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I have several devices and flash drives that need to be secured. I haven't been able to find any good physical security measures to protect those devices. What are my options? What do you use, if someone is in a similar situation?

Answers to Mike Ounsworth's ssd.eff.org/en/module/assessing-your-risks

  1. I want to protect the files from copying and the devices from being compromised.

  2. There's more than one adversary and their resources are moderate to high. They can easily break into safes and locks if they aren't secure enough.

  3. The adversary would steal the data and use it in a negative way.
  4. The risk is certain.
  5. I'd do anything I can to try to prevent, there are no social constraints, no technical constraints and finanical constraints under $1,000 USD would be ideal.
  • It is important that the devices are secured or it is important that the data on the devices are secured? If securing the data is the one you are looking for then simply encrypt the data on the devices. – Steffen Ullrich Oct 14 '18 at 16:38
  • It's important that the devices are secured. The devices are already encrypted but encryption doesn't secure the data if the devices are compromised. They can get the encryption passwords when decrypted after compromising the devices. – Perry Oct 14 '18 at 17:31
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    "They can get the encryption passwords when decrypted after compromising the devices" - I have no idea what kind of devices these are but what I meant was encryption where the password is of course not somewhere stored on the device, i.e. veracrypt, bitlocker, encrypted ZIP files ... In these cases it is not sufficient to steal the device since the passphrase for encryption is unknown to the attacker. – Steffen Ullrich Oct 14 '18 at 17:37
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    "... they can compromise the device and get the flash drive to store the password... " - how that? The encryption is not done on the flash drive, i.e. only encrypted data are stored on the drive and the password is never given to the flash drive. Or do you fear that the attackers can modify the flash drives so that it will compromise any system where the flash drive gets inserted and thus the attackers get access to the unencrypted data and also to the password? – Steffen Ullrich Oct 14 '18 at 17:58
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    The only way this could be done is either of the control your computer already (you've lost then anyway) or if they can employ a bug to make your system execute code when the USB stick is inserted. In this case: why do you want to store the data on an USB stick in the first place? Why not for example store the encrypted data somewhere in the network or in the cloud? Then the risk associated with inserting a potentially compromised stick is gone. – Steffen Ullrich Oct 14 '18 at 18:18
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The answer is found in Karate Kid II:

Remember, best block, no be there.

You cannot break a device that you don't have, and you cannot steal something of which you don't know the location. You cannot decrypt a no-sucks encryption without knowing the key. You cannot coerce someone telling you the key if the person doesn't know the key. You cannot restore a split secret if you don't have all the parts (or a certain number, at least).

So the first obvious defense is to make the location of the devices/disks unknown and the disks physically inaccessible. Make sure it is unfeasible to get all parts.

Rent a couple of bank safes, do not tell anyone where. Put one disk with one shared-secret part in each. Depending on your paranoia level, you might even have two or three trusted people independently rent safes (and possibly more than you need), and give one or several of them a share of the secret (without knowing where they've rented the safes yourself, or in which one they'll place the disk).

Use a random encryption key long enough so you cannot remember it, and have another trusted person keep it. Or, have two people store half of the key each. Could give one half to your attorney in a sealed envelope like in a 1970s spy movie. Or put it in yet a different safe, hide it on a cemetary, whatever. Protected storage on a smartphone would be an option. Both Apple and Samsung offer protected storage and will wipe memory after so and so many failures if you tell the phone to do that. Give your lawyer that phone.

Adversaries might break the safe in your home or your office. But few, very few, adversaries can break into a bank safe, let alone break into several of them either at the same time or without being noticed. As soon as one bank safe is broken into, you will be informed. So they must be really, really, really, powerful to pull that off.
If the possibility of your adversary breaking into 3 or 4 banks simultaneously and figuring out the encryption key is a realistic concern, please stop reading now, buy some rope, and hang yourself. Or, just give them your secret.

An adversary who doesn't have all split-secret devices cannot use the data even if he has some of them (well, it depends on the scheme you use, could need N-1 or N-2, or N/2). An adversary who doesn't know the encryption key has nothing but rubbish in his hands, even if he has all the parts. So... make sure that cannot happen.

The easiest, most obvious split-secret scheme that you can use without special tools is RAID-0 (or RAID-5 if you want to trade the small risk of the adversary finding N-1 disks against the risk of losing one by accident) in combination with any kind of does-not-totally-suck file-based encryption. Anything that does any kind of block chaining and uses a no-joke algorithm. Which is pretty much what every standard archive tool like 7z or such does. If you have reason to believe your adversary can conceivably break the encryption that 7z uses, then see above, buy rope and hang yourself. Nobody can help you.

As an alternative if RAID sounds too scary to you (never heard of?), you could add some random data to an (encrypted) archive (doesn't matter what, just to guarantee a minimum archive size to start with), then add all your secret data, and finally select the "split archive to..." option to generate several small sub-archives. All but the first part (which contains only random noise) are worthless without all of the preceding parts. Put one on each disk, done. You can even add some redundancy by hand if you like, in case one disk is lost. Only need to create more segments and be sure each is on at least two disks.
Or, create an encrypted archive, split in two, add the parts in reverse order to another encrypted archive which you then split in N parts (don't need random noise then, cannot decode any part, including the first, without the other parts).

If an adversary doesn't have all the disks, then whatever he has... it's completely worthless. There's no way he will decrypt the archive, or part of it, even if he knows the decryption key.

If the paranoia level is at its top, you could complement the whole thing by further adding filesystem-level encryption. But that's actually kinda silly.

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If this is your risk profile, then you may just want to copy the strategies that are used for protecting gov/military/finance info.

Don't use normal flash drives, use FIPS 140-2 compliant drives such as those made by ironkey. IronKey also makes hard drives in addition to USB drives.

These devices are secure even against advanced threats. You may have the drive stolen, but there's almost no chance of the data being compromised if you used a good password.

  • Would you trust Ironkey enough to decrypt it if they had access to the device? – Perry Oct 14 '18 at 17:32
  • If you use an unmanaged ironkey encrypted disk, then yes. There's 2 types of IronKey devices. One for companies who want to prevent the risk of disclosure, but still manage their devices. They use managed drives. What you want is one that's not managed. This means nobody will have a backdoor, and ironkey themselves can not decrypt the device. – Daisetsu Oct 14 '18 at 17:34
  • Thanks. If you still wanted to keep those flash drives inside a small safe, what would you use? – Perry Oct 14 '18 at 17:53
  • How often do you need access to your drives? Bank safety deposit box is are going to be much safer than your home in terms of burglary. Home residential safes are often a joke. Here's a reviewer of many small residential safes that may fit your need handgunsaferesearch.com – Daisetsu Oct 14 '18 at 18:02
  • The suggestion to use only FIPS 140-2 compliant drives is not necessary if the encryption is done prior to placement on the flash drive. Since only encrypted data is on the flash drive it is as secure as the encryption method. – zaph Oct 25 '18 at 9:24
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The best physical security for a small USB flash drive is to keep it with you at all times, so that it is under your immediate physical control. In order for an adversary to obtain the device, they would need to assault you and remove the object by force. Keeping a small USB flash drive in a zippered pocket or wearing it on a chain around your neck are options that help you maintain physical control over the device. Avoid keeping sensitive devices in backpacks, purses, briefcases or other containers as those are more easily removed from your control.

For storage outside your personal control you would need a floor safe embedded in concrete. The purchase and installation of such a safe will cost more thank your stated limit. The only other storage option then is to use a reputable third party for storage, as suggested in the comments, bank deposit boxes are considered secure.

Additionally you may format USB flash drives with an encrypted filesystem: BitLocker under Windows, FileVault for Mac OS X, and cryptsetup under Linux.

Try to limit the number of devices and drives by purchasing devices and storage volumes with large capacities, and securly disposing of previous devices. The more devices and storage volumes you have the more difficult it will be to keep all of them protected and secure.

  • The term encrypted filesystem is problematic, it is best if the encryption is done off the device so the device never has access to the encryption key. I guess the flash drive needs to be water resistant or the user must not bath. Sleeping might also be a problem. – zaph Oct 25 '18 at 9:30
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Using a standard flash drive where the data is encrypted on the computer and only the encrypted data is on the device is secure. This is predicated a secure and properly used encryption method such as AES, a random IV and a mode such as CBC or GCM. Zip file encryption probably is not safe.

The weak point is the encryption key, how secure it is and how it is protected. If a password/passphrase it must be good, make sure it is not on a list of frequent passwords such as SecLists.

The encryption key must be derived from the passphrase with a secure key derivation method with a random salt and a CPU utilization of 100ms duration or greater. Use a function such as PBKDF2, Rfc2898DeriveBytes, Argon2i, password_hash, Bcrypt or similar functions. The point is to make the attacker spend substantial time finding passwords by brute force.

Keep in mind that whenever the data is accessed there is an un-encrypted version on the computer and that will most likely not be erased when the access is terminated. This is a small window of opportunity but does exist. Do not access the unencrypted data on a computer that is connected to a network and at a minimum re-boot after accessing, even then there may be a copy of the data on an OS cache file.

But beware of rubber hose cryptanalysis, see XKCD.

A physical option is a class C floor safe secured in concrete which should run about $1000 installed but even then the security is limited but destructive access will generally be apparent however an extremely skilled safe cracker might be able to access the safe.

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