A paper written
by Scott A. Craver
, John R McGregor
, Min Wu
, Bede Liu
, Adam Stubblefield
, Ben Swartzlander
, Dan S. Wallach
and Edward W. Felten
. It chronicles
, reverse engineering
of the proposed audio watermarking schemes presented
on the HackSDMI challenge
The team has decided to forfeit the monies paid for winning the challenge in order to present their works but yet are still legally threatened by the SDMI under the Digital Millennium Copyright Act.
(2001/04/26) The presentation of this paper has officially been withdrawn from the IHW2001 conference. DeCSS-like cease-and-desist requests will probably follow ...
Reading Between the Lines:
Lessons from the SDMI Challenge
Scott A. Craver1, John R McGregor1, Min Wu1,
Adam Stubblefield2, Ben Swartzlander2, Dan S.
Drew Dean3, and Edward W. Felten4
1 Dept. of Electrical Engineering, Princeton University
2 Dept. of Computer Science, Rice University
3 Computer Science Laboratory, Xerox Palo Alto Research Center
4 Dept. of Computer Science, Princeton University
Abstract. The Secure Digital Music Initiative is a consortium of parties
interested in preventing piracy of digital music, and to this end they are
developing architectures] for content protection on untrusted platforms. SDMI
recently held a challenge to test the strength of 4 watermarking technologies,
and 2 other security technologies. No documentation explained the implementations
of the technologies, and neither watermark embedding nor detecting software
was directly accessible to challenge participants. We nevertheless accepted
the challenge, and learned a great deal about the inner workings of the
technologies. We report on our results here.
The Secure Digital Music Initiative (SDMI), a consortium of music-industry
companies, is working to develop and standardize technologies that give music
publishers more control over what consumers can do with recorded music that
they buy. SDMI has been a somewhat secretive organization, releasing little
information to the public about its goals, deliberations, and technology.
In September 2000, SDMI announced a "public challenge" in which it invited
members of the public to try to break certain data-encoding technologies
that SDMI had developed 3. The challenge offered a valuable window into
SDMI, not only into its technologies but also into its plans and goals. We
decided to use the challenge to learn as much as we could about SDMI. This
paper is the result of our study.1 Section 2 presents an overview
of the HackSDMI challenge. Section 3 analyzes the watermark challenges. Section
4 analyzes the non-watermark challenges. Finally, we present our conclusions
in section 5.
1 The SDMI challenge offered a small cash payment to be shared among
everyone who broke at least one of the technologies and was willing to sign
a confidentiality agreement giving up all rights to discuss their findings.
The cash prize amounted to the price of a few days of time from a skilled
computer security consultant, and it was to be split among all successful
entrants, a group that we suspected might be significant in size. We chose
to forego the payment and retain our right to publish this paper.
2 The SDMI Challenge
The SDMI challenge extended over roughly a three-week period, from September
15, 2000 until October 8, 2000. The challenge actually consisted of six
sub-challenges, named with the letters A through F, each involving a different
technology developed by SDMI. We believe these challenges correspond to
submissions to the SDMI's Call for Proposals for Phase II Screening Technology
4. According to this proposal, the watermark's purpose is to restrict an
audio clip which is compressed or has previously been compressed. That is,
if the watermark is present an audio clip may yet be admitted into an SDMI
device, but only if it has not been degraded by compression. For each challenge,
SDMI provided some information about how a technology worked, and then challenged
the public to create an object with a certain property. The exact information
provided varied among the challenges. We note, though, that in all six cases
SDMI provided less information than a music pirate would have access to in
2.1 Watermark Challenges
Four of the challenges (A, B, C, and F), involved watermarking technologies,
in which subtle modifications are made to an audio file, to encode copyright
control information without perceptible change in how the file sounds. Watermarks
can be either robust or fragile. Robust watermarks are designed to survive
common transformations like digital-to-audio conversion, compression and
decompression, and the addition of small amounts of noise to the file. Fragile
watermarks do not survive such transformations, and are used to indicate
modification of the file. For each of the four watermark challenges, SDMI
provided three files:
- File 1: an unwatermarked song;
- File 2: File 1, with a watermark added; and
- File 3: another watermarked song.
The challenge was to produce a file that sounded just like File 3 but did
not have a watermark -- in other words, to remove the watermark from File
SDMI provided an on-line "oracle" for each challenge. Entrants could email
a file to the oracle, and the oracle would tell them whether their submission
satisfied the challenge, that is, whether it contained no detectable watermark
while still sounding like File 3. Entrants were given no information about
how watermark information was stored in the file or how the oracle detected
watermarks, beyond the information that could be deduced from inspection
of the three provided files.
2.2 Challenges D and E
Challenge D concerned a technology designed to prevent a song from being
separated from the album in which it was issued. Normally, every Compact
Disc contains a table of contents, indicating the offsets and lengths of
each audio track, followed by the audio data itself. Challenge D adds an
"authenticator" track (approximately 50ms of very quiet audio,) a digital
signature derived from the table of contents, which is supposed to be difficult
to compute for an arbitrary CD. Challenge D is discussed in more detail in
Challenge E involved a technology similar to D, but one which would be immune
the obvious attack on technology D, in which one compiled an unauthorized
CD with the same table of contents as an authorized one, for which the
authenticator track is given. Unfortunately, this challenge was constructed
in a way that made it impossible to even start analyzing the technology.
SDMI provided an oracle for this challenge, but unfortunately provided no
music samples of any kind, so there was no way to determine what the oracle
might be testing for.
Given these facts, we decided not to analyze Challenge E. It is discussed
briefly in Section 4.2.
3 The Watermarking Schemes
In this section, we describe our attack(s) on each of the four watermark
challenges (A,B,C,F). Our success was confirmed by emails received from SDMI's
Fig. 1. The SDMI watermark attack problem. For
each of the four watermark challenges, Sample-1, sample-2, and sample-3 are
provided by SDMI sample-4 is generated by participants in the challenge and
submitted to SDMI oracle for testing.
Figure 1 provides an overview of the challenge goal. As mentioned earlier,
there are three audio files per watermark challenge: an original and watermarked
version of one clip, and then a watermarked version of a second clip, from
which the mark is to be removed. All clips were 2 minutes long, sampled at
44.1kHz with 16-bit precision.
The reader should note one serious flaw with this challenge arrangement.
The goal is to remove a robust mark, while these proposals appear to be Phase
II watermark screening technologies 4. As we mentioned earlier, a Phase
II screen is intended to reject audio clips if they have been compressed,
and presumably compression degrades a fragile component of the watermark.
An attacker need not remove the robust watermark to foil the Phase II screen,
but could instead repair the modified fragile component in compressed audio.
This attack was not possible under the challenge setup.
3.1 Attack and Analysis of Technology A
A reasonable first step in analyzing watermarked content with original, unmarked
samples is differencing the original and marked versions in some way. Initially,
we used sample-by-sample differences in order to determine roughly what kinds
of watermark- ing methods were taking place. Unfortunately, technology A
involved a slowly varying phase distortion which masked any other cues in
a sample-by-sample difference. We ultimately decided this distortion was
a pre-processing separate from the watermark, in part because undoing the
distortion alone did not foil the oracle.
The phase distortion nevertheless led us to attempt an attack in which both
the phase and magnitude change between sample 1 and sample 2 is applied to
sample 3. This attack was confirmed by SDMI's oracle as successful, and
illustrates the general attack approach of imposing the difference in an
original-watermark pair upon another media clip. Here, the "difference" is
taken in the FFT domain rather than the time domain, based on our suspicions
regarding the domain of embedding. Note that this attack did not require
much information about the watermarking scheme itself, and conversely did
not provide much extra insight into its workings.
A next step, then, is to compute the frequency response H(w) =
W(w)/O(w) of the watermarking process for segments of audio,
and observe both |H(w)| and the corresponding impulse response
h(t). If the watermark is based on some kind of linear filter,
whose properties change slowly enough relative to the size of a frame of
samples, then this approach is ideal.
Figure 2 illustrates one frequency response and impulse response about 0.3
seconds into the music. These responses are based on FFTs of 882 samples,
or one fiftieth second of music. As can be clearly seen, a pair of sinusoidal
ripples are present within a certain frequency band, approximately 8-16Khz.
Ripples in the frequency domain are indicative of echoes in the time domain,
and a sum of sinusoids suggested the presence of multiple echoes. The
corresponding impulse response h(t) confirms this. This pattern
of ripples changes quite rapidly from frame to frame.
Thus, we had reason to suspect a complex echo hiding system, involving multiple
time-varying echoes. It was at this point that we considered a patent search,
knowing enough about the data hiding method that we could look for specific
search terms, and we were pleased to discover that this particular scheme
appears to be listed as an alternative embodiment in US patent number 05940135,
awarded to Aris corporation, now part of Verance 5. This provided us with
little more detail than we had already discovered, but confirmed that we
were on the right track, as well as providing the probable identity of the
company which developed the scheme. It also spurred no small amount of discussion
of the validity of Kerckhoffs's criterion, the driving principle in security
that one must not rely upon the obscurity of an algorithm. This is, surely,
doubly true when the algorithm is patented.
Fig. 2. A short-term complex echo. Above, the
frequency response between the watermarked and original music, taken over
1/50 second, showing a sinusoidal ripple between 8 and 16 KHz. Below, the
corresponding impulse response. The sinusoidal pattern in the frequency domain
corresponds to a pair of echoes in the time domain.
The most useful technical detail provided by the patent was that the "delay
hopping" pattern was likely discrete rather than continuous, allowing us
to search for appropriate frame sizes during which the echo parameters were
constant. Data collection from the first second of audio showed a frame size
of approximately 882 samples, or 1/50 second. We also observed that the mark
did not begin until 10 frames after the start of the music, and that activity
also existed in a band of lower frequency, approximately 4-8 Khz. This could
be the same echo obscured by other operations, or could be a second band
used for another component in the watermarking scheme. A very clear ripple
in this band, indicating a single echo with a delay of about 34 samples,
appears shortly before the main echo-hopping pattern begins.
The next step in our analysis was the determination of the delay hopping
pattern used in the watermarking method, as this appeared to be the "secret
key" of the data embedding scheme. It is reasonable to suspect that the pattern
repeats itself in short order, since a watermark detector should be able
to find a mark in a subclip of music, without any assistance initially aligning
the mark with the detector's hopping pattern. Again, an analysis of the first
second revealed a pattern of echo pairs that appeared to repeat every 16
frames, as outlined in figure 3. The delays appear to fall within six general
categories, each delay approximately a multiple of 1/4 millisecond. The exact
values of the delays vary slightly, but this could be the result of the phase
distortion present in the music.
Fig. 3. The hypothesized delay hopping pattern
of technology A. Here two stretches of 16 frames are illustrated side-by-side,
with observed echoes in each frame categorized by six distinct delays: 2,
3, 4, 5, 6 or 7 times 0.00025 sec. Aside from several missing echoes, a pattern
appears to repeat every 16 frames. Note also that in each frame the echo
gain is the same for both echoes.
The reader will also note that in apparently two frames there is only one
echo. If this pattern were the union of two pseudorandom patterns chosen
from six possible delay choices, two "collisions" would be within what is
expected by chance.
Next, there is the issue of the actual encoded bits. Further work shows the
sign of the echo gain does not repeat with the delay-hopping pattern, and
so is likely at least part of an embedded message. Extracting such data without
the help of an original can be problematic, although the patent, of course,
outlines numerous detector structors which can be used to this end. We developed
several tools for cepstral analysis to assist us in the process. See 2
for in introduction to cepstral analysis; Anderson and Petitcolas 1 illustrate
its use in attacks on echo hiding watermark systems.
With a rapidly changing delay, normal cepstral analysis does not seem a good
choice. However, if we know that the same echo is likely to occur at multiples
of 16/50 of a second, we can improve detector capability by combining the
information of multiple liftered2 log spectra.
2 in accordance with the flopped vocabulary used with cepstral analysis,
"liftering" refers to the process of filtering data in the frequency domain
rather than the time domain. Similarly, "quefrencies" are frequencies of
ripples which occur in the frequency domain rather than the time domain.
Three detector structures are shown in figure 4. In all three, a collection
of frames are selected for which the echo delays are believed to be the same.
For each, the liftered log of an FFT or PSD of the frame is taken. In the
first two structures, we compute a cepstrum, for each frame, then either
average their squared magnitudes, or simply their squares, in hopes that
a spike of the appropriate quefrency will be clear in the combination. The
motivation for merely squaring the spectral coefficients comes from the
observation that a spike due to an echo will either possess a phase of
theta or theta + pi for some value theta. Squaring
without taking magnitudes can cause the echo phases to reinforce, whilst
still permitting other elements to combine destructively.
Fig. 4. Three cepstral detector structures.
In each case we have a collection of distinct frames, each believed to possess
echoes of the same delay. The first two compute cepstral data for each frame,
and sum their squares (or squared magnitudes) to constructively combine the
echo signal in all frames. The third structure illustrates a method for testing
a hypothesized pattern of positive and negative gains, possibly useful for
brute-forcing or testing for the presence of a known "ciphertext."
In the final structure, one cepstrum. is taken using a guess of the gain
sign for each suspect frame. With the correct guess, the ripple should be
strongest, resulting in the largest spike from the cepstral detector. Figure
5 shows the output of this detector on several sets of suspect frames. While
this requires an exponential amount of work for a given amount of frames,
it has a different intended purpose: this is a brute-forcing tool, a utility
for determining the most probable among a set of suspected short strings
of gain signs as an aid to extracting possible ciphertext values.
Fig. 5. Detection of an echo. A screenshot of
our CepstroMatic utility shows a combination of 4 separate frames of music,
each a fiftieth of a second long, in which the same echo delay was believed
to exist. Their combination shows a very clear ripple on the right, corresponding
to a clear cepstral spike on the left. This is a single echo at a delay of
33 samples, the delay suggested for these intervalus by the hypothesized
Finally, there is the issue of what this embedded watermark means. Again,
we are uncertain about a possible signalling band below 8Khz. This could
be a robust mark, signalling presence of a fragile mark of echoes between
8 and 16 KHz. The 8-16KHz band does seem like an unusual place to hide robust
data, unless it does indeed extend further down, and so this could very easily
be hidden information whose degredation is used to determine if music has
already been compressed.
Of course, knowledge of either the robust or fragile component of
the mark is enough for an attacker to circumvent the scheme, because one
can either remove the robust mark, or repair or reinstate the fragile mark
after compression has damaged it. As mentioned earlier, this possible attack
of repairing the fragile component appears to have been ruled out by the
nature of the SDMI challenge oracles. One must wait and see if real-world
attackers will attempt such an approach, or resort to more brute methods
or oracle attacks to remove the robust component.
3.2 Attack on Challenge B
We analyzed samp1b.wav and samp2b.wav using short-time FFT. Shown in Fig.
6 are the two FFT magnitudes for 1000 samples at 98.67 sec. Also shown is
the difference of the two magnitudes. A spectrum notch around 2800Hz is observed
for some segments of samp2b.wav and another notch around 3500Hz is observed
for some other segments of samp2b.wav. Similar notches are observed in
samp3b.wav. The attack fills in those notches of samp3b.wav with random but
bounded coefficient values. We also submitted a variation of this attack
involving different parameters for notch description. Both attacks were confirmed
by SDMI oracle as successful.
Fig. 6. Technology-B: FFT magnitudes of samp1b.wav
and samp2b.wav and their difference for 1000 samples at 98.67 sec.
3.3 Attacks on Challenge C
By taking the difference of samp1c.wav and samp2c.wav, bursts of narrowband
signal are observed, as shown in Fig. 7. These narrow band bursts appear
to be centered around 1350 Hz. Two different attacks were applied to Challenge
C. In the first at- tack, we shifted the pitch of the audio by about a
quartertone. In the second attack, we passed the signal through a bandstop
filter centered around 1350Hz. Our submissions were confirmed by SDMI oracle
as successful. In addition, the perceptual quality of both attacks has passed
the "golden ear" testing conducted by SDMI after the 3-week challenge.
Fig. 7. Challenge-C: Waveform of the difference
between samp1c.wav and samp2c.wav.
3.4 Attack on Challenge F
For Challenge F, we warped the time axis, by inserting a periodically varying
delay. The delay function comes from our study on Technology-A, and was in
fact initially intended to undo the phase distortion applied by technology
A. Therefore the perceptual quality of our attacked audio is expected to
be better than or comparable to that of the audio watermarked by Technology-A.
We also submitted variations of this at- tack involving different warping
parameters and different delay function. They were confirmed by SDMI oracle
4 The Non-Watermark Technologies
The HackSDMI challenge contained two "non-watermark" technologies. Together,
they appear to be intended to prevent the creation of "mix" CDs, where a
consumer might compile audio files from various locations to a writable CD.
This would be enforced by universally embedding SMDI logic into consumer
audio CD players.
4.1 Technology D
According to SDMI, Technology D was designed to require "the presence of
a CD in order to 'rip' or extract a song for SDMI purposes." The technology
aimed to accomplish this by adding a 53.3 ms audio track (four blocks of
CD audio), which we will refer to as the authenticator, to each CD.
The authenticator, combined with the CD's table of contents (TOC), would
allow a SDMI device to recognize SDMI compliant CDs. For the challenge, SDMI
provided 100 different "correct" TOC-authenticator pairs as well as 20 "rogue
tracks". A rogue track is a track length that does not match any of the track
lengths in the 100 provided TOCs. The goal of the challenge was to submit
to the SDMI oracle a correct authenticator for a TOC that contained at least
one of the rogue tracks.
The oracle for Technology D allowed several different query types. In the
first type, an SDMI provided TOC-authenticator combination is submitted so
a that user can "understand and verify the Oracle." According to SDMI, the
result of this query should either be "admit" for a correct pair or "reject"
for an incorrect pair. When we attempted this test a SDMI-provided pair,
the oracle responded that the submission was "invalid." After verifying that
we had indeed submitted a correct pair, we attempted several other submissions
using different TOC-authenticator pairs as well as different browsers and
operating systems3. We also submitted some pairs that the oracle
should have rejected; these submissions were also declared "invalid." Though
we alerted SDMI to this problem during the challenge, the oracle was never
repaired. For this reason, our analysis of Technology D is incomplete and
we lack definitive proof that it is correct. That having been said, we think
that what we learned about this technology, even without the benefit of a
correctly functioning oracle, is interesting.
3 Specifically, Netscape Navigator and Mozilla under Linux, Netscape
Navigator under Windows NT, and Internet Explorer under Windows 98 and 2000.
Analyzing the Signal Upon examination of the authenticator audio files,
we discovered several patterns. First, the left and right channels contain
the same information. The two channels differ by a "noise vector" u,
which is a vector of small integer values that range from -8 and 8. Since
the magnitude of the noise is so small, the noise vector does not significantly
affect the frequency characteristics of the signal. The noise values appear
to be random, but the noise vector is the same for each of the 100 provided
authenticator files. In other other words, in any authenticator file, the
difference between the left and right channels of the ith sample is
a constant fixed value ui. This implies that the noise vector
u does not encode any TOC-specific information.
Second, the signal repeats with a period of 1024 samples. Because the full
signal is 2352 samples long, the block repeats approximately 1.3 times. Similarly
to the left and right channels of the signal, the first two iterations of
the repeating signal differ by a constant noise vector v. The difference
between the ith sample of the first iteration and the ith sample
of the second iteration differ by a small (and apparently random) integer
value vi ranging from -15 to 15. In addition, v is
the same for each of the provided authenticator files, so v does not
encode any TOC-specific information.
Third, the first 100 samples and last 100 samples of the full signal are
faded in and faded out, respectively. This is illustrated in Figure 8. The
fade-in and fade-out are meaningless, however, because they simply destroy
data that is repeated in the middle of the file. We conjecture that this
fade-in and fade-out are included so that the audio signal does not sound
offensive to a human ear.
Fig. 8. In a Technology D Authenticator, the
signal fades in, repeats, and fades out.
Extracting the Data Frequency analysis on the 1024 sample block shows
that almost all of the signal energy is concentrated in the 16-20kHz range,
as shown in Figure 9. We believe this range was chosen because these frequencies
are less audible to the human ear. Closer examination shows that this l6-20kHz
range is divided up into 80 discrete bins, each of which appears to carry
one bit of information. As shown in Figure 10, these bits can be manually
counted by a human using a graph of the magnitude of signal in the frequency
Fig. 9. Magnitude vs. Frequency of Technology
Fig. 10. Individual Bits From a Technology D
Close inspection and pattern matching on these 80 bits of information reveals
that there are only 16 bits of information repeated 5 times using different
permutations. using the letters A-P to symbolize the 16 bits, these 5
permutations are described in Figure 11.
Fig. 11. The encoding of the 16 bits of data
in Technology D
Because of the malfunctioning oracle, we were unable to determine the function
used to map TOCs to authenticators, but given an actual SDMI device, it would
be trivial to brute force all 216 possibilities. Likewise, without
the oracle, we could not determine if there was any other signal present
in the authenticator (e.g., in the phase of the frequency components
with nonzero magnitude).
For the moment, let us assume that the hash function used in Technology D
has only 16 bits of output. Given the number of distinct CDs available, an
attacker should be able to acquire almost, if not all, of the authenticators.
We note that at 9 kilobytes each, a collection of 65,536 files would fit
nicely on a single CD. Many people have CD collections of 300+ discs, which
by the birthday paradox makes it more likely than not that there is a hash
collision among their own collection.
Our results indicated that the hash function used in Technology D could be
weak or may have less than 16 bits of output. In the 100 authenticator samples
provided in the Technology D challenge, there were 2 pairs of 16-bit hash
collisions. We will not step through the derivation here, but the probability
of two or more collisions occurring in n samples of X equally
likely possibilities is:
If the 16-bit hash function output has 16 bits of entropy, the probability
of 2 collisions occurring in n = 100 samples of X =
216 possibilities is 0.00254 (by the above 1.5 equation). If
X ~ 211.5, the chances of two collisions occurring is about
even. This suggests that either 4 bits of the 16-bit hash output may be outputs
of functions of the other 12 bits or the hash function used to generate the
16-bit signature is weak. It is also possible that the challenge designers
purposefully selected TOCs that yield collisions. The designers could gauge
the progress of the contestants by observing whether anyone submits authenticator
A with TOC B to the oracle, where authenticator A is equal to authenticator
B. Besides the relatively large number of collisions in the provided
authenticators, it appears that there are no strong biases in the authenticator
bits such as significantly more or less 1's than 0's.
4.2 Technology E
Technology E is designed to fix a specific bug in Technology D: the TOC only
mentions the length of each song but says nothing about the contents
of that song. As such, an attacker wishing to produce a mix CD would only
need to find a TOC approximately the same as the desired mix CD, then copy
the TOC and authenticator from that CD onto the mix CD. If the TOC does not
perfectly match the CD, the track skipping functionality will still work
but will only get "close" to track boundaries rather than reaching them
precisely. Likewise, if a TOC specified a track length longer than the track
we wished to put there, we could pad the track with digital silence (or properly
SDMI-watermarked silence, copied from another valid track). Regardless, a
mix CD played from start to end would work perfectly. Technology E is designed
to counter this attack, using the audio data itself as part of the authentication
The Technology E challenge presented insufficient information to be properly
studied. Rather than giving us the original audio tracks (from which we might
study the unspecified watermarking scheme), we were instead given the tables
of contents for 1000 CDs and a simple scripting language to specify a
concatenation of music clips from any of these CDs. 'Me oracle would process
one of these scripts and then state whether the resulting CD would be rejected.
While we could have mounted a detailed statistical analysis, submitting hundreds
or thousands of queries to the oracle, we believe the challenge was fundamentally
flawed. In practice, given a functioning SDMI device and actual SDMI-protected
content, we could study the audio tracks in detail and determine the structure
of the watermarking scheme.
In this paper, we have presented an analysis of the technology challenges
issued by the Secure Digital Music Initiative. Each technology challenge
described a specific goal (e.g., remove a watermark from an audio
track) and offered a Web-based oracle that would confirm whether the challenge
was successfully defeated.
We have reverse-engineered and defeated all four of their audio watermarking
technologies. We have studied and analyzed both of their "non-watermarking"
technologies to the best of our abilities given the lack of information available
to us and given a broken oracle in one case.
Some debate remains on whether our attacks damaged the audio beyond standards
measured by "golden ear" human listeners. Given a sufficient body of
SDMI-protected content using the watermark schemes presented here, we are
confident we could refine our attacks to introduce distortion no worse than
the watermarks themselves introduce to the the audio. Likewise, debate remains
on whether we have truly defeated technologies D and E. Given a functioning
implementation of these technologies, we are confident we can defeat them.
Do we believe we can defeat any audio protection scheme? Certainly, the technical
details of any scheme will become known publicly through reverse engineering.
Using the techniques we have presented here, we believe no public watermark-based
scheme intended to thwart copying will succeed. Other techniques may or may
not be strong against attacks. For example, the encryption used to protect
consumer DVDs was easily defeated. Ultimately, if it is possible for a consumer
to hear or see protected content, then it will be technically possible for
the consumer to copy that content.
1. R. J. ANDERSON, AND F. A. P. PETITCOLAS. On the limits of steganography.
IEEE Journal of Selected Areas in Communications 16,4 (May 1998),474-481.
2. R. P. BOGERT, M., AND J. W. TUKEY. The quefrency alanysis of time series
for echoes: Cepstrum, pseudo-autocovariance, cross-ceptsrum and saphe-cracking.
In Proceedings of the Symposium on Time Series Analysis (Brown University,
June 1962), pp. 209-243.
3. R. PETROVIC, J. M. WINOGRAD, K., AND E. METOIS. Apparatus and method for
encoding and decoding information in analog signals, Aug. 1999. US Patent
4. SECURE DIGITAL MUSIC INITIATIVE. Call for Proposals for Phase II Screening
Technology, Version 1.0, Feb. 2000.
5. SECURE DIGITAL MUSIC INITIATIVE. SDMI public challenge, Sept. 2000.