Re: crypting filesystems

Wiktor
2005-04-04T20:54:44Z

Hi,

I'm using the following method and it seems to be working fine
(involving crypto-loop):

i have normal ext3 /boot partition, where i store kernel image & initrd.
after lilo boots the kernel, initrd sets up /dev/loop0 to be
crypto-loop/blowfish for /dev/hda1 (losetup /dev/loop0 /dev/hda1 -e
blowfish). losetup asks for passphrase, and (if entered correctly),
/dev/loop0 is mounted as root filesystem (it can be done also by simple
mount call: mount /dev/hda1 /some-place -o rw,encryption=blowfish). for
encrypting more filesystems with one passphrase, you can read it in
shell script in non-echo-mode (if such exists, i'm not sure), and pass
it to mount or losetup. crypto-loop makes possible to switch encryption
type without modifying whole initrd.

Regarding your questions:

> 1. In order to put in the passphrase just once a time at booting, I
put the passphrase in a gpg-crypted file (cipher AES256 and 256Bit key
size), which is decrypted at boot-time to /tmp (-> tmpfs) and
immediately removed with shred, after activating the three partitions.
Is it possible to see the cleartext password after this action in tmpfs?

Disk encryption usually protects from hardware-attacks (when hacker has
physical access to the hardware). if you keep passphrase
reversible-encrypted, attacker can read it and run brute-force attack
using some huge-computing-capacity. is this what you want?

> 2. Is it possible to gain the passphrase from the active encrypted
partitions (because the passphrase is somewhere held in the RAM)?

Only when attacker has root privileges. But i'm not sure if it is
possible to extract passphrase knowing both encrypted and not encrypted
data. What i mean is that usually each filesystem begins with
filesystem-specyfic-header, which is constant or similar to each other.
so, if attacker has encrypted form of this header and can estimate
unencryptes form, it can possibly gain the passphrase. (but therse are
only my ideas, i don't know how the encryptino-algorithm works).

> 4. Are there any master keys existing, which could be used to open
every encrypted filesystem?

We all wish they are no such 'features'.

--
wixor
May the Source be with you.