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SAT-Usenet 34/96

SAT-Usenet 
34/96
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Hotbird 2 is now (3.12.96) on 29o East
With picture on trp 53 and 56
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SPECIAL EDITION ON:
- HOW DOES EUROCRYPT WORK ???

Roger Dahl wrote:
> 
> I'm wondering how eurocrypt works. That is, what kind of signals and
> information goes where.
> 
> I know that the video signal comes from the satellite in a form where
> the individual scan lines in the image are "rolled" to random positions,
> rendering the image unviewable. So the first question would be where is
> the information that tells the decoder the original position of the
> lines so it can fix it. My first guess would be that there is numbers
> coded into each image line somehow, and these numbers are the basis for
> calculating the original location of the line. But after thinking about
> this for a while I realised that if that was the case, the decoder would
> sometimes misread this constant stream of numbers, and single lines
> would me placed wrong in the image and I've never seen that. The image
> is either perfect or unviewable. So my next guess would be that a single
> number is sent once each, say, 1000 images, and the number is then run
> through some itterative formula to calculate the position of the next
> rolling. That makes sence because I've seen the image suddently turn
> unviewable for a short while and then come back again. That would happen
> if the number was interpreted wrongly.

There is a PSNG that generats that correct stream of numbers when it is
loaded with the correct seed. The decoder gets the seed from the
smartcard.
New encrypted seeds is sent out with a few seconds between them. They
are
decrypted with a operational key. 

> 
> And then there is the security and crypting itself. I know there is a
> DES key system in there somewhere but where does it fit in? It could be
> that the single number that's itterated is sent in DES crypted form and
> the key is needed to decrypt it.

The seed is DES encrypted with a operational key.

> 
> Which brings me to the smart card. I know that the smart card somehow
> holds the key to decrypting the image. But if the smart card holds the
> DES key needed to decrypt the numbers that explain how to decode
> ("unroll") the image lines, wouldn't it be very easy to snatch this key
> up when the tv company changes keys and broadcasts the new key to its
> subscriber's smart cards? So that makes me wonder, is the DES key that's
> broadcasted itself DES crypted? If so, there is a super-secret DES key
> inside the legal smart cards. That sounds like a vulnerable system to
> me.

When a key is sent out to a card it is DES encrypted. Yes it is
vulnerable
because if the managment keys gets known then the security breaks down.
Managment keys are keys stored in the card.

> 
> And finally I'm wondering about how the DES keys are cracked? I mean,
> DES is strong encryption so it can't simply be cracked. Yet, these
> familiar sequences of eight 4-digit numberes are usually posted soon
> after a channel changes it's keys. My guess is that these numbers are
> simply found by watching what a legal smart card sends to the decoder
> main unit after it has (legally) been updated with new keys. But then
> again, this sounds so easy that why do we some times have to wait rather
> long for new keys (like the latest TV1000 keys). If it was as easy as
> that, someone would set up an automatic system to continously update a
> web page with the current keys based on what numbers are going between
> the a legal smart card and the decoder at any given time.

Some people in europe got some managment keys so they can decrypt the
key when it is sent out to the cards. I dont know how they got the keys,
but i want to know.

> 
> I'm also wondering how the system of "turning on/off" new/expired cards
> works. Where and in what form are these data broadcast? I guess DES is
> used in this as well, or people would make hardware to "turn on" their
> own expired smart cards.

The card do DES hashing on the managment data. Eurocrypt cards are not
turned on or off. The get a subscription for a limited time sometimes 3
weeks.

> 
> Many questions... Anyone care to enlighten me?
> 
> - Roger


/ Joakim
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