[deliverable/linux.git] / Documentation / device-mapper / dm-crypt.txt

dm-crypt
=========

Device-Mapper's "crypt" target provides transparent encryption of block devices
using the kernel crypto API.

Parameters: <cipher> <key> <iv_offset> <device path> \
	      <offset> [<#opt_params> <opt_params>]

<cipher>
    Encryption cipher and an optional IV generation mode.
    (In format cipher[:keycount]-chainmode-ivopts:ivmode).
    Examples:
       des
       aes-cbc-essiv:sha256
       twofish-ecb

    /proc/crypto contains supported crypto modes

<key>
    Key used for encryption. It is encoded as a hexadecimal number.
    You can only use key sizes that are valid for the selected cipher.

<keycount>
    Multi-key compatibility mode. You can define <keycount> keys and
    then sectors are encrypted according to their offsets (sector 0 uses key0;
    sector 1 uses key1 etc.).  <keycount> must be a power of two.

<iv_offset>
    The IV offset is a sector count that is added to the sector number
    before creating the IV.

<device path>
    This is the device that is going to be used as backend and contains the
    encrypted data.  You can specify it as a path like /dev/xxx or a device
    number <major>:<minor>.

<offset>
    Starting sector within the device where the encrypted data begins.

<#opt_params>
    Number of optional parameters. If there are no optional parameters,
    the optional paramaters section can be skipped or #opt_params can be zero.
    Otherwise #opt_params is the number of following arguments.

    Example of optional parameters section:
        1 allow_discards

allow_discards
    Block discard requests (a.k.a. TRIM) are passed through the crypt device.
    The default is to ignore discard requests.

    WARNING: Assess the specific security risks carefully before enabling this
    option.  For example, allowing discards on encrypted devices may lead to
    the leak of information about the ciphertext device (filesystem type,
    used space etc.) if the discarded blocks can be located easily on the
    device later.

Example scripts
===============
LUKS (Linux Unified Key Setup) is now the preferred way to set up disk
encryption with dm-crypt using the 'cryptsetup' utility, see
http://code.google.com/p/cryptsetup/

[[
#!/bin/sh
# Create a crypt device using dmsetup
dmsetup create crypt1 --table "0 `blockdev --getsize $1` crypt aes-cbc-essiv:sha256 babebabebabebabebabebabebabebabe 0 $1 0"
]]

[[
#!/bin/sh
# Create a crypt device using cryptsetup and LUKS header with default cipher
cryptsetup luksFormat $1
cryptsetup luksOpen $1 crypt1
]]
Commit	Line	Data
e3dcc5a3 MB	1	dm-crypt
	2	=========
	3
	4	Device-Mapper's "crypt" target provides transparent encryption of block devices
	5	using the kernel crypto API.
	6
772ae5f5 MB	7	Parameters: <cipher> <key> <iv_offset> <device path> \
772ae5f5 MB	8	<offset> [<#opt_params> <opt_params>]
e3dcc5a3 MB	9
	10	<cipher>
	11	Encryption cipher and an optional IV generation mode.
d1f96423	12	(In format cipher[:keycount]-chainmode-ivopts:ivmode).
e3dcc5a3 MB	13	Examples:
	14	des
	15	aes-cbc-essiv:sha256
	16	twofish-ecb
	17
	18	/proc/crypto contains supported crypto modes
	19
	20	<key>
	21	Key used for encryption. It is encoded as a hexadecimal number.
	22	You can only use key sizes that are valid for the selected cipher.
	23
d1f96423 MB	24	<keycount>
	25	Multi-key compatibility mode. You can define <keycount> keys and
	26	then sectors are encrypted according to their offsets (sector 0 uses key0;
	27	sector 1 uses key1 etc.). <keycount> must be a power of two.
	28
e3dcc5a3 MB	29	<iv_offset>
	30	The IV offset is a sector count that is added to the sector number
	31	before creating the IV.
	32
	33	<device path>
	34	This is the device that is going to be used as backend and contains the
	35	encrypted data. You can specify it as a path like /dev/xxx or a device
	36	number <major>:<minor>.
	37
	38	<offset>
	39	Starting sector within the device where the encrypted data begins.
	40
772ae5f5 MB	41	<#opt_params>
	42	Number of optional parameters. If there are no optional parameters,
	43	the optional paramaters section can be skipped or #opt_params can be zero.
	44	Otherwise #opt_params is the number of following arguments.
	45
	46	Example of optional parameters section:
	47	1 allow_discards
	48
	49	allow_discards
	50	Block discard requests (a.k.a. TRIM) are passed through the crypt device.
	51	The default is to ignore discard requests.
	52
	53	WARNING: Assess the specific security risks carefully before enabling this
	54	option. For example, allowing discards on encrypted devices may lead to
	55	the leak of information about the ciphertext device (filesystem type,
	56	used space etc.) if the discarded blocks can be located easily on the
	57	device later.
	58
e3dcc5a3 MB	59	Example scripts
	60	===============
	61	LUKS (Linux Unified Key Setup) is now the preferred way to set up disk
	62	encryption with dm-crypt using the 'cryptsetup' utility, see
adc0485b	63	http://code.google.com/p/cryptsetup/
e3dcc5a3 MB	64
	65	[[
	66	#!/bin/sh
	67	# Create a crypt device using dmsetup
	68	dmsetup create crypt1 --table "0 `blockdev --getsize $1` crypt aes-cbc-essiv:sha256 babebabebabebabebabebabebabebabe 0 $1 0"
	69	]]
	70
	71	[[
	72	#!/bin/sh
	73	# Create a crypt device using cryptsetup and LUKS header with default cipher
	74	cryptsetup luksFormat $1
	75	cryptsetup luksOpen $1 crypt1
	76	]]