快猫短视频

LAST October, Mark Russinovich stumbled across a mysterious program concealed deep within his computer鈥檚 hard disc. Not only did it seem to open the door to viruses, but when he tried to remove the program it crashed his computer. The real shock came, however, when he discovered that it had been distributed intentionally by entertainment giant Sony BMG.

In an attempt to stop music pirates, the company had added 鈥渆xtended copy protection鈥� software to around 50 of its music releases 鈥� about 5 million CDs in total. Play one on your computer and the software installs itself invisibly. Its purpose is to stop you making illegal copies, but Sony BMG had not expected the side effects.

After Russinovich revealed his findings in a blog, Sony BMG came under intense criticism and several law suits were filed against it. Eventually the company recalled all affected CDs, offered compensation to anyone who had purchased one and announced that it would re-examine its anti-piracy strategy.

This is not the first time that software designed to prevent unauthorised copying has had unwelcome consequences, and it is unlikely to be the last. The entertainment industry is about to break new ground in its attempts to control what you can and cannot enjoy at home.

Over the next few months, and with much fanfare, movie fans will be introduced to a new format 鈥� the blue-laser disc. Unlike DVDs, the disc is capable of storing movies with high-definition picture quality. The format is expected to be a big hit, and will probably have the publicity to match. However, you might not hear much at all about a piece of technology that will accompany it.

Called renewable copy protection, it is designed to deal pirates and hackers a death blow. For the first time, Hollywood studios will be able to update the copy protection in the disc player in your living room, with no need for an internet connection or phone line. The secret lies in the discs themselves. Every time you play one, it will copy encrypted data onto your machine and update anti-piracy software you might not even have known was there.

If the technology works, it will be noticed only by would-be pirates. Yet some think the technology is doomed to lose the fight against illegal copying. And if anything does go wrong, they warn, innocent consumers might find that their new blue-laser disc players will refuse to play favourite discs, or simply won鈥檛 access new ones.

Lasers to the rescue

The entertainment industry has a lot riding on high-definition television and blue-laser discs. HDTV is seen as the future of broadcasting. It is already in use in the US and Japan, and the technology is being rolled out across Europe.

Its backers know that if HDTV is to be a success, consumers need a way to record and play back movies. That鈥檚 where the new blue-laser disc comes in (See 鈥淪tory of the blues鈥�). It will be available in HD-DVD and Blu-ray disc formats, which look much like conventional DVDs but hold about five times as much data. To Hollywood however, they are a double-edged sword. By giving consumers blue-laser discs, players and recorders, the studios worry that they could be handing pirates access to a technology that can play, record and repeatedly copy a movie at near cinema quality. This could create a rerun of problems the industry faced with DVD technology.

Conventional DVDs use the Content Scrambling System (CSS), encryption technology designed to stop people using a computer to make clones of movie discs on blank DVDs. In 1999, two years after the launch of DVDs, the secret encryption keys used by CSS were posted on the internet in a program called DeCSS. Norwegian teenager Jon Johansen was prosecuted for computer hacking, but a verdict of not guilty was returned. The DeCSS keys have since been included in more than a hundred consumer programs that make it easy to copy DVDs.

Though illegal in many countries, these programs are available on the internet. That leaves the entertainment industry in a no-win situation: if it changes the encryption keys on new DVDs to defeat DeCSS, several hundred million legitimate DVD players around the world will be unable to play new movies.

The lesson of DeCSS spurred Disney, Intel, Matsushita/Panasonic, Microsoft, Warner, IBM, Toshiba and Sony to join forces in 2002 to develop the Advanced Access Content System that is to be used on all blue-laser discs. Two years in the making, AACS is a clever cryptographic system, pragmatically designed on the assumption that sooner or later someone will defeat it. 鈥淣othing can be made impervious,鈥� says Alan Bell, responsible for advanced technology at Warner Brothers. 鈥淵ou only need one brain, a PC and a lot of time 鈥� which is what students have.鈥�

The way AACS works means its defeat should only be temporary. Each blue-laser disc player and recorder is given a unique identity code that is concealed in a secure chip inside the machine. This also contains secret keys used for encryption and decryption that are mathematically linked to the device鈥檚 ID code.

Whenever Hollywood releases a new movie on a blue-laser disc, the pictures are encrypted using a secret key and this 鈥渄isc key鈥� is itself encrypted and written onto the disc along with the movie. To play the disc back, a player uses one of its own secret keys to unlock the disc key, which then unlocks the movie.

So how does this help prevent piracy? It won鈥檛 usually be possible to use a recorder to copy a movie disc. But if a hacker gets hold of secret keys and modifies a machine so it can make unauthorised copies of a movie, the studios will need to change the keys used to make future movie discs so that the hacked keys no longer work.

Every machine will store an upgradeable list of the secret keys used to unlock discs. The challenge to AACS has been to find a way to update this list in all players. This is relatively straightforward for blue-laser disc players built into computers connected to the internet. Owners of these players will be forced to upgrade their software every 18 months or so, or whenever significant numbers of pirate discs are released. The upgrades will contain a code that enables the use of new keys. It can also revoke known 鈥渂ad鈥� keys used on pirated movies.

Spotting the hacks

But what about people who don鈥檛 use a computer to watch their films? Most of us have DVD players in our living rooms that connect to our TVs but not to phone lines or the internet, and this is unlikely to change with the next generation of disc players.

To solve this, the consortium has come up with a clever trick: the blue-laser disc itself will carry the updates. Every new movie released on disc will hold an updated list of keys. Whenever a new blue-laser disc is played 鈥� whether you buy the disc on the high street or borrow it from your local rental store 鈥� software will compare the list on the disc with that stored in the player鈥檚 memory. Any keys in the player鈥檚 memory that have been linked with piracy will be deleted, while new secure keys will be uploaded. From then on the machine should continue to play legitimate discs, whether old or new, but will refuse to play pirate discs with the compromised codes.

At least that鈥檚 the theory. However, not everyone thinks the scheme will be successful. Edward Felten, a computer scientist at Princeton University, accepts that AACS is well-designed but is convinced that pirates will work around it. 鈥淲e can expect new, reverse-engineered machine IDs to pop up from time to time, with each one making all existing discs available for copying,鈥� he says. Inevitably, there will be a delay between the release of a pirated disc and its code being earmarked for removal from the list, during which time it can still be played or copied. Other means of copying content, such as capturing the analogue output of a player or getting an insider to infiltrate the production process, will be unaffected by AACS. 鈥淥nce again, digital rights management software will limit competition without reducing infringement,鈥� Felten says. 鈥淐ompanies are welcome to try tactics like these. But why should the public support them?鈥�

鈥淐ompanies are welcome to try tactics like these. But why should the public support them?鈥�

If something goes wrong, there is a risk that some machines could unpredictably stop playing discs. What happens, for example, if errors occur in the updating process, either accidentally or if a hacker adds incorrect data to a pirate disc. No one knows the answer, and won鈥檛 know until the technology is in widespread use. Although the AACS consortium has published detailed specifications of its system, it has decided not to open up the technology to scientific challenge before launch, as the music industry did with its Secure Digital Music Initiative (SDMI) copy-protection system five years ago (快猫短视频, 17 February 2001, p 34). The consortium refused to tell 快猫短视频 why it has not done this.

Many music lovers already know what can happen when complex protection systems are used. Record companies have tried to add copy-protection technology to ordinary music CDs so they cannot be copied using a CD recorder on a computer. Early systems obstructed innocent play on some home and in-car players. Last year, Sony BMG鈥檚 attempt to protect music titles with the extended copy protection (XCP) also backfired when it turned out that the software made computers vulnerable to viruses, as Russinovich discovered. A later protection system used by Sony BMG also had to be modified when it emerged that a file installed on users鈥� computers by the software could allow hackers to gain control of the computer.

The Electronic Frontier Foundation, a US-based organisation that campaigns for digital rights, is concerned that AACS may not work as smoothly as claimed. 鈥淐ustomers have two things to worry about,鈥� says Seth Schoen at the EFF. 鈥淔irst, the risk of a mishap that breaks your equipment, and second, the risk that entertainment companies will decide to take away functionality that the product had when you bought it.鈥� The recent experience with Sony BMG shows that entertainment companies could end up sacrificing ordinary caution and sound engineering in the name of enforcing their copy control rules, he says.

Cryptography Research, a software company based in San Francisco, has spent two years warning of the side effects of renewable protection. Its solution is an alternative system called Self-Protecting Digital Content, which can either be used on its own or in conjunction with AACS.

SPDC hides a computer program on every blue-laser disc. Before the movie is played, the program interrogates the hardware and software inside the player. If it finds anything untoward, such as modified software that might allow illegal copying or the playback of pirated discs 鈥� machine IDs or keys from pirated discs, for example 鈥� it temporarily halts playback and puts a message on the screen giving contact details for the movie studio. 鈥淧irates will not phone,鈥� says Paul Kocher, CRI鈥檚 president. 鈥淥wners of legitimate but bugged players will get a free fix.鈥�

Backers of the Blu-ray laser disc format recently licensed CRI鈥檚 technology to use alongside AACS. At last year鈥檚 IFA international electronics show in Germany, the group revealed that it fears a 鈥渃lass attack鈥�, in which a design flaw or hacker puts a wide range of disc keys at risk. Revoking all these might stop legitimate players working, and alienate consumers. The group think that SPDC will help prevent this by giving people the chance to stop their players being blacklisted by responding to the message. 鈥淚t鈥檚 our last chance to get copyright protection right,鈥� says Andy Setos, head of engineering at 20th Century Fox, which backs Blue-ray.

Setos counters suggestion that Sony BMG鈥檚 high-profile problems with XCP are a foretaste of what might happen with AACS. 鈥淴CP is a completely different animal. Blu-ray security is a total system 鈥� you can鈥檛 access its code.鈥�

Meanwhile Blu-ray鈥檚 competitor HD-DVD, is using AACS alone. 鈥淲e spent 18 months talking to CRI,鈥� Bell said at a meeting for HD-DVD backers in Paris, France, in October last year. 鈥淲e didn鈥檛 see what extra SPDC offered.鈥�

Whatever happens, the stakes are high for all concerned. If pirates crack AACS, they will be able to clone the highest-quality copies of Hollywood鈥檚 movies, equivalent to the master negatives stored in the companies鈥� vaults. If a malicious hacker fools players into revoking good keys, perhaps by distributing a deliberately faulty disc, players in homes may stop playing. If one of the competing blue-laser systems suffers these problems, while the other does not, the battle to create a de facto standard for HDTV movies will be quickly over. But if both systems hit trouble, Hollywood and consumers alike might stick with the conventional DVD system and the whole HDTV revolution will grind to a halt.