[mythtv-users] Time Warner & Firewire

Mark Knecht markknecht at gmail.com
Tue May 17 22:46:18 UTC 2005


On 5/17/05, Lane Schwartz <dowobeha at gmail.com> wrote:

> Not that I would do this, nor recommend that others try.... :)
> 
> But how severe are the technical (not the legal) barriers to putting a
> bus sniffer between the cable box and a blessed device, and observing
> how the blessed device replies to the cable box's initial inquiries?

Getting the data isn't the hard part. It's available on the 1394
cable. What's hard is getting the info out of the data.

5C is basically a public/private key exchange every 30 seconds with a
proprietary encryption algorithm that (at the time) you had to sign an
NDA to get ahold of.  We did not sign it as we didn't need to go that
far for the work we were doing. Most folks were going to buy chips
from one of the five C's so they didn't actually need to know how it
was being encoded. Data is encrypted based on the most recently
exchanged key.

While breaking the encryption algorithm is not unheard of, I think
that breaking the current public/private key in under 30 seconds and
getting the data in real time sounded pretty much impossible to me at
the time. It would be more likely that you might find some piece of
equipment that you could modify and dig the data out of but the 5 C's
and Hollywood were being very careful about where the data could be in
a digital format and in an unencrypted state.

> 
> It's possible that the behavior of the cable box goes as follows:
> notice that a device has been attached, make an initial query to see
> if the attached device is blessed, if attached device is not blessed
> then terminate connection, else if attached device is convinces the
> cable box that it's blessed then send data to it.

Basically correct (with 'blessed' meaning that the device does some
preliminary transactions to show it's compliant with the spec) but
even once the data is sent from the DVD or cable box to the TV/monitor
it's still encrypted as per the above.

> 
> Of course it's also possible that the cable box periodically queries
> the attached device &/or actually encrypts the data.
> 
> Just some thoughts...
> 
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