The RISKS Digest
Volume 2 Issue 47

Thursday, 1st May 1986

Forum on Risks to the Public in Computers and Related Systems

ACM Committee on Computers and Public Policy, Peter G. Neumann, moderator

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o HBO hacking
Phil R. Karn
Dan Franklin
o What are the limits to simulation?
Herb Lin
o Strategic Systems Reliability Testing
Herb Lin
o Correction on Challenger Discussion
Jeff Siegal
o Info on RISKS (comp.risks)

HBO hacking

Phil R. Karn <>
Wed, 30 Apr 86 17:58:40 edt
Satellite transponders used by the cable TV industry to relay programs are
"bent pipes", that is, they simply repeat whatever they hear.  The M/A-Com
scrambler equipment is all on the ground. However, the descramblers will
switch to "pass through" mode if a nonscrambled signal is received.
Therefore, when Captain Midnite sent his unencoded signal, the descramblers
simply passed the signal straight through to the various cable systems.

The transmitter power available on a satellite is very limited (5-10 watts).
Even with a very large receiver dish, the raw carrier-to-noise ratio is far
too low for acceptable picture quality if a linear modulation scheme (such
as VSB AM, used for ordinary TV broadcasting) were used.  Therefore,
satellite TV transmissions are instead sent as wideband FM in a 40 MHz
bandwidth.  Since the baseband video signal is only 5 MHz wide, this results
in a fairly large "FM improvement ratio" and a pronounced "capture" effect.
Full receiver capture occurs at about a 10 dB S/N ratio, and this figure is
essentially the same whether the "noise" is in fact thermal noise or another
uplink signal.  So for the purposes of fully overriding another uplink your
signal must be about 10 dB stronger (10 times the power).

The latest transponders are much more sensitive than those on the earliest
C-band domestic satellites launched 12 years ago.  Most of the 6 Ghz High
Power Amplifiers (HPAs) in use at uplink stations are therefore capable of
several kilowatts of RF output, but are actually operated at only several
hundred watts.  So Captain Midnite could have easily captured the HBO uplink
if he had access to a "standard" uplink station (capable of several
kilowatts into a 10 meter dish) or equivalent.

I happened to turn on HBO in my Dayton, Ohio hotel room at about 1AM, half
an hour after the incident occurred, and noticed lots of "sparklies" (FM
noise) in the picture. At the time I grumbled something about having to pay
$90/night for a hotel that couldn't even keep their dish pointed at the
satellite, but I now suspect that the pirate was still on the air but that
HBO had responded by cranking up the wick on their own transmitter.  Because
they were unable to run 10 dB above the pirate's power level, they were
unable to fully recapture the transponder, hence the sparklies.  (Can anyone
else confirm seeing this, proving that my hotel wasn't in fact at fault?)

Even though each transponder has a bandwidth of 40 MHz, it is separated by
only 20 MHz from its neighbors. Alternating RF polarization is used to
reduce "crosstalk" below the FM capture level. Polarization "diversity"
isn't perfect, though, so it is possible in such a "power war" that the
adjacent transponders could be interfered with, requiring *their* uplinks
to compensate, which would in turn require *their* neighbors to do the same,
and so on.  So Captain Midnite could cause quite a bit of trouble for
all the users of the satellite, not just HBO.

Captain Midnite could have been anywhere within the Continental US, Southern
Canada, Northern Mexico, the Gulf of Mexico, etc.  In the worst case, it
could be practically impossible to locate him.  If he is caught, it will be
either because he shoots off his mouth, arouses suspicion among his
neighbors (or fellow workers, if a commercial uplink station), or transmits
something (distinctive character generator fonts, etc) that gives him away.
Only the NSA spooksats would be capable of locating him from his
transmissions alone, and I suspect even they would require much on-air time
to pinpoint the location accurately enough to begin an aerial search.

Phil Karn

HBO hacking

Dan Franklin <>
Wed, 30 Apr 86 18:11:02 EDT
Re the interception of HBO's uplink by "Captain Midnight": I understand
that the video scrambling is indeed pretty simple, consisting of reversing
black and white on some "randomly-chosen" scan lines.  It's easy to build
a box that will undo this scrambling.  The sound is much harder; it uses
DES.  In the accounts I read, Captain Midnight just put up a still video
picture with no sound, which would make sense assuming that the uplink is
encoded; he could easily encode his video but not his sound.

Nicholas Spies seems to feel that the scrambling was purely an act of
malice against individuals with dishes.  Not so; according to a recent
issue of Forbes, when HBO started scrambling, a number of CABLE TV
OPERATORS they'd never heard of signed up for the decoders!  If cable TV
operators can charge their customers for HBO, why should they get it for free?

I had some other comments about what the FCC Communications Act really
says and what "public" means, but this is getting awfully far from Risks...
"Telecom" and "poli-sci" are no doubt more appropriate.

    Dan Franklin (

     [Thanks for the restraint.  However, the relevance of the HBO case to
      RISKS is clear.  Various risks exist — but have been customarily
      ignored: easy free reception and spoofing without scrambling,
      video spoofing and denial of service even with scrambling.  PGN]

What are the limits to simulation?

Thu, 1 May 86 10:43:02 EDT
    From: eugene at AMES-NAS.ARPA (Eugene Miya)

    I really wonder what simulation's various limits are.

I believe it was Eddington that said "The Universe is not only
stranger than we imagine, but it is stranger than we can imagine."

Strategic Systems Reliability Testing

Thu, 1 May 86 10:41:18 EDT
    From: ball at mitre.ARPA (Dan Ball)

    I'm relatively certain that the numbers of warheads actually reaching
    the target following the initiation of an attack would be far less
    than the numbers in the inventories.

Probably true, if what you mean by target is a hardened silo.  But if
you aim at the center of a city, and you miss by a mile, that's still
"reaching the target" too.  And THAT is what the SDI is supposed to
protect us against.

    Finally, the briefing from SDI office that I heard didn't promise
    perfection.  Unlike some of the political supporters who promise that
    it will be safe for children to play outside during a nuclear
    exchange, the SDI technical types were talking about the impact it
    would have on the numbers and required modifications to the Soviet
    ICBMs that would be required for them to maintain the same confidence
    of assured first strike destruction of the US.

None of the technical supporters believe in near-perfect defense.  But
the political supporters do, and they are lying to the public.

Correction on Challenger Discussion (RISKS-2.46)

Thu 1 May 86 18:15:43-EDT
    >     "... Dr. William Doering, professor of chemistry at Harvard, pointed
    >   out that ... was not an explosion at all. 'It is best described
    >   as a fast fire ... If the fuel tank had exploded ... it would be
    >   producing something much bigger ... "

    [...]  Also, why did he
    wait until the crew module was found?  Why didn't he say after seeing the
    pictures, "That's not an explosion, it's just a fast fire."

It is stated in the original column that Dr. Doering's observation
_was_ made when he watched the videotape, not months later, as Mr.
Moore claims.

Jeff Siegal

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