Espionage by CRT mirroring

After searching a bit more thanks to Eugene's comment, I found this, where it states it can be done from hundred meters, even without very expensive equipment.

See Wim van Eck's legacy.

Fragment:

Oscillating electric currents within your monitor produce radio frequency electromagnetic radiation (EMR) that correlate to what the monitor displays. In cooperation with the BBC in February 1985, van Eck was able to confirm through experimental proof of concept that this form of electronic eavesdropping is possible from distances of up to several hundred meters.

While such danger to information security was already known at the time of van Eck's paper, it was generally believed that such eavesdropping was prohibitively difficult for amateurs — meaning, for the most part, non-military personnel — and would require extremely expensive, specialized, restricted equipment. Wim van Eck's research showed that it can be accomplished with nothing that isn't readily available on the open market — that, in fact, "In the case of eavesdropping on a video display unit, this can be a normal TV broadcast receiver."

You could start reading here for a reasonable coverage of the problem.

I worked on an early color CRT (Data General CRT terminal) that included Tempest rating. There we encoded the signals to the guns (grids) from the motherboard to the neck of the tube, decoding them right on the CRT neck. The tube was encased in extensive shielding.

Update: While some make fun of this potential compromise in the comments, there were serious implications. You don't need to RX/decode a whole screen. The biggest problem was with logon screens. Well documented and easily discerned. you only need decode user names and potentially password. I do remember that we altered our logon screens to never echo the password in any way. Many, particularly Unix based systems of the time used to flash the character you typed and then backspace and overwrite it with an Asterisk. Very poor security.

The pixel signal current ( image raster display) can be radiated easily in CRT type displays in uV /m field strength and is tested by EMI “Tempest Level screening” criteria much lower than FCC Class B.

I briefly observed such testing hidden by security drop sheets when I was doing similar tests to magnetic HDD’s on interface cables at a Burroughs test faculty in Paoli, PA, USA in the early 80’s.