[dm-crypt] Pass+keyfile

Sven Eschenberg sven at eschenberg.eu
Mon Dec 1 17:49:40 CET 2014

On Mon, December 1, 2014 17:39, Arno Wagner wrote:
> On Mon, Dec 01, 2014 at 15:54:45 CET, 0x14 at unseen.is wrote:
>> >This construction is redundant and does not provide any
>> >additional security as compared to passphrase alone,
>> >assuming that your passphrase is secure.
>> Additional security, as I see it, will be:
>> 1. If you have knowledge about encrypted device, you need to not
>> only know the passphrase, but also have the keyfile (have physical
>> access to it). Some sort of 2-FA
> And now do a scenario where an attacke has the passphrase, but
> not root access and not the keyfile. There are not many
> possibilities for that to happen and most are unrealistic.

The only setup I can imagine is having a keyfile and have that encrypted
with a password and keep it separate (as in physical). This way the
password gives you the opportunity to decrypt the keyfile which you don't
have at your fingertips.

The OPs example makes only sense in respect to the fact that the first
container can be closed and is not readable anymore, yet with root privs
the actual mkey can be derived from mem anyway.

> The typical way somebody gets your passphrase is by hacking
> your box. Then they also get the master key, the keyfile
> and anything else they want. Disk encryption does not protect
> against getting hacked.
>> 2. Separate keyfile may be easier to physical destruction, it may be
>> crucial when you are in hurry to do that - small microsd card with
>> keyfile VS encrypted harddrive. If I get it right, bigger encrypted
>> containers in plain mode are harder to destroy also.
> No. The SD card is a lot _harder_ to destroy than the LUKS header.
> The LUKS header is gone after a single overwrite of 2MiB of data.
> The SD card needs very careful physical destruction.
> Arno
>> Isn`t it somehow comparable to having LUKS header on separate device
>> (--header option)?
>> Am I wrong?
>> Thanks for patience in advance :)
>> P.S. Writing form another mail, sorry for possible confusion.
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> --
> Arno Wagner,     Dr. sc. techn., Dipl. Inform.,    Email: arno at wagner.name
> GnuPG: ID: CB5D9718  FP: 12D6 C03B 1B30 33BB 13CF  B774 E35C 5FA1 CB5D
> 9718
> ----
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