[dm-crypt] [ANNOUNCE] cryptsetup 2.0.0-rc0 (TESTING version with experimental features)
gmazyland at gmail.com
Fri Sep 29 10:18:43 CEST 2017
The TESTING cryptsetup 2.0.0-rc0 release is available at
Please note that release packages are located on kernel.org
Feedback and bug reports are welcomed.
The latest stable version is still 1.7.5.
Cryptsetup 2.0.0 RC0 Release Notes
Release candidate with experimental features.
This version introduces a new on-disk LUKS2 format.
The legacy LUKS (referenced as LUKS1) will be fully supported
forever as well as a traditional and fully backward compatible format.
NOTE: This version changes soname of libcryptsetup library and increases
major version for all public symbols.
Most of the old functions are fully backward compatible, so only
recompilation of programs should be needed.
Please note that authenticated disk encryption, noncryptographic
data integrity protection (dm-integrity), use of Argon2 Password-Based
Key Derivation Function and the LUKS2 on-disk format itself are new
features and can contain some bugs.
Please do not use it without properly configured backup or in
Until final 2.0 version is released, the new API calls or LUKS2 format
could still change if a major problem is found.
* New command integritysetup: support for the new dm-integrity kernel target.
The dm-integrity is a new kernel device-mapper target that introduces
software emulation of per-sector integrity fields on the disk sector level.
It is available since Linux kernel version 4.12.
The provided per-sector metadata fields can be used for storing a data
integrity checksum (for example CRC32).
The dm-integrity implements data journal that enforces atomic update
of a sector and its integrity metadata.
Integritysetup is a CLI utility that can setup standalone dm-integrity
devices (that internally check integrity of data).
Integritysetup is intended to be used for settings that require
non-cryptographic data integrity protection with no data encryption.
Fo setting integrity protected encrypted devices, see disk authenticated
Note that after formatting the checksums need to be initialized;
otherwise device reads will fail because of integrity errors.
Integritysetup by default tries to wipe the device with zero blocks
to avoid this problem. Device wipe can be time-consuming, you can skip
this step by specifying --no-wipe option.
(But note that not wiping device can cause some operations to fail
if a write is not multiple of page size and kernel page cache tries
to read sectors with not yet initialized checksums.)
The default setting is tag size 4 bytes per-sector and CRC32C protection.
To format device with these defaults:
$ integritysetup format <device>
$ integritysetup open <device> <name>
Note that used algorithm (unlike tag size) is NOT stored in device
kernel superblock and if you use different algorithm, you MUST specify
it in every open command, for example:
$ integritysetup format <device> --tag-size 32 --integrity sha256
$ integritysetup open <device> <name> --integrity sha256
For more info, see integrity man page.
* Veritysetup command can now format and activate dm-verity devices
that contain Forward Error Correction (FEC) (Reed-Solomon code is used).
This feature is used on most of Android devices already (available since
Linux kernel 4.5).
There are new options --fec-device, --fec-offset to specify data area
with correction code and --fec-roots that set Redd-Solomon generator roots.
This setting can be used for format command (veritysetup will calculate
and store RS codes) or open command (veritysetup configures kernel
dm-verity to use RS codes).
For more info see veritysetup man page.
* Support for larger sector sizes for crypt devices.
LUKS2 and plain crypt devices can be now configured with larger encryption
sector (typically 4096 bytes, sector size must be the power of two,
maximal sector size is 4096 bytes for portability).
Large sector size can decrease encryption overhead and can also help
with some specific crypto hardware accelerators that perform very
badly with 512 bytes sectors.
Note that if you configure such a larger sector of the device that does use
smaller physical sector, there is a possibility of a data corruption during
power fail (partial sector writes).
WARNING: If you use different sector size for a plain device after data were
stored, the decryption will produce garbage.
For LUKS2, the sector size is stored in metadata and cannot be changed later.
LUKS2 format and features
The LUKS2 is an on-disk storage format designed to provide simple key
management, primarily intended for Full Disk Encryption based on dm-crypt.
The LUKS2 is inspired by LUKS1 format and in some specific situations (most
of the default configurations) can be converted in-place from LUKS1.
The LUKS2 format is designed to allow future updates of various
parts without the need to modify binary structures and internally
uses JSON text format for metadata. Compilation now requires the json-c library
that is used for JSON data processing.
On-disk format provides redundancy of metadata, detection
of metadata corruption and automatic repair from metadata copy.
NOTE: For security reasons, there is no redundancy in keyslots binary data
(encrypted keys) but the format allows adding such a feature in future.
NOTE: to operate correctly, LUKS2 requires locking of metadata.
Locking is performed by using flock() system call for images in file
and for block device by using a specific lock file in /run/lock/cryptsetup.
This directory must be created by distribution (do not rely on internal
fallback). For systemd-based distribution, you can simply install
scripts/cryptsetup_tmpfiles.conf into tmpfiles.d directory.
For more details see LUKS2-format.txt and LUKS2-locking.txt in the docs
directory. (Please note this is just overview, there will be more formal
LUKS2 allows using all possible configurations as LUKS1.
To format device as LUKS2, you have to add "--type luks2" during format:
$ cryptsetup luksFormat --type luks2 <device>
All commands issued later will recognize the new format automatically.
The newly added features in LUKS2 include:
* Authenticated disk (sector) encryption (EXPERIMENTAL)
Legacy Full disk encryption (FDE), for example, LUKS1, is a length-preserving
encryption (plaintext is the same size as a ciphertext).
Such FDE can provide data confidentiality, but cannot provide sound data
Full disk authenticated encryption is a way how to provide both
confidentiality and data integrity protection. Integrity protection here means
not only detection of random data corruption (silent data corruption) but also
prevention of an unauthorized intentional change of disk sector content.
NOTE: Integrity protection of this type cannot prevent a replay attack.
An attacker can replace the device or its part of the old content, and it
cannot be detected.
If you need such protection, better use integrity protection on a higher layer.
For data integrity protection on the sector level, we need additional
per-sector metadata space. In LUKS2 this space is provided by a new
device-mapper dm-integrity target (available since kernel 4.12).
Here the integrity target provides only reliable per-sector metadata store,
and the whole authenticated encryption is performed inside dm-crypt stacked
over the dm-integrity device.
For encryption, Authenticated Encryption with Additional Data (AEAD) is used.
Every sector is processed as a encryption request of this format:
|----- AAD -------|------ DATA -------|-- AUTH TAG --|
| (authenticated) | (auth+encryption) | |
| sector_LE | IV | sector in/out | tag in/out |
AEAD encrypts the whole sector and also authenticates sector number
(to detect sector relocation) and also authenticates Initialization Vector.
AEAD encryption produces encrypted data and authentication tag.
The authenticated tag is then stored in per-sector metadata space provided
Most of the current AEAD algorithms requires IV as a nonce, value that is
never reused. Because sector number, as an IV, cannot be used in this
environment, we use a new random IV (IV is a random value generated by system
RNG on every write). This random IV is then stored in the per-sector metadata
Because the authentication tag (and IV) requires additional space, the device
provided for a user has less capacity. Also, the data journalling means that
writes are performed twice, decreasing throughput.
This integrity protection works better with SSDs. If you want to ignore
dm-integrity data journal (because journalling is performed on some higher
layer or you just want to trade-off performance to safe recovery), you can
switch journal off with --integrity-no-journal option.
(This flag can be stored persistently as well.)
Note that (similar to integritysetup) the device read will fail if
authentication tag is not initialized (no previous write).
By default cryptsetup run wipe of a device (writing zeroes) to initialize
authentication tags. This operation can be very time-consuming.
You can skip device wipe using --integrity-no-wipe option.
To format LUKS2 device with integrity protection, use new --integrity option.
For now, there are very few AEAD algorithms that can be used, and some
of them are known to be problematic. In this release we support only
a few of AEAD algorithms (options are for now hard coded), later this
extension will be completely algorithm-agnostic.
For testing of authenticated encryption, these algorithms work for now:
1) aes-xts-random with hmac-sha256 or hmac-sha512 as the authentication tag.
(Authentication key for HMAC is independently generated. This mode is very slow.)
$ cryptsetup luksFormat --type luks2 <device> --cipher aes-xts-random --integrity hmac-sha256
2) aes-gcm-random (native AEAD mode)
DO NOT USE in production. The GCM mode uses only 96-bit nonce,
and possible collision means fatal security problem.
GCM mode has very good hardware support through AES-NI, so it is useful
for performance testing.
$ cryptsetup luksFormat --type luks2 <device> --cipher aes-gcm-random --integrity aead
3) ChaCha20 with Poly1305 authenticator (according to RFC7539)
$ cryptsetup luksFormat --type luks2 <device> --cipher chacha20-random --integrity poly1305
To specify AES128/AES256 just specify proper key size (without possible
authentication key). Other symmetric ciphers, like Serpent or Twofish,
should work as well. The mode 1) and 2) should be compatible with IEEE 1619.1
You can also store only random IV in tag without integrity protection.
Note that using random IV forces the system to pseudorandomly change the whole
sector on every write without removing parallel processing of XTS mode.
In cryptography, we can say that this will provide indistinguishability under
chosen plaintext attack (IND-CPA) that cannot be achieved in legacy
FDE systems. On the other side, if stored random IV is corrupted, the sector
is no longer decrypted properly.
To use only random IV (no integrity protection), just specify "none" integrity.
$ cryptsetup luksFormat --type luks2 <device> --cipher aes-xts-random --integrity none
FDE authenticated encryption is not a replacement for filesystem layer
authenticated encryption. The goal is to provide at least something because
data integrity protection is often completely ignored in today systems.
* New memory-hard PBKDF
LUKS1 introduced Password-Based Key Derivation Function v2 as a tool to
increase attacker cost for a dictionary and brute force attacks.
The PBKDF2 uses iteration count to increase time of key derivation.
Unfortunately, with modern GPUs, the PBKDF2 calculations can be run
in parallel and PBKDF2 can no longer provide the best available protection.
Increasing iteration count just cannot prevent massive parallel dictionary
password attacks in long-term.
To solve this problem, a new PBKDF, based on so-called memory-hard functions
can be used. Key derivation with memory-hard function requires a certain
amount of memory to compute its output. The memory requirement is very
costly for GPUs and prevents these systems to operate ineffectively,
increasing cost for attackers.
LUKS2 introduces support for Argon2i and Argon2id as a PBKDF.
Argon2 is the winner of Password Hashing Competition and is currently
in final RFC draft specification.
For now, libcryptsetup contains the embedded copy of reference implementation
of Argon2 (that is easily portable to all architectures).
Later, once this function is available in common crypto libraries, it will
switch to external implementation. (This happened for LUKS1 and PBKDF2
as well years ago.)
With using reference implementation (that is not optimized for speed), there
is some performance penalty. However, using memory-hard PBKDF should still
significantly complicate GPU-optimized dictionary and brute force attacks.
The Argon2 uses three costs: memory, time (number of iterations) and parallel
(number of threads).
Note that time and memory cost highly influences each other (accessing a lot
of memory takes more time).
There is a new benchmark that tries to calculate costs to take similar way as
in LUKS1 (where iteration is measured to take 1-2 seconds on user system).
Because now there are more cost variables, it prefers time cost (iterations)
and tries to find required memory that fits. (IOW required memory cost can be
lower if the benchmarks are not able to find required parameters.)
The benchmark cannot run too long, so it tries to approximate next step
For now, default LUKS2 PBKDF algorithm is Argon2i (data independent variant)
with memory cost set to 128MB, time to 800ms and parallel thread according
to available CPU cores but no more than 4.
All default parameters can be set during compile time and also set on
the command line by using --pbkdf, --pbkdf-memory, --pbkdf-parallel and
(Or without benchmark directly by using --pbkdf-force-iterations, see below.)
You can still use PBKDF2 even for LUKS2 by specifying --pbkdf pbkdf2 option.
(Then only iteration count is applied.)
* Use of kernel keyring
Kernel keyring is a storage for sensitive material (like cryptographic keys)
inside Linux kernel.
LUKS2 uses keyring for two major functions:
- To store volume key for dm-crypt where it avoids sending volume key in
every device-mapper ioctl structure. Volume key is also no longer directly
visible in a dm-crypt mapping table. The key is not available for the user
after dm-crypt configuration (obviously except direct memory scan).
Use of kernel keyring can be disabled in runtime by --disable-keyring option.
- As a tool to automatically unlock LUKS device if a passphrase is put into
kernel keyring and proper keyring token is configured.
This allows storing a secret (passphrase) to kernel per-user keyring by
some external tool (for example some TPM handler) and LUKS2, if configured,
will automatically search in the keyring and unlock the system.
For more info see Tokens section below.
* Persistent flags
The activation flags (like allow-discards) can be stored in metadata and used
automatically by all later activations (even without using crypttab).
To store activation flags permanently, use activation command with required
flags and add --persistent option.
For example, to mark device to always activate with TRIM enabled,
use (for LUKS2 type):
$ cryptsetup open <device> <name> --allow-discards --persistent
You can check persistent flags in dump command output:
$ cryptsetup luksDump <device>
* Tokens and auto-activation
A LUKS2 token is an object that can be described "how to get passphrase or key"
to unlock particular keyslot.
(Also it can be used to store any additional metadata, and with
the libcryptsetup interface it can be used to define user token types.)
Cryptsetup internally implements keyring token. Cryptsetup tries to use
available tokens before asking for the passphrase. For keyring token,
it means that if the passphrase is available under specified identifier
inside kernel keyring, the device is automatically activated using this
Example of using LUKS2 keyring token:
# Adding token to metadata with "my_token" identifier (by default it applies to all keyslots).
$ cryptsetup token add --key-description "my_token" <device>
# Storing passphrase to user keyring (this can be done by an external application)
$ echo -n <passphrase> | keyctl padd user my_token @u
# Now cryptsetup activates automatically if it finds correct passphrase
$ cryptsetup open <device> <name>
The main reason to use tokens this way is to separate possible hardware
handlers from cryptsetup code.
* Keyslot priorities
LUKS2 keyslot can have a new priority attribute.
The default is "normal". The "prefer" priority tell the keyslot to be tried
before other keyslots. Priority "ignore" means that keyslot will never be
used if not specified explicitly (it can be used for backup administrator
passwords that are used only situations when a user forgets own passphrase).
The priority of keyslot can be set with new config command, for example
$ cryptsetup config <device> --key-slot 1 --priority prefer
Setting priority to normal will reset slot to normal state.
* LUKS2 label and subsystem
The header now contains additional fields for label and subsystem (additional
label). These fields can be used similar to filesystem label and will be
visible in udev rules to possible filtering. (Note that blkid do not yet
contain the LUKS scanning code).
By default both labels are empty. Label and subsystem are always set together
(no option means clear the label) with the config command:
$ cryptsetup config <device> --label my_device --subsystem ""
* In-place conversion form LUKS1
To allow easy testing and transition to the new LUKS2 format, there is a new
convert command that allows in-place conversion from the LUKS1 format and,
if there are no incompatible options, also conversion back from LUKS2
to LUKS1 format.
Note this command can be used only on some LUKS1 devices (some device header
sizes are not supported).
This command is dangerous, never run it without header backup!
If something fails in the middle of conversion (IO error), the header
is destroyed. (Note that conversion requires move of keyslot data area to
a different offset.)
To convert header in-place to LUKS2 format, use
$ cryptsetup convert <device> --type luks2
To convert it back to LUKS1 format, use
$ cryptsetup convert <device> --type luks1
You can verify LUKS version with luksDump command.
$ cryptsetup luksDump <device>
Note that some LUKS2 features will make header incompatible with LUKS1 and
conversion will be rejected (for example using new Argon2 PBKDF or integrity
extensions). Some minor attributes can be lost in conversion.
* Explicit KDF iterations count setting
With new PBKDF interface, there is also the possibility to setup PBKDF costs
directly, avoiding benchmarks. This can be useful if device is formatted to be
primarily used on a different system.
The option --pbkdf-force-iterations is available for both LUKS1 and LUKS2
format. Using this option can cause device to have either very low or very
high PBKDF costs.
In the first case it means bad protection to dictionary attacks, in the second
case, it can mean extremely high unlocking time or memory requirements.
Use only if you are sure what you are doing!
Not that this setting also affects iteration count for the key digest.
For LUKS1 iteration count for digest will be approximately 1/8 of requested
value, for LUKS2 and "pbkdf2" digest minimal PBKDF2 iteration count (1000)
will be used. You cannot set lower iteration count than the internal minimum
(1000 for PBKDF2).
To format LUKS1 device with forced iteration count (and no benchmarking), use
$ cryptsetup luksFormat <device> --pbkdf-force-iterations 22222
For LUKS2 it is always better to specify full settings (do not rely on default
For example, we can set to use Argon2id with iteration cost 5, memory 128000
and paralell set 1:
$ cryptsetup luksFormat --type luks2 <device> \
--pbkdf argon2id --pbkdf-force-iterations 5 --pbkdf-memory 128000 --pbkdf-parallel 1
* VeraCrypt PIM
Cryptsetup can now also open VeraCrypt device that uses Personal Iteration
Multiplier (PIM). PIM is an integer value that user must remember additionally
to passphrase and influences PBKDF2 iteration count (without it VeraCrypt uses
a fixed number of iterations).
To open VeraCrypt device with PIM settings, use --veracrypt-pim (to specify
PIM on the command line) or --veracrypt-query-pim to query PIM interactively.
* Support for plain64be IV
The plain64be is big-endian variant of plain64 Initialization Vector. It is
used in some images of hardware-based disk encryption systems. Supporting this
variant allows using dm-crypt to map such images through cryptsetup.
* Deferral removal
Cryptsetup now can mark device for deferred removal by using a new option
--deferred. This means that close command will not fail if the device is still
in use, but will instruct the kernel to remove the device automatically after
use count drops to zero (for example, once the filesystem is unmounted).
* A lot of updates to man pages and many minor changes that would make this
release notes too long ;-)
Libcryptsetup API changes
These API functions were removed, libcryptsetup no longer handles password
retries from terminal (application should handle terminal operations itself):
This call is removed (no need to keep typo backward compatibility,
the proper function is crypt_set_iteration_time :-)
These calls were removed because are not safe, use per-context
error callbacks instead:
The PBKDF benchmark was replaced by a new function that uses new KDF structure
crypt_benchmark_pbkdf; (new API call)
These new calls are now exported, for details see libcryptsetup.h:
Unfinished things & TODO for next RC or future
* There will be better documentation and examples.
* There will be some more formal definition of the threat model for integrity
protection. (And a link to some papers discussing integrity protection,
once it is, hopefully, accepted and published.)
* Offline re-encrypt tool supports only LUKS1 format for now (patches are
on the way).
* There will be online LUKS2 re-encryption tool in future.
* Authenticated encryption will use new algorithms from CAESAR competition,
once these algorithms are available in kernel.
* Authenticated encryption do not set encryption for dm-integrity journal.
While it does not influence data confidentiality or integrity protection,
an attacker can get some more information from data journal or cause that
system will corrupt sectors after journal replay. (That corruption will be
* Some utilities (blkid, systemd-cryptsetup) will need small updates to support
* There are some examples of user-defined tokens inside misc/luks2_keyslot_example
directory (like a simple external program that uses libssh to unlock LUKS2
using remote keyfile).
We will document these examples later in release notes for next RC.
* The distribution archive is now very big because of some testing images that do not compress
well. Some cleaning is needed here.
* A lot of ideas are hidden inside the LUKS2 design that is not yet used or
described here, let's try if the basics work first :-)
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