Create & mount 'encrypted filesystem' on a partition or file

In order to create a new encrypted filing system managed by cryptmount,
       you can use the supplied 'cryptmount-setup' program, which can be  used
       by the superuser to interactively configure a basic setup.

       Alternatively,  suppose  that  we  wish to setup a new encrypted filing
       system, that will have a target-name of "opaque".  If we  have  a  free
       disk partition available, say /dev/hdb63, then we can use this directly
       to store the encrypted filing system.  Alternatively,  if  we  want  to
       store  the  encrypted filing system within an ordinary file, we need to
       create space using a recipe such as:

$dd if=/dev/zero of=/home/opaque.fs bs=1M count=512

       and then replace all occurences of '/dev/hdb63' in the  following  with
       '/home/opaque.fs'.   (/dev/urandom  can  be used in place of /dev/zero,
       debatably for extra security, but is rather slower.)

       First,  we  need  to  add  an  entry  in  /etc/cryptmount/cmtab,  which
       describes  the  encryption  that will be used to protect the filesystem
       itself and the access key, as follows:

           opaque {
               dev=/dev/hdb63 dir=/home/crypt
               fstype=ext2 mountoptions=defaults cipher=twofish
               keyfile=/etc/cryptmount/opaque.key
               keyformat=builtin
           }

       Here, we will be using the "twofish" algorithm to  encrypt  the  filing
       system  itself, with the built-in key-manager being used to protect the
       decryption key (to be stored in /etc/cryptmount/opaque.key).

       In  order  to  generate  a  secret  decryption  key   (in   /etc/crypt‐
       mount/opaque.key)  that  will  be  used  to  encrypt  the filing system
itself, we can execute, as root:

$cryptmount --generate-key 32 opaque

       This will generate a 32-byte (256-bit) key, which is known to  be  sup‐
       ported  by the Twofish cipher algorithm, and store it in encrypted form
       after asking the system administrator for a password.

       If we now execute, as root:

$cryptmount --prepare opaque

       we will then be asked for the password that we  used  when  setting  up
       /etc/cryptmount/opaque.key,  which  will  enable  cryptmount to setup a
       device-mapper target (/dev/mapper/opaque).  (If you  receive  an  error
       message  of the form device-mapper ioctl cmd 9 failed: Invalid argument
       , this may mean that you have chosen a key-size that isn't supported by
       your chosen cipher algorithm.  You can get some information about suit‐
       able key-sizes by checking the output  from  "more  /proc/crypto",  and
       looking at the "min keysize" and "max keysize" fields.)

       We  can  now  use  standard tools to create the actual filing system on
       /dev/mapper/opaque:

$mke2fs /dev/mapper/opaque

       (It may be advisable, after the filesystem is first mounted,  to  check
       that  the  permissions of the top-level directory created by mke2fs are
       appropriate for your needs.)

       After executing

$cryptmount --release opaque
$mkdir /home/crypt

       the encrypted filing system is ready for use.  Ordinary users can mount
       it by typing

$cryptmount -m opaque

       or

           cryptmount opaque

       and unmount it using

$cryptmount -u opaque