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<?xml version="1.0"?>
<!--*-nxml-*-->
<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
<!--
SPDX-License-Identifier: LGPL-2.1-or-later
This is based on crypttab(5) from Fedora's initscripts package, which in
turn is based on Debian's version.
The Red Hat version has been written by Miloslav Trmac <mitr@redhat.com>.
-->
<refentry id="crypttab" conditional='HAVE_LIBCRYPTSETUP' xmlns:xi="http://www.w3.org/2001/XInclude">
<refentryinfo>
<title>crypttab</title>
<productname>systemd</productname>
</refentryinfo>
<refmeta>
<refentrytitle>crypttab</refentrytitle>
<manvolnum>5</manvolnum>
</refmeta>
<refnamediv>
<refname>crypttab</refname>
<refpurpose>Configuration for encrypted block devices</refpurpose>
</refnamediv>
<refsynopsisdiv>
<para><filename>/etc/crypttab</filename></para>
</refsynopsisdiv>
<refsect1>
<title>Description</title>
<para>The <filename>/etc/crypttab</filename> file describes
encrypted block devices that are set up during system boot.</para>
<para>Empty lines and lines starting with the <literal>#</literal>
character are ignored. Each of the remaining lines describes one
encrypted block device. Fields are delimited by white space.</para>
<para>Each line is in the form<programlisting><replaceable>volume-name</replaceable> <replaceable>encrypted-device</replaceable> <replaceable>key-file</replaceable> <replaceable>options</replaceable></programlisting>
The first two fields are mandatory, the remaining two are
optional.</para>
<para>Setting up encrypted block devices using this file supports four encryption modes: LUKS, TrueCrypt,
BitLocker and plain. See <citerefentry
project='die-net'><refentrytitle>cryptsetup</refentrytitle><manvolnum>8</manvolnum></citerefentry> for
more information about each mode. When no mode is specified in the options field and the block device
contains a LUKS signature, it is opened as a LUKS device; otherwise, it is assumed to be in raw dm-crypt
(plain mode) format.</para>
<para>The four fields of <filename>/etc/crypttab</filename> are defined as follows:</para>
<orderedlist>
<listitem><para>The first field contains the name of the resulting volume with decrypted data; its
block device is set up below <filename>/dev/mapper/</filename>.</para></listitem>
<listitem><para>The second field contains a path to the underlying block
device or file, or a specification of a block device via
<literal>UUID=</literal> followed by the UUID.</para></listitem>
<listitem><para>The third field specifies an absolute path to a file with the encryption
key. Optionally, the path may be followed by <literal>:</literal> and an
<filename>/etc/fstab</filename> style device specification (e.g. starting with
<literal>LABEL=</literal> or similar); in which case the path is taken relative to the specified
device's file system root. If the field is not present or is <literal>none</literal> or
<literal>-</literal>, a key file named after the volume to unlock (i.e. the first column of the line),
suffixed with <filename>.key</filename> is automatically loaded from the
<filename>/etc/cryptsetup-keys.d/</filename> and <filename>/run/cryptsetup-keys.d/</filename>
directories, if present. Otherwise, the password has to be manually entered during system boot. For
swap encryption, <filename>/dev/urandom</filename> may be used as key file, resulting in a randomized
key.</para>
<para>If the specified key file path refers to an <constant>AF_UNIX</constant> stream socket in the
file system, the key is acquired by connecting to the socket and reading it from the connection. This
allows the implementation of a service to provide key information dynamically, at the moment when it is
needed. For details see below.</para></listitem>
<listitem><para>The fourth field, if present, is a comma-delimited list of options. The supported
options are listed below.</para></listitem>
</orderedlist>
</refsect1>
<refsect1>
<title>Key Acquisition</title>
<para>Six different mechanisms for acquiring the decryption key or passphrase unlocking the encrypted
volume are supported. Specifically:</para>
<orderedlist>
<listitem><para>Most prominently, the user may be queried interactively during volume activation
(i.e. typically at boot), asking them to type in the necessary passphrases.</para></listitem>
<listitem><para>The (unencrypted) key may be read from a file on disk, possibly on removable media. The third field
of each line encodes the location, for details see above.</para></listitem>
<listitem><para>The (unencrypted) key may be requested from another service, by specifying an
<constant>AF_UNIX</constant> file system socket in place of a key file in the third field. For details
see above and below.</para></listitem>
<listitem><para>The key may be acquired via a PKCS#11 compatible hardware security token or
smartcard. In this case an encrypted key is stored on disk/removable media, acquired via
<constant>AF_UNIX</constant>, or stored in the LUKS2 JSON token metadata header. The encrypted key is
then decrypted by the PKCS#11 token with an RSA key stored on it, and then used to unlock the encrypted
volume. Use the <option>pkcs11-uri=</option> option described below to use this mechanism.</para></listitem>
<listitem><para>Similarly, the key may be acquired via a FIDO2 compatible hardware security token
(which must implement the "hmac-secret" extension). In this case a key generated randomly during
enrollment is stored on disk/removable media, acquired via <constant>AF_UNIX</constant>, or stored in
the LUKS2 JSON token metadata header. The random key is hashed via a keyed hash function (HMAC) on the
FIDO2 token, using a secret key stored on the token that never leaves it. The resulting hash value is
then used as key to unlock the encrypted volume. Use the <option>fido2-device=</option> option
described below to use this mechanism.</para></listitem>
<listitem><para>Similarly, the key may be acquired via a TPM2 security chip. In this case a (during
enrollment) randomly generated key — encrypted by an asymmetric key derived from the TPM2 chip's seed
key — is stored on disk/removable media, acquired via <constant>AF_UNIX</constant>, or stored in the
LUKS2 JSON token metadata header. Use the <option>tpm2-device=</option> option described below to use
this mechanism.</para></listitem>
</orderedlist>
<para>For the latter five mechanisms the source for the key material used for unlocking the volume is
primarily configured in the third field of each <filename>/etc/crypttab</filename> line, but may also
configured in <filename>/etc/cryptsetup-keys.d/</filename> and
<filename>/run/cryptsetup-keys.d/</filename> (see above) or in the LUKS2 JSON token header (in case of
the latter three). Use the
<citerefentry><refentrytitle>systemd-cryptenroll</refentrytitle><manvolnum>1</manvolnum></citerefentry>
tool to enroll PKCS#11, FIDO2 and TPM2 devices in LUKS2 volumes.</para>
</refsect1>
<refsect1>
<title>Supported Options</title>
<para>The following options may be used in the fourth field of each line:</para>
<variablelist class='fstab-options'>
<varlistentry>
<term><option>cipher=</option></term>
<listitem><para>Specifies the cipher to use. See <citerefentry
project='die-net'><refentrytitle>cryptsetup</refentrytitle><manvolnum>8</manvolnum></citerefentry>
for possible values and the default value of this option. A cipher with unpredictable IV values, such
as <literal>aes-cbc-essiv:sha256</literal>, is recommended. Embedded commas in the cipher
specification need to be escaped by preceding them with a backslash, see example below.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>discard</option></term>
<listitem><para>Allow discard requests to be passed through the encrypted block
device. This improves performance on SSD storage but has security implications.
</para></listitem>
</varlistentry>
<varlistentry>
<term><option>hash=</option></term>
<listitem><para>Specifies the hash to use for password
hashing. See
<citerefentry project='die-net'><refentrytitle>cryptsetup</refentrytitle><manvolnum>8</manvolnum></citerefentry>
for possible values and the default value of this
option.</para></listitem>
</varlistentry>
<varlistentry>
<term><option>header=</option></term>
<listitem><para>Use a detached (separated) metadata device or
file where the LUKS header is stored. This option is only
relevant for LUKS devices. See
<citerefentry project='die-net'><refentrytitle>cryptsetup</refentrytitle><manvolnum>8</manvolnum></citerefentry>
for possible values and the default value of this
option.</para>
<para>Optionally, the path may be followed by <literal>:</literal> and an
<filename>/etc/fstab</filename> device specification (e.g. starting with <literal>UUID=</literal> or
similar); in which case, the path is relative to the device file system root. The device gets mounted
automatically for LUKS device activation duration only.</para></listitem>
</varlistentry>
<varlistentry>
<term><option>keyfile-offset=</option></term>
<listitem><para>Specifies the number of bytes to skip at the
start of the key file. See
<citerefentry project='die-net'><refentrytitle>cryptsetup</refentrytitle><manvolnum>8</manvolnum></citerefentry>
for possible values and the default value of this
option.</para></listitem>
</varlistentry>
<varlistentry>
<term><option>keyfile-size=</option></term>
<listitem><para>Specifies the maximum number of bytes to read
from the key file. See
<citerefentry project='die-net'><refentrytitle>cryptsetup</refentrytitle><manvolnum>8</manvolnum></citerefentry>
for possible values and the default value of this option. This
option is ignored in plain encryption mode, as the key file
size is then given by the key size.</para></listitem>
</varlistentry>
<varlistentry>
<term><option>keyfile-erase</option></term>
<listitem><para>If enabled, the specified key file is erased after the volume is activated or when
activation fails. This is in particular useful when the key file is only acquired transiently before
activation (e.g. via a file in <filename>/run/</filename>, generated by a service running before
activation), and shall be removed after use. Defaults to off.</para></listitem>
</varlistentry>
<varlistentry>
<term><option>key-slot=</option></term>
<listitem><para>Specifies the key slot to compare the
passphrase or key against. If the key slot does not match the
given passphrase or key, but another would, the setup of the
device will fail regardless. This option implies
<option>luks</option>. See
<citerefentry project='die-net'><refentrytitle>cryptsetup</refentrytitle><manvolnum>8</manvolnum></citerefentry>
for possible values. The default is to try all key slots in
sequential order.</para></listitem>
</varlistentry>
<varlistentry>
<term><option>keyfile-timeout=</option></term>
<listitem><para> Specifies the timeout for the device on
which the key file resides or the device used as the key file,
and falls back to a password if it could not be accessed. See
<citerefentry><refentrytitle>systemd-cryptsetup-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>
for key files on external devices.
</para></listitem>
</varlistentry>
<varlistentry>
<term><option>luks</option></term>
<listitem><para>Force LUKS mode. When this mode is used, the
following options are ignored since they are provided by the
LUKS header on the device: <option>cipher=</option>,
<option>hash=</option>,
<option>size=</option>.</para></listitem>
</varlistentry>
<varlistentry>
<term><option>bitlk</option></term>
<listitem><para>Decrypt BitLocker drive. Encryption parameters
are deduced by cryptsetup from BitLocker header.</para></listitem>
</varlistentry>
<varlistentry>
<term><option>_netdev</option></term>
<listitem><para>Marks this cryptsetup device as requiring network. It will be
started after the network is available, similarly to
<citerefentry><refentrytitle>systemd.mount</refentrytitle><manvolnum>5</manvolnum></citerefentry>
units marked with <option>_netdev</option>. The service unit to set up this device
will be ordered between <filename>remote-fs-pre.target</filename> and
<filename>remote-cryptsetup.target</filename>, instead of
<filename>cryptsetup-pre.target</filename> and
<filename>cryptsetup.target</filename>.</para>
<para>Hint: if this device is used for a mount point that is specified in
<citerefentry project='man-pages'><refentrytitle>fstab</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
the <option>_netdev</option> option should also be used for the mount
point. Otherwise, a dependency loop might be created where the mount point
will be pulled in by <filename>local-fs.target</filename>, while the
service to configure the network is usually only started <emphasis>after</emphasis>
the local file system has been mounted.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>noauto</option></term>
<listitem><para>This device will not be added to <filename>cryptsetup.target</filename>.
This means that it will not be automatically unlocked on boot, unless something else pulls
it in. In particular, if the device is used for a mount point, it'll be unlocked
automatically during boot, unless the mount point itself is also disabled with
<option>noauto</option>.</para></listitem>
</varlistentry>
<varlistentry>
<term><option>nofail</option></term>
<listitem><para>This device will not be a hard dependency of
<filename>cryptsetup.target</filename>. It'll still be pulled in and started, but the system
will not wait for the device to show up and be unlocked, and boot will not fail if this is
unsuccessful. Note that other units that depend on the unlocked device may still fail. In
particular, if the device is used for a mount point, the mount point itself also needs to
have the <option>nofail</option> option, or the boot will fail if the device is not unlocked
successfully.</para></listitem>
</varlistentry>
<varlistentry>
<term><option>offset=</option></term>
<listitem><para>Start offset in the backend device, in 512-byte sectors. This
option is only relevant for plain devices.</para></listitem>
</varlistentry>
<varlistentry>
<term><option>plain</option></term>
<listitem><para>Force plain encryption mode.</para></listitem>
</varlistentry>
<varlistentry>
<term><option>read-only</option></term><term><option>readonly</option></term>
<listitem><para>Set up the encrypted block device in read-only
mode.</para></listitem>
</varlistentry>
<varlistentry>
<term><option>same-cpu-crypt</option></term>
<listitem><para>Perform encryption using the same CPU that IO was submitted on. The default is to use
an unbound workqueue so that encryption work is automatically balanced between available CPUs.</para>
<para>This requires kernel 4.0 or newer.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>submit-from-crypt-cpus</option></term>
<listitem><para>Disable offloading writes to a separate thread after encryption. There are some
situations where offloading write requests from the encryption threads to a dedicated thread degrades
performance significantly. The default is to offload write requests to a dedicated thread because it
benefits the CFQ scheduler to have writes submitted using the same context.</para>
<para>This requires kernel 4.0 or newer.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>no-read-workqueue</option></term>
<listitem><para>Bypass dm-crypt internal workqueue and process read requests synchronously. The
default is to queue these requests and process them asynchronously.</para>
<para>This requires kernel 5.9 or newer.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>no-write-workqueue</option></term>
<listitem><para>Bypass dm-crypt internal workqueue and process write requests synchronously. The
default is to queue these requests and process them asynchronously.</para>
<para>This requires kernel 5.9 or newer.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>skip=</option></term>
<listitem><para>How many 512-byte sectors of the encrypted data to skip at the
beginning. This is different from the <option>offset=</option> option with respect
to the sector numbers used in initialization vector (IV) calculation. Using
<option>offset=</option> will shift the IV calculation by the same negative
amount. Hence, if <option>offset=<replaceable>n</replaceable></option> is given,
sector <replaceable>n</replaceable> will get a sector number of 0 for the IV
calculation. Using <option>skip=</option> causes sector
<replaceable>n</replaceable> to also be the first sector of the mapped device, but
with its number for IV generation being <replaceable>n</replaceable>.</para>
<para>This option is only relevant for plain devices.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>size=</option></term>
<listitem><para>Specifies the key size in bits. See
<citerefentry project='die-net'><refentrytitle>cryptsetup</refentrytitle><manvolnum>8</manvolnum></citerefentry>
for possible values and the default value of this
option.</para></listitem>
</varlistentry>
<varlistentry>
<term><option>sector-size=</option></term>
<listitem><para>Specifies the sector size in bytes. See
<citerefentry project='die-net'><refentrytitle>cryptsetup</refentrytitle><manvolnum>8</manvolnum></citerefentry>
for possible values and the default value of this
option.</para></listitem>
</varlistentry>
<varlistentry>
<term><option>swap</option></term>
<listitem><para>The encrypted block device will be used as a
swap device, and will be formatted accordingly after setting
up the encrypted block device, with
<citerefentry project='man-pages'><refentrytitle>mkswap</refentrytitle><manvolnum>8</manvolnum></citerefentry>.
This option implies <option>plain</option>.</para>
<para>WARNING: Using the <option>swap</option> option will
destroy the contents of the named partition during every boot,
so make sure the underlying block device is specified
correctly.</para></listitem>
</varlistentry>
<varlistentry>
<term><option>tcrypt</option></term>
<listitem><para>Use TrueCrypt encryption mode. When this mode
is used, the following options are ignored since they are
provided by the TrueCrypt header on the device or do not
apply:
<option>cipher=</option>,
<option>hash=</option>,
<option>keyfile-offset=</option>,
<option>keyfile-size=</option>,
<option>size=</option>.</para>
<para>When this mode is used, the passphrase is read from the
key file given in the third field. Only the first line of this
file is read, excluding the new line character.</para>
<para>Note that the TrueCrypt format uses both passphrase and
key files to derive a password for the volume. Therefore, the
passphrase and all key files need to be provided. Use
<option>tcrypt-keyfile=</option> to provide the absolute path
to all key files. When using an empty passphrase in
combination with one or more key files, use
<literal>/dev/null</literal> as the password file in the third
field.</para></listitem>
</varlistentry>
<varlistentry>
<term><option>tcrypt-hidden</option></term>
<listitem><para>Use the hidden TrueCrypt volume. This option
implies <option>tcrypt</option>.</para>
<para>This will map the hidden volume that is inside of the
volume provided in the second field. Please note that there is
no protection for the hidden volume if the outer volume is
mounted instead. See
<citerefentry project='die-net'><refentrytitle>cryptsetup</refentrytitle><manvolnum>8</manvolnum></citerefentry>
for more information on this limitation.</para></listitem>
</varlistentry>
<varlistentry>
<term><option>tcrypt-keyfile=</option></term>
<listitem><para>Specifies the absolute path to a key file to
use for a TrueCrypt volume. This implies
<option>tcrypt</option> and can be used more than once to
provide several key files.</para>
<para>See the entry for <option>tcrypt</option> on the
behavior of the passphrase and key files when using TrueCrypt
encryption mode.</para></listitem>
</varlistentry>
<varlistentry>
<term><option>tcrypt-system</option></term>
<listitem><para>Use TrueCrypt in system encryption mode. This
option implies <option>tcrypt</option>.</para></listitem>
</varlistentry>
<varlistentry>
<term><option>tcrypt-veracrypt</option></term>
<listitem><para>Check for a VeraCrypt volume. VeraCrypt is a fork of
TrueCrypt that is mostly compatible, but uses different, stronger key
derivation algorithms that cannot be detected without this flag.
Enabling this option could substantially slow down unlocking, because
VeraCrypt's key derivation takes much longer than TrueCrypt's. This
option implies <option>tcrypt</option>.</para></listitem>
</varlistentry>
<varlistentry>
<term><option>timeout=</option></term>
<listitem><para>Specifies the timeout for querying for a
password. If no unit is specified, seconds is used. Supported
units are s, ms, us, min, h, d. A timeout of 0 waits
indefinitely (which is the default).</para></listitem>
</varlistentry>
<varlistentry>
<term><option>tmp=</option></term>
<listitem><para>The encrypted block device will be prepared for using it as
<filename>/tmp/</filename>; it will be formatted using <citerefentry
project='man-pages'><refentrytitle>mkfs</refentrytitle><manvolnum>8</manvolnum></citerefentry>. Takes
a file system type as argument, such as <literal>ext4</literal>, <literal>xfs</literal> or
<literal>btrfs</literal>. If no argument is specified defaults to <literal>ext4</literal>. This
option implies <option>plain</option>.</para>
<para>WARNING: Using the <option>tmp</option> option will destroy the contents of the named partition
during every boot, so make sure the underlying block device is specified correctly.</para></listitem>
</varlistentry>
<varlistentry>
<term><option>tries=</option></term>
<listitem><para>Specifies the maximum number of times the user
is queried for a password. The default is 3. If set to 0, the
user is queried for a password indefinitely.</para></listitem>
</varlistentry>
<varlistentry>
<term><option>headless=</option></term>
<listitem><para>Takes a boolean argument, defaults to false. If true, never query interactively
for the password/PIN. Useful for headless systems.</para></listitem>
</varlistentry>
<varlistentry>
<term><option>verify</option></term>
<listitem><para>If the encryption password is read from console, it has to be entered twice to
prevent typos.</para></listitem>
</varlistentry>
<varlistentry>
<term><option>password-echo=yes|no|masked</option></term>
<listitem><para>Controls whether to echo passwords or security token PINs
that are read from console. Takes a boolean or the special string <literal>masked</literal>.
The default is <option>password-echo=masked</option>.</para>
<para>If enabled, the typed characters are echoed literally. If disabled,
the typed characters are not echoed in any form, the user will not get
feedback on their input. If set to <literal>masked</literal>, an asterisk
(<literal>*</literal>) is echoed for each character typed. Regardless of
which mode is chosen, if the user hits the tabulator key (<literal>↹</literal>)
at any time, or the backspace key (<literal>⌫</literal>) before any other
data has been entered, then echo is turned off.</para></listitem>
</varlistentry>
<varlistentry>
<term><option>pkcs11-uri=</option></term>
<listitem><para>Takes either the special value <literal>auto</literal> or an <ulink
url="https://tools.ietf.org/html/rfc7512">RFC7512 PKCS#11 URI</ulink> pointing to a private RSA key
which is used to decrypt the encrypted key specified in the third column of the line. This is useful
for unlocking encrypted volumes through PKCS#11 compatible security tokens or smartcards. See below
for an example how to set up this mechanism for unlocking a LUKS2 volume with a YubiKey security
token.</para>
<para>If specified as <literal>auto</literal> the volume must be of type LUKS2 and must carry PKCS#11
security token metadata in its LUKS2 JSON token section. In this mode the URI and the encrypted key
are automatically read from the LUKS2 JSON token header. Use
<citerefentry><refentrytitle>systemd-cryptenroll</refentrytitle><manvolnum>1</manvolnum></citerefentry>
as simple tool for enrolling PKCS#11 security tokens or smartcards in a way compatible with
<literal>auto</literal>. In this mode the third column of the line should remain empty (that is,
specified as <literal>-</literal>).</para>
<para>The specified URI can refer directly to a private RSA key stored on a token or alternatively
just to a slot or token, in which case a search for a suitable private RSA key will be performed. In
this case if multiple suitable objects are found the token is refused. The encrypted key configured
in the third column of the line is passed as is (i.e. in binary form, unprocessed) to RSA
decryption. The resulting decrypted key is then Base64 encoded before it is used to unlock the LUKS
volume.</para>
<para>Use <command>systemd-cryptenroll --pkcs11-token-uri=list</command> to list all suitable PKCS#11
security tokens currently plugged in, along with their URIs.</para>
<para>Note that many newer security tokens that may be used as PKCS#11 security token typically also
implement the newer and simpler FIDO2 standard. Consider using <option>fido2-device=</option>
(described below) to enroll it via FIDO2 instead. Note that a security token enrolled via PKCS#11
cannot be used to unlock the volume via FIDO2, unless also enrolled via FIDO2, and vice
versa.</para></listitem>
</varlistentry>
<varlistentry>
<term><option>fido2-device=</option></term>
<listitem><para>Takes either the special value <literal>auto</literal> or the path to a
<literal>hidraw</literal> device node (e.g. <filename>/dev/hidraw1</filename>) referring to a FIDO2
security token that implements the <literal>hmac-secret</literal> extension (most current hardware
security tokens do). See below for an example how to set up this mechanism for unlocking an encrypted
volume with a FIDO2 security token.</para>
<para>If specified as <literal>auto</literal> the FIDO2 token device is automatically discovered, as
it is plugged in.</para>
<para>FIDO2 volume unlocking requires a client ID hash (CID) to be configured via
<option>fido2-cid=</option> (see below) and a key to pass to the security token's HMAC functionality
(configured in the line's third column) to operate. If not configured and the volume is of type
LUKS2, the CID and the key are read from LUKS2 JSON token metadata instead. Use
<citerefentry><refentrytitle>systemd-cryptenroll</refentrytitle><manvolnum>1</manvolnum></citerefentry>
as simple tool for enrolling FIDO2 security tokens, compatible with this automatic mode, which is
only available for LUKS2 volumes.</para>
<para>Use <command>systemd-cryptenroll --fido2-device=list</command> to list all suitable FIDO2
security tokens currently plugged in, along with their device nodes.</para>
<para>This option implements the following mechanism: the configured key is hashed via they HMAC
keyed hash function the FIDO2 device implements, keyed by a secret key embedded on the device. The
resulting hash value is Base64 encoded and used to unlock the LUKS2 volume. As it should not be
possible to extract the secret from the hardware token, it should not be possible to retrieve the
hashed key given the configured key — without possessing the hardware token.</para>
<para>Note that many security tokens that implement FIDO2 also implement PKCS#11, suitable for
unlocking volumes via the <option>pkcs11-uri=</option> option described above. Typically the newer,
simpler FIDO2 standard is preferable.</para></listitem>
</varlistentry>
<varlistentry>
<term><option>fido2-cid=</option></term>
<listitem><para>Takes a Base64 encoded FIDO2 client ID to use for the FIDO2 unlock operation. If
specified, but <option>fido2-device=</option> is not, <option>fido2-device=auto</option> is
implied. If <option>fido2-device=</option> is used but <option>fido2-cid=</option> is not, the volume
must be of LUKS2 type, and the CID is read from the LUKS2 JSON token header. Use
<citerefentry><refentrytitle>systemd-cryptenroll</refentrytitle><manvolnum>1</manvolnum></citerefentry>
for enrolling a FIDO2 token in the LUKS2 header compatible with this automatic
mode.</para></listitem>
</varlistentry>
<varlistentry>
<term><option>fido2-rp=</option></term>
<listitem><para>Takes a string, configuring the FIDO2 Relying Party (rp) for the FIDO2 unlock
operation. If not specified <literal>io.systemd.cryptsetup</literal> is used, except if the LUKS2
JSON token header contains a different value. It should normally not be necessary to override
this.</para></listitem>
</varlistentry>
<varlistentry>
<term><option>tpm2-device=</option></term>
<listitem><para>Takes either the special value <literal>auto</literal> or the path to a device node
(e.g. <filename>/dev/tpmrm0</filename>) referring to a TPM2 security chip. See below for an example
how to set up this mechanism for unlocking an encrypted volume with a TPM2 chip.</para>
<para>Use <option>tpm2-pcrs=</option> (see below) to configure the set of TPM2 PCRs to bind the
volume unlocking to. Use
<citerefentry><refentrytitle>systemd-cryptenroll</refentrytitle><manvolnum>1</manvolnum></citerefentry>
as simple tool for enrolling TPM2 security chips in LUKS2 volumes.</para>
<para>If specified as <literal>auto</literal> the TPM2 device is automatically discovered. Use
<command>systemd-cryptenroll --tpm2-device=list</command> to list all suitable TPM2 devices currently
available, along with their device nodes.</para>
<para>This option implements the following mechanism: when enrolling a TPM2 device via
<command>systemd-cryptenroll</command> on a LUKS2 volume, a randomized key unlocking the volume is
generated on the host and loaded into the TPM2 chip where it is encrypted with an asymmetric
"primary" key pair derived from the TPM2's internal "seed" key. Neither the seed key nor the primary
key are permitted to ever leave the TPM2 chip — however, the now encrypted randomized key may. It is
saved in the LUKS2 volume JSON token header. When unlocking the encrypted volume, the primary key
pair is generated on the TPM2 chip again (which works as long as the chip's seed key is correctly
maintained by the TPM2 chip), which is then used to decrypt (on the TPM2 chip) the encrypted key from
the LUKS2 volume JSON token header saved there during enrollment. The resulting decrypted key is then
used to unlock the volume. When the randomized key is encrypted the current values of the selected
PCRs (see below) are included in the operation, so that different PCR state results in different
encrypted keys and the decrypted key can only be recovered if the same PCR state is
reproduced.</para></listitem>
</varlistentry>
<varlistentry>
<term><option>tpm2-pcrs=</option></term>
<listitem><para>Takes a <literal>+</literal> separated list of numeric TPM2 PCR (i.e. "Platform
Configuration Register") indexes to bind the TPM2 volume unlocking to. This option is only useful
when TPM2 enrollment metadata is not available in the LUKS2 JSON token header already, the way
<command>systemd-cryptenroll</command> writes it there. If not used (and no metadata in the LUKS2
JSON token header defines it), defaults to a list of a single entry: PCR 7. Assign an empty string to
encode a policy that binds the key to no PCRs, making the key accessible to local programs regardless
of the current PCR state.</para></listitem>
</varlistentry>
<varlistentry>
<term><option>tpm2-pin=</option></term>
<listitem><para>Takes a boolean argument, defaults to <literal>false</literal>. Controls whether
TPM2 volume unlocking is bound to a PIN in addition to PCRs. Similarly, this option is only useful
when TPM2 enrollment metadata is not available.</para></listitem>
</varlistentry>
<varlistentry>
<term><option>tpm2-signature=</option></term>
<listitem><para>Takes an absolute path to a TPM2 PCR JSON signature file, as produced by the
<citerefentry><refentrytitle>systemd-measure</refentrytitle><manvolnum>1</manvolnum></citerefentry>
tool. This permits locking LUKS2 volumes to any PCR values for which a valid signature matching a
public key specified at key enrollment time can be provided. See
<citerefentry><refentrytitle>systemd-cryptenroll</refentrytitle><manvolnum>1</manvolnum></citerefentry>
for details on enrolling TPM2 PCR public keys. If this option is not specified but it is attempted to
unlock a LUKS2 volume with a signed TPM2 PCR enrollment a suitable signature file
<filename>tpm2-pcr-signature.json</filename> is searched for in <filename>/etc/systemd/</filename>,
<filename>/run/systemd/</filename>, <filename>/usr/lib/systemd/</filename> (in this
order).</para></listitem>
</varlistentry>
<varlistentry>
<term><option>tpm2-measure-pcr=</option></term>
<listitem><para>Controls whether to measure the volume key of the encrypted volume to a TPM2 PCR. If
set to "no" (which is the default) no PCR extension is done. If set to "yes" the volume key is
measured into PCR 15. If set to a decimal integer in the range 0…23 the volume key is measured into
the specified PCR. The volume key is measured along with the activated volume name and its UUID. This
functionality is particularly useful for the encrypted volume backing the root file system, as it
then allows later TPM objects to be securely bound to the root file system and hence the specific
installation.</para></listitem>
</varlistentry>
<varlistentry>
<term><option>tpm2-measure-bank=</option></term>
<listitem><para>Selects one or more TPM2 PCR banks to measure the volume key into, as configured with
<option>tpm2-measure-pcr=</option> above. Multiple banks may be specified, separated by a colon
character. If not specified automatically determines available and used banks. Expects a message
digest name (e.g. <literal>sha1</literal>, <literal>sha256</literal>, …) as argument, to identify the
bank.</para></listitem>
</varlistentry>
<varlistentry>
<term><option>token-timeout=</option></term>
<listitem><para>Specifies how long to wait at most for configured security devices (i.e. FIDO2,
PKCS#11, TPM2) to show up. Takes a time value in seconds (but other time units may be specified too,
see <citerefentry><refentrytitle>systemd.time</refentrytitle><manvolnum>7</manvolnum></citerefentry>
for supported formats). Defaults to 30s. Once the specified timeout elapsed authentication via
password is attempted. Note that this timeout applies to waiting for the security device to show up —
it does not apply to the PIN prompt for the device (should one be needed) or similar. Pass 0 to turn
off the time-out and wait forever.</para></listitem>
</varlistentry>
<varlistentry>
<term><option>try-empty-password=</option></term>
<listitem><para>Takes a boolean argument. If enabled, right before asking the user for a password it
is first attempted to unlock the volume with an empty password. This is useful for systems that are
initialized with an encrypted volume with only an empty password set, which shall be replaced with a
suitable password during first boot, but after activation.</para></listitem>
</varlistentry>
<varlistentry>
<term><option>x-systemd.device-timeout=</option></term>
<listitem><para>Specifies how long systemd should wait for a block device to show up before
giving up on the entry. The argument is a time in seconds or explicitly specified units of
<literal>s</literal>, <literal>min</literal>, <literal>h</literal>, <literal>ms</literal>.
</para></listitem>
</varlistentry>
<varlistentry>
<term><option>x-initrd.attach</option></term>
<listitem><para>Setup this encrypted block device in the initrd, similarly to
<citerefentry><refentrytitle>systemd.mount</refentrytitle><manvolnum>5</manvolnum></citerefentry>
units marked with <option>x-initrd.mount</option>.</para>
<para>Although it's not necessary to mark the mount entry for the root file system with
<option>x-initrd.mount</option>, <option>x-initrd.attach</option> is still recommended with
the encrypted block device containing the root file system as otherwise systemd will
attempt to detach the device during the regular system shutdown while it's still in
use. With this option the device will still be detached but later after the root file
system is unmounted.</para>
<para>All other encrypted block devices that contain file systems mounted in the initrd should use
this option.</para>
</listitem>
</varlistentry>
</variablelist>
<para>At early boot and when the system manager configuration is
reloaded, this file is translated into native systemd units by
<citerefentry><refentrytitle>systemd-cryptsetup-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>.</para>
</refsect1>
<refsect1>
<title><constant>AF_UNIX</constant> Key Files</title>
<para>If the key file path (as specified in the third column of <filename>/etc/crypttab</filename>
entries, see above) refers to an <constant>AF_UNIX</constant> stream socket in the file system, the key
is acquired by connecting to the socket and reading the key from the connection. The connection is made
from an <constant>AF_UNIX</constant> socket name in the abstract namespace, see <citerefentry
project='man-pages'><refentrytitle>unix</refentrytitle><manvolnum>7</manvolnum></citerefentry> for
details. The source socket name is chosen according the following format:</para>
<programlisting><constant>NUL</constant> <replaceable>RANDOM</replaceable> /cryptsetup/ <replaceable>VOLUME</replaceable></programlisting>
<para>In other words: a <constant>NUL</constant> byte (as required for abstract namespace sockets),
followed by a random string (consisting of alphanumeric characters only), followed by the literal
string <literal>/cryptsetup/</literal>, followed by the name of the volume to acquire they key
for. For example, for the volume <literal>myvol</literal>:</para>
<programlisting>\0d7067f78d9827418/cryptsetup/myvol</programlisting>
<para>Services listening on the <constant>AF_UNIX</constant> stream socket may query the source socket
name with <citerefentry
project='man-pages'><refentrytitle>getpeername</refentrytitle><manvolnum>2</manvolnum></citerefentry>,
and use this to determine which key to send, allowing a single listening socket to serve keys for
multiple volumes. If the PKCS#11 logic is used (see above), the socket source name is picked in similar
fashion, except that the literal string <literal>/cryptsetup-pkcs11/</literal> is used. And similarly for
FIDO2 (<literal>/cryptsetup-fido2/</literal>) and TPM2 (<literal>/cryptsetup-tpm2/</literal>). A different
path component is used so that services providing key material know that the secret key was not requested
directly, but instead an encrypted key that will be decrypted via the PKCS#11/FIDO2/TPM2 logic to acquire
the final secret key.</para>
</refsect1>
<refsect1>
<title>Examples</title>
<example>
<title>/etc/crypttab example</title>
<para>Set up four encrypted block devices. One using LUKS for normal storage, another one for usage as
a swap device and two TrueCrypt volumes. For the fourth device, the option string is interpreted as two
options <literal>cipher=xchacha12,aes-adiantum-plain64</literal>,
<literal>keyfile-timeout=10s</literal>.</para>
<programlisting>luks UUID=2505567a-9e27-4efe-a4d5-15ad146c258b
swap /dev/sda7 /dev/urandom swap
truecrypt /dev/sda2 /etc/container_password tcrypt
hidden /mnt/tc_hidden /dev/null tcrypt-hidden,tcrypt-keyfile=/etc/keyfile
external /dev/sda3 keyfile:LABEL=keydev keyfile-timeout=10s,cipher=xchacha12\,aes-adiantum-plain64
</programlisting>
</example>
<example>
<title>Yubikey-based PKCS#11 Volume Unlocking Example</title>
<para>The PKCS#11 logic allows hooking up any compatible security token that is capable of storing RSA
decryption keys for unlocking an encrypted volume. Here's an example how to set up a Yubikey security
token for this purpose on a LUKS2 volume, using <citerefentry
project='debian'><refentrytitle>ykmap</refentrytitle><manvolnum>1</manvolnum></citerefentry> from the
yubikey-manager project to initialize the token and
<citerefentry><refentrytitle>systemd-cryptenroll</refentrytitle><manvolnum>1</manvolnum></citerefentry>
to add it in the LUKS2 volume:</para>
<programlisting><xi:include href="yubikey-crypttab.sh" parse="text" /></programlisting>
<para>A few notes on the above:</para>
<itemizedlist>
<listitem><para>We use RSA2048, which is the longest key size current Yubikeys support</para></listitem>
<listitem><para>We use Yubikey key slot 9d, since that's apparently the keyslot to use for decryption purposes,
<ulink url="https://developers.yubico.com/PIV/Introduction/Certificate_slots.html">see
documentation</ulink>.</para></listitem>
</itemizedlist>
</example>
<example>
<title>FIDO2 Volume Unlocking Example</title>
<para>The FIDO2 logic allows using any compatible FIDO2 security token that implements the
<literal>hmac-secret</literal> extension for unlocking an encrypted volume. Here's an example how to
set up a FIDO2 security token for this purpose for a LUKS2 volume, using
<citerefentry><refentrytitle>systemd-cryptenroll</refentrytitle><manvolnum>1</manvolnum></citerefentry>:</para>
<programlisting><xi:include href="fido2-crypttab.sh" parse="text" /></programlisting>
</example>
<example>
<title>TPM2 Volume Unlocking Example</title>
<para>The TPM2 logic allows using any TPM2 chip supported by the Linux kernel for unlocking an
encrypted volume. Here's an example how to set up a TPM2 chip for this purpose for a LUKS2 volume,
using
<citerefentry><refentrytitle>systemd-cryptenroll</refentrytitle><manvolnum>1</manvolnum></citerefentry>:</para>
<programlisting><xi:include href="tpm2-crypttab.sh" parse="text" /></programlisting>
</example>
</refsect1>
<refsect1>
<title>See Also</title>
<para>
<citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
<citerefentry><refentrytitle>systemd-cryptsetup@.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
<citerefentry><refentrytitle>systemd-cryptsetup-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
<citerefentry><refentrytitle>systemd-cryptenroll</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
<citerefentry project='man-pages'><refentrytitle>fstab</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
<citerefentry project='die-net'><refentrytitle>cryptsetup</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
<citerefentry project='man-pages'><refentrytitle>mkswap</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
<citerefentry project='man-pages'><refentrytitle>mke2fs</refentrytitle><manvolnum>8</manvolnum></citerefentry>
</para>
</refsect1>
</refentry>