Reading Between
the Lines:
Lessons from the SDMI Challenge
Scott A. Craver1, John R McGregor1,
Min Wu1, Bede Liu1,
Adam Stubblefield2, Ben Swartzlander2, Dan S. Wallach2,
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.
1 Introduction
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.
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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 forgo 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 practice.
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 3.
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 Section 4.1.
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
oracles.
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 intervals
by the hypothesized delay-hopping pattern.
Finally, there is the issue of what this embedded watermark means. Again,
we are uncertain about a possible signaling band below 8Khz. This could be a
robust mark, signaling 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 degradation 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 as successful.
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 writeable 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 u[i]. 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 v[i] 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 domain.
Fig. 9. Magnitude vs. Frequency
of Technology D Authenticator
Fig. 10. Individual Bits From a
Technology D Authenticator
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.
ABCDEFGHIJKLMNOP
OMILANHGPBDCKJFE
PKINHODFMJBCAGLE
FCKLGMEPNOADJBHI
PMGHLECAKDONIFJB
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 process.
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.
5 Conclusion
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.
References
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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 No 05940135 http://www.delphion.com/details?pn=US05940135__.
4. SECURE DIGITAL MUSIC INITIATIVE. Call for Proposals
for Phase II Screening Technology, Version 1.0, Feb. 2000. http://www.sdmi.org/download/FRWG00022401-Ph2_CFPv1.0.PDF.
5. SECURE DIGITAL MUSIC INITIATIVE. SDMI public challenge,
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