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:
8https://gitlab.com/cryptsetup/cryptsetup/wikis/DMCrypt
9
10Parameters: <cipher> <key> <iv_offset> <device path> \
11	      <offset> [<#opt_params> <opt_params>]
12
13<cipher>
14    Encryption cipher, encryption mode and Initial Vector (IV) generator.
15
16    The cipher specifications format is:
17       cipher[:keycount]-chainmode-ivmode[:ivopts]
18    Examples:
19       aes-cbc-essiv:sha256
20       aes-xts-plain64
21       serpent-xts-plain64
22
23    Cipher format also supports direct specification with kernel crypt API
24    format (selected by capi: prefix). The IV specification is the same
25    as for the first format type.
26    This format is mainly used for specification of authenticated modes.
27
28    The crypto API cipher specifications format is:
29        capi:cipher_api_spec-ivmode[:ivopts]
30    Examples:
31        capi:cbc(aes)-essiv:sha256
32        capi:xts(aes)-plain64
33    Examples of authenticated modes:
34        capi:gcm(aes)-random
35        capi:authenc(hmac(sha256),xts(aes))-random
36        capi:rfc7539(chacha20,poly1305)-random
37
38    The /proc/crypto contains a list of curently loaded crypto modes.
39
40<key>
41    Key used for encryption. It is encoded either as a hexadecimal number
42    or it can be passed as <key_string> prefixed with single colon
43    character (':') for keys residing in kernel keyring service.
44    You can only use key sizes that are valid for the selected cipher
45    in combination with the selected iv mode.
46    Note that for some iv modes the key string can contain additional
47    keys (for example IV seed) so the key contains more parts concatenated
48    into a single string.
49
50<key_string>
51    The kernel keyring key is identified by string in following format:
52    <key_size>:<key_type>:<key_description>.
53
54<key_size>
55    The encryption key size in bytes. The kernel key payload size must match
56    the value passed in <key_size>.
57
58<key_type>
59    Either 'logon' or 'user' kernel key type.
60
61<key_description>
62    The kernel keyring key description crypt target should look for
63    when loading key of <key_type>.
64
65<keycount>
66    Multi-key compatibility mode. You can define <keycount> keys and
67    then sectors are encrypted according to their offsets (sector 0 uses key0;
68    sector 1 uses key1 etc.).  <keycount> must be a power of two.
69
70<iv_offset>
71    The IV offset is a sector count that is added to the sector number
72    before creating the IV.
73
74<device path>
75    This is the device that is going to be used as backend and contains the
76    encrypted data.  You can specify it as a path like /dev/xxx or a device
77    number <major>:<minor>.
78
79<offset>
80    Starting sector within the device where the encrypted data begins.
81
82<#opt_params>
83    Number of optional parameters. If there are no optional parameters,
84    the optional paramaters section can be skipped or #opt_params can be zero.
85    Otherwise #opt_params is the number of following arguments.
86
87    Example of optional parameters section:
88        3 allow_discards same_cpu_crypt submit_from_crypt_cpus
89
90allow_discards
91    Block discard requests (a.k.a. TRIM) are passed through the crypt device.
92    The default is to ignore discard requests.
93
94    WARNING: Assess the specific security risks carefully before enabling this
95    option.  For example, allowing discards on encrypted devices may lead to
96    the leak of information about the ciphertext device (filesystem type,
97    used space etc.) if the discarded blocks can be located easily on the
98    device later.
99
100same_cpu_crypt
101    Perform encryption using the same cpu that IO was submitted on.
102    The default is to use an unbound workqueue so that encryption work
103    is automatically balanced between available CPUs.
104
105submit_from_crypt_cpus
106    Disable offloading writes to a separate thread after encryption.
107    There are some situations where offloading write bios from the
108    encryption threads to a single thread degrades performance
109    significantly.  The default is to offload write bios to the same
110    thread because it benefits CFQ to have writes submitted using the
111    same context.
112
113integrity:<bytes>:<type>
114    The device requires additional <bytes> metadata per-sector stored
115    in per-bio integrity structure. This metadata must by provided
116    by underlying dm-integrity target.
117
118    The <type> can be "none" if metadata is used only for persistent IV.
119
120    For Authenticated Encryption with Additional Data (AEAD)
121    the <type> is "aead". An AEAD mode additionally calculates and verifies
122    integrity for the encrypted device. The additional space is then
123    used for storing authentication tag (and persistent IV if needed).
124
125sector_size:<bytes>
126    Use <bytes> as the encryption unit instead of 512 bytes sectors.
127    This option can be in range 512 - 4096 bytes and must be power of two.
128    Virtual device will announce this size as a minimal IO and logical sector.
129
130iv_large_sectors
131   IV generators will use sector number counted in <sector_size> units
132   instead of default 512 bytes sectors.
133
134   For example, if <sector_size> is 4096 bytes, plain64 IV for the second
135   sector will be 8 (without flag) and 1 if iv_large_sectors is present.
136   The <iv_offset> must be multiple of <sector_size> (in 512 bytes units)
137   if this flag is specified.
138
139Example scripts
140===============
141LUKS (Linux Unified Key Setup) is now the preferred way to set up disk
142encryption with dm-crypt using the 'cryptsetup' utility, see
143https://gitlab.com/cryptsetup/cryptsetup
144
145[[
146#!/bin/sh
147# Create a crypt device using dmsetup
148dmsetup create crypt1 --table "0 `blockdev --getsz $1` crypt aes-cbc-essiv:sha256 babebabebabebabebabebabebabebabe 0 $1 0"
149]]
150
151[[
152#!/bin/sh
153# Create a crypt device using dmsetup when encryption key is stored in keyring service
154dmsetup create crypt2 --table "0 `blockdev --getsize $1` crypt aes-cbc-essiv:sha256 :32:logon:my_prefix:my_key 0 $1 0"
155]]
156
157[[
158#!/bin/sh
159# Create a crypt device using cryptsetup and LUKS header with default cipher
160cryptsetup luksFormat $1
161cryptsetup luksOpen $1 crypt1
162]]
163