The swap partition can hold a lot of unencrypted confidential information and the fact that it persists after shutting down the computer can be a problem.

Encrypting a swap partition however is slightly tricky if one wants to also support suspend-to-disk (also called hibernation). Here's a procedure that worked for me on both Debian Lenny and Ubuntu 7.10 (Gutsy Gibbon):

  1. Install the cryptsetup package:

     apt-get install cryptsetup
    
  2. Setup the encrypted partition as root:

     swapoff -a
     cryptsetup -h sha256 -c aes-cbc-essiv:sha256 -s 256 luksFormat /dev/hda2
     cryptsetup luksOpen /dev/hda2 cswap
     mkswap /dev/mapper/cswap
    
  3. Add this line to /etc/crypttab:

     cswap /dev/hda2 none swap,luks,timeout=30
    
  4. Set the swap partition to be this in /etc/fstab:

     /dev/mapper/cswap none swap sw 0 0
    
  5. Configure uswsusp to use /dev/mapper/cswap and write unencrypted data

     dpkg-reconfigure -plow uswsusp
    

You will of course want to replace /dev/hda2 with the partition that currently holds your unencrypted swap.

(This is loosely based on a similar procedure for Ubuntu 6.10.)