I’m fairly sure it is also solved by having a decoupling capacitor for the LED too…

I'm guessing a capacitor would help negate this issue but it won't be possible to entirely eliminate the issue

    // TODO remove ductape; temp fix of https://ia.cr/2023/923

Edit: Joking aside, capacitors and black tape are actual countermeasures:

    Fig. 15. A circuit that leaks information via its power LED (a). Counter-
    measures using a capacitor (b), an additional OPAMP (c), and an existing

OPAMP (d).

And:

    Consumer Side Methods. The attack can also be prevented
    by placing black tape over a device’s power LED.

or just snipping it.

Or simply manufacturers not using LEDs to indicate activity.

This is not an intentional indication of activity, but rather just of power.

They don't. The LEDs fluctuations are a by-product of power rail fluctuation and this in turn can be detected to discover some elements of the computations involved.

It's pretty subtle. This isn't like a flashing harddrive LED, it is an LED that to all intents and purposes is burning steadily. Until you look at it in more detail and you realize that it isn't quite as steady as it should be.

I'd at least solder another diode across the terminals if I did that. An LED is a diode at the end of the day and breaking the circuit might break the device.

Not if it is just a power LED. No circuit that I'm aware of would break if the power LED wasn't drawing power. You'd have to go out of your way to make it that way.

Thank you for the clarification. I've not soldered anything for a couple of decades so my knowledge is poor.

I did think that your approach is very thorough, 'change nothing' is a good principle.

A capacitor is a few extra cents on the bom which is a no go for profit maximizing companies.

Dyson batteries are a good example:

Dyson vacuum batteries are designed to fail

* Series battery cells in a battery pack inevitably become imbalanced. This is extremely common and why cell balancing was invented.

* Dyson uses a very nice ISL94208 battery management IC that includes cell balancing. It only requires 6 resistors that cost $0.00371 each, or 2.2 cents in total for six.

* Dyson did not install these resistors. (They even designed the V6 board, PCB 61462, to support them. They just left them out.)

* Rather than letting an unbalanced pack naturally result in lower usable capacity, when the cells go moderately (300mV) out of balance (by design, see step 3) Dyson programmed the battery to stop working...permanently. It will give you the 32 red blinks of death and will not charge or discharge again. It could not be fixed. Until now.

https://github.com/tinfever/FU-Dyson-BMS

That is not about saving cents tough.

No, it will completely eliminate it, if the capacitor is large enough. If you're super paranoid you can do the maths and compute how much does it cut off at such frequency ranges. It's likely to be a ridiculous value. And one does not have an infinite amplitude resolution in any measurement device.

Also, you can just power the LED from a separate voltage regulator. Which is somewhat likely anyways, as the IC will probably want a lower voltage.

(Don't get me wrong, it's cool stuff, but there's also a very easy solution.)

It's a harder EE problem than you think. For example the Samsung Galaxy S8 was attacked by analyzing video footage of the power LED of Logitech Z120 USB speakers. Those were most certainly on a different power supply and the two were connected by a long wire.

There are circuit level solutions but the solution is not a $0.01 MLCC.

And once you solve the LED problem remember: The S8 was attacked by plugging in a peripheral to it’s 5V USB supply. Imagine if the peripheral was instead a high speed ADC that just measured 5V USB directly…

Best solutions are the simplest ones, and this is the right one! It's an electrical coupling issue so I think it should be fixed electrically.