crypttab(5) — Linux manual page

NAME | SYNOPSIS | DESCRIPTION | EXAMPLES | SEE ALSO | NOTES | COLOPHON

CRYPTTAB(5)                       crypttab                       CRYPTTAB(5)

NAME         top

       crypttab - Configuration for encrypted block devices

SYNOPSIS         top

       /etc/crypttab

DESCRIPTION         top

       The /etc/crypttab file describes encrypted block devices that are set
       up during system boot.

       Empty lines and lines starting with the "#" character are ignored.
       Each of the remaining lines describes one encrypted block device.
       Fields are delimited by white space.

       Each line is in the form

           volume-name encrypted-device key-file options

       The first two fields are mandatory, the remaining two are optional.

       Setting up encrypted block devices using this file supports three
       encryption modes: LUKS, TrueCrypt and plain. See cryptsetup(8) 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.

       The first field contains the name of the resulting encrypted volume;
       its block device is set up below /dev/mapper/.

       The second field contains a path to the underlying block device or
       file, or a specification of a block device via "UUID=" followed by
       the UUID.

       The third field specifies an absolute path to a file with the
       encryption key. Optionally, the path may be followed by ":" and an
       fstab device specification (e.g. starting with "LABEL=" or similar);
       in which case the path is taken relative to the device file system
       root. If the field is not present or is "none" or "-", a key file
       named after the volume to unlock (i.e. the first column of the line),
       suffixed with .key is automatically loaded from the
       /etc/cryptsetup-keys.d/ and /run/cryptsetup-keys.d/ directories, if
       present. Otherwise, the password has to be manually entered during
       system boot. For swap encryption, /dev/urandom may be used as key
       file.

       The fourth field, if present, is a comma-delimited list of options.
       The following options are recognized:

       cipher=
           Specifies the cipher to use. See cryptsetup(8) for possible
           values and the default value of this option. A cipher with
           unpredictable IV values, such as "aes-cbc-essiv:sha256", is
           recommended. Embedded commas in the cipher specification need to
           be escaped by preceding them with a backslash, see example below.

       discard
           Allow discard requests to be passed through the encrypted block
           device. This improves performance on SSD storage but has security
           implications.

       hash=
           Specifies the hash to use for password hashing. See cryptsetup(8)
           for possible values and the default value of this option.

       header=
           Use a detached (separated) metadata device or file where the LUKS
           header is stored. This option is only relevant for LUKS devices.
           See cryptsetup(8) for possible values and the default value of
           this option.

           Optionally, the path may be followed by ":" and an fstab device
           specification (e.g. starting with "UUID=" 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.

       keyfile-offset=
           Specifies the number of bytes to skip at the start of the key
           file. See cryptsetup(8) for possible values and the default value
           of this option.

       keyfile-size=
           Specifies the maximum number of bytes to read from the key file.
           See cryptsetup(8) 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.

       keyfile-erase
           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 /run/, generated by a service running before
           activation), and shall be removed after use. Defaults to off.

       key-slot=
           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 luks. See cryptsetup(8) for possible values. The
           default is to try all key slots in sequential order.

       keyfile-timeout=
           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 systemd-cryptsetup-generator(8) for key files on external
           devices.

       luks
           Force LUKS mode. When this mode is used, the following options
           are ignored since they are provided by the LUKS header on the
           device: cipher=, hash=, size=.

       bitlk
           Decrypt Bitlocker drive. Encryption parameters are deduced by
           cryptsetup from Bitlocker header.

       _netdev
           Marks this cryptsetup device as requiring network. It will be
           started after the network is available, similarly to
           systemd.mount(5) units marked with _netdev. The service unit to
           set up this device will be ordered between remote-fs-pre.target
           and remote-cryptsetup.target, instead of cryptsetup-pre.target
           and cryptsetup.target.

           Hint: if this device is used for a mount point that is specified
           in fstab(5), the _netdev 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 local-fs.target, while the
           service to configure the network is usually only started after
           the local file system has been mounted.

       noauto
           This device will not be added to cryptsetup.target. 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 noauto.

       nofail
           This device will not be a hard dependency of cryptsetup.target.
           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 nofail option, or the boot will fail if the
           device is not unlocked successfully.

       offset=
           Start offset in the backend device, in 512-byte sectors. This
           option is only relevant for plain devices.

       plain
           Force plain encryption mode.

       read-only, readonly
           Set up the encrypted block device in read-only mode.

       same-cpu-crypt
           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.

           This requires kernel 4.0 or newer.

       submit-from-crypt-cpus
           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.

           This requires kernel 4.0 or newer.

       skip=
           How many 512-byte sectors of the encrypted data to skip at the
           beginning. This is different from the offset= option with respect
           to the sector numbers used in initialization vector (IV)
           calculation. Using offset= will shift the IV calculation by the
           same negative amount. Hence, if offset=n is given, sector n will
           get a sector number of 0 for the IV calculation. Using skip=
           causes sector n to also be the first sector of the mapped device,
           but with its number for IV generation being n.

           This option is only relevant for plain devices.

       size=
           Specifies the key size in bits. See cryptsetup(8) for possible
           values and the default value of this option.

       sector-size=
           Specifies the sector size in bytes. See cryptsetup(8) for
           possible values and the default value of this option.

       swap
           The encrypted block device will be used as a swap device, and
           will be formatted accordingly after setting up the encrypted
           block device, with mkswap(8). This option implies plain.

           WARNING: Using the swap option will destroy the contents of the
           named partition during every boot, so make sure the underlying
           block device is specified correctly.

       tcrypt
           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: cipher=, hash=,
           keyfile-offset=, keyfile-size=, size=.

           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.

           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 tcrypt-keyfile= to
           provide the absolute path to all key files. When using an empty
           passphrase in combination with one or more key files, use
           "/dev/null" as the password file in the third field.

       tcrypt-hidden
           Use the hidden TrueCrypt volume. This option implies tcrypt.

           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 cryptsetup(8) for more information on this
           limitation.

       tcrypt-keyfile=
           Specifies the absolute path to a key file to use for a TrueCrypt
           volume. This implies tcrypt and can be used more than once to
           provide several key files.

           See the entry for tcrypt on the behavior of the passphrase and
           key files when using TrueCrypt encryption mode.

       tcrypt-system
           Use TrueCrypt in system encryption mode. This option implies
           tcrypt.

       tcrypt-veracrypt
           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 tcrypt.

       timeout=
           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).

       tmp=
           The encrypted block device will be prepared for using it as
           /tmp/; it will be formatted using mkfs(8). Takes a file system
           type as argument, such as "ext4", "xfs" or "btrfs". If no
           argument is specified defaults to "ext4". This option implies
           plain.

           WARNING: Using the tmp option will destroy the contents of the
           named partition during every boot, so make sure the underlying
           block device is specified correctly.

       tries=
           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.

       verify
           If the encryption password is read from console, it has to be
           entered twice to prevent typos.

       pkcs11-uri=
           Takes a RFC7512 PKCS#11 URI[1] pointing to a private RSA key
           which is used to decrypt the key specified in the third column of
           the line. This is useful for unlocking encrypted volumes through
           security tokens or smartcards. See below for an example how to
           set up this mechanism for unlocking a LUKS volume with a YubiKey
           security token. 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 key configured in the third column is
           passed as is to RSA decryption. The resulting decrypted key is
           then base64 encoded before it is used to unlock the LUKS volume.

       try-empty-password=
           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.

       x-systemd.device-timeout=
           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 "s", "min", "h", "ms".

       x-initrd.attach
           Setup this encrypted block device in the initramfs, similarly to
           systemd.mount(5) units marked with x-initrd.mount.

           Although it's not necessary to mark the mount entry for the root
           file system with x-initrd.mount, x-initrd.attach 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.

           All other encrypted block devices that contain file systems
           mounted in the initramfs should use this option.

       At early boot and when the system manager configuration is reloaded,
       this file is translated into native systemd units by
       systemd-cryptsetup-generator(8).

EXAMPLES         top

       Example 1. /etc/crypttab example

       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 "cipher=xchacha12,aes-adiantum-plain64",
       "keyfile-timeout=10s".

           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

       Example 2. Yubikey-based Volume Unlocking Example

       The PKCS#11 logic allows hooking up any compatible security token
       that is capable of storing RSA decryption keys. Here's an example how
       to set up a Yubikey security token for this purpose, using ykmap(1)
       from the yubikey-manager project:

           # Make sure no one can read the files we generate but us
           umask 077

           # Destroy any old key on the Yubikey (careful!)
           ykman piv reset

           # Generate a new private/public key pair on the device, store the public key in 'pubkey.pem'.
           ykman piv generate-key -a RSA2048 9d pubkey.pem

           # Create a self-signed certificate from this public key, and store it on the
           # device. The "subject" should be an arbitrary string to identify the token in
           # the p11tool output below.
           ykman piv generate-certificate --subject "Knobelei" 9d pubkey.pem

           # Check if the newly create key on the Yubikey shows up as token in PKCS#11. Have a look at the output, and
           # copy the resulting token URI to the clipboard.
           p11tool --list-tokens

           # Generate a (secret) random key to use as LUKS decryption key.
           dd if=/dev/urandom of=plaintext.bin bs=128 count=1

           # Encode the secret key also as base64 text (with all whitespace removed)
           base64 < plaintext.bin | tr -d '\n\r\t ' > plaintext.base64

           # Encrypt this newly generated (binary) LUKS decryption key using the public key whose private key is on the
           # Yubikey, store the result in /etc/cryptsetup-keys.d/mytest.key, where we'll look for it during boot.
           mkdir -p /etc/cryptsetup-keys.d
           sudo openssl rsautl -encrypt -pubin -inkey pubkey.pem -in plaintext.bin -out /etc/cryptsetup-keys.d/mytest.key

           # Configure the LUKS decryption key on the LUKS device. We use very low pbkdf settings since the key already
           # has quite a high quality (it comes directly from /dev/urandom after all), and thus we don't need to do much
           # key derivation. Replace /dev/sdXn by the partition to use (e.g. sda1)
           sudo cryptsetup luksAddKey /dev/sdXn plaintext.base64 --pbkdf=pbkdf2 --pbkdf-force-iterations=1000

           # Now securely delete the plain text LUKS key, we don't need it anymore, and since it contains secret key
           # material it should be removed from disk thoroughly.
           shred -u plaintext.bin plaintext.base64

           # We don't need the public key anymore either, let's remove it too. Since this one is not security
           # sensitive we just do a regular "rm" here.
           rm pubkey.pem

           # Test: Let's run systemd-cryptsetup to test if this all worked. The option string should contain the full
           # PKCS#11 URI we have in the clipboard; it tells the tool how to decipher the encrypted LUKS key. Note that
           # systemd-cryptsetup automatically searches for the encrypted key in /etc/cryptsetup-keys.d/, hence we do
           # not need to specify the key file path explicitly here.
           sudo systemd-cryptsetup attach mytest /dev/sdXn - 'pkcs11-uri=pkcs11:...'

           # If that worked, let's now add the same line persistently to /etc/crypttab, for the future.
           sudo bash -c 'echo "mytest /dev/sdXn - \'pkcs11-uri=pkcs11:...\'" >> /etc/crypttab'

       A few notes on the above:

       •   We use RSA (and not ECC), since Yubikeys support PKCS#11
           Decrypt() only for RSA keys

       •   We use RSA2048, which is the longest key size current Yubikeys
           support

       •   LUKS key size must be shorter than 2048bit due to RSA padding,
           hence we use 128 bytes

       •   We use Yubikey key slot 9d, since that's apparently the keyslot
           to use for decryption purposes, see documentation[2].

SEE ALSO         top

       systemd(1), systemd-cryptsetup@.service(8),
       systemd-cryptsetup-generator(8), fstab(5), cryptsetup(8), mkswap(8),
       mke2fs(8)

NOTES         top

        1. RFC7512 PKCS#11 URI
           https://tools.ietf.org/html/rfc7512

        2. see documentation
           https://developers.yubico.com/PIV/Introduction/Certificate_slots.html

COLOPHON         top

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       ⟨https://github.com/systemd/systemd.git⟩ on 2020-11-01.  (At that
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systemd 247                                                      CRYPTTAB(5)

Pages that refer to this page: 30-systemd-environment-d-generator(7)systemd.directives(7)systemd.index(7)systemd-cryptsetup(8)systemd-cryptsetup-generator(8)systemd-cryptsetup.service(8)systemd-cryptsetup@.service(8)