Date: Tue, 5 Sep 89 21:24:24 -0700
From: email@example.com (Brian Kantor)
Subject: Re: medical systems and RF interference (Ranzenbach, RISKS-9.21)
If your radio was part of an 800MHz "trunked" radio system, it may have
transmitted even when you were not keying the microphone.
Trunked radio systems work by sharing a pool of channels between many users.
When a user transmits, he briefly interrogates the controller and is
temporarily assigned one of the pool of channels for his use. After some idle
time, the channel is considered available for use by other members of the
trunked system. Normally each mobile (which includes portables, etc) is
monitoring a "home" channel, but when any unit in the system transmits, the
repeater controller sends a signal out that directs ALL the units of that
customer to switch to the channel chosen, typically for the duration of the
dispatch. The mobiles normally do NOT acknowledge this switching, so they
don't transmit for that reason. In this way they differ from a cellular
However, it is possible to equip mobiles with unit status reporting that allows
the base or repeater to poll each unit for its current status and can even be
used (in wide-area multiple-receiver trunked systems) to guess a rough location
of the unit. If your system were equipped with such an option, it might well
cause your walkie to key up even if you aren't talking on it.
There are, of course, other reasons for radios to key inadvertently. I recall
a prime example when the local fire department kept getting jammed; it turns
out that water from the fire hoses was spraying into a gap in the external-
microphone plug in their walkies and keying the transmitter. Of course it only
happened when they needed a clear channel most: at a working fire.
However, NO piece of life-support equipment should be as RF sensitive as you
describe that respirator to be. There is simply too much RF in the world
around us to allow that kind of shoddy design.
Date: Mon, 18 Oct 93 23:56:28 -0700
From: Phil Karn <firstname.lastname@example.org>
Subject: Re: Porn, Wiretapping, DES, HERF (RISKS-15.14)
Several items in this digest struck my interest.
First was John Gray's comments on "porn" accidentally making it onto a
CATV "children's channel".
>Have you ever wondered how much trust you place on what you see on television?
>Not only that broadcasters will show "appropriate" programs but that the
>service will provide information when you need it.
Very little. And I do not expect this to change. I really do wish the
population at large would discover the "end to end principle" for
itself. They should stop demanding that the CATV companies, satellite
uplinkers, broadcasters, video store owners, the government, i.e.,
anyone and everyone but themselves, be responsible for controlling
what they and their children watch.
I have this product idea should make me millions once I patent and sell it to
all those easily offended households: it's call an "off switch".
>From email@example.com's comment on Denning's wiretap article:
>In other words, these wiretapping capabilities are not being used against real
>crimes, but against actions that are defined as criminal for no better reason
>than that Leviathan has a boot with which to stamp, and we have faces to be
Bingo! This was one of the things that convinced me that the
widespread use of strong cryptography to defeat wiretapping will on
balance be a Good Thing. But to be honest, when this happens (and it
will, whether the government likes it or not) it will admittedly
become more difficult, though not impossible, to prosecute a few
crimes that actually ought to *be* crimes. Foremost among them is
influence peddling and bribery among government officials.
I had resigned myself to this as an unfortunate consequence of an
otherwise positive development. But then it occurred to me: the only
reason crimes like influence peddling and bribery are possible is
because the public has granted government officials so much trust and
power in the first place! Who knows? Perhaps one of the consequences
of universal cryptography will be a lessening of the power of
centralized government and the delegation of much less personal
authority to those within it.
Re Kevin Burfitt's note on a new Australian cipher to replace DES,
does anyone know if the algorithm will be publicly available?
|> Isn't part of the security with DES its slowness, which implies that this
|> new encryption method will be inherently risky because of its speed ?
Not necessarily. DES was originally designed for hardware
implementation, and many of its operations are inherently slow in
software. A good example are the initial and final permutations, which
consist simply of renumbering the input and output bits. This is
trivial in hardware but a real pain in software. Some even suspect
that these permutations were added solely to sabotage efficient
software implementation, as they contribute nothing to the strength of
the algorithm. Certainly not to a brute-force keysearch attack, which
can be conducted after the permutations have been "factored out".
A new encryption algorithm designed specifically for efficient
software implementation could run much faster than software DES
without necessarily being less secure. It would use the native
operations and native data sizes found on most modern computers.
Examples include IDEA and MD5 (although MD5 is not, strictly speaking,
a cipher, it does have a cipher-like structure).
|> Subject: The FAA and HERF
Winn Schwartau's article on "The FAA and HERF" is exactly the kind of article
we've been seeing far too many of in the media lately. Not because the subject
isn't worth investigating, but because the article is long on scary anecdotes,
impressive sounding jargon and calls for action, and short on cold,
quantitative information and logical reasoning.
The term "High Energy RF" is something I'd associate with broadcast
transmitters, long range radars and microwave ovens, not your average laptop
computer. Exactly what constitutes "high energy"? A few orders of magnitude
would be good enough.
And there are quite a few radionavigation systems in use by commercial
aviation, each with its own uses, strengths and weaknesses, including
vulnerability to interference. Which ones are we talking about? Over land,
VOR and DME are the most common. And they work by two very different
principles on widely separated radio frequencies. DME is inherently much more
resistant to interference than VOR. ILS (instrument landing system), is a
cousin to VOR. It probably has about the same susceptibility to interference,
but in a situation with a much smaller margin for error -- which is why many
airlines now ban electronics during landing, even though it may not be
strictly necessary. And over the oceans you have Omega, operating at VLF
frequencies, usually combined with an Inertial Navigation System (INS). (GPS
is not yet permitted as a primary navigation reference, and LORAN-C is common
in US private planes and helicopters but rare in commercial aircraft.)
So exactly which system was in use by the 747-400 in question? Chances are it
was an INS, found on almost all commercial transoceanic aircraft. And INS's
main feature is that it lacks a radio receiver, making it virtually immune to
radio interference! This makes the anecdote just a *little* less credible.
Again, I'm not trying to belittle those concerned about interference to
aviation navigation. I myself fly frequently with a laptop. If there really
were a hazard, believe me, I'd want to know about it. But what we need are
some carefully controlled tests producing reliable, quantitative information.
The closest I've seen to this appears in the October 1993 issue of PC
Computing magazine. They actually measured the RF emissions from a variety of
personal electronic devices, including cellular phones, AM/FM broadcast
radios, walkmans, laptop computers, CD players and handheld games. Their
"...it was highly unlikely for laptops and most PEDs [portable electronic
devices] to cause navigational interference. Of the devices tested, nearly
half produced signals so weak they couldn't be measured above the baseline
noise present on all radio frequencies... In general, we were unable to
produce any real VOR interference except when we used FM receivers and
cellular phones, and when we placed other devices unrealistically close --
within 6 to 12 inches of the VOR receiver antenna."