Hashing a hard drive before shutdown

Question

Problem: Currently have no way to verify integrity of my offline encrypted system. I would like to ensure the system hasn't been physically tampered with while I've been away from it.

(To clarify:while I am aware that the system cannot be modified without gaining access to the my encryption key. This question is to address the off-chance that the encryption is somehow bypassed. i.e. They gain access to my encryption key.)

Proposed solution: Hash the contents of the hard drive and have that hash added to a thumbdrive that I will maintain control of.

Question:

1. How difficult would it be to obtain a hash for a harddrive before every system shutdown and have that hash added to a log on a thumbdrive that can be validated after every login?

2. Would I need to have the system unmounted to ensure this method is valid?

3. Is there already a prepackaged solution that you are aware of for this problem?

Considerations:

1. I would need to ensure physical security of the thumbdrive to make sure the checksums match is valid (maintaining physical security of a keydrive is much easier than carrying a computer around with me)

2. This method would probably be more easily implemented on virtual machines (though that doesn't do anything if the host is compromised).

3. Having a Live Linux USB on the thumbdrive to create the hash on the unmounted drive may be a more simplistic method, although not quite as streamlined.

Other thoughts: I am aware that system encryption offers some level of integrity assurance. But in the off chance that encryption becomes violated, I would like to know.

OS: Arch Linux

Answer 1

Near impossible, or at least very difficult. First, there is too much data. It would take a VERY long time to compute a hash of the entire HDD every shutdown and again for every boot. You could alternately use chaining of hashes though to get around that, but it would require a specialized file system. Second however, even if you were willing to wait for a full hash or use a file system that does chaining like that, values are going to change during the boot process itself. Temp files will be created, paging files will change, etc, so it won't work on a system drive without a specialized boot loader on the USB stick that could validate from the earliest stages.

Either way, the hash is irrelevant if the encryption key is securely maintained. You can't meaningfully change encrypted data if you don't have the encryption key. You can corrupt it, but a basic parity check would find that kind of tampering. If the encryption key becomes compromised, you are screwed either way as your data could be read without altering the drive.

An elaborate attacker could also do something like loading in to firmware or memory before your hash can run and then replacing itself after your check, thus avoiding detection by this means (and altering the hash after the first time) unless you do some really elaborate custom mounting that will process everything from the HDD as untrusted and only load things which pass validation chaining.

It also is a lot easier if you are mounting it as an offline drive mounted after boot and can trust the hardware you are connecting to it since that doesn't require a trusted bootloader then. If you can't trust the hardware, bios and other system firmwares also become a concern.

Not much is out there to try to handle this situation as just about every threat model considers the system broken if the key is compromised.

Answer 2

An attacker would have to have access to the encryption key to decrypt the volume, modify a file, then re-encrypt. An attacker couldn't just modify the straight encrypted data. Then when you went to decrypt the data, the decryption would fail. As long as the encryption key is secure you shouldn't have any problems.

If you're worried about file integrity then there are programs like TripWire that can tell you when specific files have been modified on disk. It keeps hashes of files you tell it to care about. If one of them is changed on disk then this is logged. You would be able to check whenever you mount the volume if any files were changed. I think this would be your best option. Keep in mind that if the attacker has access to encryption keys, all of this can be modified while the drive is online.

To answer your questions specifically:

How difficult would it be to obtain a hash for a harddrive before every system shutdown and have that hash added to a log on a thumbdrive that can be validated after every login?

This would be severely impractical. You would have to hash the entire volume which is essentially an entire volume scan. This could take from 30 minutes to hours depending on the size of the drive.

Would I need to have the system unmounted to ensure this method is valid?

For the integrity check to be valid the drive would have to be online and mounted. However, an attacker could hide themselves while the drive is online. So they would be included in the integrity check. Kind of a who's on first situation there.

Is there already a prepackaged solution that you are aware of for this problem?

The short answer is no, not that I'm aware. To ensure the integrity of the drive it must be online and decrypted. But an attacker can hide themselves in the integrity check this way. You would want to do an offline integrity check, but then the drive is encrypted. I think you either choose the encryption, or choose the offline integrity. I believe it's impractical to have both.

I would be satisfied with the encryption option myself. Then I only have myself to blame if the encryption keys are compromised.