Provided by: systemd-cryptsetup_256.5-2ubuntu3.1_amd64 
NAME
systemd-cryptenroll - Enroll PKCS#11, FIDO2, TPM2 token/devices to LUKS2 encrypted volumes
SYNOPSIS
systemd-cryptenroll [OPTIONS...] [DEVICE]
DESCRIPTION
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.
Specifically, it supports tokens and credentials of the following kind to be enrolled:
1. PKCS#11 security tokens and smartcards that may carry an RSA or EC key pair (e.g.
various YubiKeys)
2. FIDO2 security tokens that implement the "hmac-secret" extension (most FIDO2 keys,
including YubiKeys)
3. TPM2 security devices
4. Regular passphrases
5. Recovery keys. These are similar to regular passphrases, however are randomly
generated on the computer and thus generally have higher entropy than user-chosen
passphrases. Their character set has been designed to ensure they are easy to type in,
while having high entropy. They may also be scanned off screen using QR codes.
Recovery keys may be used for unlocking LUKS2 volumes wherever passphrases are
accepted. They are intended to be used in combination with an enrolled hardware
security token, as a recovery option when the token is lost.
In addition, the tool may be used to enumerate currently enrolled security tokens and wipe
a subset of them. The latter may be combined with the enrollment operation of a new
security token, in order to update or replace enrollments.
The tool supports only LUKS2 volumes, as it stores token meta-information in the LUKS2
JSON token area, which is not available in other encryption formats.
systemd-cryptsetup operates on the device backing /var/ if no device is specified
explicitly, and no wipe operation is requested. (Note that in the typical case where /var/
is on the same file system as the root file system, this hence enrolls a key into the
backing device of the root file system.)
TPM2 PCRs and policies
PCRs allow binding of the encryption of secrets to specific software versions and system
state, so that the enrolled key is only accessible (may be "unsealed") if specific trusted
software and/or configuration is used. Such bindings may be created with the option
--tpm2-pcrs= described below.
Secrets may also be bound indirectly: a signed policy for a state of some combination of
PCR values is provided, and the secret is bound to the public part of the key used to sign
this policy. This means that the owner of a key can generate a sequence of signed
policies, for specific software versions and system states, and the secret can be
decrypted as long as the machine state matches one of those policies. For example, a
vendor may provide such a policy for each kernel+initrd update, allowing users to encrypt
secrets so that they can be decrypted when running any kernel+initrd signed by the vendor.
Such bindings may be created with the options --tpm2-public-key=, --tpm2-public-key-pcrs=,
--tpm2-signature= described below.
See Linux TPM PCR Registry[1] for an authoritative list of PCRs and how they are updated.
The table below contains a quick reference, describing in particular the PCRs modified by
systemd.
Table 1. Well-known PCR Definitions
┌────┬─────────────────────┬──────────────────────────┐
│PCR │ name │ Explanation │
├────┼─────────────────────┼──────────────────────────┤
│0 │ platform-code │ Core system firmware │
│ │ │ executable code; changes │
│ │ │ on firmware updates │
├────┼─────────────────────┼──────────────────────────┤
│1 │ platform-config │ Core system firmware │
│ │ │ data/host platform │
│ │ │ configuration; typically │
│ │ │ contains serial and │
│ │ │ model numbers, changes │
│ │ │ on basic │
│ │ │ hardware/CPU/RAM │
│ │ │ replacements │
├────┼─────────────────────┼──────────────────────────┤
│2 │ external-code │ Extended or pluggable │
│ │ │ executable code; │
│ │ │ includes option ROMs on │
│ │ │ pluggable hardware │
├────┼─────────────────────┼──────────────────────────┤
│3 │ external-config │ Extended or pluggable │
│ │ │ firmware data; includes │
│ │ │ information about │
│ │ │ pluggable hardware │
├────┼─────────────────────┼──────────────────────────┤
│4 │ boot-loader-code │ Boot loader and │
│ │ │ additional drivers, PE │
│ │ │ binaries invoked by the │
│ │ │ boot loader; changes on │
│ │ │ boot loader updates. sd- │
│ │ │ stub(7) measures system │
│ │ │ extension images read │
│ │ │ from the ESP here too │
│ │ │ (see systemd-sysext(8)). │
├────┼─────────────────────┼──────────────────────────┤
│5 │ boot-loader-config │ GPT/Partition table; │
│ │ │ changes when the │
│ │ │ partitions are added, │
│ │ │ modified, or removed │
├────┼─────────────────────┼──────────────────────────┤
│7 │ secure-boot-policy │ Secure Boot state; │
│ │ │ changes when UEFI │
│ │ │ SecureBoot mode is │
│ │ │ enabled/disabled, or │
│ │ │ firmware certificates │
│ │ │ (PK, KEK, db, dbx, ...) │
│ │ │ changes. │
├────┼─────────────────────┼──────────────────────────┤
│9 │ kernel-initrd │ The Linux kernel │
│ │ │ measures all initrds it │
│ │ │ receives into this PCR. │
├────┼─────────────────────┼──────────────────────────┤
│10 │ ima │ The IMA project measures │
│ │ │ its runtime state into │
│ │ │ this PCR. │
├────┼─────────────────────┼──────────────────────────┤
│11 │ kernel-boot │ systemd-stub(7) measures │
│ │ │ the ELF kernel image, │
│ │ │ embedded initrd and │
│ │ │ other payload of the PE │
│ │ │ image it is placed in │
│ │ │ into this PCR. systemd- │
│ │ │ pcrphase.service(8) │
│ │ │ measures boot phase │
│ │ │ strings into this PCR at │
│ │ │ various milestones of │
│ │ │ the boot process. │
├────┼─────────────────────┼──────────────────────────┤
│12 │ kernel-config │ systemd-boot(7) measures │
│ │ │ the kernel command line │
│ │ │ into this PCR. systemd- │
│ │ │ stub(7) measures any │
│ │ │ manually specified │
│ │ │ kernel command line │
│ │ │ (i.e. a kernel command │
│ │ │ line that overrides the │
│ │ │ one embedded in the │
│ │ │ unified PE image) and │
│ │ │ loaded credentials into │
│ │ │ this PCR. │
├────┼─────────────────────┼──────────────────────────┤
│13 │ sysexts │ systemd-stub(7) measures │
│ │ │ any systemd-sysext(8) │
│ │ │ images it passes to the │
│ │ │ booted kernel into this │
│ │ │ PCR. │
├────┼─────────────────────┼──────────────────────────┤
│14 │ shim-policy │ The shim project │
│ │ │ measures its "MOK" │
│ │ │ certificates and hashes │
│ │ │ into this PCR. │
├────┼─────────────────────┼──────────────────────────┤
│15 │ system-identity │ systemd-cryptsetup(8) │
│ │ │ optionally measures the │
│ │ │ volume key of activated │
│ │ │ LUKS volumes into this │
│ │ │ PCR. systemd- │
│ │ │ pcrmachine.service(8) │
│ │ │ measures the machine- │
│ │ │ id(5) into this PCR. │
│ │ │ systemd- │
│ │ │ pcrfs@.service(8) │
│ │ │ measures mount points, │
│ │ │ file system UUIDs, │
│ │ │ labels, partition UUIDs │
│ │ │ of the root and /var/ │
│ │ │ filesystems into this │
│ │ │ PCR. │
├────┼─────────────────────┼──────────────────────────┤
│16 │ debug │ Debug │
├────┼─────────────────────┼──────────────────────────┤
│23 │ application-support │ Application Support │
└────┴─────────────────────┴──────────────────────────┘
In general, encrypted volumes would be bound to some combination of PCRs 7, 11, and 14 (if
shim/MOK is used). In order to allow firmware and OS version updates, it is typically not
advisable to use PCRs such as 0 and 2, since the program code they cover should already be
covered indirectly through the certificates measured into PCR 7. Validation through
certificates hashes is typically preferable over validation through direct measurements as
it is less brittle in context of OS/firmware updates: the measurements will change on
every update, but signatures should remain unchanged. See the Linux TPM PCR Registry[1]
for more discussion.
LIMITATIONS
Note that currently when enrolling a new key of one of the five supported types listed
above, it is required to first provide a passphrase, a recovery key, a FIDO2 token, or a
TPM2 key. It's currently not supported to unlock a device with a PKCS#11 key in order to
enroll a new PKCS#11 key. Thus, if in future key roll-over is desired it's generally
recommended to ensure a passphrase, a recovery key, a FIDO2 token, or a TPM2 key is always
enrolled.
Also note that support for enrolling multiple FIDO2 tokens is currently limited. When
multiple FIDO2 tokens are enrolled, systemd-cryptsetup will perform pre-flight requests to
attempt to identify which of the enrolled tokens are currently plugged in. However, this
is not possible for FIDO2 tokens with user verification (UV, usually via biometrics), in
which case it will fall back to attempting each enrolled token one by one. This will
result in multiple prompts for PIN and user verification. This limitation does not apply
to PKCS#11 tokens.
COMPATIBILITY
Security technology both in systemd and in the general industry constantly evolves. In
order to provide best security guarantees, the way TPM2, FIDO2, PKCS#11 devices are
enrolled is regularly updated in newer versions of systemd. Whenever this happens the
following compatibility guarantees are given:
• Old enrollments continue to be supported and may be unlocked with newer versions of
systemd-cryptsetup@.service(8).
• The opposite is not guaranteed however: it might not be possible to unlock volumes
with enrollments done with a newer version of systemd-cryptenroll with an older
version of systemd-cryptsetup.
That said, it is generally recommended to use matching versions of systemd-cryptenroll and
systemd-cryptsetup, since this is best tested and supported.
It might be advisable to re-enroll existing enrollments to take benefit of newer security
features, as they are added to systemd.
OPTIONS
The following options are understood:
--password
Enroll a regular password/passphrase. This command is mostly equivalent to cryptsetup
luksAddKey, however may be combined with --wipe-slot= in one call, see below.
Added in version 248.
--recovery-key
Enroll a recovery key. 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.
Added in version 248.
--unlock-key-file=PATH
Use a file instead of a password/passphrase read from stdin to unlock the volume.
Expects the PATH to the file containing your key to unlock the volume. Currently there
is nothing like --key-file-offset= or --key-file-size= so this file has to only
contain the full key.
Added in version 252.
--unlock-fido2-device=PATH
Use a FIDO2 device instead of a password/passphrase read from stdin to unlock the
volume. Expects a hidraw device referring to the FIDO2 device (e.g. /dev/hidraw1).
Alternatively the special value "auto" may be specified, in order to automatically
determine the device node of a currently plugged in security token (of which there
must be exactly one). This automatic discovery is unsupported if --fido2-device=
option is also specified.
Added in version 253.
--unlock-tpm2-device=PATH
Use a TPM2 device instead of a password/passhprase read from stdin to unlock the
volume. Expects a device node path referring to the TPM2 chip (e.g. /dev/tpmrm0).
Alternatively the special value "auto" may be specified, in order to automatically
determine the device node of a currently discovered TPM2 device (of which there must
be exactly one).
Added in version 256.
--pkcs11-token-uri=URI
Enroll a PKCS#11 security token or smartcard (e.g. a YubiKey). Expects a PKCS#11 URI
that allows finding an X.509 certificate or a public key on the token. The URI must
also be suitable to find a related private key after changing the type of object in
it. Alternatively the special value "auto" may be specified, in order to automatically
determine the suitable URI if a single security token containing a single key pair is
plugged in. The special value "list" may be used to enumerate all suitable PKCS#11
tokens currently plugged in.
The PKCS#11 token must contain an RSA or EC key pair which will be used to unlock a
LUKS2 volume. For RSA, a randomly generated volume key is encrypted with a public key
in the token, and stored in the LUKS2 JSON token header area. To unlock a volume, the
stored encrypted volume key will be decrypted with a private key in the token. For
ECC, ECDH algorithm is used: we generate a pair of EC keys in the same EC group, then
derive a shared secret using the generated private key and the public key in the
token. The derived shared secret is used as a volume key. The generated public key is
stored in the LUKS2 JSON token header area. The generated private key is erased. To
unlock a volume, we derive the shared secret with the stored public key and a private
key in the token.
In order to unlock a LUKS2 volume with an enrolled PKCS#11 security token, specify the
pkcs11-uri= option in the respective /etc/crypttab line:
myvolume /dev/sda1 - pkcs11-uri=auto
See crypttab(5) for a more comprehensive example of a systemd-cryptenroll invocation
and its matching /etc/crypttab line.
Added in version 248.
--fido2-credential-algorithm=STRING
Specify COSE algorithm used in credential generation. The default value is "es256".
Supported values are "es256", "rs256" and "eddsa".
"es256" denotes ECDSA over NIST P-256 with SHA-256. "rs256" denotes 2048-bit RSA with
PKCS#1.5 padding and SHA-256. "eddsa" denotes EDDSA over Curve25519 with SHA-512.
Note that your authenticator may choose not to support some algorithms.
Added in version 251.
--fido2-device=PATH
Enroll a FIDO2 security token that implements the "hmac-secret" extension (e.g. a
YubiKey). Expects a hidraw device referring to the FIDO2 device (e.g. /dev/hidraw1).
Alternatively the special value "auto" may be specified, in order to automatically
determine the device node of a currently plugged in security token (of which there
must be exactly one). This automatic discovery is unsupported if
--unlock-fido2-device= option is also specified. The special value "list" may be used
to enumerate all suitable FIDO2 tokens currently plugged in. Note that many hardware
security tokens that implement FIDO2 also implement the older PKCS#11 standard.
Typically FIDO2 is preferable, given it's simpler to use and more modern.
In order to unlock a LUKS2 volume with an enrolled FIDO2 security token, specify the
fido2-device= option in the respective /etc/crypttab line:
myvolume /dev/sda1 - fido2-device=auto
See crypttab(5) for a more comprehensive example of a systemd-cryptenroll invocation
and its matching /etc/crypttab line.
Added in version 248.
--fido2-with-client-pin=BOOL
When enrolling a FIDO2 security token, controls whether to require the user to enter a
PIN when unlocking the volume (the FIDO2 "clientPin" feature). Defaults to "yes".
(Note: this setting is without effect if the security token does not support the
"clientPin" feature at all, or does not allow enabling or disabling it.)
Added in version 249.
--fido2-with-user-presence=BOOL
When enrolling a FIDO2 security token, controls whether to require the user to verify
presence (tap the token, the FIDO2 "up" feature) when unlocking the volume. Defaults
to "yes". (Note: this setting is without effect if the security token does not support
the "up" feature at all, or does not allow enabling or disabling it.)
Added in version 249.
--fido2-with-user-verification=BOOL
When enrolling a FIDO2 security token, controls whether to require user verification
when unlocking the volume (the FIDO2 "uv" feature). Defaults to "no". (Note: this
setting is without effect if the security token does not support the "uv" feature at
all, or does not allow enabling or disabling it.)
Added in version 249.
--tpm2-device=PATH
Enroll a TPM2 security chip. Expects a device node path referring to the TPM2 chip
(e.g. /dev/tpmrm0). Alternatively the special value "auto" may be specified, in order
to automatically determine the device node of a currently discovered TPM2 device (of
which there must be exactly one). The special value "list" may be used to enumerate
all suitable TPM2 devices currently discovered.
In order to unlock a LUKS2 volume with an enrolled TPM2 security chip, specify the
tpm2-device= option in the respective /etc/crypttab line:
myvolume /dev/sda1 - tpm2-device=auto
See crypttab(5) for a more comprehensive example of a systemd-cryptenroll invocation
and its matching /etc/crypttab line.
Use --tpm2-pcrs= (see below) to configure which TPM2 PCR indexes to bind the
enrollment to.
Added in version 248.
--tpm2-device-key=PATH
Enroll a TPM2 security chip using its public key. Expects a path referring to the TPM2
public key in TPM2B_PUBLIC format. This cannot be used with --tpm2-device=, as it
performs the same operation, but without connecting to the TPM2 security chip; instead
the enrollment is calculated using the provided TPM2 key. This is useful in situations
where the TPM2 security chip is not available at the time of enrollment.
The key, in most cases, should be the Storage Root Key (SRK) from a local TPM2
security chip. If a key from a different handle (not the SRK) is used, you must
specify its handle index using --tpm2-seal-key-handle=.
The systemd-tpm2-setup.service(8) service writes the SRK to
/run/systemd/tpm2-srk-public-key.tpm2b_public automatically during boot, in the
correct format.
Alternatively, you may use systemd-analyze srk to retrieve the SRK from the TPM2
security chip explicitly. See systemd-analyze(1) for details. Example:
systemd-analyze srk > srk.tpm2b_public
Added in version 255.
--tpm2-seal-key-handle=HANDLE
Configures which parent key to use for sealing, using the TPM handle (index) of the
key. This is used to "seal" (encrypt) a secret and must be used later to "unseal"
(decrypt) the secret. Expects a hexadecimal 32bit integer, optionally prefixed with
"0x". Allowable values are any handle index in the persistent
("0x81000000"-"0x81ffffff") or transient ("0x80000000"-"0x80ffffff") ranges. Since
transient handles are lost after a TPM reset, and may be flushed during TPM context
switching, they should not be used except for very specific use cases, e.g. testing.
The default is the Storage Root Key (SRK) handle index "0x81000001". A value of 0 will
use the default. For the SRK handle, a new key will be created and stored in the TPM
if one does not already exist; for any other handle, the key must already exist in the
TPM at the specified handle index.
This should not be changed unless you know what you are doing.
Added in version 255.
--tpm2-pcrs=PCR[+PCR...]
Configures the TPM2 PCRs (Platform Configuration Registers) to bind to when enrollment
is requested via --tpm2-device=. Takes a list of PCR entries, where each entry starts
with a name or numeric index in the range 0...23, optionally followed by ":" and a
hash algorithm name (specifying the PCR bank), optionally followed by "=" and a hash
digest value. Multiple PCR entries are separated by "+". If not specified, the default
is to use PCR 7 only. If an empty string is specified, binds the enrollment to no PCRs
at all. See the table above for a list of available PCRs.
Example: --tpm2-pcrs=boot-loader-code+platform-config+boot-loader-config specifies
that PCR registers 4, 1, and 5 should be used.
Example: --tpm2-pcrs=7:sha256 specifies that PCR register 7 from the SHA256 bank
should be used.
Example: --tpm2-pcrs=4:sha1=3a3f780f11a4b49969fcaa80cd6e3957c33b2275 specifies that
PCR register 4 from the SHA1 bank should be used, and a hash digest value of
3a3f780f11a4b49969fcaa80cd6e3957c33b2275 will be used instead of reading the current
PCR value.
Added in version 248.
--tpm2-with-pin=BOOL
When enrolling a TPM2 device, controls whether to require the user to enter a PIN when
unlocking the volume in addition to PCR binding, based on TPM2 policy authentication.
Defaults to "no". Despite being called PIN, any character can be used, not just
numbers.
Note that incorrect PIN entry when unlocking increments the TPM dictionary attack
lockout mechanism, and may lock out users for a prolonged time, depending on its
configuration. The lockout mechanism is a global property of the TPM,
systemd-cryptenroll does not control or configure the lockout mechanism. You may use
tpm2-tss tools to inspect or configure the dictionary attack lockout, with
tpm2_getcap(1) and tpm2_dictionarylockout(1) commands, respectively.
Added in version 251.
--tpm2-public-key=PATH, --tpm2-public-key-pcrs=PCR[+PCR...], --tpm2-signature=PATH
Configures a TPM2 signed PCR policy to bind encryption to. The --tpm2-public-key=
option accepts a path to a PEM encoded RSA public key, to bind the encryption to. If
this is not specified explicitly, but a file tpm2-pcr-public-key.pem exists in one of
the directories /etc/systemd/, /run/systemd/, /usr/lib/systemd/ (searched in this
order), it is automatically used. The --tpm2-public-key-pcrs= option takes a list of
TPM2 PCR indexes to bind to (same syntax as --tpm2-pcrs= described above). If not
specified defaults to 11 (i.e. this binds the policy to any unified kernel image for
which a PCR signature can be provided).
Note the difference between --tpm2-pcrs= and --tpm2-public-key-pcrs=: the former binds
decryption to the current, specific PCR values; the latter binds decryption to any set
of PCR values for which a signature by the specified public key can be provided. The
latter is hence more useful in scenarios where software updates shell be possible
without losing access to all previously encrypted LUKS2 volumes. Like with
--tpm2-pcrs=, names defined in the table above can also be used to specify the
registers, for instance --tpm2-public-key-pcrs=boot-loader-code+system-identity.
The --tpm2-signature= option takes a path to a TPM2 PCR signature file as generated by
the systemd-measure(1) tool. If this is not specified explicitly, a suitable signature
file tpm2-pcr-signature.json is searched for in /etc/systemd/, /run/systemd/,
/usr/lib/systemd/ (in this order) and used. If a signature file is specified or found
it is used to verify if the volume can be unlocked with it given the current PCR
state, before the new slot is written to disk. This is intended as safety net to
ensure that access to a volume is not lost if a public key is enrolled for which no
valid signature for the current PCR state is available. If the supplied signature does
not unlock the current PCR state and public key combination, no slot is enrolled and
the operation will fail. If no signature file is specified or found no such safety
verification is done.
Added in version 252.
--tpm2-pcrlock=PATH
Configures a TPM2 pcrlock policy to bind encryption to. Expects a path to a pcrlock
policy file as generated by the systemd-pcrlock(1) tool. If a TPM2 device is enrolled
and this option is not used but a file pcrlock.json is found in /run/systemd/ or
/var/lib/systemd/ it is automatically used. Assign an empty string to turn this
behaviour off.
Added in version 255.
--wipe-slot=SLOT[,SLOT...]
Wipes one or more LUKS2 key slots. Takes a comma separated list of numeric slot
indexes, or the special strings "all" (for wiping all key slots), "empty" (for wiping
all key slots that are unlocked by an empty passphrase), "password" (for wiping all
key slots that are unlocked by a traditional passphrase), "recovery" (for wiping all
key slots that are unlocked by a recovery key), "pkcs11" (for wiping all key slots
that are unlocked by a PKCS#11 token), "fido2" (for wiping all key slots that are
unlocked by a FIDO2 token), "tpm2" (for wiping all key slots that are unlocked by a
TPM2 chip), or any combination of these strings or numeric indexes, in which case all
slots matching either are wiped. As safety precaution an operation that wipes all
slots without exception (so that the volume cannot be unlocked at all anymore, unless
the volume key is known) is refused.
This switch may be used alone, in which case only the requested wipe operation is
executed. It may also be used in combination with any of the enrollment options listed
above, in which case the enrollment is completed first, and only when successful the
wipe operation executed — and the newly added slot is always excluded from the wiping.
Combining enrollment and slot wiping may thus be used to update existing enrollments:
systemd-cryptenroll /dev/sda1 --wipe-slot=tpm2 --tpm2-device=auto
The above command will enroll the TPM2 chip, and then wipe all previously created TPM2
enrollments on the LUKS2 volume, leaving only the newly created one. Combining wiping
and enrollment may also be used to replace enrollments of different types, for example
for changing from a PKCS#11 enrollment to a FIDO2 one:
systemd-cryptenroll /dev/sda1 --wipe-slot=pkcs11 --fido2-device=auto
Or for replacing an enrolled empty password by TPM2:
systemd-cryptenroll /dev/sda1 --wipe-slot=empty --tpm2-device=auto
Added in version 248.
-h, --help
Print a short help text and exit.
--version
Print a short version string and exit.
CREDENTIALS
systemd-cryptenroll supports the service credentials logic as implemented by
ImportCredential=/LoadCredential=/SetCredential= (see systemd.exec(5) for details). The
following credentials are used when passed in:
cryptenroll.passphrase, cryptenroll.new-passphrase
May contain the passphrase to unlock the volume with/to newly enroll.
Added in version 256.
cryptenroll.tpm2-pin, cryptenroll.new-tpm2-pin
May contain the TPM2 PIN to unlock the volume with/to newly enroll.
Added in version 256.
cryptenroll.fido2-pin
If a FIDO2 token is enrolled this may contain the PIN of the token.
Added in version 256.
cryptenroll.pkcs11-pin
If a PKCS#11 token is enrolled this may contain the PIN of the token.
Added in version 256.
EXIT STATUS
On success, 0 is returned, a non-zero failure code otherwise.
EXAMPLES
crypttab(5) and systemd-measure(1) contain various examples employing systemd-cryptenroll.
SEE ALSO
systemd(1), systemd-cryptsetup@.service(8), crypttab(5), cryptsetup(8), systemd-measure(1)
NOTES
1. Linux TPM PCR Registry
https://uapi-group.org/specifications/specs/linux_tpm_pcr_registry/