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Documentation / device-mapper / dm-crypt.txt




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Based on kernel version 4.13.3. Page generated on 2017-09-23 13:54 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	https://gitlab.com/cryptsetup/cryptsetup/wikis/DMCrypt
9	
10	Parameters: <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	
90	allow_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	
100	same_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	
105	submit_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	
113	integrity:<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	
125	sector_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	
130	iv_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	
139	Example scripts
140	===============
141	LUKS (Linux Unified Key Setup) is now the preferred way to set up disk
142	encryption with dm-crypt using the 'cryptsetup' utility, see
143	https://gitlab.com/cryptsetup/cryptsetup
144	
145	[[
146	#!/bin/sh
147	# Create a crypt device using dmsetup
148	dmsetup 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
154	dmsetup 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
160	cryptsetup luksFormat $1
161	cryptsetup luksOpen $1 crypt1
162	]]
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