October 20, 2017

Understanding the HDCP Master Key Leak

On Monday, somebody posted online an array of numbers which purports to be the secret master key used by HDCP, a video encryption standard used in consumer electronics devices such as DVD players and TVs. I don’t know if the key is genuine, but let’s assume for the sake of discussion that it is. What does the leak imply for HDCP’s security? And what does the leak mean for the industry, and for consumers?

HDCP is used to protect high-def digital video signals “on the wire,” for example on the cable connecting your DVD player to your TV. HDCP is supposed to do two things: it encrypts the content so that it can’t be captured off the wire, and it allows each endpoint to verify that the other endpoint is an HDCP-licensed device. From a security standpoint, the key step in HDCP is the initial handshake, which establishes a shared secret key that will be used to encrypt communications between the two devices, and at the same time allows each device to verify that the other one is licensed.

As usual when crypto is involved, the starting point for understanding the system’s design is to think about the secret keys: how many there are, who knows them, and how they are used. HDCP has a single master key, which is supposed to be known only by the central HDCP authority. Each device has a public key, which isn’t a secret, and a private key, which only that device is supposed to know. There is a special key generation algorithm (“keygen” for short) that is used to generate private keys. Keygen uses the secret master key and a public key, to generate the unique private key that corresponds to that public key. Because keygen uses the secret master key, only the central authority can do keygen.

Each HDCP device (e.g., a DVD player) has baked into it a public key and the corresponding private key. To get those keys, the device’s manufacturer needs the help of the central authority, because only the central authority can do keygen to determine the device’s private key.

Now suppose that two devices, which we’ll call A and B, want to do a handshake. A sends its public key to B, and vice versa. Then each party combines its own private key with the other party’s public key, to get a shared secret key. This shared key is supposed to be secret—i.e., known only to A and B—because making the shared key requires having either A’s private key or B’s private key.

Note that A and B actually did different computations to get the shared secret. A combined A’s private key with B’s public key, while B combined B’s private key with A’s public key. If A and B did different computations, how do we know they ended up with the same value? The short answer is: because of the special mathematical properties of keygen. And the security of the scheme depends on this: if you have a private key that was made using keygen, then the HDCP handshake will “work” for you, in the sense that you’ll end up getting the same shared key as the party on the other end. But if you tried to use a random “private key” that you cooked up on your own, then the handshake won’t work: you’ll end up with a different shared key than the other device, so you won’t be able to talk to that device.

Now we can understand the implications of the master key leaking. Anyone who knows the master key can do keygen, so the leak allows everyone to do keygen. And this destroys both of the security properties that HDCP is supposed to provide. HDCP encryption is no longer effective because an eavesdropper who sees the initial handshake can use keygen to determine the parties’ private keys, thereby allowing the eavesdropper to determine the encryption key that protects the communication. HDCP no longer guarantees that participating devices are licensed, because a maker of unlicensed devices can use keygen to create mathematically correct public/private key pairs. In short, HDCP is now a dead letter, as far as security is concerned.

(It has been a dead letter, from a theoretical standpoint, for nearly a decade. A 2001 paper by Crosby et al. explained how the master secret could be reconstructed given a modest number of public/private key pairs. What Crosby predicted—a total defeat of HDCP—has now apparently come to pass.)

The impact of HDCP’s failure on consumers will probably be minor. The main practical effect of HDCP has been to create one more way in which your electronics could fail to work properly with your TV. This is unlikely to change. Mainstream electronics makers will probably continue to take HDCP licenses and to use HDCP as they are now. There might be some differences at the margin, where manufacturers feel they can take a few more liberties to make things work for their customers. HDCP has been less a security system than a tool for shaping the consumer electronics market, and that is unlikely to change.

Will they ever learn? Hollywood still pursuing DRM

In today’s New York Times, we read that Hollywood is working on a grand unified video DRM scheme intended to allow for video portability, such as, for example, when you visit a hotel room, you’d like to have your videos with you.

What’s sad, of course, is that you can have all of this today with very little fuss. I use iTiVo to extract videos from my TiVo, transcoding them to an iPhone-compatible format. I similarly use Fairmount to rip DVDs to my hard drive, making them easy to play later without worrying about the physical media getting damaged or lost. But if I want to download video, I have no easy mechanism to download non-DRM content. BitTorrent gives access to many things, including my favorite Top Gear, which I cannot get through any other channel, but many things I’d like aren’t available, and of course, there’s the whole legality issue.

I recently bought a copy of Disney/Pixar’s Up (Blu-ray), which includes a “Digital Copy” of some sort that’s rippable, but the other ones are rippable as well (even the Bluray), so I haven’t bothered to sort out how the “Digital Copy” works.

(UPDATE: the disc contains Windows and Mac executables which will ask the user for an “activation code” which is then sent to a Disney server which responds with some sort of decryption key. The resulting file is then installed in iTunes or Windows Media Player with their native DRM restrictions. The Disney server, of course, wants you to set up an account, and they’re working up some sort of YouTube-ish streaming experiences for movies where you’ve entered an activation code.)

So what exactly are the Hollywood types cooking up? There are no technical details in the article, but the broad idea seems to be that you authenticate as yourself from any device, anywhere, and then the central server will let you at “your” content. It’s unclear the extent to which they have an offline viewing story, such as you might want to do on your computer on an airplane. One would imagine they would download an encrypted file, perhaps customized for you, along with a dedicated video player that keeps the key material hidden away through easily broken, poorly conceived mechanisms.

It’s not like we haven’t been here before. I just wonder if we’ll have a repeat of the ill-fated SDMI challenge.

DRM by any other name: The latest from Hollywood

Sunday’s New York Times had an article, Studios’ Quest for Life After DVDs. To nobody’s surprise, consumers want to have convenient access to “their” media, wherever they happen to be, without all the annoying restrictions that come into play when you add DRM to the picture. To many people’s surprise, sales of DVDs (much less Blu-ray) are in trouble.

In the third quarter, studios’ home entertainment divisions generated about $4 billion, down 3.2 percent from a year ago, according to the Digital Entertainment Group, a trade consortium. But digital distribution contributed just $420 million, an increase of 18 percent.

Given that DVDs are really a luxury good (versus, say, food or electricity), the 3.2 percent drop seems like Hollywood is getting off easy. The growth in digital distribution is clearly getting attention, though. What’s going on here? I imagine several things. People sometimes miss their shows. Maybe the cable went out. Maybe the TiVo crashed. Maybe they’re on the road. Drop $2 at the iTunes Store and you’re good to go. That’s attractive and it’s real money.

Still, the article goes on to talk about… yet more DRM.

Standing in the way are technology hurdles — how to let consumers play a video on various devices without letting them share it with 10,000 close friends on a pirate site — and the reluctance of studios to cooperate too closely with rivals for reasons of antitrust scrutiny and sheer competitiveness.

And piracy, at least conceptually, would be less of a worry. The technology [Disney’s Keychest] rests on cloud computing, in which huge troves of data are stored on remote servers so users have access from anywhere. Movies would be streamed from the cloud and never downloaded, making them harder to pirate.

Of course, this is baloney. If it’s going to work on my iPhone while I’m sitting in an airplane, the entire video needs to be stored there in advance. Furthermore, if the video is supposed to be “high definition,” that’s a bare minimum of 5 megabits/sec. (Broadcast HD is 20 megabits/sec and Blu-ray is 48 megabits/sec.) Most home DSL or cable modem connections either will never go that fast, or certainly cannot maintain those speeds without hiccups, particularly when sharing the line with other users. To do high quality video, you either have to have a real broadcast medium (cable, over-the-air, or satellite) or you have to download in advance and store on a hard drive.

And, of course, once you’ve stored the video, it’s just not that hard to extract it. And it always will be. The challenge for Hollywood is to change the incentives of the game. Maybe sell me a flat-rate subscription. Maybe bundle it with my DSL provider. But make the experience compelling enough and cheap enough, and I’ll do it. I regularly extract video from my TiVo and copy it to my iPhone via third-party software. It’s practically painless and it happens to yield files that I could share with the world, but I don’t. Why? Because there’s real downside (I’d rather not get sued, thanks), and no particular upside.

So, dearest Hollywood executive, consider that selling your content for a reduced price, with no DRM, is not the same thing as “giving it away.” If you allow third-parties to license your content and distribute it without DRM, you can still go after the “pirates”, yet you’ll allow normal people to enjoy your work without making them suffer for it. Yes, you may have kids copying content from one to the next, just like we used to do dubbing cassette tapes, but those incremental losses can and will be offset by the incremental gains of people enjoying your work and hitting the “buy” button.