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The key needs to be available to continue to be able to decrypt the data on the device. All encrypted data is not decrypted as you mount or unlock your encrypted device, that is done one the fly as you use it.
The attack you are thinking of should also not be relevant. What you worry about appears to imply that you are more concerned about the key being protected, rather than the data the key protects. You seem to wish to have your decrypted data available, but not the key.
Thank you, I realise that what I'm asking for might not be physically possible. I'm certain that RAM loses all of its contents after a loss of power, but would it be possible to pad the RAM before/during the shutdown process to make sure that nobody gets to the key?
Yes, but: somebody trying to attack your machine that way would cut the power and try to freeze your memory modules. So that mitigation wouldn't trigger.
If you think you really need to guard against that attack you'd have to look into physical security: At room temperature there's a pretty short window available for saving the contents. So if you manage to remove access of possibly used cooling agents to the memory modules you already made things quite tricky.
Now if you can make removing the memory modules hard as well, and prevent booting anything but what you want to be booted there's a decent chance it'll be impossible to recover memory contents.
If that still isn't good enough you'd have to look into providing a means of physical destruction of the memory modules triggered by a backup power source inside the case on unexpected power loss.
Thank you for the comprehensive answer. I will go through it again and attempt to implement some of these mitigations.
Thanks again, I saved your comment
Take into account that your average police raid will not attempt that - they just don't have the means for that.
If you have managed to become an important enough target that either specialists get called in, or you've managed to become target of three letter agencies or the equivalent in your country you will have been targeted by other attacks to gain access to your data, both software and hardware - and if you have to ask that kind of question here you're very unlikely to successfully defend against them.
Thanks for your reply. Fortunately, I am not a person under such scrutiny, and the only reason I ask this is because I'm paranoid.
This level of paranoia isn't really compatible with modern hardware, and requires a lot of effort.
You're pretty much limited to stuff that has open firmware available, and even then you have to hope there are no bugs or backdoors in the hardware.
For the intel world almost everything with open firmware is pretty old - some nowadays unsupported, which means no longer microcode updates. And those microcode updates also are a problem - you can't mitigate everything in kernel space, so usually you'd want them, but they'd also be an attack vector against you.
And even if you manage to trust the computer itself there are a lot of attack vectors surrounding it. Do you have anything capable of recording audio in the same room as your computer? If yes, not a good idea - it has been proven possible to extract passwords from audio recordings of a keyboard. Does the room have windows? That counts as an audio recording device.
If you got rid of that, do you have some other hardware with sensors? There's a high chance that a device placed on your desk containing an accelerometer would also be capable of extracting your password.
I'm so glad that you brought this up. As might be apparent, I have fairly strong sentiments regarding freedom of software. I won't be spilling everything I feel into a single comment, but needless to say, this is a subject I feel strongly about.
On hardware
I absolutely detest Intel and Qualcomm. I wish them the worst in their collective futures (alongside the likes of Samsung and Mediatek too, but I'll keep to the two of these for now). I have a soft spot for AMD for sticking with the FOSS community to an extent and for their affirmative action towards open silicon initialisation with OpenSIL.
I am not one to drink, but I will personally purchase an expensive bottle of wine from the nearest Costco on the occasion I feel that RISC-V has finally reached the realm of everyday computing. Unfortunately, that seems to be a while away, with SBCs being the only ones to bring the technology forward.
On software
On a related note, I am almost equally annoyed at software that has been locked down. Embedded software like the one in the EC and the PCH, alongside proprietary drivers in peripherals and microcode like you described, are something I wholly abhor.
But that was a lot of complaining. If you go through my posts, just the other day I was asking if the T440p was the last Thinkpad I could put Coreboot on (the answer is yes), alongside which I went over me_cleaner and the AMD PSP remover. I do not prefer Coreboot either, but it's the best we can do till probably 2030.
On physical security
I will be employing Faraday cages and metal shielding liberally around my electronics, in an attempt to prevent some of what you mention. Unless we're talking about undisclosed exploits in Android, removing Google and most other proprietary applications should do the trick (I doubt I can do much if the NSA really wants to listen). Of course, by that logic, me_cleaner should be good enough too, but I digress. Of course, all network traffic will be logged and I will operate on a "whitelist by case" basis.
Thank you for bringing across the point of spying using an accelerometer (I'm interested in how that would work, could you point me towards what I should look for?), I'll make sure to utilise simple and easily auditable hardware.
This is not a perfect method, and I have a lot to learn about OPSEC and cybersecurity. Thanks again for your comment.
Or, you could address that paranoia problem directly.
I'm quite happy with having proper graphics cards again thanks to AMD working with their open source driver - and also looking forward to OpenSIL. Though there's still the problem with the PSP in their CPUs.
Did you checkout heads? That's what I'm using on my x230 - seems to be currently the most sensible choice for portable hardware.
Also make sure to shield cables. There's not much public research into passive RF, but from the few people who looked into that we can say that the situation is bad, and the bad guys probably can do a lot of bad things (most likely both display signals and keystrokes from a USB or PS/2 keyboards can be recovered reasonably well from some distance by just analysing the RF sent by the cables)
Pretty much all phones sold in a bit over a decade no longer have a separate baseband. With a unified memory architecture you possibly have a remotely exploitable (remember, baseband) access to the OS memory, if you manage to bypass memory restrictions - in which case none of the mitigations in the OS will help you as it's just not aware of you being there. While this is a pretty complex attack it unfortunately has been proven in a few cases to be possible. I don't keep very important stuff on my phone - I don't consider it trustworthy.
Seems research about being able to recover a phone password/pin by using the phones accelerometer is shadowing search results - I'm pretty sure I've seen a paper about a phones accelerometer being used to reconstruct key strokes of a keyboard on the same table a few years ago - pretty much same idea as recovering the keystrokes via sound.
Also note that things like hard disks contain their own embedded computer, and in some cases contain an accelerometer. They also have DMA access...
Thank you for raising these points. I'll need to read more about DMA, Baseband and shielding.