Storing GPG Keys on an Encrypted USB Flash Drive
It
is often desirable to be able to use a GPG key on more than one
computer, for instance at home and at work, or on a desktop and a
laptop. Unfortunately, storing encryption keys where you don't have
physical control is generally a bad idea. Even storing keys on a laptop
can be troublesome--if the laptop gets stolen, so does your GPG key.
Luckily, you can probably revoke the key before anybody is able to
decrypt it because GPG keys are stored encrypted at all times by
default, but that's a hassle. What if you could securely store the key
on a device that you always have on your person?
(Note
from another reader:Although using your key on a computer that you
don't have physical control of is still dangerous, and although your key
is already encrypted with a well respected and highly secure encryption
algorithm, you may prefer the extreme security of double encryption.
There may be a huge number of other things to spend your time on that
would increase your security more, but here's how you can encrypt your
already encrypted key again, if you so desire. This Howto is very useful
just for learning how to set up an encrypted storage area on a USB
drive for general usage though.)
IMPORTANT: Make sure you make a backup copy of your ~/.gnupg directory before you do this. The last thing you want to happen is to lose your keyring because something went wrong.
dm-crypt
From
the dm-crypt website: " Device-mapper is a new infrastructure in the
Linux 2.6 kernel that provides a generic way to create virtual layers of
block devices that can do different things on top of real block devices
like striping, concatenation, mirroring, snapshotting, etc... The
device-mapper is used by the LVM2 and EVMS 2.x tools. dm-crypt is such a
device-mapper target that provides transparent encryption of block
devices using the new Linux 2.6 cryptoapi. The user can basically
specify one of the symmetric ciphers, a key (of any allowed size), an iv
generation mode and then he can create a new block device in /dev.
Writes to this device will be encrypted and reads decrypted. You can
mount your filesystem on it as usual. But without the key you can't
access your data."
This
is perfect for our needs. We will create an encrypted filesystem
inside of a regular file on the USB flash drive, where we will store
sensitive data like GnuPG keys.
Installing the Software
First, you will need to install cryptsetup:
sudo apt-get install cryptsetup
This will also pull in some other necessary dependencies.
Setting Up the Encrypted Filesystem
I
store my GPG keys on a cheap, tiny USB flash drive that fits
comfortably on my keyring. When I plug it in, it is automatically
mounted as /media/usbdisk. The following sections will assume a similar setup.
I
decided to make my encrypted filesystem live in a regular file rather
than its own partition. This requires less tweaking of the disk, and
makes mounting and unmounting the encrypted filesystem easier, as you
will see later. However, many of the steps in this tutorial can be
adapted to use a real partition instead of a regular file.
Creating the File
Before we can make a filesystem, we need a file that is large enough to hold it. This can be accomplished with dd:
dd if=/dev/zero of=/media/usbdisk/disk.img bs=1M count=16
The above command will make a 16 MB file containing only zeros. Modify the count option to get your desired encrypted filesystem size.
Setting up the Encrypted Loop Device
Before
we can actually create the filesystem on our new file, we need to
attach it to a loop device and set up a device-mapper target with
encryption. losetup -f will find the first free loop device, so we will set its output to a variable called loopdev and use it for several commands:
sudo modprobe cryptoloop sudo modprobe dm-crypt sudo modprobe aes_generic export loopdev=$(sudo losetup -f) sudo losetup $loopdev /media/usbdisk/disk.img sudo cryptsetup -c aes -s 256 -h sha256 -y create usbkey $loopdev
This will set up the file with 256-bit AES encryption, hashing the passphrase you issue through SHA-256.
After
it's set up, it's a good idea to remove the usbkey device-mapper device
and re-run cryptsetup to make sure that you didn't mistype the initial
password (I say this from experience...):
sudo cryptsetup remove usbkey sudo cryptsetup -c aes -s 256 -h sha256 create usbkey $loopdev
If all goes well, we're ready for the next step!
Creating the Actual Filesystem
This is the easiest step of all. I chose the ext3 filesystem for its journaling capability, just in case the USB drive gets removed before the filesystem is unmounted. The cryptsetup command above created the device /dev/mapper/usbkey,
which is a map through dm-crypt to the encrypted filesystem. So, this
device appears to the system as a regular old block device, like a hard
disk or partition. The following command will create an ext3 filesystem on the encrypted file:
sudo mkfs.ext3 /dev/mapper/usbkey
Now, try mounting the filesystem:
sudo mkdir -p /media/encrypted sudo mount -t ext3 /dev/mapper/usbkey /media/encrypted
Setting up GnuPG on the Encrypted Filesystem
Now, make a .gnupg directory in /media/encrypted, make it owned by your user, and link it to your own ~/.gnupg (if you already have a .gnupg directory, move it out of the way first):
sudo mkdir /media/encrypted/.gnupg sudo chown $UID.$UID /media/encrypted/.gnupg chmod 0700 /media/encrypted/.gnupg ln -s /media/encrypted/.gnupg ~/.gnupg
Now, create a GnuPG key as described in GPGKey or, if you already have a key, move the files in your old .gnupg directory into the new one, possibly using shred or wipe to securely remove the old files.
Making Things Easier
Simplifying the Mount Process
It's
not really fun to type three or four commands each time you want to
mount your encrypted filesystem. So, I wrote two really simple scripts
for mounting and unmounting. Before using these, you should unmount
your filesystem and detach the loop device:
sudo umount /media/encrypted sudo cryptsetup remove usbkey sudo losetup -d $loopdev
Now, save the following as mount.sh in the root of your USB drive (not in the encrypted filesystem!):
dir=`dirname $0` loopdev=$(sudo losetup -f) sudo -p "Password (sudo): " modprobe cryptoloop && \ sudo modprobe dm-crypt && \ sudo modprobe aes_generic && \ sudo mkdir -p /media/encrypted && \ sudo losetup $loopdev $dir/disk.img && \ sudo cryptsetup -c aes -s 256 -h sha256 create usbkey $loopdev && \ sudo mount -t ext3 /dev/mapper/usbkey /media/encrypted && \ sudo chown -R $UID.$UID /media/encrypted/
Then, save the following as umount.sh in the same place:
loopdev=$(sudo cryptsetup status usbkey | grep device | sed -e "s/ *device:[ \t]*//") sync sudo umount /media/encrypted sudo cryptsetup remove usbkey sudo losetup -d $loopdev
You
may not be able to execute these scripts directly, since the default
auto-mounting options prohibit running executables. But, since they are
shell scripts, you can simply pass them on to sh. So, once the USB drive has been mounted, you can simply type:
sh /media/usbdisk/mount.sh
and
all the work will be done for you! (Of course, you will need the
encryption password, and you may be asked for a password for sudo.)
Verifying PGP Signatures Without the Encrypted Filesystem
You
might want to be able to verify a signed message without needing to
mount the encrypted filesystem. To facilitate this, simply copy the
public keyring and the trust database file to the "real" .gnupg directory:
cp /media/encrypted/.gnupg/{pubring,trustdb}.gpg /tmp
sh /media/usbdisk/umount.sh
sudo mv /tmp/{pubring,trustdb}.gpg ~/.gnupg
Now, when the encrypted filesystem is not mounted, you will see those files in your .gnupg directory, so that gpg --verify will work. But when it is mounted, you will see the files that are actually in the encrypted filesystem. 
