systemd-cryptenroll

Related articles
dm-crypt
Smartcards
Universal 2nd Factor
Trusted Platform Module
Unified Extensible Firmware Interface/Secure Boot
From
systemd-cryptenroll(1)
:
systemd-cryptenroll is a tool for enrolling hardware security tokens and devices into a LUKS2 encrypted volume, which may then be used to unlock the volume during boot.
systemd-cryptenroll
allows enrolling
smartcards
,
FIDO2
tokens and
Trusted Platform Module
security chips into
LUKS
devices, as well as regular passphrases. These devices are later unlocked by
systemd-cryptsetup@.service(8)
, using the enrolled tokens.
Installation
systemd-cryptenroll
is part of and packaged with
systemd
. However, extra packages are required to use hardware devices as keys:
To use PKCS#11 tokens,
install
libp11-kit
, you may also need
opensc
and
opensc-p11-kit-module
AUR
[
broken link
: package not found]
.
To use FIDO2 tokens, install
libfido2
.
To use TPM2 devices, install
tpm2-tss
.
List keyslots
systemd-cryptenroll
can list the keyslots in a LUKS device, similar to
cryptsetup
luksDump
, but in a more user-friendly format.
# systemd-cryptenroll /dev/
disk
SLOT TYPE    
   0 password
   1 recovery
   2 tpm2
Erasing keyslots
# systemd-cryptenroll /dev/
disk
--wipe-slot=
SLOT
Where
SLOT
can be:
A single keyslot index, as represented in
#List keyslots
A type of keyslot, which will erase all keyslots of that type. Valid types are
empty
,
password
,
recovery
,
pkcs11
,
fido2
,
tpm2
A combination of all of the above, separated by commas
The string
all
, which erases all keyslots on the device. This option can only be used when enrolling another device or passphrase at the same time.
The
--wipe-slot
operation can be used in combination with all enrollment options, which is useful to update existing device enrollments:
# systemd-cryptenroll /dev/
disk
--wipe-slot=fido2 --fido2-device=auto
Enrolling passphrases
Regular password
This is equivalent to
cryptsetup luksAddKey
.
# systemd-cryptenroll /dev/
disk
--password
Recovery key
From
systemd-cryptenroll(1)
:
Recovery keys are mostly identical to passphrases, but are computer-generated instead of being chosen by a human, and thus have a guaranteed high entropy. The key uses a character set that is easy to type in, and may be scanned off screen via a QR code.
A recovery key is designed to be used as a fallback if the hardware tokens are unavailable, and can be used in place of regular passphrases whenever they are required.
# systemd-cryptenroll /dev/
disk
--recovery-key
Enrolling hardware devices
The
--
type
-device
options must point to a valid device path of their respective type. A list of available devices can be obtained by passing the
list
argument to this option. Alternatively, if you only have a single device of the desired type connected, the
auto
option can be used to automatically select it.
Note
After enrolling the hardware tokens into the LUKS2 volumes, you must configure your system to use them when appropriate. See
dm-crypt/System configuration#Trusted Platform Module and FIDO2 keys
for volumes that should be unlocked in early userspace like the root filesystem, and
dm-crypt/System configuration#Unlocking in late userspace
for other partitions.
PKCS#11 tokens or smartcards
The token or smartcard must contain a RSA key pair, which will be used to encrypt the generated key that will be used to unlock the volume.
# systemd-cryptenroll /dev/
disk
--pkcs11-token-uri=
device
FIDO2 tokens
Any FIDO2 token that supports the "hmac-secret" extension can be used with
systemd-cryptenroll
. The following example would enroll a FIDO2 token to an encrypted LUKS2 block device, requiring only user presence as authentication.
# systemd-cryptenroll /dev/
disk
--fido2-device=
device
--fido2-with-client-pin=no
In addition,
systemd-cryptenroll
supports using the token's built-in user verification methods:
--fido2-with-user-presence
defines whether to verify the user presence (i.e. by tapping the token) before unlocking, defaults to yes
--fido2-with-user-verification
defines whether to require user verification before unlocking, defaults to no
Note
These options will have no effect if the token does not support these features.
See
User Presence vs User Verification
for more information on the difference between the two.
By default, the cryptographic algorithm used when generating a FIDO2 credential is
es256
which denotes Elliptic Curve Digital Signature Algorithm (ECDSA) over NIST P-256 with SHA-256. If desired and provided by the FIDO2 token, a different cryptographic algorithm can be specified during enrollment.
Note
This may also be desirable for those concerned with ECDSA. See
SSH keys#ECDSA
for details.
Suppose that a previous FIDO2 token has already been enrolled and the user wishes to enroll another, the following generates an
eddsa
credential which denotes
EdDSA
over Curve25519 with SHA-512 and authenticates the device with a previous enrolled token instead of a password.
# systemd-cryptenroll /dev/
disk
--fido2-device=
device
--fido2-credential-algorithm=eddsa --unlock-fido2-device=auto
Note
Both tokens must be plugged in to the system for successful enrollment.
Trusted Platform Module
This article or section needs expansion.
Reason:
Document
--tpm2-seal-key-handle
--tpm2-device-key
--tpm2-pcrlock
(Discuss in
Talk:Systemd-cryptenroll
)
systemd-cryptenroll
has native support for enrolling LUKS keys in TPMs. It requires the following:
tpm2-tss
must be
installed
,
A LUKS2 device (currently the default type used by
cryptsetup
),
If you intend to use this method on your root partition, some tweaks need to be made to the
initramfs
(see
dm-crypt/System configuration#Using systemd-cryptsetup-generator
for advanced configuration) :
mkinitcpio
users: enable the
systemd
and
sd-encrypt
hooks
.
Note
The order of the entries in the hooks is important. A nonstandard ordering
can make the system unbootable
(you will need to
regenerate the initramfs
from within
arch-chroot
to recover). See
dm-crypt/System configuration#Examples
for an example of the correct order.
dracut
users: enable the
tpm2-tss
module
.
To begin, run the following command to list your installed TPMs and the driver in use:
$ systemd-cryptenroll --tpm2-device=list
Tip
If your computer has multiple TPMs installed, specify the one you wish to use with
--tpm2-device=
/path/to/tpm2_device
in the following steps.
Consider using
PCR policies
instead of binding secrets to raw PCR values.
A key may be enrolled in both the TPM and the LUKS volume using only one command. The following example generates a new random key, adds it to the volume so it can be used to unlock it in addition to the existing keys, and binds this new key to PCR 7 (
Secure Boot
state):
# systemd-cryptenroll --tpm2-device=auto --tpm2-pcrs=7 /dev/
sdX
where
/dev/
sdX
is the full path to the encrypted LUKS volume. Use
--unlock-key-file=
/path/to/keyfile
if the LUKS volume is unlocked by a keyfile instead of a passphrase.
Refer to
systemd-cryptenroll(1)
and
Trusted Platform Module#Accessing PCR registers
for common PCR measurements in Linux. Adjust
--tpm2-pcrs=7
as necessary (parameters are separated by the
+
symbol).
Warning
Make sure
Secure Boot
is active and in user mode when binding to PCR 7, otherwise, unauthorized boot devices could unlock the encrypted volume.
The state of PCR 7 can change if firmware certificates change, which can risk locking the user out. This can be implicitly done by
fwupd
[1]
or explicitly by rotating Secure Boot keys.
Only binding to PCRs measured pre-boot (PCRs 0-7) opens a vulnerability from rogue operating systems. A rogue partition with metadata copied from the real root filesystem (such as partition UUID) can mimic the original partition. Then, initramfs will attempt to mount the rogue partition as the root filesystem (decryption failure will fall back to password entry), leaving pre-boot PCRs unchanged. The rogue root filesystem with files controlled by an attacker is still able to receive the decryption key for the real root partition. See
Brave New Trusted Boot World
and
BitLocker documentation
for additional information.
A solution for the root volume is to bind to an empty PCR 15 using
--tpm2-pcrs=
other_pcrs
+15:sha256=0000000000000000000000000000000000000000000000000000000000000000
. If you set any
rd.luks
kernel parameters or use
/etc/crypttab.initramfs
, additionally add the
tpm2-measure-pcr=yes
option to
rd.luks.options=
or the fourth field in
/etc/crypttab.initramfs
; this is not required when relying on
GPT partition automounting
. After the root volume is unlocked in early userspace, PCR 15 will change and the enrolled key will no longer be retrievable.
Another cleaner solution is described in
dm-crypt/System configuration#Pinning a LUKS volume
.
The combination of PCRs to bind to depends on the individual case to balance usability and lock-down. For example, you may require UEFI firmware updates without manual intervention to the
Secure Boot
state, or different boot devices. As another example, Microsoft's
Bitlocker
prefers PCR
7+11
, but may also use other PCR combinations.
Note
It is possible to require a PIN to be entered in addition to the TPM state being correct. Simply add the option
--tpm2-with-pin=yes
to the command above and enter the PIN when prompted.
systemd-cryptenroll
does not check the TPM measurement before asking for the PIN, therefore consider using a unique PIN since the environment may be untrustworthy.
To check that the new key was enrolled, dump the LUKS configuration and look for a
systemd-tpm2
token entry, as well as an additional entry in the
Keyslots
section:
# cryptsetup luksDump /dev/sdX
To test that the key works, run the following command while the LUKS volume is closed:
# systemd-cryptsetup attach
mapping_name
/dev/
sdX
none tpm2-device=auto
where
mapping_name
is your chosen name for the volume once opened.
See
dm-crypt/System configuration#crypttab
and
dm-crypt/System configuration#Trusted Platform Module and FIDO2 keys
in order to unlock the volume at boot time.
Note
While you may specify the UUID of your LUKS volume in place of the pathname in
/etc/crypttab
, the
systemd-cryptenroll
command itself currently only supports path names.
See
systemd-cryptenroll(1)
and
crypttab(5)
for more information and examples.
See also
Lennart's blog: Unlocking LUKS2 volumes with TPM2, FIDO2, PKCS#11 Security Hardware on systemd 248