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Here is a quick rundown on how to create a cryptfs partition that mounts during boot with Fedora Core 4.

Configure the kernel (this may already be done for you):

Device Drivers --->
  Multiple devices driver support (RAID and LVM) --->
    [*] Multiple devices driver support (RAID and LVM)
      < >   RAID support
      <*>   Device mapper support
      <*>     Crypt target support

Cryptographic options --->
  <M>   MD5 digest algorithm
  <M>   SHA1 digest algorithm
  <M>   AES cipher algorithms
   .... 

Install the userland tools:

yum install cryptsetup

Create the filesystem and format it:

cryptsetup -c blowfish -s 64 create fs_name /dev/sda2
mkfs.ext3 /dev/mapper/fs_name

Create /etc/init.d/crytptinit:

if [ -b /dev/mapper/fs_name ]; then
  /sbin/cryptsetup remove fs_name
fi
/sbin/cryptsetup -c blowfish -s 64  create fs_name /dev/sda2

Run it at the runlevels you want:

cd /etc/rc3.d
ln -s ../init.d/cryptinit S08cryptinit

Create the mount point:

mkdir /mount_point

Edit /etc/fstab:

/dev/mapper/fs_name /mount_point               ext3   defaults 0 0

Reboot. You will be asked for your passphrase when the machine boots.

Information in this post was gleaned from several places. Here is one
that matches closely.