<?xml version="1.0"?>

<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"

"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">

<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 passphrase(s).</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>Similar, the key may be acquired via a FIDO2 compatible hardware security token (which

must implement the "hmac-secret" extension). In this case a (during enrollment) randomly generated key

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>Similar, 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 and falls back to a password if

it could not be mounted. 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 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>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 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 initramfs, 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 initramfs

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> <literal>/cryptsetup/</literal> <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. Example (for a volume <literal>myvol</literal>):</para>

<example><programlisting>\0d7067f78d9827418/cryptsetup/myvol</programlisting></example>

<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 it to determine which key to send, allowing a single listening socket to serve keys for a

multitude of volumes. If the PKCS#11 logic is used (see above) the socket source name is picked in

identical fashion, except that the literal string <literal>/cryptsetup-pkcs11/</literal> is used (similar

for FIDO2: <literal>/cryptsetup-fido2/</literal> and TPM2: <literal>/cryptsetup-tpm2/</literal>). This is

done so that services providing key material know that not a secret key is requested but 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>