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We would like to provide a flash drive to a client, that they can boot to, which includes a read-only operating system of our own customization, probably Linux. Every boot, it starts from clean slate.

With the current setup, a person with physical access to the drive could switch the write protect switch, inject some code that calls back their own server, flip the switch back, and we may or may not be able to detect that the code was injected, when booting, because once booted, the OS could be tampered to hide the injected files from boot integrity checksumming, or modify the checksumming routine not to check the injected files.

What I would like is to put a password on the flash drive, so the write protect can only be disabled if we enter the password.

  • Is there such a device with the additional hardware/firmware countermeasure I refer to? What would it be called, how could I find this type of product?
  • For an operating system that is required to contact our server equipment, do you have an alternate suggestion of how to detect and/or prevent operating system tamper such as this?
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You could encrypt the USB drive with truecrypt, then install your distro inside truecrypt to protect it. This wouldn't satisfy the write-protected requirement, but seems the best option considering everything else you're asking.

https://kb.berkeley.edu/jivekb/servlet/KbServlet/download/1625-102-55/

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(The write protect switch on the model drive I have used is discrete like a reset button, you need to stick a pen or something into the slot in order to reach the switch and flip it)

The best solution I have so far - that still satisfies the requirement that no malicious files be downloaded by the user - is to get a flash drive that has a physical write protect switch, and apply a coating that will harden. (similar coats are on the market for covering tool handles, such as wrenches)

If the hacker has physical access, I see this setup as having 2 possible outcomes

  1. the tamper would be evident to the user (broken seal), or
  2. after tamper is complete, a new coat of similar color would have to be applied by the hacker.
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USB flash drive controllers can often be modified through software. The write protect switch is in fact probably a software feature. If access to the flash controller is possible then the write protect switch can likely be disabled in software.

As to preventing the user of the flash drive from intentionally disabling the write protection, that's rather difficult. The original xbox had an on chip ROM that verified the integrity of the system firmware before booting it and even that got cracked. You could just solder the write protect circuit to be enabled. If the user wants to desolder the flash chip and put it in another flash drive so they can write to it then I don't think you'll be able to stop them. What's to stop them from buying a new flash drive of the same model and then copying everything from the read only flash drive over to it?

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If you google for secure flash drive you will find a few. There aren't many that are commercially available that will do exactly what you want though.

Have a look at http://www.secureusb.co.uk/index.php for a catalogue of secure USB and Flash devices.

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