dm-crypt is a transparent block device encryption

subsystem A system is a group of interacting or interrelated elements that act according to a set of rules to form a unified whole. A system, surrounded and influenced by its environment, is described by its boundaries, structure and purpose and is exp ...

Linux kernel The Linux kernel is a Free and open-source software, free and open source Unix-like kernel (operating system), kernel that is used in many computer systems worldwide. The kernel was created by Linus Torvalds in 1991 and was soon adopted as the k ...

versions 2.6 and later and in

DragonFly BSD DragonFly BSD is a free and open-source Unix-like operating system forked from FreeBSD 4.8. Matthew Dillon, an Amiga developer in the late 1980s and early 1990s and FreeBSD developer between 1994 and 2003, began working on DragonFly BSD in ...

. It is part of the

device mapper The device mapper is a framework provided by the Linux kernel for mapping physical block devices onto higher-level ''virtual block devices''. It forms the foundation of the logical volume manager (LVM), software RAIDs and dm-crypt disk encrypt ...

(dm) infrastructure, and uses cryptographic routines from the kernel's Crypto API. Unlike its predecessor cryptoloop, dm-crypt was designed to support advanced modes of operation, such as XTS, LRW and ESSIV, in order to avoid

watermarking attack In cryptography, a watermarking attack is an attack on disk encryption methods where the presence of a specially crafted piece of data can be detected by an attacker without knowing the encryption key. Problem description Disk encryption suites g ...

s. In addition to that, dm-crypt addresses some reliability problems of cryptoloop. dm-crypt is implemented as a device mapper target and may be stacked on top of other device mapper transformations. It can thus encrypt whole disks (including

removable media In computing, a removable media is a data storage media that is designed to be readily inserted and removed from a system. Most early removable media, such as floppy disks and optical discs, require a dedicated read/write device (i.e. a drive) ...

), partitions, software RAID volumes,

logical volume In computer storage, logical volume management or LVM provides a method of allocating space on mass-storage devices that is more flexible than conventional partitioning schemes to store volumes. In particular, a volume manager can concatenate, ...

s, as well as files. It appears as a block device, which can be used to back file systems, swap or as an LVM

physical volume In computer storage, logical volume management or LVM provides a method of allocating space on mass-storage devices that is more flexible than conventional partitioning schemes to store volumes. In particular, a volume manager can concatenate, ...

. Some

Linux distribution A Linux distribution, often abbreviated as distro, is an operating system that includes the Linux kernel for its kernel functionality. Although the name does not imply product distribution per se, a distro—if distributed on its own—is oft ...

s support the use of dm-crypt on the root file system. These distributions use

initrd In Linux systems, initrd (''initial ramdisk'') is a scheme for loading a temporary root file system into memory, to be used as part of the Linux startup process. initrd and initramfs (from INITial RAM File System) refer to two different methods ...

to prompt the user to enter a passphrase at the console, or insert a

smart card A smart card (SC), chip card, or integrated circuit card (ICC or IC card), is a card used to control access to a resource. It is typically a plastic credit card-sized card with an Embedded system, embedded integrated circuit (IC) chip. Many smart ...

prior to the normal boot process.

Frontends

The dm-crypt device mapper target resides entirely in kernel space, and is only concerned with encryption of the

block device In Unix-like operating systems, a device file, device node, or special file is an interface to a device driver that appears in a file system as if it were an ordinary file. There are also special files in DOS, OS/2, and Windows. These spec ...

it does not interpret any data itself. It relies on

user space A modern computer operating system usually uses virtual memory to provide separate address spaces or regions of a single address space, called user space and kernel space. This separation primarily provides memory protection and hardware prote ...

front-ends to create and activate encrypted volumes, and manage authentication. At least two frontends are currently available: cryptsetup and cryptmount.

cryptsetup

The cryptsetup command-line interface, by default, does not write any headers to the encrypted volume, and hence only provides the bare essentials: encryption settings have to be provided every time the disk is mounted (although usually employed with automated scripts), and only one key can be used per volume; the

symmetric encryption Symmetric-key algorithms are algorithms for cryptography that use the same cryptographic keys for both the encryption of plaintext and the decryption of ciphertext. The keys may be identical, or there may be a simple transformation to go between t ...

key is directly derived from the supplied

passphrase A passphrase is a sequence of words or other text used to control access to a computer system, program or data. It is similar to a password in usage, but a passphrase is generally longer for added security. Passphrases are often used to control ...

. Because it lacks a "

salt In common usage, salt is a mineral composed primarily of sodium chloride (NaCl). When used in food, especially in granulated form, it is more formally called table salt. In the form of a natural crystalline mineral, salt is also known as r ...

", using cryptsetup is less secure in this mode than is the case with

Linux Unified Key Setup The Linux Unified Key Setup (LUKS) is a disk encryption specification created by Clemens Fruhwirth in 2004 and originally intended for Linux. LUKS implements a platform-independent standard on-disk format for use in various tools. This facilitate ...

(LUKS). However, the simplicity of cryptsetup makes it useful when combined with third-party software, for example, with

authentication. cryptsetup also provides commands to deal with the LUKS on-disk format. This format provides additional features such as

key management Key management refers to management of Key (cryptography), cryptographic keys in a cryptosystem. This includes dealing with the generation, exchange, storage, use, crypto-shredding (destruction) and replacement of keys. It includes cryptographic ...

and

key stretching In cryptography, key stretching techniques are used to make a possibly weak key, typically a password or passphrase, more secure against a brute-force attack by increasing the resources (time and possibly space) it takes to test each possible ke ...

(using

PBKDF2 In cryptography, PBKDF1 and PBKDF2 (Password-Based Key Derivation Function 1 and 2) are key derivation functions with a sliding computational cost, used to reduce vulnerability to brute-force attacks. PBKDF2 is part of RSA Laboratories' Public- ...

), and remembers encrypted volume configuration across reboots.

cryptmount

The cryptmount interface is an alternative to the "cryptsetup" tool that allows any user to

mount Mount is often used as part of the name of specific mountains, e.g. Mount Everest. Mount or Mounts may also refer to: Places * Mount, Cornwall, a village in Warleggan parish, England * Mount, Perranzabuloe, a hamlet in Perranzabuloe parish, ...

and unmount a dm-crypt file system when needed, without needing

superuser In computing, the superuser is a special user account used for system administration. Depending on the operating system (OS), the actual name of this account might be root, administrator, admin or supervisor. In some cases, the actual name of the ...

privileges after the device has been configured by a superuser.

Features

The fact that disk encryption (volume encryption) software like dm-crypt only deals with transparent encryption of abstract

s gives it a lot of flexibility. This means that it can be used for encrypting any disk-backed file systems supported by the

operating system An operating system (OS) is system software that manages computer hardware and software resources, and provides common daemon (computing), services for computer programs. Time-sharing operating systems scheduler (computing), schedule tasks for ...

, as well as

swap space In computer operating systems, memory paging is a memory management scheme that allows the physical memory used by a program to be non-contiguous. This also helps avoid the problem of memory fragmentation and requiring compaction to reduce fra ...

;

write barrier In operating systems, write barrier is a mechanism for enforcing a particular ordering in a sequence of writes to a storage system in a computer system. For example, a write barrier in a file system is a mechanism (program logic) that ensures that ...

s implemented by file systems are preserved. Encrypted volumes can be stored on

disk partition Disk partitioning or disk slicing is the creation of one or more regions on Computer data storage#Secondary storage, secondary storage, so that each region can be managed separately. These regions are called partitions. It is typically the first ...

s, whole disks as well as file-backed

disk image A disk image is a snapshot of a storage device's content typically stored in a file on another storage device. Traditionally, a disk image was relatively large because it was a bit-by-bit copy of every storage location of a device (i.e. every ...

s (through the use of

loop device In Unix-like operating systems, a loop device, vnd (vnode disk), or lofi (loop file interface) is a pseudo-device that makes a computer file accessible as a block device. Before use, a loop device must be connected to an existent file in the fil ...

s with the losetup utility). dm-crypt can also be configured to encrypt

RAID RAID (; redundant array of inexpensive disks or redundant array of independent disks) is a data storage virtualization technology that combines multiple physical Computer data storage, data storage components into one or more logical units for th ...

volumes and LVM physical volumes. dm-crypt can also be configured to provide pre-boot authentication through an

, thus encrypting all the data on a computer except the bootloader, the kernel and the initrd image itself. When using the

cipher block chaining In cryptography, a block cipher mode of operation is an algorithm that uses a block cipher to provide information security such as confidentiality or authenticity. A block cipher by itself is only suitable for the secure cryptographic transform ...

(CBC) mode of operation with predictable

initialization vector In cryptography, an initialization vector (IV) or starting variable is an input to a cryptographic primitive being used to provide the initial state. The IV is typically required to be random or pseudorandom, but sometimes an IV only needs to be un ...

s as other disk encryption software, the disk is vulnerable to

s. This means that an attacker is able to detect the presence of specially crafted data on the disk. To address this problem in its predecessors, dm-crypt included provisions for more elaborate, disk encryption-specific modes of operation. Support for ESSIV (encrypted salt-sector initialization vector) was introduced in Linux kernel version 2.6.10, LRW in 2.6.20 and XTS in 2.6.24. A wide-block disk encryption algorithm,

Adiantum ''Adiantum'' (), the maidenhair fern (not to be confused with the similar-looking maidenhair spleenwort fern), is a genus of about 250 species of ferns in the subfamily Vittarioideae of the family Pteridaceae, though some researchers place it ...

, was added in 5.0, and its AES-based cousin HCTR2 in 6.0. The Linux Crypto API includes support for most popular

block cipher In cryptography, a block cipher is a deterministic algorithm that operates on fixed-length groups of bits, called ''blocks''. Block ciphers are the elementary building blocks of many cryptographic protocols. They are ubiquitous in the storage a ...

s and

hash function A hash function is any Function (mathematics), function that can be used to map data (computing), data of arbitrary size to fixed-size values, though there are some hash functions that support variable-length output. The values returned by a ...

s, which are all usable with dm-crypt. Crypted FS support include LUKS (versions 1 and 2) volumes, loop-AES,

TrueCrypt TrueCrypt is a discontinued source-available freeware utility software, utility used for on-the-fly encryption (OTFE). It can create a virtual encrypted disk within a file, encrypt a Disk partitioning, partition, or encrypt the whole Data storag ...

VeraCrypt VeraCrypt is a free and open-source utility for on-the-fly encryption (OTFE). The software can create a virtual encrypted disk that works just like a regular disk but within a file. It can also encrypt a partition or (in Windows) the entire sto ...

(since Linux kernel 3.13), and

BitLocker BitLocker is a full volume encryption feature included with Microsoft Windows versions starting with Windows Vista. It is designed to protect data by providing encryption for entire volumes. By default, it uses the Advanced Encryption Standard ...

-encrypted NTFS (since cryptsetup 2.3.0). TrueCrypt/VeraCrypt (TCRYPT) and BitLocker (BITLK) support require the kernel userspace crypto API.

Compatibility

dm-crypt and LUKS encrypted disks can be accessed and used under MS Windows using the now defunct

FreeOTFE FreeOTFE is a discontinued open source computer program for on-the-fly disk encryption (OTFE). On Microsoft Windows, and Windows Mobile (using FreeOTFE4PDA), it can create a virtual drive within a file or partition, to which anything written i ...

(formerly DoxBox, LibreCrypt), provided that the filesystem used is supported by Windows (e.g.

FAT In nutrition science, nutrition, biology, and chemistry, fat usually means any ester of fatty acids, or a mixture of such chemical compound, compounds, most commonly those that occur in living beings or in food. The term often refers specif ...

/FAT32/

NTFS NT File System (NTFS) (commonly called ''New Technology File System'') is a proprietary journaling file system developed by Microsoft in the 1990s. It was developed to overcome scalability, security and other limitations with File Allocation Tabl ...

). Encrypted

ext2 ext2, or second extended file system, is a file system for the Linux kernel (operating system), kernel. It was initially designed by French software developer Rémy Card as a replacement for the extended file system (ext). Having been designed ...

and

ext3 ext3, or third extended filesystem, is a journaling file system, journaled file system that is commonly used with the Linux kernel. It used to be the default file system for many popular Linux distributions but generally has been supplanted by ...

filesystems are supported by using Ext2Fsd or so-called "Ext2 Installable File System for Windows"; FreeOTFE also supports them. Cryptsetup/LUKS and the required infrastructure have also been implemented on the DragonFly BSD operating system.

References

External links

* Official , and websites
All about dm-crypt and LUKS on one page (on archive.org)
a page covering dm-crypt/LUKS, starting with theory and ending with many practical examples about its usage. {{DEFAULTSORT:Dm-Crypt Device mapper Disk encryption Cryptographic software