I hope this is the right forum to ask this question. If not, please tell me where to go.

From my understanding most cables have an electromagnetic field around them when data passes through them and I think I remember having unexplainable issues with one of my TVs about 12 years ago where, when watching a VHS, I could still see a pale image of what was currently on TV in the background. I suspect that this had something to do with the bad insulation of the cables, but it also seems super unlikely that the image could be transmited to flalessly, so I don't know...

How conceivable would it be that, with the tech now available, we could build a device that could receive and process the data that is sent through a cable, without having physical contact with it?

I'm mostly thinking about USB and ethernet cables, but also about cables that have a lot of wires, like HDMI. And what about fiberglas cables?

Could it be possible that the NSA is spying on our ethernet cables without even thouching them?
And could you build a hardware keylogger that doesn't need to touch the keyboard's wires? (I'm obviously not talking about wireless keyboards.)

What would be a conceivable range for something like this to work in a realistic home with TVs, routers and smartphones sending their waves all day long?

How problematic is it that an HDMI cable has 19 wires? Would that interfere and make it inconceivable that someone could reproduce what is sent through the cable?

And what about injecting fake data into a cable? How conceivable is that?

  • This question might receive better answers on electronics.stackexchange.com, but I don't think it's off topic here.
    – lily
    Commented Feb 15, 2015 at 16:20
  • In short, yes, it is possible to eavesdrop on wired connections inductively, although it's not the cheapest way of eavesdropping. Jamming wired connections directly is much harder, thus the three-letter folks do it one or several levels up the stack. Commented Feb 15, 2015 at 16:40

1 Answer 1


Can This be done?

I would say yes, but with some caveats. Depending on the cable and the data, you would need some very expensive / sensitive equipment to pull this off. To me this is a similar issue to the old Van Eck Phreaking (http://en.wikipedia.org/wiki/Van_Eck_phreaking).

Intel has some tech to circumvent this kind of attack: http://en.wikipedia.org/wiki/High-bandwidth_Digital_Content_Protection

This whole line of inquiry remind me of a time I was trying to middle some RAM with a logic analyser. Ooops - RAM is way too fast for most logic analysers. Or maybe if I had a fast enough logic analyser? Wouldn't be cheap!

Can the danger be totally avoided?

In most cases yes. Just encrypt absolutely everything. There's no danger of people reading your cables if everything you send is AES256. And you protect your keys well...

Cable by Cable

  • Fiberglass - Encrypt! You should be doing that anyway.
  • USB - The attacker would need to be very close. Improve your physical security. Also Encrypt!
  • HDMI - Encrypt if you can. I can't see a situation where you would want to send sensitive data ever an HDMI cable.
  • Routers - You can 802.11? Like wifi? That's it's own security issue. Use HTTPS. Also known as: ENCRYPT
  • 3G/Other cellphone signals - No idea if and when these are encrypted. You may or may not be at the mercy of the telcoms for that one.
  • Reading the EMF from your cellphone's components - If someone could pull this off, I'd be very amazed. I doubt this is possible from a distance. Maybe if they took apart your cellphone.

  • Your T.V - If you mean the cable connection, that depends on whether your cable company is encrypting the signal, and whether your cable box encrypts signals sent upstream. Turns out they do: http://en.wikipedia.org/wiki/Television_encryption#Cable_and_early_satellite_television_encryption - but I think it's worth checking out sometime ;)

  • If you mean the EMF from your T.V Screen, yeah an attacker can grab that. See Van Eck Phreaking above.
  • USB devices which you wouldn't normally encrypt (like a keyboard) - The keylogging application might be possible. If I were an attacker, I would first look for wireless keyboards and break the protections on that. The thing to overcome here would be signal to noise ratio. I'm not saying it's impossible. However, if you were close enough to actually do this kind of attack (you'd basically have to be within a few feet) then there are other ways. I think you can buy USB cables that are have shielding, so that might mitigate the attack. I personally think there are just better ways to read keystrokes if you are that close. Shoulder surfing ya know?
  • Well, obviously encrypting is almost always a good idea, but it's not always up to you to encrypt things. A website that doesn't offer https, can't really decrypt or encrypt the traffic. And involving a third party (VPN/proxy) really doesn't solve the problem imo. And for HDMI: You send sensitive data through the cable, pretty much every time you open a sensitive file on your computer (photos, text documents, emails..). I don't think there is an encryption standard for HDMI that could be used by your monitors.
    – Forivin
    Commented Feb 15, 2015 at 19:56
  • And for keyboards, I could think of quite a few situations in which you would only have physical access for a few seconds or even never. Imagine you would want to spy on your boss, but you cannot enter his office. You could simply give him something that he would likely place on his desk, possibly near his keyboard/cable.
    – Forivin
    Commented Feb 15, 2015 at 20:05

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