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Comment by willis936 | Hacker News Reader

My first question was answered by the first answer in the FAQ:

> This is caused by the fact that the power LED is connected directly to the power line of the electrical circuit which lacks effective means (e.g., filters, voltage stabilizers) of decoupling the correlation with the power consumption.

The solution is simple: don't have crap power trees.

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Also, don't put high-power leds on everything.

I've often had to tape over LEDs on various external hardware. Some of them will light up my entire room at night.

Good advice in general. I hate turning off the lights to go to bed and seeing LEDs glaring everywhere. I've banished most such devices from my bedroom for that reason.

Honest question: is that a crap power tree because it's electrically unsound or because it exposes these attack channels? In the first case, it's really crap. In the second one, the extra cost (cents or dollars?) should be part of the the security risk evaluation made by the customer. Maybe that attack was an unknown unknown until now.

Electrically unsound. Having a LED flicker enough purely because you're adding load to the device is just poor engineering. There should be enough decoupling on the LED supply so that it doesn't flicker visibly when another system nearby is loaded.

LEDs almost never have decoupling caps. I've literally never seen a design where there was more decoupling than the minimum necessary to stabilize the power supply's control loop

...and if you do, add a capacitor.

Let your LED will take extra 500-700 ms of fading when the power goes off. It would prevent such side-channel attacks extremely cheaply though.

It's a low-pass filter. It won't eliminate side channel transmissions in very low frequencies, though.

But as the device is in the attacker's hands, even a good power supply could be compromised by replacing or removing capacitors that are used to smooth out the power rails. You'd have to open the device up to do it, but eg to get at the keys inside the secure enclave on an iphone, a couple devices could be sacrificed for the cause.

> But as the device is in the attacker's hands, even a good power supply could be compromised by replacing or removing capacitors that are used to smooth out the power rails.

slightly improving the argument, while keeping the device non-compromised, is that whatever info is coming out of the LED is probably coming out in RF, also. so getting the LED further away from the CPU's power rail(s) probably isn't going to help. they're already emitting the data.

if you can stick a camera this close to the LED, you can probably surround the device with antennae, as well.

I mean, at that point just read the key directly from the ram. The TLDR does emphasize this is for non-compromised devices

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