1========
2dm-crypt
3========
4
5Device-Mapper's "crypt" target provides transparent encryption of block devices
6using the kernel crypto API.
7
8For a more detailed description of supported parameters see:
9https://gitlab.com/cryptsetup/cryptsetup/wikis/DMCrypt
10
11Parameters::
12
13	      <cipher> <key> <iv_offset> <device path> \
14	      <offset> [<#opt_params> <opt_params>]
15
16<cipher>
17    Encryption cipher, encryption mode and Initial Vector (IV) generator.
18
19    The cipher specifications format is::
20
21       cipher[:keycount]-chainmode-ivmode[:ivopts]
22
23    Examples::
24
25       aes-cbc-essiv:sha256
26       aes-xts-plain64
27       serpent-xts-plain64
28
29    Cipher format also supports direct specification with kernel crypt API
30    format (selected by capi: prefix). The IV specification is the same
31    as for the first format type.
32    This format is mainly used for specification of authenticated modes.
33
34    The crypto API cipher specifications format is::
35
36        capi:cipher_api_spec-ivmode[:ivopts]
37
38    Examples::
39
40        capi:cbc(aes)-essiv:sha256
41        capi:xts(aes)-plain64
42
43    Examples of authenticated modes::
44
45        capi:gcm(aes)-random
46        capi:authenc(hmac(sha256),xts(aes))-random
47        capi:rfc7539(chacha20,poly1305)-random
48
49    The /proc/crypto contains a list of curently loaded crypto modes.
50
51<key>
52    Key used for encryption. It is encoded either as a hexadecimal number
53    or it can be passed as <key_string> prefixed with single colon
54    character (':') for keys residing in kernel keyring service.
55    You can only use key sizes that are valid for the selected cipher
56    in combination with the selected iv mode.
57    Note that for some iv modes the key string can contain additional
58    keys (for example IV seed) so the key contains more parts concatenated
59    into a single string.
60
61<key_string>
62    The kernel keyring key is identified by string in following format:
63    <key_size>:<key_type>:<key_description>.
64
65<key_size>
66    The encryption key size in bytes. The kernel key payload size must match
67    the value passed in <key_size>.
68
69<key_type>
70    Either 'logon', 'user' or 'encrypted' kernel key type.
71
72<key_description>
73    The kernel keyring key description crypt target should look for
74    when loading key of <key_type>.
75
76<keycount>
77    Multi-key compatibility mode. You can define <keycount> keys and
78    then sectors are encrypted according to their offsets (sector 0 uses key0;
79    sector 1 uses key1 etc.).  <keycount> must be a power of two.
80
81<iv_offset>
82    The IV offset is a sector count that is added to the sector number
83    before creating the IV.
84
85<device path>
86    This is the device that is going to be used as backend and contains the
87    encrypted data.  You can specify it as a path like /dev/xxx or a device
88    number <major>:<minor>.
89
90<offset>
91    Starting sector within the device where the encrypted data begins.
92
93<#opt_params>
94    Number of optional parameters. If there are no optional parameters,
95    the optional paramaters section can be skipped or #opt_params can be zero.
96    Otherwise #opt_params is the number of following arguments.
97
98    Example of optional parameters section:
99        3 allow_discards same_cpu_crypt submit_from_crypt_cpus
100
101allow_discards
102    Block discard requests (a.k.a. TRIM) are passed through the crypt device.
103    The default is to ignore discard requests.
104
105    WARNING: Assess the specific security risks carefully before enabling this
106    option.  For example, allowing discards on encrypted devices may lead to
107    the leak of information about the ciphertext device (filesystem type,
108    used space etc.) if the discarded blocks can be located easily on the
109    device later.
110
111same_cpu_crypt
112    Perform encryption using the same cpu that IO was submitted on.
113    The default is to use an unbound workqueue so that encryption work
114    is automatically balanced between available CPUs.
115
116submit_from_crypt_cpus
117    Disable offloading writes to a separate thread after encryption.
118    There are some situations where offloading write bios from the
119    encryption threads to a single thread degrades performance
120    significantly.  The default is to offload write bios to the same
121    thread because it benefits CFQ to have writes submitted using the
122    same context.
123
124no_read_workqueue
125    Bypass dm-crypt internal workqueue and process read requests synchronously.
126
127no_write_workqueue
128    Bypass dm-crypt internal workqueue and process write requests synchronously.
129    This option is automatically enabled for host-managed zoned block devices
130    (e.g. host-managed SMR hard-disks).
131
132integrity:<bytes>:<type>
133    The device requires additional <bytes> metadata per-sector stored
134    in per-bio integrity structure. This metadata must by provided
135    by underlying dm-integrity target.
136
137    The <type> can be "none" if metadata is used only for persistent IV.
138
139    For Authenticated Encryption with Additional Data (AEAD)
140    the <type> is "aead". An AEAD mode additionally calculates and verifies
141    integrity for the encrypted device. The additional space is then
142    used for storing authentication tag (and persistent IV if needed).
143
144sector_size:<bytes>
145    Use <bytes> as the encryption unit instead of 512 bytes sectors.
146    This option can be in range 512 - 4096 bytes and must be power of two.
147    Virtual device will announce this size as a minimal IO and logical sector.
148
149iv_large_sectors
150   IV generators will use sector number counted in <sector_size> units
151   instead of default 512 bytes sectors.
152
153   For example, if <sector_size> is 4096 bytes, plain64 IV for the second
154   sector will be 8 (without flag) and 1 if iv_large_sectors is present.
155   The <iv_offset> must be multiple of <sector_size> (in 512 bytes units)
156   if this flag is specified.
157
158Example scripts
159===============
160LUKS (Linux Unified Key Setup) is now the preferred way to set up disk
161encryption with dm-crypt using the 'cryptsetup' utility, see
162https://gitlab.com/cryptsetup/cryptsetup
163
164::
165
166	#!/bin/sh
167	# Create a crypt device using dmsetup
168	dmsetup create crypt1 --table "0 `blockdev --getsz $1` crypt aes-cbc-essiv:sha256 babebabebabebabebabebabebabebabe 0 $1 0"
169
170::
171
172	#!/bin/sh
173	# Create a crypt device using dmsetup when encryption key is stored in keyring service
174	dmsetup create crypt2 --table "0 `blockdev --getsize $1` crypt aes-cbc-essiv:sha256 :32:logon:my_prefix:my_key 0 $1 0"
175
176::
177
178	#!/bin/sh
179	# Create a crypt device using cryptsetup and LUKS header with default cipher
180	cryptsetup luksFormat $1
181	cryptsetup luksOpen $1 crypt1
182