LED Lights: Friend or Foe?

elfdump writes: "In an article (pdf) soon to be published in ACM Transactions on Information and Systems Security, security researchers have discovered that data transmitted through modems and routers can be remotely reconstructed from the equipment's LED status indicators. According to experiments, their light-to-information retrieval method is successful even when the light is captured 'at a considerable distance' from the source. If you want to prevent people from spying on your data, you may want to tape up those blinking LEDs!"

Comments

Tempest

[Bruce Perens]

Look around for info on the U.S. government's declassified Tempest program. That shows how you can really do this, by sampling the radio emissions of the equipment. Any rapid switching creates radio waves, if you don't shield them effectively you may indeed leak information off site. There have been demonstrations of reading a CRT by the video monitors radio emissions.

To do this with an LED would require that the LED be actually driven by the data signal. Most of them go on at the start of the packet or byte and go off at the end, they don't go on for 1 and off for 0. So, you might be able to do a little traffic analysis, but you would not be able to recover the data.

Bruce

Re:Tempest

[kitchen]

Tempest for home use

Re:Tempest

[fsmunoz]

Look around for info on the U.S. government's declassified Tempest program. That shows how you can really do this, by sampling the radio emissions of the equipment. Any rapid switching creates radio waves, if you don't shield them effectively you may indeed leak information off site. There have been demonstrations of reading a CRT by the video monitors radio emissions

Indeed. Here is a program that implements just that. Tempest for Eliza is an interisting program... it actually played classical music on my AM radio using the monitor color intensity! There's a mod for mp3 even. Check it out.

cheers,

fsm

Re:bullshit

[CrazyBrett]

Not necessarily BS, though it depends on the way the hardware is made. A very simple way (engineering-wise) to implement an indicator LED on a cable modem would be as follows: Whenever the modem is receiving a "1" bit, turn the LED on, otherwise, turn the LED off. Being a type of diode, LEDs are capable of extremely high switching rates (remote controls generally use infrared LEDs pulsed at 56 kHz to transmit data. They can actually switch much faster). Hence, for each packet received, the LED would actually blink dozens of times. To a person, this looks like just a single blink, but a high-speed photodetector would be able to measure the length of each pulse, and use that information to reconstruct the data that was received.

Of course, all this relies on the construction of the modem. Using a slightly less naive algorithm (when a packet arrives, turn the LED on for 1 ms and then shut it off) would defeat this unique kind of sniffing. Still, after staring at my lan hub for a few minutes, I'm wondering if it uses the former technique for flashing the light...

Move over 802.11x

[uigrad_2000]

If it can really pick up signals with few enough errors to be usable, then I want to use it for networking! Some posts here claim that it can easily do 10MBit/sec. What's stopping someone from making an array of them, for high speed wireless access?

Actually, now that I think of it, that must have been what all those big clunky lights were on ST:TOS. Networking of the future!

They may mean more than you think

[horza]

I remember when I was in the office at Acorn Computers chatting to a guy called Dave Walker. Someone walked up to his desk, plonked down an Acorn PC and said it wasn't working. He plugged it in and watched it for a moment (just the box, no monitor was plugged in). After a few seconds he pulled the top off, pushed in a certain chip (loose memory or something), put the lid on and booted... this time the PC whirred into life properly. When I asked him how he did that magic trick, he told me that when there is an error the floppy drive light blinks it out in morse code. I'd had one of these machines for years and had never known that was staring me in the face!

Phillip.