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Documentation / device-mapper / dm-crypt.txt




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Based on kernel version 3.16. Page generated on 2014-08-06 21:36 EST.

1	dm-crypt
2	=========
3	
4	Device-Mapper's "crypt" target provides transparent encryption of block devices
5	using the kernel crypto API.
6	
7	For a more detailed description of supported parameters see:
8	http://code.google.com/p/cryptsetup/wiki/DMCrypt
9	
10	Parameters: <cipher> <key> <iv_offset> <device path> \
11		      <offset> [<#opt_params> <opt_params>]
12	
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
20	
21	    /proc/crypto contains supported crypto modes
22	
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.
30	
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.
35	
36	<iv_offset>
37	    The IV offset is a sector count that is added to the sector number
38	    before creating the IV.
39	
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>.
44	
45	<offset>
46	    Starting sector within the device where the encrypted data begins.
47	
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.
52	
53	    Example of optional parameters section:
54	        1 allow_discards
55	
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.
59	
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.
65	
66	Example scripts
67	===============
68	LUKS (Linux Unified Key Setup) is now the preferred way to set up disk
69	encryption with dm-crypt using the 'cryptsetup' utility, see
70	http://code.google.com/p/cryptsetup/
71	
72	[[
73	#!/bin/sh
74	# Create a crypt device using dmsetup
75	dmsetup create crypt1 --table "0 `blockdev --getsize $1` crypt aes-cbc-essiv:sha256 babebabebabebabebabebabebabebabe 0 $1 0"
76	]]
77	
78	[[
79	#!/bin/sh
80	# Create a crypt device using cryptsetup and LUKS header with default cipher
81	cryptsetup luksFormat $1
82	cryptsetup luksOpen $1 crypt1
83	]]
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