Keys! Keys! Keys! There seems to be some confusion with keys and I want to try to simplify some of this.

Authentication Keys - This is sent by the source, the display receives it and applies an equation and sends back the answer. If the answer is good the next step begins. This is done on the DDC channel. When you read about Key selection Vector (KSV) this is what it refers to, not encryption keys. Repeaters have to identify themselves, some require keys and others don't. I'm not sure what determines if a key is required by a repeater and not just identified.

Device Keys - The number of devices the source allows up to 127 with 7 different levels (ie repeater type devices).

Encryption/cypher keys - This is what is needed to decode the TMDS data. TMDS data is encrypted if the source requires HDCP otherwise it is not. This is needed to prevent "man in the middle" from getting the data. A circuit board could be made that just read the data streams but didn't touch the DDC line and placed between two devices and the data could be stolen easily without encryption.

Now I am not sure where this key is generated and on which line it originates. HDCP uses a stream cipher, so my guess is the key is transmitted at the beginning of the data. Again, I am not sure and it really doesn't matter. But if you want more info on stream ciphers here is a link from wikipedia.
[Link: en.wikipedia.org]

Stream ciphers are a basic form of encryption, similar to locking your car but having no alarm on it. It will keep only basic thieves away.

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