On June 15, 2010 at 14:49, BobL said...
It clearly states all the KSVs of downstream devices are passed back to the source for authentication. The source uses an XOR of a calculation of the KSVs to encrypt the data. This would be Ks the secret session key and all devices have to come up with this calculation and they must match. The encryption is not set to a given key by any device but the whole process is started and controlled by the source. A repeater (AVR) cannot act independently without the sources permission which is what your examples about separate authentication/encryption keys (KSVs) would allow.
I believe you are confusing elements of the authentication process with the encryption process.
It is true that in the authentication process, the display's keys are passed back to the source by the AVR for verification.
However, this has nothing to do with the encrytion key used by the AVR when encrypting the TMDS stream for transmission out to the display.
Here is what I believe you are missing:
The key used by the AVR when encrypting the data for transmission to the display is produced by using a new shared key - one that exists between the AVR and display only. The key is produced by using the KSV of the AVR along with the private key sets of the AVR and/or Display.Note in the below excerpt from a DCP white paper that the encrypted data "can only be decrypted by the receiver." That means the stream sent by the source can only be decrypted by the AVR. That stream could not be decrypted by the display. A new stream must be created/encrypted by the AVR using a shared key with the display, so that the display can decrypt the stream.
[Link: digital-cp.com]Page 8
After authentication, the transmitter uses its HDCP cipher engine and the shared session key to create a stream of encrypted data that can only be decrypted by the receiver. The receiver uses its HDCP cipher engine and its copy of the session key to decrypt the content.
