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Comment by Jestar342 | Hacker News Reader

A friend of mine had a side-gig at an electronics factory that used to fit/repair government printers, telegram, and fax machines in the 80s. When he first started he noted that the schematics said to double up the number of capacitors. When he raised this as a potential error in the schematic with his boss, he was told that it was quite deliberate and was there to stop giving away what was being printed by anyone measuring the power fluctuations. Feels kinda obvious now but when I first heard it I had a mini-mind-blown moment.

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TEMPEST, it's as old as the Cold War. In retrospect, side-channel attacks and bugs are always the norm, a cryptographic break is rare. A good introductory article is TEMPEST: A Signal Problem written by the NSA.

https://web.archive.org/web/20130918021523/http://www.nsa.go...

A similar mind-blown moment for me was reading about foreign intelligence agencies listening for radio frequencies emitted by typewriter keys. They were able to map specific frequencies to specific letters, and could reconstruct most outgoing letters. It inspired a side-project using an SDR (software defined radio) to fingerprint specific hardware I had around the house.

Listening for radio frequencies how? Near by the building where the typewriters were being used?

This reminds me of the Gunman Project .

https://www.nsa.gov/Portals/70/documents/news-features/decla...

That seems like a good way to make a piece of equipment just not work. The stuff inside the printers also probably relies on proper capacitor values.

Seems unlikely. It's probably ~all digital, so I expect the capacitors are literally only there for filtering the power supply.

If you learn about https://en.m.wikipedia.org/wiki/Decoupling_capacitor you'll see that there is more to the consideration than "just use the highest capacitance capacitor".

Power caps. They act like a shock absorber or a very short term UPS for incoming power.

A shock absorber contains an element that does damping and a spring. The ideal capacitor can only be one of those elements in the analogy, I think.

The shock absorber isn't a great analogy for a filter cap but, a capacitor has inductance and resistance, even though it is primarily a capacitor and we usually ignore the L and R for simplicity's sake.

That's why I wrote "ideal capacitor"

You're right. From https://en.m.wikipedia.org/wiki/Decoupling_capacitor

Since capacitors differ in their high-frequency characteristics (and capacitors with good high-frequency properties are often types with small capacity, while large capacitors usually have worse high-frequency response), decoupling often involves the use of a combination of capacitors. For example in logic circuits, a common arrangement is ~100 nF ceramic per logic IC (multiple ones for complex ICs), combined with electrolytic or tantalum capacitor(s) up to a few hundred μF per board or board section.

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