Saturday, September 18, 2010

Understanding the HDCP Master Key Leak

Via Freedom to Tinker (Sept 16, 2010) -

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.


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.


he 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.

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