Ed and I are working on an academic paper, “Lessons from the Sony CD DRM Episode”, which will analyze several not-yet-discussed aspects of the XCP and MediaMax CD copy protection technologies, and will try to put the Sony CD episode in context and draw lessons for the future. We’ll post the complete paper here next Friday. Until then, we’ll post drafts of a few sections here. We have two reasons for this: we hope the postings will be interesting in themselves, and we hope your comments will help us improve the paper.
Today’s excerpt is from the middle of the paper, where we’re wading through details about the copy protection systems and the techniques they use to recognize protected CDs.
Please note that this is a draft and should not be formally quoted or cited. The final version of our entire paper will be posted here when it is ready.
Attacks on Disk Recognition
The active protection mechanisms introduced earlier selectively 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 attached to the CD-ROM and IDE devices called
cor.sys. 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 mechanism 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.
Any disc recognition system must involve detecting some identifying feature on discs protected by a particular scheme. Ideally, such a feature would satisfy these requirements:
- Uniqueness. The feature should identify protected discs without accidentally triggering the copy protection on unprotected titles.
- 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.
- Indelibility. The feature should be difficult 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.
- Unforgeability. It should be difficult to apply the feature to an unprotected album without the cooperation of the protection vendor, even if the adversary has access to protected discs.
This last requirement stems from the business strategies of the copy protection vendors. As discussed in earlier, many of these vendors are pursuing a platform building strategy. The biggest obstacle to the success of an active protection system is getting the protection software installed on client machines. Once installed, the software can regulate access to all discs protected by the scheme, even if the user learns to disable autorun or refuse future CD DRM installation requests. Thus each completed installation increases the effectiveness of the protection platform and heightens its value to the protection vendor and its music label clients.
Being widely installed adds value to these copy protection systems, but it also exposes them to a new class of attacks. The protection companies earn revenue from record labels who license their schemes, typically paying some fee per title or per copy. This revenue stream may be threatened if disc publishers can mark their discs as protected without paying.
There are advantages and disadvantages for the person placing the 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 from licensed titles. They would 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, they could invoke multiple layers of security and provide stronger protection than is available with any single technique, all without paying. (It is possible that protection producers could have legal remedies against such free riders, such as through a patented identification feature, but we are unaware of any patents that cover the identification features known to be in use. Even if some kind of legal remedy is available, it’s worth designing the mark to prevent the problem too, at least if the cost of doing so is low.) 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.
How MediaMax Recognizes Protected Discs
To find out how the disc recognition mechanisms employed by CD DRM systems stack up the ideal requirements, we examined the recognition system built into MediaMax CD-3 and MM-5 systems. The MediaMax system drew our attention because MediaMax’s creators have touted their advanced disc identification capabilities, including the ability to identify individual tracks within a compilation as protected, and well as their platform-building strategy. (The XCP scheme appears to use a less sophisticated disc recognition system based on a marker stored in the data track of protected discs. We may talk more about it later.)
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 virtual machine running on top of a Fedora Linux host system, which we modified by patching the kernel IDE-SCSI device to log all drive activity.
With this setup we observed that the MediaMax software executes a disc recognition procedure immediately upon the insertion of a CD. The MediaMax driver reads two sectors of audio data at a specific offset from the beginning of audio tracks—approximately 365 and 366 frames in (a CD frame is 1/75 second). 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. (By locating the watermark nearly five seconds after the start of the track, MediaMax reduces the likelihood that it will occur in a very quiet passage, where it might be more audible, and makes it more difficult to crop out.)
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 is a 1500 lb. granite slab unearthed by French archaeologists in Rosetta, Egypt, in 1799. A single Ptolemaic decree is written on the stone in three scripts: ancient hieroglyphics, demotic (simplified) 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 (BMG, 2004), 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 revealed 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 protection 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. 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 approximately 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 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 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) and then patching up the two other bits, according to this table:
(This design seems to be intended to lessen the audible distortion caused by 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 LSBs that makes the watermark easier to detect or remove.)
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:
The 288-bit raw watermark is detected by the active protection software only when it has certain properties, as shown in the sequence below. In the 288-bit sequence, 96 positions have fixed bit values, either 0 or 1. The trailing 32 positions have arbitrary values (as indicated by _), and can be used to store a 32-bit disc-specific value. The other 192 positions are divided into 32 groups of linked values (denoted a-z and alpha-zeta). In each group, three positions share the same value and three share the complement value. This allows the scheme to encode a second 32-bit value, though in actual discs it appears to be a different random value in each of the 30 mark clusters.
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 manipulate the audio in the frequency domain and attempt to resist removal by lossy compression, multiple conversions between digital and analog formats, and other common transformation. 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 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 MediaMax 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 LSBs of each sample–to forge it with minimal loss of fidelity. Such an attacker could transplant the three LSBs 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.
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. 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, and with CD DRM systems. Making marks that are both indelible and unforgeable is likely much more difficult. There 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, either situation leads to the same result (no protection), the attacker’s power is multiplied.
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:
- 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 SL1 := SignKS(Hash(L1)). SL1 is then stored at a second location 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.
- 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.
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 vulnerable to corruption by a single bit too while being far less resistant to forgery. Robustness to removal, while desirable in principle, is of limited value in real CD DRM applications. 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 larger.