I know all this because in my first job we did a bit of experimentation to see if it would be possible to reconstruct the encrypted images without the key, and in fact it was quite easy because images are very highly correlated from one line to the next, so you just had to rotate each line through each of the 256 combinations and compare it to the previous line, and pick the one with the least difference. Although our system was far too slow to decode a live feed (this would be early 90s), and had some other problems.
I didn't explain this very well - the Wikipedia page has a better explanation:
The technical paper from BBC R&D is quite interesting: http://downloads.bbc.co.uk/rd/pubs/reports/1995-11.pdf
There are a couple of captures of the scrambled transmission on Youtube - it's definitely far more advanced than the earlier Canal method
I remember writing a small program to decrypt image stills in the late 90s and it was surprisingly easy and straightforward, though obviously doing it in real time may have been another story...
You Never needed to actually use the cards for Nagra. But The competitor DTV system built by NDS had an ASIC on the chip, so all solutions required a card at some level. But some setups could use one slave card for multiple receivers. Even over IP.
Instead of using OEM receivers and compromised cards, they’ve gone to running emulated receivers and regular full-sub cards.
I used to be really into FTA satellite, but have not played with it in awhile.
Most likely they would use a delay line https://en.wikipedia.org/wiki/Analog_delay_line
Regular PAL (and I guess SECAM) TVs used one of those
(And I don't understand how the reduction to traveling salesman problem would help here?)
Let the similarity between line i when rotated to position k and line i+1 when rotated to position j be d_k,j. Then you want to find a sequence of rotations r_1..r_n so that
d_1,r_1,r_2 + d_2,r_2,r_3 + ... + d_(n-1),r_(n-1),r_n
is minimized. In plain terms, you want to align each line so that it's most alike the previous line you rotated, but that in turn depends on the next-to-previous line you rotated, and so on up.
This is a restricted type of traveling salesman problem: you want to visit all the nodes corresponding to each adjacent pair of rotation offsets so that the total edge cost (distance between the lines) is minimized.
Since it's 1D, it should be pretty easy to solve with some dynamic programming (straightforward memoization). Calculate the minimal cost for each line down to k. Then for every possible choice of rotating the (k+1)th line, check which order of rotations of the preceding k lines that would minimize the objective down to the (k+1)th line, given that you rotated the k+1th line in that particular manner. Then determine the minima for every k+1 position and go on to line k+2.
EDIT: It's even easier, since you can hold one line constant and rotate the next. Whenever you rotate the first line k, if you know the second line's optimal rotation for a zero rotation of the first line is j, you just rotate the second line k+j. So none of the above complexity is actually needed. I'll keep the comment to show how it could be considered a restricted TSP.
Yes, you can always reduce problems in P or in NP to a TSP. (In this case, I didn't see the reduction immediately, hence my question.)
But a reduction to the general TSP is basically never useful, since general TSP is so hard to solve. (Perhaps the reduction is useful, when you don't know whether your problem is even in NP or perhaps more complicated, yet.)
For any pair of picture data fragments, you'd calculate a goodness of fit score to placing these next to each other. E.g. a JPG of an apple would fit badly with a JPG of a car because they don't share any edges, but two consecutive car JPG fragments would fit well.
Then the problem of ordering the data fragments so that each fragment has a high goodness-of-fit (low distance) to the previous one, is a traveling salesman problem.
The paper then went on to say that nobody can exactly solve TSP instances of the size required, but gave examples where the spanning tree heuristic worked pretty well.
Integer linear programming is also in NP in general. (It has to be, since you can solve the TSP with integer linear programming.) But it's usually much easier to 'write' a linear programme than to reduce a combinatorial problem to a TSP.
Linear programming solvers are very advanced these days.
The other general workhorse I know of for these kinds of problems are SMT solvers. (https://en.wikipedia.org/wiki/Satisfiability_modulo_theories)
I always wondered how the decoding worked, and for its time, using the serial number of the equipment as part of this calculation was a good one.
Also brings to mind the stories in the US of DirecTV pirating and how they put a stop to scores of pirates on Black Sunday - https://blog.codinghorror.com/revisiting-the-black-sunday-ha...
Ahem, yes, "movies".
When BT848/878 PCI TV cards arrived, a few years later everyone tried to decode C+, for two (maybe three) reasons:
- Often, music videoclips.
The TV schedule quality of that channel was light years ahead compared to the ones you could watch in almost any European countries for free.
But I swear I was just video-taping late evening TV to get the summaries of soccer games. (Yes, video-taping. Did I mention I was old ?)
Out of curiosity I watched the 2006 world cup final https://ascii-wm.net/doc.php It's... refreshing
Also it was something "for the middle-upper class". Not for a loaded guy, but for someone with some degree of freedom in order to spend your salary, such as a College guy/group living alone with no family. If you had a TV deco, for sure you have some elder brother/sister with a job or your parents were from a relatively good position.
Everything changed in the middle-late 90's, OFC, as everyone began to buy multimedia PC's so kids at home could do their homework with word processors and the Encarta. Then the Avermedia TV arrived, cracking tools were widespread and with Linux and xawtv-nagra you had a really easy and secure way to watch a record C+ with no issues, except a huge 13GB file per hour, which you encoded into MPEG/XVID for convenience.
Spice TV: https://youtu.be/B89oftmlOuI
It also allowed us to pirate Canal+, decrypting the video stream with a program called Moo TV (if I remember correctly). The audio stream required another program.
It was all a bit janky, frames were frequently dropped, and sometimes you had to tweak a few parameters to get a proper audio stream but the fact that it was (mostly) working was simply amazing to my brother and I.
Good times :)
Meuh Meuh TV. In the earlier version you had to use TV cards with special chips to use it. Good times.
I think it pretty much died with the advent of digital TV.
That's it ! Thank you so much for the rabbit hole of memories i'm about to go in
This prevented you from copying your neighbors codes, they wouldn't work.
It’s never gotten traction on HN before either, or that specific article hasn’t anyways.
It's not the real channel obviously (with digital TV the encrypted channels are simply blacked out), but it's a nice throwback.
 https://ru.wikipedia.org/wiki/Дискотека_80-х seems to be the main one, but it has competitors.
 from the 90s, but if the instigators of the US PMRC had ever seen eurodance like https://en.wikipedia.org/wiki/E-Rotic they'd have been clutching their pearls.
(the retro-encrypt screen's "pour en savoir+" mdr)
I had that channel for a while and watching WWE with French commentary was a favorite when I was a kid :)
Those 'free viewing' (canal+ en clair) probably had one of the biggest impact on french comedy TV culture ever. Some shows of that era are still on AFAIK.
And french politics. French politics in the '90s and '00s wouldn't be the same without Les Guignols de l'Info.
Now if you tune to channel 4, you get the blurred image again...
Tu veux dire le p•rn• du premier samedi du mois ;)
If you're enjoying this, you might also enjoy Creatures Of Thought at https://technicshistory.com/. Their history of the transistor series was superb reading.
I know people bought cheap pirated decoders that decrypted the channel without the need to pay the subscruption fee.
It couldn't be the exact same system because we have PAL, but it was something very similar.
Canal Plus reacted quickly by adding a little bit of noise to the signal after the horizontal pulse.
Some very cool engineering was done without much in the way of technology being available!
If I remember correctly, audio was transmitted on a subcarrier some 6 or so MHz higher?
Edit: Later in the article it is actually stated: "A normal SECAM signal uses FM on a 6Mhz carrier"
That's why when inverted it was sounding like crickets speaking.
You could unscramble the sound just by multiplying it with the right sine wave at the right frequency.
I remember prototyping that with a modular synthesizer :-)