In the mid 80's, several national news outlets showed people able to reproduce a computers monitors screen at least 100ft away. How were early computers compromised like this?

closed as off-topic by Stephen Kitt, dirkt, Alex Hajnal, tofro, JAL Jul 11 '18 at 14:16

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    This is Van Eck phreaking. The CRT-based version works because CRT beams are driven by high-voltage, high-frequency signals, which produce electromagnetic radiation which can be picked up remotely (even using a cheap SDR). LCDs are subject to this kind of attack too (as are many, many other systems — see for example this paper describing a system which intercepts AES keys). But this isn’t retro, you should ask this on Electrical Engineering. – Stephen Kitt Jul 11 '18 at 12:31
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    I'm voting to close this question as off-topic because this isn’t about retrocomputing, and belongs instead on Electrical Engineering. – Stephen Kitt Jul 11 '18 at 12:32
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    What? It's about computers in the 80's! I always find it interesting when someones provides a answer presumably to get some points, then votes to close the question... – jwzumwalt Jul 11 '18 at 12:54
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    How am I getting points here? CRTs are still the same nowadays, the same electromagnetic principles still apply. – Stephen Kitt Jul 11 '18 at 12:59
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    ... and my main objection to having this question here isn’t whether it’s retro or not, it’s that it’s not about computing. – Stephen Kitt Jul 11 '18 at 13:16

It was a timing attack — the electron gun in a CRT is instantaneously lighting only a single point, and the region lit is only that point plus a very short slither of the screen behind it where the phosphors are not yet fully dark again. But each of those is substantially less bright than the current position. You see a full 2d image through a mixture of persistence-of-vision and psychology.

So such an attack just samples the amount of light emanating from the monitor as a whole, e.g. through the amount falling on an operator's face or anything else in the room, at a very high rate and reconstructs the original scan from that. Some human intervention may be necessary as sync signals cannot reliably be detected — no electrons are fired during those periods so you could figure it out if the screen were otherwise entirely lit but anything like text on a black background spends a lot of time not generating light for reasons other than sync.

Modern monitors light the entire surface the whole time. So they're not susceptible to the same attack.

  • Accepting this is a completely different answer from Stephen Kitt's comment, it is nevertheless one I saw demonstrated on national television so I think it's a valid alternative answer. Possibly depending on how strongly you think questions should be tied to titles, I guess. – Tommy Jul 11 '18 at 12:36
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    Relevant paper: Optical Time-Domain Eavesdropping Risks of CRT Displays – Nayuki Jul 29 '18 at 4:18

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