1 ========
2 dm-crypt
3 ========
4
5 Device-Mapper's "crypt" target provides transparent encryption of block devices
6 using the kernel crypto API.
7
8 For a more detailed description of supported parameters see:
9 https://gitlab.com/cryptsetup/cryptsetup/wikis/DMCrypt
10
11 Parameters::
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 currently 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', 'encrypted' or 'trusted' 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 parameters 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
101 allow_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
111 same_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
116 high_priority
117 Set dm-crypt workqueues and the writer thread to high priority. This
118 improves throughput and latency of dm-crypt while degrading general
119 responsiveness of the system.
120
121 submit_from_crypt_cpus
122 Disable offloading writes to a separate thread after encryption.
123 There are some situations where offloading write bios from the
124 encryption threads to a single thread degrades performance
125 significantly. The default is to offload write bios to the same
126 thread because it benefits CFQ to have writes submitted using the
127 same context.
128
129 no_read_workqueue
130 Bypass dm-crypt internal workqueue and process read requests synchronously.
131
132 no_write_workqueue
133 Bypass dm-crypt internal workqueue and process write requests synchronously.
134 This option is automatically enabled for host-managed zoned block devices
135 (e.g. host-managed SMR hard-disks).
136
137 integrity:<bytes>:<type>
138 The device requires additional <bytes> metadata per-sector stored
139 in per-bio integrity structure. This metadata must by provided
140 by underlying dm-integrity target.
141
142 The <type> can be "none" if metadata is used only for persistent IV.
143
144 For Authenticated Encryption with Additional Data (AEAD)
145 the <type> is "aead". An AEAD mode additionally calculates and verifies
146 integrity for the encrypted device. The additional space is then
147 used for storing authentication tag (and persistent IV if needed).
148
149 sector_size:<bytes>
150 Use <bytes> as the encryption unit instead of 512 bytes sectors.
151 This option can be in range 512 - 4096 bytes and must be power of two.
152 Virtual device will announce this size as a minimal IO and logical sector.
153
154 iv_large_sectors
155 IV generators will use sector number counted in <sector_size> units
156 instead of default 512 bytes sectors.
157
158 For example, if <sector_size> is 4096 bytes, plain64 IV for the second
159 sector will be 8 (without flag) and 1 if iv_large_sectors is present.
160 The <iv_offset> must be multiple of <sector_size> (in 512 bytes units)
161 if this flag is specified.
162
163 integrity_key_size:<bytes>
164 Use an integrity key of <bytes> size instead of using an integrity key size
165 of the digest size of the used HMAC algorithm.
166
167
168 Module parameters::
169 max_read_size
170 Maximum size of read requests. When a request larger than this size
171 is received, dm-crypt will split the request. The splitting improves
172 concurrency (the split requests could be encrypted in parallel by multiple
173 cores), but it also causes overhead. The user should tune this parameters to
174 fit the actual workload.
175
176 max_write_size
177 Maximum size of write requests. When a request larger than this size
178 is received, dm-crypt will split the request. The splitting improves
179 concurrency (the split requests could be encrypted in parallel by multiple
180 cores), but it also causes overhead. The user should tune this parameters to
181 fit the actual workload.
182
183
184 Example scripts
185 ===============
186 LUKS (Linux Unified Key Setup) is now the preferred way to set up disk
187 encryption with dm-crypt using the 'cryptsetup' utility, see
188 https://gitlab.com/cryptsetup/cryptsetup
189
190 ::
191
192 #!/bin/sh
193 # Create a crypt device using dmsetup
194 dmsetup create crypt1 --table "0 `blockdev --getsz $1` crypt aes-cbc-essiv:sha256 babebabebabebabebabebabebabebabe 0 $1 0"
195
196 ::
197
198 #!/bin/sh
199 # Create a crypt device using dmsetup when encryption key is stored in keyring service
200 dmsetup create crypt2 --table "0 `blockdev --getsize $1` crypt aes-cbc-essiv:sha256 :32:logon:my_prefix:my_key 0 $1 0"
201
202 ::
203
204 #!/bin/sh
205 # Create a crypt device using cryptsetup and LUKS header with default cipher
206 cryptsetup luksFormat $1
207 cryptsetup luksOpen $1 crypt1
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