Re: [dm-crypt] miscellaneous dm-crypt/LUKS/cryptsetup questions

[Milan Broz <mbroz@redhat.com>]

On 05/28/2010 12:56 PM, Christoph Anton Mitterer wrote:
> On Thu, 2010-05-27 at 16:45 +0200, Milan Broz wrote:
>>> So would it be better for now to do a --master-key-file /dev/random?
>>> Or is this not as secure (as urandom) in this case?
>>
>> --master-key-file intentionally can read only regular file.

master-key-file is new option which allows using pre-generated master key.

Please use it only if you must - it was designed for Key Escrow functions,
(which uses libcryptsetup directly anyway) not for normal use.

>>
>> (read from /dev/random can return if there is not enough entropy, it needs
>> implement some more code to use.)
> Is it then secure (an better than using /dev/urandom) to do something
> like:
> 
> mount -t tmpfs foo /mnt
> cd /mnt
> dd if=/dev/random of=mk bs=1 count=64
> cryptsetup --master-key-file mk -s 512 -c aes-xts-plain --key-file
> someKeyFile luksFormat /dev/blafasl
> 
> ?
> The master key should not be leaked to disk,... it 64 bytes large, and a
> keyfile is used to encrypt it.
> 
> Or better stick with urandom?

See above, do not use it at all just run luksFormat in your case which
generate key from /dev/urandom.

>> This is LUKS (keyslot) hash algorithm used (together with PBKDF2) when
>> unlocking keyslot.
>> (see documentation again, project site http://code.google.com/p/cryptsetup/
>> -> Specification)
> Can/should I change it to something "better" (e.g. SHA512)?
> I guess this could be done by simply exchanging the key slots and I do
> not have to reencrypt the whole disk?

First read archive discussion about that - there is no problem with SHA1 in this use.
You can define hash during luksFormat (-h <alg>) with cryptsetup 1.1.x, but if you select
somethin other than sha1, it will be incompatible with old distros.

You cannot change it later (only by reformatting header).

Milan