----- Original Message ----- X-Loop: openpgp.net From: Lucky Green <shamrock@cypherpunks.to> Subject: RE: Burglar Politics, Tempesting PC's that watch TV and DVD regions

Sunder wrote, quoting

...

http://www.theregister.co.uk/content/1/13863.html <SNIP> [...] It's my understanding that TV detector vans work by picking up the radiation emitted by cathode ray tube TVs - which should mean that, if you're rich enough to run an LCD monitor they'll never know you're a secret Paxman admirer.

A popular, but false, myth. The video cards radiate more than the CRT's. Laptops tend to be the worst offenders.

--Lucky Green <shamrock@cypherpunks.to>

As to the video cards... Sorry, Lucky, but you're going to have to support this a little better. Emissions are a function of the signal voltage in a conductor, and the extent that this conductor is free to emit. The latter is strongly related to (among other things) the wavelength of the signal versus the conductor length: To emit "well" requires an "antenna" length a significant fraction of 1/4 wave, and that would be about 40 cm for 150 MHz video signals. (I am assuming a signal velocity of 0.8C, since it's PC-board insulated.) These days it's hard to find a video card as long as 10 cm, and the path length from the D/A to the output connector is probably 3 cm or so. (And that is a pc board trace which is on a 4 (or more) layer PC board, not exactly conducive to unintended radiation. Besides, the voltage on that signal is probably around 1 volt, quite low. And, remember that this video card is, itself, stuck inside an enclosure which is at least intended to shield the outside world from the signals, at least more than it already is.. Compare this to the monitor: Those low-level video signals are amplified a few times to drive the electron guns, and they modulate the passage of electrons over a 24,000-volt circuit, in a not-particularly-well-shielded (and huge) vacuum tube. The length of that tube is close to 50 cm, just right to emit ca. 150 MHz. The two ends of the tube must be insulated from each other, of course. Naturally, the case of that monitor is...plastic. As for the laptops: I am unfamiliar with the way laptops drive their LCD displays, but if they don't possess a SVGA-type (X/Y scanned) signal internally, perhaps they can avoid emitting a useful signal. CRT's "write" with an electron beam and must time-multiplex their video information. LCD's (particularly TFT's)COULD be implemented so that the actual display is scanned on only one axis, making it virtually impossible to "read." The data being sent to that LCD driver would have to be some sort of parallel digital, I suppose. Jim Bell, N7IJS ..

Its good to have you back Jim... one of your anonymous fans... BTW in a small startup I worked for for 2 years(it never came to fruition) developing anonymous digital cash technologies with chaumian technologies, the expression "good enough for Assasination Politics" came to be regarded as high praise for digital cash systems that were tight enough for privacy/anonymity...

Here's an empirical result, if we can ignore theory a minute :-) A few years ago, I was using my laptop a few feet away from my parents' TV set, and text from my laptop showed up on the screen. It was shredded into a couple of pieces, because the sync was hosed, but it was quite identifiable as my text, so a spook with good equipment shouldn't have much trouble reading it. If you want more details, dredge the cypherpunks archives. One of the issues is that most laptops have video ports on the back to allow you to plug in real monitors, and if you don't have anything plugged in, they're sitting there with raw pins pointing out. I'm not sure if my PC was in "use both displays" mode or "only use the LCD" mode - most laptops don't have an indicator other than "the LCD is dark"... Among other things, most laptops are designed so that the PC model of display card interface is maintained, so it's transparent to software that's poking around where it shouldn't. Palmtops probably behave differently, but I wouldn't trust them either. At 11:31 AM 10/11/00 -0400, Ray Dillinger wrote:

On Wed, 11 Oct 2000, jim bell wrote:

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A popular, but false, myth. The video cards radiate more than the CRT's. Laptops tend to be the worst offenders. --Lucky Green <shamrock@cypherpunks.to>

As to the video cards... Sorry, Lucky, but you're going to have to support this a little better. Emissions are a function of the signal voltage in a conductor, and the extent that this conductor is free to emit.

Given that a laptop uses an LCD display, there's really no good reason, electronically speaking, why its video hardware should have to do the ((scan+horizontal_retrace)*+vertical_retrace) sequence that the technology for getting a coherent signal relies upon.

But the fact is, laptop hardware does write bits in a predefined order, (in fact the same order as CRT-based machines) so it's a worthwhile question whether anyone can figure the order and pick up the emissions from the video hardware.

This looks like the sort of thing that can be resolved by experiment though; Anybody got enough DSP smarts to put an induction coil next to a laptop monitor and *see* whether they can read the darn thing?

Also, it looks like the sort of thing that could be designed around. If someone were building a "secure laptop" they could make a video system and drivers that wrote the bits in a different, randomized order each time, and which only wrote the changed bits. If anybody is actually making a product like this, it would be a strong indication that *somebody* with money to spend on R&D considers it a valid threat model, because nobody makes products without a market.

Bear

Thanks! Bill Bill Stewart, bill.stewart@pobox.com PGP Fingerprint D454 E202 CBC8 40BF 3C85 B884 0ABE 4639

At 10:31 AM 10/11/2000, Ray Dillinger wrote: ...

Also, it looks like the sort of thing that could be designed around. If someone were building a "secure laptop" they could make a video system and drivers that wrote the bits in a different, randomized order each time, and which only wrote the changed bits. If anybody is actually making a product like this, it would be a strong indication that *somebody* with money to spend on R&D considers it a valid threat model, because nobody makes products without a market.

This could easily be the recipe for a flickering monitor. First of all, the refresh rate needs to be probably in excess of twice what the human eye can see, just so a random signal can be detected before the old signal has expired. if you can count to 4 in n length of time, then you can only reliably provide all the information in that time by keeping to the same order. 1,2,3,4. or 4,3,2,1 or 1,3,2,4, etc. But not 1,2,3,4,3,2,4,1. By the time it rewrote 1, the old 1 would be out of date, and it would appear that you had video problems, (shortly followed by a headache). If you can count to 16, but only need to count to 4, then your options are tremendous. 1,2,3,4,3,4,2,4,2,3,1,2,4, 11 commands from 1 to the next 1 in this example. from line to that same line. This would allow for a secure monitor that didn't invite eyestrain. Good luck, Sean