware.
6.1
Attacks on Players
The XCP and MediaMax version 5 players were designed to enforce usage restrictions specified by content
providers. In practice, they provide minimal security because there are many ways that users can bypass
the limitations. Perhaps the most interesting class of attacks targets the limited number of burned copies
permitted by the players. Both players are designed to enforce this limit without communicating with any
networked server; thus, the player must keep track of how many allowed copies remain by storing state on
the local machine.
It is well known that DRM systems like this are vulnerable to rollback attacks. A rollback attack backs
up the state of the machine before performing the limited operation (in this case, burning the copy). When
the operation is complete, the old system state is restored, and the DRM software is not able to determine
that the operation has occurred. This kind of attack is easy to perform with virtual machine software like
VMWare, which allows the entire state of the system to be saved or restored in a few clicks. XCP and
MediaMax both fail under this attack, which allows unlimited copies to be burned with their players.
A refined variation of this attack targets only the specific pieces of state that the DRM system uses to
remember the number of copies remaining. The XCP player uses a single file, %windir%\system32\
$sys$filesystem\$sys$parking, to record how many copies remain for every XCP album that
has been used on the system.9 Rolling back this file after a disc copy operation would restore the original
number of copies remaining.
9This file is hidden and protected by the XCP rootkit. Before the user can access the file, the rootkit must be disabled, as
described in Section 7.2. We did not determine how the MediaMax player stores the number of copies remaining.
14

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A more advanced attacker can go further and modify the $sys$parking file to set the counter to an
arbitrary value. The file consists of a 16 byte header followed by a series of 177 byte structures. For each
XCP disc used on the machine, the file contains a whole-disc structure and an individual structure for each
track. Each disc structure stores the number of permitted copies remaining for the disc as a 32-bit integer
beginning 100 bytes from the start of the structure.
The file is protected by primitive encryption. Each structure is XORed with a repeating 256-bit pad.
The pad--a single pad is used for all structures--is randomly chosen when XCP is first installed and stored
in the system registry in the key HKLM\SOFTWARE\$sys$reference\ClassID. Note that this key,
which is hidden by the rootkit, is intentionally misnamed "ClassID" to confuse investigators. Instead of a
ClassID, it contains the 32 bytes of pad data.
Hiding the pad actually doesn't increase the security of the design. An attacker who knows only the
format of the $sys$parking file and the current number of copies remaining can change the counter
to an arbitrary value without needing to know the pad. Say the counter indicates that there are x copies
remaining and the attacker wants to set it to y copies remaining. Without decrypting the structure, she can
XOR the padded bytes where the counter is stored with the value x y. If the original value was padded
with p, the new value is (x p) (x y) = (y p), y padded with p.
6.1.1
iPod Compatibility
Ironically, Sony itself furnishes directions for carrying out another kind of attack on the player DRM. Con-
spicuously absent from the XCP and MediaMax players is support for the Apple iPod--by far the most
popular portable music player. A Sony FAQ blames Apple for this shortcoming and urges users to direct
complaints to them: "Unfortunately, in order to directly and smoothly rip content into iTunes it [sic.] re-
quires the assistance of Apple. To date, Apple has not been willing to cooperate with our protection vendors
to make ripping to iTunes and to the iPod a simple experience." [32]. Strictly speaking, it is untrue that Sony
requires Apple's cooperation to work with the iPod, as the iPod can import MP3s and other open formats.
What Sony has difficulty doing is moving music to the iPod while keeping it wrapped in copy protection.
This is because Apple has so far refused to support interoperation with its FairPlay DRM.
Yet so great is consumer demand for iPod compatibility that Sony gives out--to any customer who fills
out a form on its web site [31]--instructions for working around its own copy protection and transforming
the music into a DRM-free format that will work with the iPod. The procedure is simple but cumbersome:
users are directed to use the player software to rip the songs into Windows Media DRM files; use Windows
Media Player to burn the files to a blank CD, which will be free of copy protection; and then use iTunes to
rip the songs once more and transfer them to the iPod.
6.2
XCP's Hidden iPod Support
A further irony came to light in the weeks following the public disclosure of the XCP rootkit when it was
discovered that XCP itself infringes on the copyrights to several open source software projects. In one
case, Sam Hocevar found conclusive evidence [14] that part of XCP's code was copied from a program
called DRMS [18], which he co-authored with Jon Lech Johansen and released under the terms of the
GNU General Public License (GPL). This was particularly curious, because the purpose of DRMS is to
break Apple's FairPlay DRM. Its presence in XCP is interesting enough to warrant a digression from our
discussion of player-related attacks.
We discovered that XCP utilizes the DRMS code not to remove Apple DRM but to add it, as part of a
hidden XCP feature that provides iTunes and iPod compatibility. This functionality shipped on nearly every
XCP CD, but it was never enabled or made visible in the XCP user interface. Despite being inactive, the
code appears to be fully functional and was compatible with the current version of iTunes when the first
15

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XCP CDs were released.10 This strongly suggests that the infringing DRMS code was deliberately copied
by XCP's creator, First4Internet, rather than accidentally included as part of a more general purpose media
library used for other functions in the copy protection system.
This isn't the first time another vendor has tried to make its DRM compatible with FairPlay. FairPlay
is the only DRM compatible with the iPod, and Apple has declined to license it to rival music distributors,
effectively locking rivals out from the iPod platform (at least as long as the rivals insist on using DRM). In
2004, RealNetworks attempted to work around Apple and reverse engineered FairPlay so that Real Player
could create FairPlay files for use with the iPod [9]. Apple responded by making vague legal threats and
updating iTunes to break this compatibility. The evidence suggests that First4Internet wanted to create their
own iPod compatibility system, but rather than take the time to reverse engineer FairPlay themselves, they
copied critical pieces of code from DRMS in violation of the GPL license.
Understanding how XCP uses code from DRMS requires some background information about FairPlay.
When a customer purchases a song from the iTunes Music Store, she receives a FairPlay encrypted audio
file that can only be played with knowledge of a secret key assigned to her by Apple. iTunes retrieves this
key from an Apple server and stores it on the hard drive in an encrypted key database (a file called SC
Info.sidb). When the user plays the song again, or if she copies it to an iPod, iTunes reads her key from
the database instead of reconnecting to the server [34].
FairPlay's security depends on the encrypted key database being difficult for anyone but Apple to deci-
pher, so it is protected using a proprietary encryption method and a system-dependent secret key.11 iTunes
encrypts the key database using a two step process. First it pads the plaintext database by XORing it with
the output of a proprietary pseudorandom number generator (PRNG) using a system-dependent seed; then
it applies AES encryption in ECB mode with a system-dependent key. As a consequence of this design, the
code for the PRNG is exactly the same whether the file is being encrypted or decrypted. To decrypt, iTunes
applies AES decryption, then XORs the same PRNG output again. This explains why parts of the DRMS
code--in particular, a function called DoShuffle, which computes the PRNG's output--are useful for
encryption as well as their original purpose, decryption.
The proprietary PRNG must have been especially difficult to reverse engineer. Rather than expend this
effort themselves, XCP's authors appear to have lifted the DoShuffle code verbatim from DRMS. XCP uses
this code to manipulate the iTunes key database in the process of adding FairPlay protection. Starting with
an unencrypted audio file, such as a track from a protected CD, XCP applies AAC compression in memory,
then encrypts using the same algorithm as FairPlay. Instead of using an Apple-assigned user key, XCP
creates a new random user key and, with the help of the DRMS code, adds it to the iTunes key database.
This ensures that the song file can only be used on the computer where it was created.
The XCP FairPlay compatibility code is contained in a file named ECDPlayerControl.ocx that is
installed the first time an XCP CD is played. The code can be tested by jumping to a function at debugger
offset 0x10010380 (apparently the start of a thread for transferring music to iTunes). This function takes
one parameter, a wide character string of the form <MP3><"C:\test.mp3">. This syntax causes the
function to convert an MP3 file to a FairPlay-protected AAC file. Variations can be used to specify other
audio sources: WAV files, raw audio files, standard unprotected audio CDs, and XCP copy-protected CDs.
Before calling the function, it is necessary to initialize a Windows CriticalSection object and set the
ECX register to the object's address minus 0x6C.
The entry function calls a subroutine (offset 0x10027D20) that converts an audio file into a FairPlay-
protected AAC file. This call another subroutine (offset 0x1008A470) that reads the iTunes key database,
10XCP's FairPlay-compatibility code works with iTunes up to iTunes version 4.8. iTunes 4.9, released June 28, 2005, includes
changes unrelated to FairPlay that cause the XCP code to fail. XCP CDs released after this date do not appear to contain an updated
version of the code.
11As security experts predicted, this protection was quickly broken. Today DRMS is able to defeat FairPlay because Jon Lech
Johansen reverse engineered the database decryption code in iTunes.
16

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decrypts it, and, if necessary, adds the XCP user key to the database and re-saves it in encrypted form. The
iTunes database encryption function (0x1008A0C0) and decryption function (0x1008A300) both make use
of the DoShuffle routine (0x10089E00) taken from DRMS.
6.3
Attacks on the Backdoor
We note that the DRM vendor must include some kind of limited back door so that the vendor's own CD-
reading software can reliably access the music on the disc. The active protection software will offer some
kind of interface that can be called to get access to the raw data from the disc, or to deactivate (temporarily)
the active protection. This back door interface must be protected so that an ordinary program cannot use the
back door to access the music. There are three ways to protect the back door.
1. Secret interface. The back door interface can be kept secret so that ordinary programmers do not
know how to call it. This method is disfavored because its security by obscurity method violates
Kerckhoffs's Principle [19].
2. Secret key passed as argument. There can be a secret key that must be passed to the back door interface,
and the active protection software can be programmed to ignore requests that do not contain the correct
key. This is superior to the secret interface method because it relies on a randomly generated key rather
than secrecy of algorithm information. But an adversary who can interpose himself into the back door
interface can observe the key, and can act as a man in the middle to modify the calls being made to
the active protection software.
3. Cryptographic protocol. The vendor's application software can use a cryptographic protocol to com-
municate with the active protection software. The sort of protocol used for secure remote procedure
call on a network would be suitable, with the network messages replaced by calls across the back
door interface, so that each back-and-forth pair of messages in the protocol was replaced by a single
call and return. This approach makes interface snooping and interposition attacks useless (assuming
the protocol is properly secure). However, it cannot stop an adversary from reverse engineering the
vendor's application-level software to learn the protocol and extract any keys.
6.4
MediaMax Player Security Risks
Besides suffering from several kinds of attacks that expose the music content to copying, the MediaMax
version 5 player makes the user's system more vulnerable to attack. When a MediaMax CD is inserted
into a computer, Windows autorun launches an installer from the disc. Even before displaying a license
agreement, MediaMax copies almost twelve megabytes of files and data related to the MediaMax player
to the hard disk and stores them in a folder named %programfiles%\Common Files\SunnComm
Shared. Jesse Burns and Alex Stamos of iSec Partners discovered that the MediaMax installer sets file
permissions that allow any user to modify its code directory and the files and programs in it [5].
As Burns and Stamos realized, the lax permissions allow a non-privileged user to replace the executable
code in the MediaMax player files with malicious code. The next time a user plays a MediaMax-protected
CD, the attack code will be executed with that user's security privileges. The MediaMax player requires
Power User or Administrator privileges to run, so it's likely that the attacker's code will run with almost
complete control of the system.
Normally, this problem could be fixed by manually correcting the errant permissions. However, Media-
Max aggressively updates the installed player code each time the software on a protected disc autoruns or
is launched manually. As part of this update, the permissions on the installation directory are reset to the
insecure state.
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We discovered a variation of the attack suggested by Burns and Stamos that allows the attack code
to be installed even if the user has never consented to the installation of MediaMax, and to be triggered
immediately whenever the user inserts a MediaMax CD. In the original attack, the user needs to accept the
MediaMax license agreement before attack code can be inserted or executed, because the code is placed
in a file called MMX.EXE that is not copied to the system until after the agreement is accepted. In our
attack, the attacker places hostile code in the DllMain procedure of a code file called MediaMax.dll,
which MediaMax installs even before displaying the EULA. The next time a MediaMax CD is inserted, the
installer autoruns and immediately attempts to check the version of the installed MediaMax.dll file. To
do this, the installer calls the Windows LoadLibrary function on the DLL file, which causes the file's
DllMain procedure to execute, together with any attack code inserted there.
This problem is exacerbated because parts of the MediaMax software are installed automatically and
without consent. Users who declined the EULA would likely assume that MediaMax was not installed,
and so most would be unaware that they were vulnerable. The same installer code performs the dangerous
version check as soon as the CD is inserted. A CD that prompted the user to accept a license before installing
code would give the user a chance to head off the attack.
Fixing this problem permanently without losing the use of protected discs requires installing a patch
from MediaMax. Unfortunately, as we discovered, the initial patch released by Sony-BMG in response to
the iSec report was capable of triggering precisely the kind of attack it was supposed to prevent. In the
process of updating MediaMax, the patch checked the version of MediaMax.dll just like the MediaMax
installer does. If this file was already modified by an attacker, the process of applying the security patch
would execute the attack code. Prior versions of the MediaMax uninstaller had the same vulnerability,
though both the uninstaller and the patch have since been replaced with versions that do not suffer from this
problem.
7
Deactivation
Active protection methods install and run software components that interfere with accesses to a CD. Users
can remove or deactivate the active protection software by using standard system administration tools that are
designed to find, characterize, and control the programs installed on a machine. Deactivating the protection
will enable arbitrary use or ripping of the music, and it is difficult to stop if the user has system administrator
privileges. In this section, we discuss how active protection may be deactivated.
7.1
Deactivating MediaMax
The MediaMax active protection software is simple to deactivate since it is a single device driver with a
consistent service name, sbcphid. The service can be manipulated using the Windows XP command line
utility sc. To check the status of the service, a user can open a command prompt windows and issue the
command sc query sbcphid; if the reported state is "RUNNING" then the MediaMax driver is active.
It can be deactivated using the command sc stop sbcphid. To permanently remove it, a user can issue
the command sc delete sbcphid, then delete %windir%\system32\drivers\sbcphid.sys,
the driver's program file. Once the driver is deactivated, MediaMax-protected albums can be accessed as if
they were unprotected.
7.2
Defenses Against Deactivation
To counter deactivation attempts, a vendor might try technical tricks to evade detection and frustrate removal
of the active protection software. An example is the rootkit-like behavior of XCP, discovered by Mark
Russinovich [29]. When XCP installs its active protection software, it also installs a second program--the
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rootkit--that conceals any file, process, or registry key whose name begins with the prefix $sys$. The
result is that XCP's main installation directory, and most of its registry keys, files, and processes, become
invisible to normal programs and administration tools.
The rootkit is a kernel-level driver named $sys$aries that is set to automatically load early in the boot
process. When the rootkit starts, it hooks several Windows system calls by modifying the system service
dispatch table (the kernel's KeServiceDescriptorTable structure) which is an array of pointers to
the kernel functions that implement basic system calls. The rootkit changes five of these addresses to point
to functions within the rootkit, so that calls to the patched system calls are handled by the rootkit rather than
the original kernel function. The rootkit calls the real kernel function with the same parameters and filters
the results before returning them to the application.
The system calls intercepted by the rootkit are:
1. NtQueryDirectoryFile ­ This function is used to list the contents of a directory; the rootkit ver-
sion filters out directory entries that begin with $sys$, rendering such files and directories invisible
to applications.
2. NtCreateFile ­ This call is used for creating and opening files. The rootkit version returns an
invalid filename error when programs attempt to open existing files with names starting with $sys$,
protecting XCP's files from reading or writing by other programs.
3. NtQuerySystemInformation ­ One use of this function is to obtain a list of running processes.
The rootkit filters out any processes with names prefixed by $sys$, making them invisible to other
applications.
4. NtEnumerateKey ­ This function returns a list of the subkeys of a registry key. The rootkit fil-
ters the results to remove subkeys with names starting with $sys$. Note that it does not conceal
individual fields within the registry ("values" in Windows parlance) with names starting with $sys$.
5. NtOpenKey ­ This function opens a registry key for reading or modifying. The rootkit intercepts this
function call but does not alter its behavior. Its authors may have intended to restrict access to hidden
registry keys in the same way that the hooked NtQueryDirectoryFile call restricts access to
hidden files, but they did not ship a working implementation of this behavior.
On intercepting a function call, the rootkit checks the name of the calling process. If the name of
the calling process begins with $sys$, the rootkit returns the results of the real kernel function without
alteration so that XCP's own processes have an accurate view of the system.
The XCP rootkit increases users' vulnerability to attack by allowing any software to hide--not just XCP.
Malware authors can exploit the fact that any files, registry keys, or processes with names beginning in
$sys$ will be hidden, thereby saving the trouble of installing their own rootkits. Malware that lacks the
privileges to install its own rootkit can still rely on XCP's rootkit.
Only kernel-level processes can patch the Windows system service dispatch table, and only privileged
users--normally, members of the Administrators or Power Users groups--can install such processes. (XCP
itself requires these privileges to install.) Malicious code running as an unprivileged user can't normally
install a rootkit that intercepts system calls. But if the XCP rootkit is installed, it will hide all programs
that adopt the $sys$ prefix so that even privileged users will be unable to see them. This vulnerability has
already been exploited by at least two Trojan horses seen in the wild [21, 17].
Another privilege escalation attack facilitated by the XCP rootkit allows an unprivileged application to
crash the system. Russinovich demonstrated this problem using an automated testing program he created
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called NTCRASH2 [30]. This utility makes repeated system calls with randomly generated invalid parame-
ters. The original Windows kernel functions handle invalid inputs correctly and the system remains stable,
but with the XCP rootkit installed, certain invalid inputs result in a system crash.
We investigated the specific circumstances when these crashes occur. The rootkit's implementation of
NtCreateFile can cause a crash if it is passed an invalid pointer as its ObjectAttributes argument,
or if it is passed a valid ObjectAttributes structure that points to a ObjectName structure with
an invalid Buffer pointer. We do not believe that an attacker could exploit these flaw to execute code;
however, they do allow an unprivileged user to bring the system to a halt. As Russinovich and others have
pointed out, these problem illustrates the security danger of installing software in secret. Users experiencing
system instability due to these rootkit bugs would have great difficulty diagnosing the problem, since they
likely would be unaware of the rootkit's presence.
7.3
Deactivating XCP
XCP's active protection is more complicated to deactivate than MediaMax's, because it comprises several
processes that are more deeply entangled in the system configuration and are hidden by the XCP rootkit.
Deactivation requires a three-step procedure, which we describe here in detail so that affected users can
decontaminate their systems.
1. The first step is to remove the rootkit. From the command prompt, run sc delete $sys$aries.
Delete the rootkit's program file %windir%\system32\$sys$filesystem\aries.sys, and
reboot the system. Disabling the rootkit exposes the previously hidden files, registry entries, and
processes.
2. Next, edit the system registry to remove references to XCP's filter drivers and CoDeviceInstallers.
XCP uses the Windows filter driver facility to intercept commands to the CD drives and IDE bus.
If these filter drivers are not removed, the CD and IDE device drivers will fail to initialize after the
program files for the filter drivers are deleted. This can cause the CD drives to malfunction, or, worse,
cause the system to fail to boot because the IDE device driver is disabled.
First remove references to the $sys$cor filter driver, which intercepts commands sent to the IDE
device. Use the Windows Registry Editor to search for occurences of $sys$cor in registry entries
named UpperFilters. Edit each list of filters to remove the reference to $sys$cor. (You will
need to temporarily change the security permissions on the enclosing registry key to grant yourself
permission to edit the filters list.) References to this filter driver may occur in multiple registry keys;
be sure to remove them all.
Repeat this step to remove references to the $sys$crater filter driver, which intercepts commands
sent to the CD drive. This filter driver appears in devices' LowerFilters lists. Be sure to remove
all occurences.
Search the registry once again for $sys$caj.dll. This file is configured as a CoDeviceInstaller
for the CD-ROM and IDE devices. It installs the filter drivers when any new CD drive or IDE bus
device is configured. Remove the lines from any list of CoDeviceInstallers in which they appear:
$sys$caj.dll,CoInstallCdrom, $sys$caj.dll,CoInstallPC.
3. The next step is to delete the XCP services and remove the XCP program files. Open a command
prompt and issue these commands:
sc delete $sys$crater
sc delete $sys$lim
sc delete $sys$oct
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sc delete cd proxy
sc delete $sys$drmserver
sc delete $sys$cor
del %windir%\system32\$sys$filesystem\crater.sys
del %windir%\system32\$sys$filesystem\lim.sys
del %windir%\system32\$sys$filesystem\oct.sys
del %windir%\system32\drivers\$sys$cor.sys
del %windir%\system32\$sys$caj.dll
del %windir%\system32\$sys$upgtool.exe
Reboot and remove two remaining XCP program files:
del %windir%\CDProxyServ.exe
del %windir%\system32\$sys$filesystem\$sys$DRMServer.exe
Performing these steps will deactivate the XCP active protection, leaving only the passive protection on
XCP CDs in force. The procedure could easily be automated to create a point-and-click removal tool.
7.4
Impact of Spyware Tactics
The use of rootkits and other spyware tactics harms users by undermining their ability to manage their
computers. If users lose effective control over which programs run on their computers, they can no longer
patch malfunctioning programs or remove unneeded programs. Managing a system securely is difficult
enough without spyware tactics making it even harder.
Though it is no surprise that spyware tactics would be attractive to DRM designers, it is a bit surprising
that mass-market DRM vendors chose to use those tactics despite their impact on users. If only one vendor
had chosen to use such tactics, we could write it off as an aberration. But two vendors made that choice,
which is probably not a coincidence. We suspect that the vendors let the lure of platform building override
the risk to users.
7.5
Summary of Deactivation Attacks
Ultimately, there is little a CD DRM vendor can do to stop users from deactivating active protection software.
Vendors' attempts to frustrate users' control of their machines are harmful and will trigger a strong backlash
from users. In practice, vendors will probably have to provide some kind of uninstaller--users will insist on
it, and some users will need it to deal with the bugs and incompatibilities that crop up inevitably in complex
software. Once an uninstaller is released, users can use it to remove the DRM software. Determined users
will be able to keep CD DRM software off their machines.
8
Uninstallation
The DRM vendors responded to user complaints about spyware-like behavior by offering uninstallers that
would remove their software from users' systems. Uninstallers had been available before but had been very
difficult to acquire. For example, to get the original XCP uninstaller, a user had to fill out an online form
involving personal information, then wait a few days for a reply email, then fill out another online form and
install some software, then wait a few days for yet another email, and finally click a URL in the last email.
It is hard to explain this complexity of this procedure, except as a way to deter users from uninstalling XCP.
The uninstallers, when users did manage to get them, did not behave like ordinary software uninstallers.
Normal uninstallers are programs that can be acquired and used by any user who has the software. The
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first XCP uninstaller was customized for each user so that it would only work for a limited time and only
on the computer on which the user had filled out the second form. This meant, for example, that if a user
uninstalled XCP but it was reinstalled later--say, if the user inserted an XCP CD--the user could not use
the same uninstaller again but would have to go through the entire process again to request a new one.
Customizing the uninstaller is more difficult, compared to a traditional uninstaller, for both vendor and
user, so it must benefit the vendor somehow. Probably the benefit is to the vendor's platform building
strategy, which takes a step backward every time a user uninstalls the software. Customizing the uninstaller
allows the vendor to control who receives the uninstaller and to change the terms under which it is delivered.
As user complaints mounted, Sony-BMG announced that unrestricted uninstallers for both XCP and
MediaMax would be released from the vendors' web sites. Both vendors chose to make these uninstallers
available as ActiveX controls. By an unfortunate coincidence, both uninstallers turned out to open the same
serious vulnerability on any computer where they were used.
8.1
MediaMax Uninstaller Vulnerability
The MediaMax uninstaller used a proprietary ActiveX control, called AxWebRemove.ocx, created and
signed by SunnComm. Users visiting the MediaMax uninstaller web page were prompted to install the
control, then the web page uninstalled MediaMax by invoking one of the control's methods. This method,
Remove, took two parameters: key, and validate url. When Remove was called it issued an HTTP
GET to validate url to validate key. If key was valid, the server at validate url would respond
with the message true,<uninstall url>, where uninstall url was the URL of a DLL file containing code
to uninstall MediaMax. The control would retrieve this DLL file from the Internet and save it to a temporary
location, then call a function in the DLL named ECF7 to perform the uninstallation. If the function returned
success, the control would issue a second HTTP GET request to validate url to report that the uninstall
was successful and that the single-use key should be retired.
This design is vulnerable because the control accepts an arbitrary validate url parameter and does
not check that the DLL supplied by the server at that URL is authentic. The ActiveX control is not itself
removed during the uninstallation process, so its methods can be invoked later by any web page without
further browser security warnings. An attacker can create a web page that invokes the Remove method and
provides a validate url pointing to a page under the attacker's control. This page can accept whatever
key is presented and return an uninstall url pointing to a DLL created by the attacker. When the
MediaMax control executes the uninstall function in this file, arbitrary attacker code will execute on the
user's machine.
8.2
XCP Uninstaller Vulnerability
The XCP uninstaller contains the same design flaw and is only slightly more difficult to exploit. XCP's
ActiveX-based uninstaller invokes a proprietary ActiveX control named CodeSupport.ocx. (Early ver-
sions of XCP's rootkit removal patch utilized the same control.) Usually this control is installed in the
second step of the three-step XCP uninstall process. In this step, a pseudorandom code generated by the
ActiveX control is sent to the XCP server. The same code is written to the system registry. Eventually the
user receives an email with a link to another web page that uses the ActiveX control to remove XCP, but only
after verifying that the correct code is in the registry on the local system. This check tethers the uninstaller
to the machine from which the uninstallation request was made. Due to this design, the vulnerable control
may be present on a user's system even if she never performed the step in the uninstallation process where
XCP is removed.
Matti Nikki first noted that the XCP ActiveX control contains suspiciously-named methods, including
InstallUpdate(url), Uninstall(url), and RebootMachine() [25]. He demonstrated that
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the control was still present after the XCP uninstallation was complete, and that its methods (including one
that rebooted the computer) were scriptable from any web page without further browser security warnings.
We found that the InstallUpdate and Uninstall methods have an even more serious flaw. Each
takes as an argument a URL pointing to a specially formatted archive that contains updater or uninstaller
code and data files. When these methods are invoked, the archive is retrieved from the provided URL and
stored in a temporary location. For the InstallUpdate method, the ActiveX control extracts from the
archive a file named InstallLite.dll and calls a function in this DLL named InstallXCP.
Like the MediaMax ActiveX control, the XCP control does not validate the download URL or the
downloaded archive. The only barrier to using the control to execute arbitrary code is the proprietary format
of the archive file. We determined the format by disassembling the control. The archive file consists of
several blocks of gzip-compressed data, each storing a separate file and preceded with a short header. At
the end of the archive, a catalog structure lists metadata for each of the blocks, including a 32-bit CRC. The
control verifies this CRC before executing code from the DLL.
With knowledge of this file format, we constructed an archive containing benign proof-of-concept ex-
ploit code. The most difficult detail was the CRC, which is computed with an apparently proprietary algo-
rithm that proved tedious to reverse engineer. We saved the trouble by having the ActiveX control compute
the CRC for us. The control checks the CRC by computing a CRC for the file data in the archive and verify-
ing that it matches the CRC specified in the archive catalog. We inserted a break point where the comparison
occurs and ran the control on an archive containing code we prepared. We then took the CRC computed by
the control and placed it in the archive catalog. Thus modified, the archive passed the CRC check and the
ActiveX control executed our code. (This illustrated why digital signatures, rather than CRCs, must be used
to validate code from untrusted sources.)
This procedure would allow a malicious web site to execute arbitrary code on the user's machine. Like
the MediaMax uninstaller flaw, it is especially dangerous because users who have completed the XCP unin-
stallation may not be aware that they are still vulnerable.
Obviously, these vulnerabilities could have been prevented by careful design and programming. But they
were only possible at all because the vendors chose to deliver the uninstallers via this ActiveX method rather
than using an ordinary download. We conjecture that the vendors made this choice because they wanted to
retain the ability to rewrite, modify, or cancel the uninstaller later, in order to further their platform building
strategy.
9
Compatibility and Software Updates
Compared to other media on which software is distributed, compact discs have a very long life. Many
compact discs will still be inserted into computers and other players twenty years or more after they are first
bought. If a particular version of DRM software is shipped on a new CD, that software version may well try
to install and run decades after it was developed. The same is not true of most software, even when shipped
on a CD-ROM. Very few if any of today's Windows XP CDs will be inserted into computers in 2026; but
today's music CDs will be, so their DRM software must be designed carefully for future compatibility.
The software should be designed for safety, so as not to cause crashes or malfunction of other software,
and may be designed for efficacy, to ensure that its anti-copying features remain effective.
9.1
Supporting Safety by Deactivating Old Software
Safety is easier to achieve, and probably more important. One approach is to design the DRM software to
be inert and harmless on future systems. Both XCP and MediaMax do this by relying on Windows autorun,
which is likely to be disabled in future versions of Windows for security reasons. If the upcoming Windows
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Vista disables autorun by default, XCP and MediaMax will be inert on most Vista systems. Perhaps XCP
and MediaMax used autorun for safety reasons; but more likely, this choice was expedient for other reasons.
Another safety technique is to build in a sunset date, after which the software will make itself inert. A
sunset would improve safety but would have relatively little effect on record label revenue for most discs, as
we expect nearly all revenue from the disc to have been extracted from the customer in the first three years
after she buys the disc. If more copies of the disc were pressed, with updated DRM software, these could
have a later sunset.
9.2
Updating the Software
When a new version of DRM software is released, it can be shipped on newly pressed CDs, but existing
CDs cannot be modified retroactively. Updates for existing users can be delivered either by download or
on new CDs. Downloads are faster but require an Internet connection; CD delivery is slower but can reach
non-networked machines.
Users will generally cooperate with updates that help them, by improving safety, or otherwise making
the software more useful. But updates to retain the efficacy of the software's usage controls will not be
welcomed by users. Usage controls provide no value to individual user, but only reduce what the user can do
with music from a disc. Because usage controls typically control some uses that are allowed under copyright
law, even law-abiding users would prefer to avoid usage controls, and therefore would not welcome updates
that prolonged their efficacy.
Users have many ways to stop updates from downloading or installing, for example by write-protecting
the software's code so that it cannot be updated, or using a personal firewall to block network connections
to the vendor's download servers. System security tools, which are designed generally to stop unwanted
network connections, downloads, and code installation, can be set to treat CD DRM software as malware. If
users want to block updates, makers of system security tools will have an incentive to provide tools capable
of doing so.
A DRM vendor who wants to deliver unwanted updates has two options. First, the vendor can simply
offer updates and hope some users will not bother to block them. For the vendor and record label, this is
better than nothing. Alternatively, the vendor can try to force users to accept updates.
9.3
Forcing Updates
If a user has the ability to block DRM software updates, a vendor who wants an update must somehow
convince the user that updating is in her best interest. One approach is to make a non-updated system
painful to use.
Ruling out dangerous and legally risky tactics such as logic bombs that destroy the user's system or hold
her (unrelated) data hostage, the vendor's strongest tactic for forcing updates is to make the DRM software
block all access to protected CDs until the user accepts an update. The DRM software might check with
a network server, which would produce periodically a digitally signed and dated certificate listing allowed
versions of the DRM software. If the software on the user's system found that its version number was not
on the list (or if it could not get a recent list), it would block all access to protected discs. The user would
then have to update to a new version to get access to her protected CDs.
This approach would convince some users to update, and would thereby prolong the DRM's efficacy for
those users. But it has several drawbacks. If the computer is not networked, the software will eventually
lock down because it cannot get certificates. (If the software kept working in this case, users could avoid
updates by preventing the DRM software from making network connections.) A bug in the software could
cause an accidental but irreversible lockdown. The software could lock itself down if the vendor's Internet
site is shut down, for example if the vendor goes bankrupt.
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Strong-arm tactics can also be counterproductive, by giving the user further reason to defeat or remove
the DRM software.12 The software is more likely to remain on the user's system if it does not behave
annoyingly. Trying to force updates can reduce the DRM system's efficacy if it convinces users to remove
the DRM altogether.
Users will tend to be suspicious of software updates in any case. From the user's standpoint, every
software update is a security risk, which could carry hostile or buggy code onto the user's system. Users
will worry that the update adds a security hole or backdoor into the vendor's software, or that the encryption
key that the vendor uses to sign updates has been compromised. Careful users try to minimize the amount
of new software coming onto their systems, and so they will tend to resist software updates, especially
mandatory ones.
Given the problems with forced updates, and the user backlash they likely would have triggered, we are
not surprised that neither XCP nor MediaMax tried to force updates.
10
Conclusion
Our analysis of Sony-BMG's CD DRM carries wider lessons for content companies, DRM vendors, policy-
makers, end users, and the security community. We draw six main conclusions.
First, the design of DRM systems is driven strongly by the incentives of the content distributor and the
DRM vendor, but these incentives are not always aligned. Where they differ, the DRM design will not
necessarily serve the interests of copyright owners, not to mention artists.
Second, DRM, even if backed by a major content distributor, can expose users to significant security
and privacy risks. Incentives for aggressive platform building drive vendors toward spyware tactics that
exacerbate these risks.
Third, there can be an inverse relation between the efficacy of DRM and the user's ability to defend the
computer from unrelated security and privacy risks. The user's best defense is rooted in understanding and
controlling which software is installed on the computer, but many DRM systems rely on undermining the
user's understanding and control.
Fourth, CD DRM systems are mostly ineffective at controlling uses of content. Major increases in
complexity have not increased their effectiveness over that of early schemes, and may in fact have made
things worse by creating more avenues for attack. We think it unlikely that future CD DRM systems will do
better.
Fifth, the design of DRM systems is only weakly connected to the contours of copyright law. The
systems make no pretense of enforcing copyright law as written, but instead seek to enforce rules dictated
by the label's and vendor's business models. These rules, and the technologies that try to enforce them,
implicate other public policy concerns, such as privacy and security.
Finally, the stakes are high. Bad DRM design choices can seriously harm users, create major liability
for copyright owners and DRM vendors, and ultimately reduce artists' incentive to create.
Acknowledgments
We are grateful for the expert legal advice of Deirdre Mulligan and her colleagues at U.C. Berkeley: Aaron
Perzanowski, Sara Adibisedeh, Azra Medjedovic, Brian W. Carver, Jack Lerner, and Joseph Lorenzo Hall.
We are also grateful to Clayton Marsh at Princeton. Sadly, research of this type does seem to require support
from a team of lawyers.
12Users could also mislead the DRM software about the date and time, but most users with the inclination to do that would
probably just remove the DRM software altogether.
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We thank the readers of Freedom to Tinker for their comments on partial drafts that we posted there;
thanks especially to C. Scott Ananian, Randall Chertkow, Tim Howland, Edward Kuns, Jim Lyon, Tobias
Robison, Adam Shostack, Ned Ulbricht, and several pseudonymous commenters. Shirley Gaw, Janek Klawe,
and Harlan Yu provided helpful last-minute suggestions. Thanks to Claire Felten for help with copy-editing.
This material is based upon work supported under a National Science Foundation Graduate Research
Fellowship. Any opinions, findings, conclusions or recommendations expressed in this publication are those
of the authors and do not necessarily reflect the views of the National Science Foundation.
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[19] Auguste Kerckhoffs. La cryptographie militaire. J. des Sciences Militaires, 9:161­191, 1883.
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