File: [cvs.NetBSD.org] / htdocs / docs / guide / en / chap-cgd.xml (download)
Revision 1.6, Fri Nov 11 16:37:41 2016 UTC (7 years, 5 months ago) by leot
Branch: MAIN
Changes since 1.5: +1 -14
lines
Delete `Example: encrypted CDs/DVDs - Introduction' section.
It was a bit confusing and self-referential without providing useful
information. The chapter provides a good introduction to cgd(4)
and vnd(4) usage in the paragraph can be considered pretty self
consistent.
Pointed out by visitor via PR misc/46012.
|
<!-- $NetBSD: chap-cgd.xml,v 1.6 2016/11/11 16:37:41 leot Exp $ -->
<!-- from %Geek: cgdhack.txt,v 1.7 2004/02/21 00:46:58 dan Exp % -->
<chapter id="chap-cgd">
<title>The cryptographic device driver (CGD)</title>
<para>The <devicename>cgd</devicename> driver provides functionality
which allows you to use disks or partitions for encrypted storage.
After providing the appropriate key, the encrypted partition is
accessible using <devicename>cgd</devicename> pseudo-devices.</para>
<sect1 id="chap-cgd-overview">
<title>Overview</title>
<para>People often store sensitive information on their hard disks and
are concerned about this information falling into the wrong hands.
This is particularly relevant to users of laptops and other
portable devices, or portable media, which might be stolen or
accidentally misplaced.</para>
<sect2 id="chap-cgd-overview-why">
<title>Why use disk encryption?</title>
<para>File-oriented encryption tools like
<application>GnuPG</application> are great for encrypting
individual files, which can then be sent across untrusted
networks as well as stored encrypted on disk. But sometimes
they can be inconvenient, because the file must be decrypted
each time it is to be used; this is especially cumbersome when
you have a large collection of files to protect. Any time a
security tool is cumbersome to use, there's a chance you'll
forget to use it properly, leaving the files unprotected for
the sake of convenience.</para>
<para>Worse, readable copies of the encrypted contents might still exist
on the hard disk. Even if you overwrite these files (using
<command>rm -P</command>) before unlinking them, your application
software might make temporary copies you don't know about, or have
been paged to swapspace - and even your hard disk might have
silently remapped failing sectors with data still in them.</para>
<para>The solution is to simply never write the information unencrypted
to the hard disk. Rather than taking a file-oriented approach to
encryption, consider a block-oriented approach - a virtual hard
disk, that looks just like a normal hard disk with normal
filesystems, but which encrypts and decrypts each block on the way
to and from the real disk.</para>
</sect2>
<sect2 id="chap-cgd-overview-logicaldriver">
<title>Logical Disk Drivers</title>
<para>The <devicename>cgd</devicename> device looks and behaves to the rest of
the operating system like any other disk driver. Rather than
driving real hardware directly, it provides a logical function
layered on top of another block device. It has a special
configuration program, <command>cgdconfig</command>, to create and
configure a <devicename>cgd</devicename> device and point it at the
underlying disk device that will hold the encrypted data.</para>
<para>&os; includes several other similar logical block devices, each
of which provides some other function where <devicename>cgd</devicename>
provides encryption. You can stack several of these logical block
devices together:
<!--
vnd is presently broken, and putting cgd on top of it exposes the
issue; don't suggest this usage yet
<devicename>cgd</devicename> on top of
<devicename>vnd</devicename> is handy to make an encrypted volume in a
regular file without repartitioning, or
-->
you can make an encrypted
<devicename>raid</devicename> to protect your encrypted data against
hard disk failure as well.</para>
<para>Once you have created a <devicename>cgd</devicename> disk, you can
use <command>disklabel</command> to divide it up into
partitions, <command>swapctl</command> to enable swapping to
those partitions or <command>newfs</command> to make
filesystems, then <command>mount</command> and use those
filesystems, just like any other new disk.</para>
</sect2>
<sect2 id="chap-cgd-overview-availability">
<title>Availability</title>
<para>The <devicename>cgd</devicename> driver was written by Roland
C. Dowdeswell, and introduced in the NetBSD 2.0 release.</para>
</sect2>
</sect1>
<sect1 id="chap-cgd-components">
<title>Components of the Crypto-Graphic Disk system</title>
<para>A number of components and tools work together to make the
<devicename>cgd</devicename> system effective.</para>
<sect2 id="chap-cgd-component-kernel">
<title>Kernel driver pseudo-device</title>
<para>To use <devicename>cgd</devicename> you need a kernel with support
for the <devicename>cgd</devicename> pseudo-device. Make sure the
following line is in the kernel configuration file:</para>
<!-- leave indentation for existing text alone for now, to make diffs saner -->
<programlisting>pseudo-device cgd 4 # cryptographic disk driver</programlisting>
<para>The number specifies how many <devicename>cgd</devicename>
devices may be configured at the same time. After configuring
the <devicename>cgd</devicename> pseudo-device you can recompile
the kernel and boot it to enable <devicename>cgd</devicename>
support.</para>
</sect2>
<!-- add subsection on cgdconfig(8) -->
<!-- add subsection on parameters files -->
<sect2 id="chap-cgd-components-ciphers">
<title>Ciphers</title>
<para>The <devicename>cgd</devicename> driver provides the following
encryption algorithms:</para>
<variablelist>
<title>Encryption Methods</title>
<varlistentry>
<term><literal>aes-cbc</literal></term>
<listitem>
<para>AES (Rijndael). AES uses a 128 bit blocksize and
accepts 128, 192 or 256 bit keys.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>blowfish-cbc</literal></term>
<listitem>
<para>Blowfish uses a 64 bit blocksize and accepts 128 bit
keys</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>3des-cbc</literal></term>
<listitem>
<para>Triple DES uses a 64 bit blocksize and accepts
192 bit keys (only 168 bits are actually used for encryption)</para>
</listitem>
</varlistentry>
</variablelist>
<para>All three ciphers are used in CBC mode. This means each block
is XORed with the previous encrypted block before
encryption. This reduces the risk that a pattern can be found,
which can be used to break the encryption.</para>
</sect2>
<sect2 id="chap-cgd-overview-verification">
<title>Verification Methods</title>
<para>Another aspect of <devicename>cgd</devicename> that needs some
attention are the verification methods
<command>cgdconfig</command> provides. These verification
methods are used to verify the passphrase is correct. The
following verification methods are available:</para>
<variablelist>
<title>Verification Methods</title>
<varlistentry>
<term><literal>none</literal></term>
<listitem>
<para>no verification is performed. This can be dangerous,
because the key is not verified at all. When a wrong key
is entered <command>cgdconfig</command> configures the
<devicename>cgd</devicename> device as normal, but data
which was available on the volume will be destroyed
(decrypting blocks with a wrong key will result in
random data, which will result in a regeneration of the
disklabel with the current key).</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>disklabel</literal></term>
<listitem>
<para><command>cgdconfig</command> scans for a valid
disklabel. If a valid disklabel is found with the key
that is provided authentication will succeed.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>ffs</literal></term>
<listitem>
<para><command>cgdconfig</command> scans for a valid FFS file
system. If a valid FFS file system is found with the key
that is provided authentication will succeed.</para>
</listitem>
</varlistentry>
<!-- add re-enter verify method -->
</variablelist>
</sect2>
</sect1>
<sect1 id="chap-cgd-example">
<title>Example: encrypting your disk</title>
<para>This section works through a step-by-step example of converting
an existing system to use <devicename>cgd</devicename>,
performing the following actions:</para>
<orderedlist>
<listitem>
<para>Preparing the disk and partitions</para>
</listitem>
<listitem>
<para>Scrub off all data</para>
</listitem>
<listitem>
<para>Create the cgd</para>
</listitem>
<listitem>
<para>Adjust config-files</para>
</listitem>
<listitem>
<para>Restoring your backed-up files to the encrypted disk</para>
</listitem>
</orderedlist>
<sect2 id="chap-cgd-example-prepare">
<title>Preparing the disk</title>
<para>First, decide which filesystems you want to move to an encrypted
device. You're going to need to leave at least the small root
(<filename>/</filename>) filesystem unencrypted, in order to load
the kernel and run <command>init</command>,
<command>cgdconfig</command> and the <command>rc.d</command>
scripts that configure your <devicename>cgd</devicename>. In this
example, we'll encrypt everything except the root
(<filename>/</filename>) filesystem.</para>
<para>We are going to delete and re-make partitions and filesystems,
and will require a backup to restore the data. So make sure
you have a current, reliable backup stored on a different disk
or machine. Do your backup in single-user mode, with the
filesystems unmounted, to ensure you get a clean
<command>dump</command>. Make sure you back up the disklabel
of your hard disk as well, so you have a record of the
partition layout before you started.</para>
<para>With the system at single user, <filename>/</filename> mounted
read-write and everything else unmounted, use
<command>disklabel</command> to delete all the data partitions
you want to move into <devicename>cgd</devicename>.</para>
<para>Then make a single new partition in all the space you just
freed up, say, <replaceable>wd0e</replaceable>. Set the
partition type for this partition to <devicename>cgd</devicename>
Though it doesn't really matter what it is, it will help remind
you that it's not a normal filesystem later. When finished,
label the disk to save the new partition table.</para>
</sect2>
<sect2 id="chap-cgd-example-scrubbing">
<title>Scrubbing the disk</title>
<para>We have removed the partition table information, but the
existing filesystems and data are still on disk. Even after
we make a <devicename>cgd</devicename> device, create filesystems,
and restore our data, some of these disk blocks might not yet
be overwritten and still contain our data in plaintext. This
is especially likely if the filesystems are mostly empty. We
want to scrub the disk before we go further.</para>
<para>We could use <command>dd</command> to copy
<filename>/dev/zero</filename> over the new
<replaceable>wd0e</replaceable> partition, but this will leave
our disk full of zeros, except where we've written encrypted
data later. We might not want to give an attacker any clues
about which blocks contain real data, and which are free
space, so we want to write "noise" into all the disk
blocks. So we'll create a temporary <devicename>cgd</devicename>,
configured with a random, unknown key.</para>
<para>First, we configure a <devicename>cgd</devicename> to use a random key:</para>
<screen>&rprompt; <userinput>cgdconfig -s cgd0 /dev/wd0e aes-cbc 128 < /dev/urandom </userinput></screen>
<para>Now we can write zeros into the raw partition of our
<devicename>cgd</devicename> (<filename>/dev/rcgd0d</filename> on
NetBSD/i386, <filename>/dev/rcgd0c</filename> on most other
platforms):</para>
<screen>&rprompt; <userinput>dd if=/dev/zero of=/dev/rcgd0d bs=32k</userinput></screen>
<para>The encrypted zeros will look like random data on disk. This might
take a while if you have a large disk. Once finished, unconfigure the
random-key <devicename>cgd</devicename>:</para>
<screen>&rprompt; <userinput>cgdconfig -u cgd0</userinput></screen>
</sect2>
<sect2 id="chap-cgd-example-creating">
<title>Creating the <devicename>cgd</devicename></title>
<para>The <command>cgdconfig</command> program, which manipulates
<devicename>cgd</devicename> devices, uses parameters files to store
such information as the encryption type, key length, and a
random password salt for each <devicename>cgd</devicename>. These
files are very important, and need to be kept safe - without
them, you will not be able to decrypt the data!</para>
<para>We'll generate a parameters file and write it into the default
location (make sure the directory
<filename>/etc/cgd</filename> exists and is mode 700):</para>
<screen>&rprompt; <userinput>cgdconfig -g -V disklabel -o /etc/cgd/wd0e aes-cbc 256</userinput></screen>
<para>This creates a parameters file
<filename>/etc/cgd/wd0e</filename> describing a
<devicename>cgd</devicename> using the
<literal>aes-cbc</literal> cipher method, a key
verification method of <literal>disklabel</literal>,
and a key length of <literal>256</literal>
bits. It will look something like this:</para>
<programlisting>algorithm aes-cbc;
iv-method encblkno;
keylength 256;
verify_method disklabel;
keygen pkcs5_pbkdf2/sha1 {
iterations 6275;
salt AAAAgHTg/jKCd2ZJiOSGrgnadGw=;
};</programlisting>
<note>
<para>Remember, you'll want to save this file somewhere safe
later.</para>
</note>
<tip>
<para>When creating the parameters file,
<command>cgdconfig</command> reads from
<filename>/dev/random</filename> to create the password
salt. This read may block if there is not enough collected
entropy in the random pool. This is unlikely, especially if
you just finished overwriting the disk as in the previous
step, but if it happens you can press keys on the console
and/or move your mouse until the
<devicename>rnd</devicename> device gathers enough
entropy.</para>
</tip>
<para>Now it's time to create our <devicename>cgd</devicename>, for which
we'll need a passphrase. This passphrase needs to be entered
every time the <devicename>cgd</devicename> is opened, which is
usually at each reboot. The encryption key is derived from this
passphrase and the salt. Make sure you choose something you won't
forget, and others won't guess.</para>
<para>The first time we configure the <devicename>cgd</devicename>, there
is no valid disklabel on the logical device, so the validation
mechanism we want to use later won't work. We override it this
one time:</para>
<screen>&rprompt; <userinput>cgdconfig -V re-enter cgd0 /dev/wd0e</userinput></screen>
<para>This will prompt twice for a matching passphrase, just in case
you make a typo, which would otherwise leave you with a
<devicename>cgd</devicename> encrypted with a passphrase that's
different to what you expected.</para>
<para>Now that we have a new <devicename>cgd</devicename>, we need to
partition it and create filesystems. Recreate your previous
partitions with all the same sizes, with the same letter
names.</para>
<tip>
<para>Remember to use the <command>disklabel -I</command>
argument, because you're creating an initial label for a new
disk.</para>
</tip>
<note>
<para>Although you want the sizes of your new partitions to be
the same as the old, unencrypted ones, the offsets will be
different because they're starting at the beginning of this
virtual disk.</para>
</note>
<para>Then, use <command>newfs</command> to create filesystems on
all the relevant partitions. This time your partitions will
reflect the <devicename>cgd</devicename> disk names, for example:</para>
<screen>&rprompt; <userinput>newfs /dev/rcgd0h</userinput></screen>
</sect2>
<sect2 id="chap-cgd-example-configfiles">
<title>Modifying configuration files</title>
<para>We've moved several filesystems to another (logical) disk, and
we need to update <filename>/etc/fstab</filename>
accordingly. Each partition will have the same letter (in this
example), but will be on <devicename>cgd0</devicename> rather than
<devicename>wd0</devicename>. So you'll have
<filename>/etc/fstab</filename> entries something like this:</para>
<programlisting>/dev/wd0a / ffs rw 1 1
/dev/cgd0b none swap sw 0 0
/dev/cgd0b /tmp mfs rw,-s=132m 0 0
/dev/cgd0e /var ffs rw 1 2
/dev/cgd0f /usr ffs rw 1 2
/dev/cgd0h /home ffs rw 1 2</programlisting>
<note>
<para><filename>/tmp</filename> should be a separate filesystem,
either <devicename>mfs</devicename> or <devicename>ffs</devicename>,
inside the <devicename>cgd</devicename>, so that your temporary
files are not stored in plain text in the
<filename>/</filename> filesystem.</para>
</note>
<para>Each time you reboot, you're going to need your
<devicename>cgd</devicename> configured early, before
<command>fsck</command> runs and filesystems are mounted.</para>
<para>Put the following line in
<filename>/etc/cgd/cgd.conf</filename>:</para>
<programlisting>cgd0 /dev/wd0e</programlisting>
<para>This will use <filename>/etc/cgd/wd0e</filename> as config
file for <devicename>cgd0</devicename>.</para>
<para>To finally enable cgd on each boot, put the following line
into <filename>/etc/rc.conf</filename>:</para>
<programlisting>cgd=YES</programlisting>
<para>You should now be prompted for
<filename>/dev/cgd0</filename>'s passphrase whenever
<filename>/etc/rc</filename> starts.</para>
</sect2>
<sect2 id="chap-cgd-example-restore">
<title>Restoring data</title>
<para>Next, <command>mount</command> your new filesystems, and
<command>restore</command> your data into them. It often helps
to have <filename>/tmp</filename> mounted properly first, as
<command>restore</command> can use a fair amount of temporary
space when extracting a large dumpfile.</para>
<para>To test your changes to the boot configuration,
<command>umount</command> the filesystems and unconfigure the
<devicename>cgd</devicename>, so when you exit the single-user
shell, <command>rc</command> will run like on a clean boot,
prompting you for the passphrase and mounting your filesystems
correctly. Now you can bring the system up to multi-user, and
make sure everything works as before.</para>
</sect2>
</sect1>
<sect1 id="cryptocds">
<title>Example: encrypted CDs/DVDs</title>
<sect2 id="cryptocds-create">
<title>Creating an encrypted CD/DVD</title>
<para>cgd(4) provides highly secure encryption of whole partitions
or disks. Unfortunately, creating "normal" CDs is not
disklabeling something and running newfs on it. Neither can you
just put a CDR into the drive, configure cgd and assume it to
write encrypted data when syncing. Standard CDs contain at
least an ISO-9660 filesystem created with mkisofs(8) from the
<filename role="pkg">sysutils/cdrtools</filename> package.
ISO images may <emphasis>not</emphasis> contain disklabels or
cgd partitions.</para>
<para>But of course CD reader/writer hardware doesn't care about
filesystems at all. You can write raw data to the CD if you
like - or an encrypted FFS filesystem, which is what we'll do
here. But be warned, there is NO way to read this CD with any
OS except NetBSD - not even other BSDs due to the lack of cgd.</para>
<para>The basic steps when creating an encrypted CD are:</para>
<itemizedlist>
<listitem><para>Create an (empty) imagefile</para></listitem>
<listitem><para>Register it as a virtual disk using vnd(4)</para></listitem>
<listitem><para>Configure cgd inside the vnd disk</para></listitem>
<listitem><para>Copy content to the cgd</para></listitem>
<listitem><para>Unconfigure all (flush!)</para></listitem>
<listitem><para>Write the image on a CD</para></listitem>
</itemizedlist>
<para>The first step when creating an encrypted CD is to create a
single image file with dd. The image may not grow, so make it
large enough to allow all CD content to fit into. Note that
the whole image gets written to the CD later, so creating a
700 MB image for 100 MB content will still require a 700 MB
write operation to the CD. Some info on DVDs here: DVDs are
only 4.7 GB in marketing language. 4.7GB = 4.7 x 1024 x 1024 x
1024 = 5046586573 bytes. In fact, a DVD can only
approximately hold 4.7 x 1000 x 1000 x 1000 = 4700000000
bytes, which is about 4482 MB or about 4.37 GB. Keep this in
mind when creating DVD images. Don't worry for CDs, they hold
"real" 700 MB (734003200 Bytes).</para>
<para>Invoke all following commands as root!</para>
<para>For a CD:</para>
<screen>&rprompt; <userinput>dd if=/dev/zero of=image.img bs=1m count=700</userinput></screen>
<para>or, for a DVD:</para>
<screen>&rprompt; <userinput>dd if=/dev/zero of=image.img bs=1m count=4482</userinput></screen>
<para>Now configure a &man.vnd.4;-pseudo disk with the image:</para>
<screen>&rprompt; <userinput>vnconfig vnd0 image.img</userinput></screen>
<para>In order to use cgd, a so-called parameter file, describing
encryption parameters and a containing "password salt" must be
generated. We'll call it <filename>/etc/cgd/image</filename>
here. You can use one parameter file for several encrypted
partitions (I use one different file for each host and a
shared file <filename>image</filename> for all removable
media, but that's up to you).</para>
<para>I'll use AES-CBC with a keylength of 256 bits. Refer to
&man.cgd.4; and &man.cgdconfig.8; for details and alternatives.</para>
<para>The following command will create the parameter file as
<filename>/etc/cgd/image</filename>. <emphasis>YOU DO NOT WANT
TO INVOKE THE FOLLOWING COMMAND AGAIN</emphasis> after you
burnt any CD, since a recreated parameter file is a lost
parameter file and you'll never access your encrypted CD again
(the "salt" this file contains will differ among each
call). Consider this file being <emphasis>HOLY, BACKUP
IT</emphasis> and <emphasis>BACKUP IT AGAIN!</emphasis> Use
switch -V to specify verification method "disklabel" for the CD
(cgd cannot detect whether you entered a valid password for the
CD later when mounting it otherwise).</para>
<screen>&rprompt; <userinput>cgdconfig -g -V disklabel aes-cbc 256 > /etc/cgd/image</userinput></screen>
<para>Now it's time to configure a cgd for our vnd drive. (Replace
slice "d" with "c" for all platforms that use "c" as the whole
disk (where "<command>sysctl kern.rawpartition</command>"
prints "2", not "3"); if you're on i386 or amd64, "d" is OK
for you):</para>
<screen>&rprompt; <userinput>cgdconfig -V re-enter cgd1 /dev/vnd0d /etc/cgd/image</userinput></screen>
<para>The "<option>-V re-enter</option>" option is necessary
as long as the
cgd doesn't have a disklabel yet so we can access and
configure
it. This switch asks for a password twice and uses it for
encryption.</para>
<para>Now it's time to create a disklabel inside the cgd. The
defaults of the label are ok, so invoking disklabel with</para>
<screen>&rprompt; <userinput>disklabel -e -I cgd1</userinput></screen>
<para>and leaving vi with "<command>:wq</command>"
immediately will do.</para>
<para>Let's create a filesystem on the cgd, and finally mount it
somewhere:</para>
<screen>&rprompt; <userinput>newfs /dev/rcgd1a</userinput>
&rprompt; <userinput>mount /dev/cgd1a /mnt</userinput></screen>
<para>The cgd is alive! Now fill <filename>/mnt</filename> with
content. When finished, reverse the configuration process. The
steps are:</para>
<orderedlist>
<listitem>
<para>Unmounting the cgd1a:</para>
<screen>&rprompt; <userinput>umount /mnt</userinput></screen>
</listitem>
<listitem>
<para>Unconfiguring the cgd:</para>
<screen>&rprompt; <userinput>cgdconfig -u cgd1</userinput></screen>
</listitem>
<listitem>
<para>Unconfiguring the vnd: </para>
<screen>&rprompt; <userinput>vnconfig -u vnd0</userinput></screen>
</listitem>
</orderedlist>
<para>The following commands are examples to burn the images on CD
or DVD. Please adjust the <parameter>dev=</parameter> for
cdrecord or the <filename>/dev/rcd0d</filename> for
growisofs. Note the
"<emphasis>r</emphasis><filename>cd0d</filename>"
<emphasis>is</emphasis> necessary with NetBSD. Growisofs is
available in the <filename role="pkg">sysutils/dvd+rw-tools</filename>
package. Again, use "<filename>c</filename>" instead of
"<filename>d</filename>" if this is the raw partition on your
platform.</para>
<para>Finally, write the image file to a CD:</para>
<screen>&rprompt; <userinput>cdrecord dev=/dev/rcd0d -v image.img</userinput></screen>
<para>...or to a DVD:</para>
<screen>&rprompt; <userinput>growisofs -dvd-compat -Z /dev/rcd0d=image.img</userinput></screen>
<para>Congratulations! You've just created a really secure CD!</para>
</sect2>
<sect2 id="cryptocds-use">
<title>Using an encrypted CD/DVD</title>
<para>After creating an encrypted CD as described above, we're not
done yet - what about mounting it again? One might guess,
configuring the cgd on <filename>/dev/cd0d</filename> is
enough - no, it is not.</para>
<para>NetBSD cannot access FFS file systems on media that is not 512
bytes/sector format. It doesn't matter that the cgd on the CD
is, since the CD's disklabel the cgd resides in has 2048
bytes/sector.</para>
<para>But the CD driver cd(4) is smart enough to grant "write"
access to the (emulated) disklabel on the CD. So before
configuring the cgd, let's have a look at the disklabel and
modify it a bit:</para>
<screen>&rprompt; <userinput>disklabel -e cd0</userinput>
# /dev/rcd0d:
type: ATAPI
disk: mydisc
label: fictitious
flags: removable
bytes/sector: 2048 <userinput># -- Change to 512 (= orig / 4)</userinput>
sectors/track: 100 <userinput># -- Change to 400 (= orig * 4)</userinput>
tracks/cylinder: 1
sectors/cylinder: 100 <userinput># -- Change to 400 (= orig * 4)</userinput>
cylinders: 164
total sectors: 16386 <userinput># -- Change to value of slice "d" (=65544)</userinput>
rpm: 300
interleave: 1
trackskew: 0
cylinderskew: 0
headswitch: 0 # microseconds
track-to-track seek: 0 # microseconds
drivedata: 0
4 partitions:
# size offset fstype [fsize bsize cpg/sgs]
a: 65544 0 4.2BSD 0 0 0 # (Cyl. 0 - 655+)
d: 65544 0 ISO9660 0 0 # (Cyl. 0 - 655+)</screen>
<para>If you don't want to do these changes every time by hand, you
can use Florian Stoehr's tool <emphasis
role="bold">neb-cd512</emphasis> which is (at time of writing
this) in pkgsrc-wip and will move to
<filename role="pkg">sysutils/neb-cd512</filename> soon.
You can also download the neb-cd512 source from <ulink
url="http://sourceforge.net/projects/neb-stoehr/">
http://sourceforge.net/projects/neb-stoehr/</ulink> (be sure
to use neb-cd512, not neb-wipe!).</para>
<para>It is invoked with the disk name as parameter, by root:</para>
<screen>&rprompt; <userinput>neb-cd512 cd0</userinput></screen>
<para>Now as the disklabel is in 512 b/s format, accessing the CD
is as easy as:</para>
<screen>&rprompt; <userinput>cgdconfig cgd1 /dev/cd0d /etc/cgd/image</userinput>
&rprompt; <userinput>mount -o ro /dev/cgd1a /mnt</userinput></screen>
<para>Note that the cgd <emphasis>MUST</emphasis> be mounted read-only
or you'll get illegal command errors from the cd(4) driver which
can in some cases make even mounting a CD-based cgd impossible!</para>
<para>Now we're done! Enjoy your secure CD!</para>
<screen>&rprompt; <userinput>ls /mnt</userinput></screen>
<para>Remember you have to reverse all steps to remove the CD:</para>
<screen>&rprompt; <userinput>umount /mnt</userinput>
&rprompt; <userinput>cgdconfig -u cgd1</userinput>
&rprompt; <userinput>eject cd0</userinput></screen>
</sect2>
</sect1>
<sect1 id="chap-cgd-suggestions">
<title>Suggestions and Warnings</title>
<para>You now have your filesystems encrypted within a
<devicename>cgd</devicename>. When your machine is shut down, the data
is protected, and can't be decrypted without the passphrase.
However, there are still some dangers you should be aware of,
and more you can do with <devicename>cgd</devicename>. This section
documents several further suggestions and warnings that will
help you use <devicename>cgd</devicename> effectively.</para>
<itemizedlist>
<listitem>
<para>Use multiple <devicename>cgd</devicename>'s for different kinds of
data, one mounted all the time and others mounted only when
needed.</para>
</listitem>
<listitem>
<para>Use a <devicename>cgd</devicename> configured on top of a
<devicename>vnd</devicename> made from a file on a remote network
fileserver (NFS, SMBFS, CODA, etc) to safely store private data
on a shared system. This is similar to the procedure for
using encrypted CDs and DVDs described in <xref
linkend="cryptocds" />.</para>
</listitem>
</itemizedlist>
<sect2 id="chap-cgd-swap-encryption">
<title>Using a random-key cgd for swap</title>
<para>You may want to use a dedicated random-key
<devicename>cgd</devicename> for swap space, regenerating the key
each reboot. The advantage of this is that once your machine
is rebooted, any sensitive program memory contents that may
have been paged out are permanently unrecoverable, because the
decryption key is never known to you.</para>
<para>We created a temporary <devicename>cgd</devicename> with a random
key when scrubbing the disk in the example above, using a
shorthand <command>cgdconfig -s</command> invocation to avoid
creating a parameters file.</para>
<para>The <command>cgdconfig</command> params file includes a
<quote>randomkey</quote> keygen method. This is more
appropriate for "permanent" random-key configurations, and
facilitates the easy automatic configuration of these volumes
at boot time.</para>
<para>For example, if you wanted to convert your existing
<filename>/dev/wd0b</filename> partition to a dedicated
random-key cgd1, use the following command to generate
<filename>/etc/cgd/wd0b</filename>:</para>
<screen>&rprompt; <command>cgdconfig -g -o /etc/cgd/wd0b -V none -k randomkey blowfish-cbc</command></screen>
<para>When using the randomkey keygen method, only verification
method "none" can be used, because the contents of the new
<devicename>cgd</devicename> are effectively random each time (the
previous data decrypted with a random key). Likewise, the new
disk will not have a valid label or partitions, and
<command>swapctl</command> will complain about configuring
swap devices not marked as such in a disklabel.</para>
<para>In order to automate the process of labeling the disk,
prepare an appropriate disklabel and save it to a file, for
example <filename>/etc/cgd/wd0b.disklabel</filename>. Please
refer to &man.disklabel.8; for information about
how to use <command>disklabel</command> to set up a swap
partition.</para>
<para>On each reboot, to restore this saved label to the new
<devicename>cgd</devicename>, create the
<filename>/etc/rc.conf.d/cgd</filename> file as below:</para>
<programlisting>swap_device="cgd1"
swap_disklabel="/etc/cgd/wd0b.disklabel"
start_postcmd="cgd_swap"
cgd_swap()
{
if [ -f $swap_disklabel ]; then
disklabel -R -r $swap_device $swap_disklabel
fi
}</programlisting>
<para>The same technique could be extended to encompass using
<command>newfs</command> to re-create an
<devicename>ffs</devicename> filesystem for
<filename>/tmp</filename> if you didn't want to use
<devicename>mfs</devicename>.</para>
</sect2>
<sect2 id="chap-cgd-suggestions-warnings">
<title>Warnings</title>
<para>Prevent cryptographic disasters by making sure you can always
recover your passphrase and parameters file. Protect the
parameters file from disclosure, perhaps by storing it on
removable media as above, because the salt it contains helps
protect against dictionary attacks on the passphrase.</para>
<para>Keeping the data encrypted on your disk is all very well, but
what about other copies? You already have at least one other
such copy (the backup we used during this setup), and it's not
encrypted. Piping <command>dump</command> through file-based
encryption tools like <command>gpg</command> can be one way of
addressing this issue, but make sure you have all the keys and
tools you need to decrypt it to <command>restore</command>
after a disaster.</para>
<para>Like any form of software encryption, the
<devicename>cgd</devicename> key stays in kernel memory while the
device is configured, and may be accessible to privileged
programs and users, such as <filename>/dev/kmem</filename>
grovellers. Taking other system security steps, such as
running with elevated securelevel, is highly recommended.</para>
<para>Once the <devicename>cgd</devicename> volumes are mounted as normal
filesystems, their contents are accessible like any other
file. Take care of file permissions and ensure your running
system is protected against application and network security
attack.</para>
<para>Avoid using suspend/resume, especially for laptops with a BIOS
suspend-to-disk function. If an attacker can resume your
laptop with the key still in memory, or read it from the
suspend-to-disk memory image on the hard disk later, the whole
point of using <devicename>cgd</devicename> is lost.</para>
</sect2>
</sect1>
<sect1 id="chap-cgd-further">
<title>Further Reading</title>
<para>
The following resources contain more information on CGD:
</para>
<bibliography id="chap-cgd-bibliography">
<biblioentry id="smackie-cgd">
<title><ulink
url="http://www.bsdguides.org/guides/netbsd/misc/cgd_setup.php">NetBSD CGD Setup</ulink></title>
<author>
<surname>Mackie</surname>
<firstname>Stuart</firstname>
</author>
</biblioentry>
<biblioentry id="nycbug-cgd">
<title><ulink url="http://genoverly.com/articles/view/5/">
I want my cgd</ulink> aka: I want an encrypted pseudo-device on my laptop</title>
</biblioentry>
<biblioentry id="elric-cgd">
<title>The original paper on <ulink url="http://www.imrryr.org/~elric/cgd/cgd.pdf">
The CryptoGraphic Disk Driver</ulink></title>
<authorgroup>
<author>
<surname>Dowdeswell</surname>
<firstname>Roland</firstname>
</author>
<author>
<surname>Ioannidis</surname>
<firstname>John</firstname>
</author>
</authorgroup>
</biblioentry>
<biblioentry id="biancuzzi-cgd">
<title><ulink
url="http://onlamp.com/pub/a/bsd/2005/12/21/netbsd_cgd.html">Inside NetBSD's CGD</ulink> - an interview with CGD creator Roland Dowdeswell</title>
<author>
<surname>Federico</surname>
<firstname>Biancuzzi</firstname>
</author>
</biblioentry>
<biblioentry id="hubertf-cgd">
<title><ulink
url="http://www.feyrer.de/NetBSD/blog.html/nb_20060823_2311.html">CryptoGraphicFile (CGF)</ulink>, or how to keep sensitive data on your laptop</title>
<author>
<surname>Hubert</surname>
<firstname>Feyrer</firstname>
</author>
</biblioentry>
</bibliography>
</sect1>
</chapter>
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