[dm-crypt] Passphrase protected key file?

Laurence Darby ldarby at tuffmail.com
Wed Aug 3 13:35:55 CEST 2011

Hello again,

Arno Wagner wrote:

> > Alternatively, I could just do this:
> > 
> > ( cat ~/pass_key ; cat ) | cryptsetup luksOpen --key-file
> > - /dev/loop1 loop1
> > 
> > so I still have to provide both the key and passphrase, terminated
> > with Ctrl-D.  Any thoughts?
> Yes, why do you not use the passphrase entry function of cryptsetup
> directly? Without a specific and credible risk, there is no
> reason to do anything of what you describe here...

Ok, but I may have a reason I need to do this anyway, that probably no
one else has - these disks are external usb connected disks and they are
noisy, so I keep them powered off unless backing up to them. Their power
management is really broken, or maybe linux is, so I connected them to
a relay on the parallel port to properly power them off. That setup has
been working for years, and to keep it automated with encryption, the
key has to be stored somewhere, doesn't it?

I'm not sure I understand the point of having a key file, if that key
file isn't protected somehow - an attacker would have access to the
machine that stores the key as well.  (I'm not going to spend €50000 on
a HSM for this, that would be overamplifing risks by a very long way)

An attacker would have to break into the system after I've entered the
passphrase, without powering it off (notwithstanding cold-boot
attacks), but that's the same case as with normal disk encryption,
isn't it? If the disks were kept powered on, I would enter the
passphrase once at boot up, and keep the disk mounted.  Isn't that what
everyone else does? To get the same but with the disks powering off, I
would decrypt the passphrase to a ramfs (not /dev/shm, as that can get
written to swap), and make cryptsetup read it from there. 

> I would suggest you read up a bit more on cryptography. 
> "Cryptography Engineering" by Schneier et al. is a good book for
> example, to get a good understanding of cryto technology
> and risks.
> You are at the moment in this dangerous "half-knowledge" state, 
> were you see some risks and overamplify them, while you completely 
> miss others. It is normal to go through this stage, but make sure 
> you leave it behind. 

Yes, well, I know that, and really if I didn't overamplify some
risks, then I would probably just not bother with disk encryption at
all, but that doesn't achieve or teach me anything. Anyway, I live in
the UK which has the RIPA act, so they send people to prison simply for
not handing over the keys. Check the references on


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