dm_crypt_partition.howto

dm-crypt with LUKS
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LUKS with dm-crypt has better encryption and makes it possible to have multiple passphrase for the same partition or to change the password easily. To test if LUKS is available, simply type # cryptsetup --help, if nothing about LUKS shows up, use the instructions below Without LUKS. First create a partition if necessary: fdisk /dev/sdc.
Create encrypted partition

# dd if=/dev/urandom of=/dev/sda3          # Optional. For paranoids only (takes days)
# cryptsetup -y luksFormat /dev/sda3       # This destroys any data on sda3
# cryptsetup luksOpen /dev/sda3 sda3
# mkfs.ext3 /dev/mapper/sda3               # create ext3 file system
# mount -t ext3 /dev/mapper/sda3 /mnt
# umount /mnt
# cryptsetup luksClose sda3                # Detach the encrypted partition
Attach

# cryptsetup luksOpen /dev/sda3 sda3
# mount -t ext3 /dev/mapper/sda3 /mnt
Detach

# umount /mnt
# cryptsetup luksClose sda3
dm-crypt without LUKS

# cryptsetup -y create sda3 /dev/sda3      # or any other partition like /dev/loop0
# dmsetup ls                               # check it, will display: sda3 (254, 0)
# mkfs.ext3 /dev/mapper/sda3               # This is done only the first time!
# mount -t ext3 /dev/mapper/sda3 /mnt
# umount /mnt/
# cryptsetup remove sda3                   # Detach the encrypted partition


Taken/inspired from

http://sleepyhead.de/howto/?href=cryptpart