Pam_mount and encrypted filesystem keys

["W. Michael Petullo" <mike@flyn.org>]

I have been working a lot on Elvis Pfützenreuter's pam_mount PAM module.
This module allows one to mount, among other things, loopback encrypted
filesystems at login time.  The module makes it easy for a user to have
an encrypted home directory.  The module is available at
http://www.flyn.org but is not quite ready for prime time.

I recently added the concept of an encrypted filesystem key, similar to
that which loop-AES uses.  For example, given:

sk
	system key, the key or password used to log into the system

fsk
	filesystem key, the key that allows you to use the filesystem
	you wish pam_mount to mount for you

E and D
	an openssl supported synchronous encryption/decryption algorithm

efsk
	encrypted filesystem key, efsk = E_sk (fsk), stored somewhere
	on the local filesystem (ie: /home/user.key)

Pam_mount will read efsk from the local filesystem, perform 
fsk = D_sk (efsk) and use fsk to mount the filesystem. If you change
your system password, simply regenerate efsk using efsk = E_sk  (fsk).

Does this technique appear to make sense from a security point of view?

I am using the following technique to create encrypted volumes with a
random key, encrypted with the user's system password and stored on the
local filesystem:

a.  dd if=/dev/urandom of=/home/user.img bs=1M count=<image size in MB>
b.  dd if=/dev/urandom bs=1c count=<keysize / 8> | openssl enc \
    -<fs key cipher> > /home/user.key
    Encrypt this file using the user's login password as the key.
c.  openssl enc -d -<fs key cipher> -in /home/user.key | losetup -e aes \
    -k <keysize> -p0 /dev/loop0 /home/user.img
d.  mkfs -t ext2 /dev/loop0
 ...
n - 1.  umount /dev/loop0
n.  losetup -d /dev/loop0

Again, does this technique also make sense?

-- 
Mike

:wq
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Linux-crypto:  cryptography in and on the Linux system
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