Text 10766, 44 rader
Skriven 2006-04-17 18:01:36 av Mike '/m' (1:379/45)
Ärende: Freedom to Tinker
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From: Mike '/m' <mike@barkto.com>


http://www.freedom-to-tinker.com/?p=1005

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Making and Breaking HDCP Handshakes

I wrote yesterday about the HDCP/HDMI technology that Hollywood wants to use to
restrict the availability of very high-def TV content. Today I want to go under
the hood, explaining how the key part of HDCP, the handshake, works. I’ll leave
out some mathematical niceties to simplify the explanation; full details are in
a 2001 paper by Crosby et al.

Suppose you connect an HDMI-compliant next-gen DVD player to an HDMI-compliant
TV, and you try to play a disc. Before sending its highest-res digital video to
the TV, the player will insist on doing an HDCP handshake. The purpose of the
handshake is for the two devices to authenticate each other, that is, to verify
that the other device is an authorized HDCP device, and to compute a secret
key, known to both devices, that can be used to encrypt the video as it is
passed across the HDMI cable.

Every new HDCP device is given two things: a secret vector, and an addition
rule. The secret vector is a sequence of 40 secret numbers that the device is
not supposed to reveal to anybody. The addition rule, which is not a secret,
describes a way of adding up numbers selected from a vector. Both the secret
vector and the addition rule are assigned by HDCP’s central authority. (I like
to imagine that the central authority occupies an undersea command center
worthy of Doctor Evil, but it’s probably just a nondescript office suite in
Burbank.)

An example will help to make this clear. In the example, we’ll save space by
pretending that the vectors have four secret numbers rather than forty, but the
idea will be the same. Let’s say the central authority issues the following
values:....

[read the full article, I didn't copy the tables...]

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 /m

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