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Documentation / device-mapper / dm-crypt.txt

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Based on kernel version 4.0. Page generated on 2015-04-14 21:20 EST.

1	dm-crypt
2	=========
4	Device-Mapper's "crypt" target provides transparent encryption of block devices
5	using the kernel crypto API.
7	For a more detailed description of supported parameters see:
8	http://code.google.com/p/cryptsetup/wiki/DMCrypt
10	Parameters: <cipher> <key> <iv_offset> <device path> \
11		      <offset> [<#opt_params> <opt_params>]
13	<cipher>
14	    Encryption cipher and an optional IV generation mode.
15	    (In format cipher[:keycount]-chainmode-ivmode[:ivopts]).
16	    Examples:
17	       des
18	       aes-cbc-essiv:sha256
19	       twofish-ecb
21	    /proc/crypto contains supported crypto modes
23	<key>
24	    Key used for encryption. It is encoded as a hexadecimal number.
25	    You can only use key sizes that are valid for the selected cipher
26	    in combination with the selected iv mode.
27	    Note that for some iv modes the key string can contain additional
28	    keys (for example IV seed) so the key contains more parts concatenated
29	    into a single string.
31	<keycount>
32	    Multi-key compatibility mode. You can define <keycount> keys and
33	    then sectors are encrypted according to their offsets (sector 0 uses key0;
34	    sector 1 uses key1 etc.).  <keycount> must be a power of two.
36	<iv_offset>
37	    The IV offset is a sector count that is added to the sector number
38	    before creating the IV.
40	<device path>
41	    This is the device that is going to be used as backend and contains the
42	    encrypted data.  You can specify it as a path like /dev/xxx or a device
43	    number <major>:<minor>.
45	<offset>
46	    Starting sector within the device where the encrypted data begins.
48	<#opt_params>
49	    Number of optional parameters. If there are no optional parameters,
50	    the optional paramaters section can be skipped or #opt_params can be zero.
51	    Otherwise #opt_params is the number of following arguments.
53	    Example of optional parameters section:
54	        3 allow_discards same_cpu_crypt submit_from_crypt_cpus
56	allow_discards
57	    Block discard requests (a.k.a. TRIM) are passed through the crypt device.
58	    The default is to ignore discard requests.
60	    WARNING: Assess the specific security risks carefully before enabling this
61	    option.  For example, allowing discards on encrypted devices may lead to
62	    the leak of information about the ciphertext device (filesystem type,
63	    used space etc.) if the discarded blocks can be located easily on the
64	    device later.
66	same_cpu_crypt
67	    Perform encryption using the same cpu that IO was submitted on.
68	    The default is to use an unbound workqueue so that encryption work
69	    is automatically balanced between available CPUs.
71	submit_from_crypt_cpus
72	    Disable offloading writes to a separate thread after encryption.
73	    There are some situations where offloading write bios from the
74	    encryption threads to a single thread degrades performance
75	    significantly.  The default is to offload write bios to the same
76	    thread because it benefits CFQ to have writes submitted using the
77	    same context.
79	Example scripts
80	===============
81	LUKS (Linux Unified Key Setup) is now the preferred way to set up disk
82	encryption with dm-crypt using the 'cryptsetup' utility, see
83	http://code.google.com/p/cryptsetup/
85	[[
86	#!/bin/sh
87	# Create a crypt device using dmsetup
88	dmsetup create crypt1 --table "0 `blockdev --getsize $1` crypt aes-cbc-essiv:sha256 babebabebabebabebabebabebabebabe 0 $1 0"
89	]]
91	[[
92	#!/bin/sh
93	# Create a crypt device using cryptsetup and LUKS header with default cipher
94	cryptsetup luksFormat $1
95	cryptsetup luksOpen $1 crypt1
96	]]
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