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I would like to set up my computer's full disk encryption to unlock with a key file (preferably one that uses the SD card interface, but I'm not particularly picky) because it's annoying to enter in long passwords every time I boot my computer, but I don't like the idea of anyone with the "key" being able to open my computer.

My preferred compromise would be a memory card / stick that has a small key file on it, with a random 100-200 character password that I could easily and securely wipe by flipping a switch. This way, I could keep the password backed up for manual entry elsewhere, and use the key for convenience during day-to-day life, if I feel like I'm going to be in a situation where someone might demand the key of me (or if I'm otherwise going to be separated from the key), I could easily and surreptitiously wipe its memory. Ideally, it would err on the side of failing (e.g. fail closed rather than open), since it's not the only place the information is stored.

For hopefully obvious reasons, it would need to be something that I can wipe without putting it in a computer, so something like a powered device that stores the key in volatile memory would work well, I imagine, or something that has a small battery that can be used to erase itself.

Does anyone know if such a device exists, and if so, what are they called?

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    Unfortunately, product recommendations are off-topic here. Plus, your requirements are awfully specific.
    – schroeder ♦
    May 10, 2016 at 23:58
  • 4
    Have you looked at yubikey?
    – schroeder ♦
    May 11, 2016 at 0:10
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    @schroeder I think there's a lot of "flavor text" that makes it seem like it's a product recommendation, but I'm just wondering if these sorts of devices exist and what they are called. Boiled down to the essentials, I'm just asking if there is a class of storage device that exists that can be wiped offline in a secure manner.
    – Paul
    May 11, 2016 at 0:13
  • @schroeder As for Yubikeys, I don't think there are any that have the "quick erase" property, I'd have to physically destroy it to avoid key compromise.
    – Paul
    May 11, 2016 at 0:16
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    @schroeder I disagree in the sense that there is a correct answer to this, unlike product recommendations which are opinion based. If I were to ask the question, "Is there a function that always evaluates to the same output given the same inputs, but from which you cannot retrieve the inputs, given the outputs?", I think it would be clearly on-topic, and yet conceptually this is no different. As for it being security-related, I think the issue that it is about handling keying materials securely puts it in the security realm.
    – Paul
    May 11, 2016 at 0:22

4 Answers 4

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Try to make your own key from ATMega/STM32 ARM chip:

  • you make it as an USB slave (peripherial) device
  • it has an EEPROM with the key, so you have a jumper you're removing after storing the key
  • it has a battery inside
  • an "emergency wipe button* starts an EEPROM filling with random data circuitry
  • (optional) after some wiping cycles it uses an overvoltage techniques to burn an EEPROM chip to hell and/or overheating its case top surface to make it unrecoverable even with a newly described method of removing a chip's top cover layer by layer

It's not an obvious Google-it task, but should be no problem. Good idea, by the way! Maybe I'll make such a device myself!

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    This seems to be the closest to the right answer, but could you possibly address the actual question of "does this exist / what is it called"? I'm guessing the answer is "it does not already exist" / "it has no name". I'll give it a week for someone to come up with an actual name, then accept this answer.
    – Paul
    May 12, 2016 at 14:46
  • By the way, this is the "active erase" version - the "dead man's switch" version is probably more in line with my original question, which is that while the device has power, it stores the key in volatile memory. This has the benefit of "failing closed", since a failure of the power supply or button destroys the key. The downside is presumably power consumption.
    – Paul
    May 12, 2016 at 14:48
  • @Paul you should see a fail-tolerant mechanical switches, they do act like buttons, but don't look like. Some of them a look like a valve, i.e. you can break an acrylic protector and turn your key's case's tail 180 degrees to activate it. of course, there always a fault probability - and it will be always : ther's no 100% fail-safe devices in this world, including the world itself =) Such things do exist, but according to the ones I saw myself(and made myself) - they are built-to-order in small batches for particular buyer, no mass-production: it won't be a secret ability then
    – Alexey Vesnin
    May 12, 2016 at 18:39
  • @Paul <continuing>: The key goal and advantage of this kind of devices is that it's self-destruction way is secret, i.e. nobody can expect, that a single and ordinary action actually is a command to self-destruct. And one of my customers used to ask for a multi-component thing, i.e. a killswitch will not trigger if it's no RFID label around. This is an actual example of a real customer request. So - use your imagination, and remember : stock ones are forced to have a backdoors for governments, even this requirement is not always stated clearly in any doc, but they "won't pass a tests"..
    – Alexey Vesnin
    May 12, 2016 at 19:05
  • I agree, but my suggestion about failing closed is that if, for example, the thing runs out of batteries, your decryption key is still on it. If it's stored in volatile memory, if the thing runs out of batteries, the volatile memory resets and the information is lost. Either way, I think an ATMega chip should work if you store the key in SRAM rather than EEPROM.
    – Paul
    May 12, 2016 at 21:24
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A smartcard with encrypted storage might be able to satisfy your requirement.

While not being used, the smartcard will have the key encrypted in its persistent storage, it is inaccessible in its encrypted form.

You enter the decryption key once when you plug the smartcard into a computer.

When you unplug the card, the smartcard loses power and this wipes off the decryption key, rendering the key inaccessible.

This far should be possible using off the shelf PGP smartcard.

If you need to use the key across replugs without reauthenticating, then the card would need to have auxiliary power/battery, you'd then be able to wipe off the smartcard by unplugging the auxiliary power/battery. I don't know of any off the shelf component that have a battery switch, but it may be possible to assemble one yourself.

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  • I think this provides the functionality I don't need (acting as a hardware token), but lacks functionality I do need (secure offline erase).
    – Paul
    May 11, 2016 at 13:28
  • @Paul If you have physical access to a smartcard, it's easy to erase — cut the chip with scissors, or break it by folding hard enough at the right place. It might not be as discreet as you like, but nothing is completely invisible.
    – Gilles 'SO- stop being evil'
    May 12, 2016 at 17:53
  • Hmm... On second read, I think I misunderstood this answer the first time I read it. I think I was thrown by the fact that the "smart card" properties of the smart card are not being used. This is functionally equivalent to Alexey's answer, except that it does provide the fail closed behaviour by default.
    – Paul
    May 12, 2016 at 21:31
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    @Paul: I suggested smartcard/HSM here instead of a microcontroller because smartcard/HSM is designed to be tamper resistant and have all the components needed to fail closed. While smart cards are usually programmed to do cryptographic operations on the card, as that is the most secure way to implement HSM-based authentication, many can be programmed to expose the decrypted passwords directly if you prefer it that way.
    – Lie Ryan
    May 13, 2016 at 6:47
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The instances I know of that use quick-erase are built into special purpose, tamper-detecting hardware. If someone attempts to open the case, cut a hole in the case, disassemble the case, or even drill a hole through the printed circuit board, the detectors trigger. This sends a signal to a small special purpose CPU. Its only job is to wait for a tamper signal, then overwrite the bytes stored in a small CMOS chip that contain the private keys. There is a coin cell on the PC board that powers Vcc on the CPU and the CMOS, but is not connected to the common power rails. To further make tampering difficult, the chips are soldered on using BGA, eliminating easy access via "chip clips".

These are a small part of the overall device that performs encryption. All of the device exists under the protection of the tamper-detecting hardware. Only the data and power lines leave the protected zone of the PC board, and they connect to a less-protected area of the board that hosts the USB chips, USB sockets, and power circuitry.

It's important to understand that tamper-resistance is not the primary purpose of these devices. Their goal is to perform a certain cryptographic function. But as tamper-resistance is an integral requirement of the device, that means it's inseparable from the main device. In other words, tamper resistance has to be designed in; as tamper resistance requires quick-erase functionality, quick erase isn't normally delivered as a separate module you just buy.

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  • I'm not sure I understand the part about tamper resistance being built in. The device you describe is an automated system. The one I described is more akin to revocable keys. Just because tamper resistant circuits use quick erase doesn't mean that quick erasing a chunk of memory can only be used for tamper resistance.
    – Paul
    May 12, 2016 at 16:48
  • Absolutely true. It was my mistake in presuming your need for fast erase key storage device included tamper resistance. But as most of the commercial needs for secure storage include tamper protection, and that always requires a custom solution, that's probably why you're not finding a pre-built chip that meets your exact needs.
    – John Deters
    May 12, 2016 at 17:45
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You could setup with your usb/SD card or any memory device make sure your machine would recognize device.

In windows, use Bitlocker and it also share key with removable device and printer too.

You could do this, and setup your own security make sure it would not erase for that you could disable write pin from device

In pendrive 4 pins:

  • 1 pin 5v power
  • 2 pin for write data
  • 3 pin for read data
  • 4 pin for ground

You could disable it.

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  • This answer is very difficult to understand. It also does not seem to meet the requirements (erasure).
    – schroeder ♦
    May 11, 2016 at 14:44
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    There's no such thing as "pin for write data". The two data wires in USB forms a differential pair and must be both connected for the device to work.
    – billc.cn
    May 11, 2016 at 15:00
  • @billc.cn good catch. Pins 2 and 3 are D- and D+, not "read and "write". This answer is completely factually wrong.
    – schroeder ♦
    May 12, 2016 at 16:38

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