Unscrambling pay TV’s new descramblers

Unscrambling pay TV’s new descramblers – HBO’s new encryption system, Videocipher II

News item: On Jan. 15 Home Box Office (HBO) began scrambling the signal by which it transmits programming via satellite to cable companies for local distribution. In that way HBO hoped to ensure that dish owners–1.5 million at latest count–couldn’t pluck a clear picture of its programs out of the air for free. Other cable programmers will soon follow suit. According to Ed Horowitz, HBO’s senior vice president for technology and operations, it took four years to develop a descrambler for local cable operators and dish owners who subscribe to the pay service.

What HBO came up with is a complex encryption system called Videocipher II. Developed by M/A-COM, an electronics company in Burlington, Mass., it distorts the video and converts audio signals into digital information–a series of 1’s and 0’s–and then scrambles these numbers so that they can be decrypted only with a special descrambler.

HBO’s digitized information is en crypted using the data encryption standard (DES), which was developed to protect unclassified federal information. In DES, once the signal is digitized, it’s translated into an unintelligible signal called cipher. The form of cipher is determined by an algorithm for mixing up a digitized signal and a key, a 56-digit string of 1’s and 0’s that works like a password.

This password directs the algorithm. If, say, the password is 111100011. . ., the algorithm will scramble the signal one way, but if it’s 111100010. . ., it will scramble it another. The password makes the scrambling unique. Even though a person may know the algorithm, he can’t decipher the code unless he also has the password. And he isn’t likely to guess it, since there are 72 quad- rillion possible 56-digit passwords.

The tricky part of HBO’s system isn’t in the scrambling, though,but in the descrambling. First, HBO, which is owned by Time Inc., the publisher of DISCOVER, sends out a unique signal via satellite to every local cable operator’s or subscriber’s descrambling box– 20,000-plus locations as we went to press. Then each of these descramblers deciphers its signal with its unique pass- word. In other words, a descrambler can only receive a message sent specially to it. But even when this message has been decrypted, a clear transmission of HBO’ program doesn’t come through, because the message isn’t the program itself. Rather, the message tells the descram bling unit which cable services it’s entitled to receive and gives the unit yet another password, called the monthly key.

Like the first password, the new key is a 56-digit series of 1’sand 0’s. But unlike it, the monthly key is the same for all cable operators and, as its name implies, is changed every month. This way, a de scrambling unit with an outdated monthly key won’t be able to descram ble any more information sent by HBO. However, if the descrambler has the right key, it receives yet another mes- sage from HBO, which contains information about the program being shown at that moment.

Before the program comes on, one more major decryption must takeplace. And before that can be done, the de scrambling unit must determine whether the customer is entitled to receive whatever HBO happens to be sending at that moment. If he is, his descrambling unit gets the program password, a key that changes every hour. This password descrambles the last code that HBO sends, and the program comes in loud and clear.

Since HBO started scrambling, many dish owners have been up in arms. While some are challenging the legality of scrambling, others are no doubt dreaming up new ways to pirate HBO’s signal. According to Mark Medress, assistant vice president for M/A-COM, someone with a large mainframe com- puter could go through and test each of the 72 quadrillion possible key configurations and (assuming it takes roughly ten microseconds to test each key) unlock the program once every 10,000 years. However, since the program key changes every hour, the viewer could only decrypt the audio for one show before searching for the next program key.

One more way to pirate the signal would be to obtain the unique de scrambler key for a certain box. ”Then one could build clones, all with the same secret information,” says Medress. ”But we don’t believe that’s possible,” he says, because the microchip with the de scrambler key on it is designed so that the information can’t be copied.

For the time being HBO has the upper hand. But some industry experts believe it’s only a matter of time before the de scramblers catch up with the scramblers, and the poaching begins anew.

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