Friday, April 21, 2006

HDCP: Why So Weak?

Professor Felten makes a very interesting point about the encryption used in next generation cables boxes, televisions, and high definition media formats (such as HDDVD and Blu-Ray). The encryption being added to these products is insufficient to stop hackers from breaking it, or to stop companies from releasing software or hardware to circumvent it.

Instead, this protection is only there to allow lawsuits under the DMCA (under the anti-circumvention provision) to keep products out of the marketplace (that haven't been authorized for interoperability by the HDCP cartel).

So just like with the DVD cartel, the law of technology is being bent (dare I say it is being used as intended) to control players in the HDCP affiliated industries.

Question: Is the weakest link here the anti-circumvention provision of the DMCA?

Purportedly Via

HDCP: Why So Weak?: "

Today I want to wrap up (I think) the discussion on security weaknesses in HDCP, the encryption scheme used for sending very high-def video from a device like a next-gen DVD player to a TV monitor. I wrote previously (1, 2, 3) about how HDCP will inevitably fail — catastrophically — when somebody manages to recover the master secrets that are the source of all power in the system, and publishes those secrets on the Internet. I wrote, too, about how this problem could have been avoided by using standard cryptographic primitives rather than custom-designed ones.

It seems very likely that the people in charge of HDCP knew what they were doing, and made a deliberate choice to use the less-secure scheme rather than the more secure, standard one. (I don’t have definite proof that they knew about the security problems, but it’s pretty hard to believe that their engineers failed to notice it.) Why did they choose the weak system?

The academic paper on HDCP, by Crosby et al., says that HDCP’s designers were given a ‘budget’ of 10,000 gates. (Gates are one of the basic building blocks from which digital chips are designed.) Crosby estimates that a more secure design would have required about 30,000 gates, to fix the vulnerability I discussed earlier and some smaller vulnerabilities. How much does it cost to add gates to a design? That depends — the high end of the cost range is around $100 per 10,000 gates, but the low end might be much lower.

There are really two questions here. (1) Why did they think it was worth paying for 10,000 extra gates to have the weak system, rather than no encryption at all? (2) Why did they think it wasn’t worth 20,000 gates to have a stronger system, rather than the weak system? Let’s consider these questions in order.

First: Why is the weak system worth spending 10,000 gates for? The answer doesn’t lie in platitudes about speedbumps or raising the bar — any technical bumps or bars will be obliterated when the master secrets are published. It’s worth noting, too, that the data stream they are protecting — uncompressed super high-def (1080i) video — blasts so much data so fast that there’s no affordable way for a would-be pirate to capture it, at least today. About all that can be done with such data streams today, at reasonable cost, is to display them, or to run them through simple format converter boxes. In future years, capturing the video stream will become a viable piracy strategy, but by then the master secrets will almost certainly have been published. So temporary piracy prevention doesn’t seem like a good explanation.

A much more plausible answer is that HDCP encryption exists only as a hook on which to hang lawsuits. For example, if somebody makes unlicensed displays or format converters, copyright owners could try to sue them under the DMCA for circumventing the encryption. (Also, converter box vendors who accepted HDCP’s license terms might sue vendors who didn’t accept those terms.) The price of enabling these lawsuits is to add the cost of 10,000 gates to every high-def TV or video source, and to add another way in which high-def video devices can be incompatible.

The second question is why they weren’t willing to spend an extra 20,000 gates to use a more secure crypto scheme. Doing so would have reduced, in the long run, some types of P2P infringement. They apparently felt this would not be a good investment, presumably because other infringment scenarios were more troublesome. Why spend money strengthening one link in a chain, when other links are already weaker?

The bottom line is clear. In HDCP, ‘security’ technologies serve not to disable pirates but to enable lawsuits. When you buy an HDCP-enabled TV or player, you are paying for this — your device will cost more and do less.


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