Monday, February 18, 2008

Keeping sensitive data on the crypto disks

Previously, I described how to create one or more crytpographic partitions. The data stored on those partitions is not retrievable without the 32-digit hexadecimal key that protects it, the key being constructed from a passphrase input by the user. It may seem that this is sufficient to protect sensitive data, making sure simply to create and edit your files only in that partition. However, there are some subtle details that have to be kept in mind.

Information stored on an unencrypted ext2 or ext3 partition has an unknown persistence. A file that was stored there, and later deleted, may be partially or fully recoverable at some time in the future. To be sure of the confidentiality of your data, you have to make sure that it has never been stored to an unencrypted partition.

If you start up your favourite text editor, telling it to create a new file in some place, let's call it /crypto/sensitive.txt, and then start typing, you may expect that the data never lands on an unencrypted partition. However, there are at least four things to be careful of:
  1. The editor may store information in your home directory, which may not be on the encrypted partition. It might store some of the file contents there, or it might store file metadata. Your editor may keep a table of filenames recently visited in /home, with information about the line number last visited. Your editor might be configured to store crash-recovery autosave files in a directory under your /home directory.
  2. The editor may sometimes store the contents of a working buffer to a file in /tmp.
  3. The computer may come under memory pressure, resulting in some of your data being sent to the swap device.
  4. Your backups may not be as well protected as the files on the cryptographic disk.
The first two points are probably best addressed by ensuring that all of the directories writable by the unprivileged user are on cryptographic partitions. If you only have write permission to the crypto drives, you won't store any files in plaintext. Note, however, that you typically need /tmp to exist and be writable during the bootup of your system, so that partition can't be protected with a passphrase if you care about the system successfully performing an unattended reboot.

So, what do we do about /tmp? Well, one simple solution is an overmount. While you normally mount a partition onto an empty directory, it is legal to mount onto a directory that is not empty. The files that were present in that directory are mostly inaccessible after that (a process with access to file descriptors that it opened before the mount will still be able to operate on those files, but they will be invisible to new open operations by pathname).

We're assuming you have at least one cryptographic partition. So, create a directory on that partition, let's say /crypto/tmp. After you have formatted and mounted your cryptographic partition, run this command. You only have to do this once, the first time you set up cryptographic disks.
mkdir --mode=01777 /crypto/tmp

Now, you can add the following command to the end of the script in the previous post, the script that mounts your formatted disks:
mount --bind /crypto/tmp /tmp

After you've done this, the system will still boot up as usual, using its unencrypted /tmp partition. Then, the root user can run the script from the previous post, now modified to have this extra mount line on the end of it. After entering the passphrase the script will do its work and exit, at which time your /tmp partition will have been replaced with the one in /crypto. Note that if your system starts up in X, with a graphical login screen, you will have to restart it after you have overmounted /tmp, otherwise you will find that X programs fail to work at all. I usually restart X by issuing a simple "killall X" command, and letting the xdm or gdm program start it back up again. This is a lot of trouble, but all manner of things can be stored on your /tmp disk. Firefox will store downloaded files such as PDFs there when there is a helper application ready to use them.

That leaves us with swap. Encrypting the swap space is actually very easy:

# Encrypt the swap partition
hashed=`dd if=/dev/urandom bs=1 count=64 | md5sum | awk ' { print $1 } '`
dmsetup create SWP <<DONE
0 `blockdev --getsize /dev/hda6` crypt aes-plain $hashed 0 /dev/hda6 0
mkswap /dev/mapper/SWP
swapon /dev/mapper/SWP

This can run unattended during the bootup. It creates a random cryptographic key using /dev/urandom, a device especially designed to produce true random numbers even during a system bootup sequence. This random key is used to create an encrypted interface to /dev/hda6. It is formatted as a swap partition, and then enabled. A new key will be generated each time the system boots, so nothing in swap space will survive a reboot. Note that there do exist suspend-to-disk procedures for Linux that store a memory image on the swap partition. If you intend to use such a suspend system, you will have to ensure that it does not attempt to write to the cryptographic swap partition, or you'll have to defer mounting the swap partition until the root user can enter a specific passphrase, thereby allowing you to preserve the contents across a reboot. If you're supplying a passphrase to handle encryption on the swap space, you should not run mkswap, except the first time you set up the partition (think of mkswap as being a reformat).

The question of how to protect your backup copies of sensitive files is entirely dependent on what system you use for backups. You may be able to pipe your backups through the des binary, or you may be able to store the backups on encrypted filesystems, but there are too many variations for me to offer much advice here. The security of your backups is not something that can be ignored, as has been made all to obvious with the various data disclosure scares that occur with alarming regularity when shipments of tapes or CDs fail to arrive at their destinations.


See my followup article for a warning about a vulnerability in this technique.

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