LUKS with USB key authentication, how secure is it?

Question

I am thinking of creating a LUKS encryption by using a USB key instead of a passphrase to unlock my hard drive.

However some people told me that using a USB key for authentication is less secure than a passphrase. My question is: How is it less secure? And I am thinking about a scenario where I use said USB key only on my own devices and in this scenario my USB key is never stolen.

I know I could go even further and set up a two factor authentication but that is not what I am asking about.

Solution

It depends on your point of view. It's more secure since keyfiles usually are truly random while passphrases tend to be short and weak.

Then again a passphrase, on its own, takes about $5 to break, either using a keylogger dongle or modified bootloader/initramfs or the infamous XKCD decryption wrench. Same is true for keyfiles if it's only that file on the USB stick, while bootloader/initramfs is unprotected on HDD.

The USB key on the other hand, gives anyone access - if you left it plugged in instead of carrying it on your person at all times. Anyone can copy an unattended USB stick quickly and easily...

I do both... a USB key which contains bootloader, kernel, initramfs with LUKS-encrypted keyfiles which requires a passphrase to be accessed. So you need both key and passphrase to unlock. Not sure if you can call this a two factor authentication - it's about the limit of what I was willing to do while keeping things practical. (Bootable USB stick is also useful for carrying a dozen LiveCDs around).

What's your threat model anyway, did you even think about it?

If you let me move into your house, do you want me to be able to access your data with little to no effort?

If you died tomorrow, do you want your relatives to be able decrypt your stuff?

With USB key that does not take a passphrase, it would be possible - and desireable for those family photos only you have a copy of...

I have a server that decrypts itself on bootup [using hardware fingerprints such as cpu, ram, mac address, ...], no interaction at all necessary... threat model here is that someone might remove the drive and put it into another customer's box, or stuff like that.

Comments

[Mario Kamenjak]

"What's your threat model anyway, did you even think about it?" I am doing this just as a learning experience. "If you let me move into your house, do you want me to be able to access your data with little to no effort?" I am the only user of my device. So, no. Also I plan to put the USB on my keyring with my very expensive apartment key. So I will have the USB stick always with me. I may make a backup key stashed away in a very hidden place. "If you died tomorrow, do you want your relatives to be able decrypt your stuff?"

[frostschutz]

Regarding backup key, you might consider a backup passphrase too. LUKS supports up to 8 different phrases/keys.

[Gilles 'SO- stop being evil']

A passphrase takes more than $5 to break. You can get a wrench for $5 but even cheap goons cost more than that. Chamber maid attacks can be dirt cheap but they require physical access multiple times, which is a far more advanced threat than device theft.