tjh.dev
Linux kernel mirror (for testing)
git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
kernel
os
linux
1dm-crypt
2=========
3
4Device-Mapper's "crypt" target provides transparent encryption of block devices
5using the kernel crypto API.
6
7For a more detailed description of supported parameters see:
8http://code.google.com/p/cryptsetup/wiki/DMCrypt
9
10Parameters: <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
56allow_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
66Example scripts
67===============
68LUKS (Linux Unified Key Setup) is now the preferred way to set up disk
69encryption with dm-crypt using the 'cryptsetup' utility, see
70http://code.google.com/p/cryptsetup/
71
72[[
73#!/bin/sh
74# Create a crypt device using dmsetup
75dmsetup 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
81cryptsetup luksFormat $1
82cryptsetup luksOpen $1 crypt1
83]]