Hello,

I'm going to use cryptsetup on a usb stick. I've done this before and always use a passphrase. I wasn't aware that a key file can be used as well. So, which is better? Or is it possible to use both to authenticate?

thanks

I think it really depends on how you plan to use the USB stick. If you're using it for transport between known computers, each one of which would be expected to have a copy of the key-file installed, then this would be more secure than a passphrase. But you can't "type in" a key.

If your objective is simply to make a USB "useless if stolen, but still relatively easy to use," a passphrase is probably good enough.

I think I'll stick with the passphrase as a better option for transport of usb sticks. Besides, if I use a key file I would need an extra usb stick to hold the key file. Thanks sundialsvcs...

/me nods ...

Yes, this seems to me to best-suit your use case.

To illustrate the opposing case: I once did a project for an insurance company who wanted to be able to put subscriber medical-records onto a USB stick that the patient could then carry to one of their health-care centers ... recently-acquired centers in small towns that (at that time) were not fully integrated into their corporate system. It was crucial, for legal reasons, that the information must be "HIPAA-compliant" secure.

The objective was reached by using a certificate-encrypted filesystem, installing an encrypted copy of the certificate on the computers at the clinic. When the stick was inserted into one of those computers, the information could be read. But a "passphrase" did not exist: only the intended computers could retrieve the data, and we could prove this to any government security-compliance auditor.

(The insurance company's attorneys carefully scrutinized and then approved our plan, which was fully understood to be an interim measure.)

In time, the company upgraded their systems in all of their clinics so that medical records can now be securely retrieved from their secure corporate databases just by scanning a QR-code on the membership card. (This just contains a membership number and an extra "nonce.") They stopped using the sticks. But they didn't have to attempt to get people to turn the sticks in, because they remained confident – and, it was true – that the information they still contained remained secure. (Instead, they invited people to simply re-format them and use them as they pleased.)

The way we did it, you couldn't even begin to access the content of the drive: every sector was enciphered using a random key that was thousands of bits long. If you stuck the thing into an ordinary computer, there was no readable file system at all. (Windows would "helpfully" offer to format it for you, and some customers did. ) But the bottom line was that the information which could be easily read at a clinic could not be accessed otherwise – even to this day – and we could prove it.