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 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 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 116high_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 121submit_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 129no_read_workqueue 130 Bypass dm-crypt internal workqueue and process read requests synchronously. 131 132no_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 137integrity:<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 149integrity_key_size:<bytes> 150 Optionally set the integrity key size if it differs from the digest size. 151 It allows the use of wrapped key algorithms where the key size is 152 independent of the cryptographic key size. 153 154sector_size:<bytes> 155 Use <bytes> as the encryption unit instead of 512 bytes sectors. 156 This option can be in range 512 - 4096 bytes and must be power of two. 157 Virtual device will announce this size as a minimal IO and logical sector. 158 159iv_large_sectors 160 IV generators will use sector number counted in <sector_size> units 161 instead of default 512 bytes sectors. 162 163 For example, if <sector_size> is 4096 bytes, plain64 IV for the second 164 sector will be 8 (without flag) and 1 if iv_large_sectors is present. 165 The <iv_offset> must be multiple of <sector_size> (in 512 bytes units) 166 if this flag is specified. 167 168integrity_key_size:<bytes> 169 Use an integrity key of <bytes> size instead of using an integrity key size 170 of the digest size of the used HMAC algorithm. 171 172 173Module parameters:: 174 max_read_size 175 Maximum size of read requests. When a request larger than this size 176 is received, dm-crypt will split the request. The splitting improves 177 concurrency (the split requests could be encrypted in parallel by multiple 178 cores), but it also causes overhead. The user should tune this parameters to 179 fit the actual workload. 180 181 max_write_size 182 Maximum size of write requests. When a request larger than this size 183 is received, dm-crypt will split the request. The splitting improves 184 concurrency (the split requests could be encrypted in parallel by multiple 185 cores), but it also causes overhead. The user should tune this parameters to 186 fit the actual workload. 187 188 189Example scripts 190=============== 191LUKS (Linux Unified Key Setup) is now the preferred way to set up disk 192encryption with dm-crypt using the 'cryptsetup' utility, see 193https://gitlab.com/cryptsetup/cryptsetup 194 195:: 196 197 #!/bin/sh 198 # Create a crypt device using dmsetup 199 dmsetup create crypt1 --table "0 `blockdev --getsz $1` crypt aes-cbc-essiv:sha256 babebabebabebabebabebabebabebabe 0 $1 0" 200 201:: 202 203 #!/bin/sh 204 # Create a crypt device using dmsetup when encryption key is stored in keyring service 205 dmsetup create crypt2 --table "0 `blockdev --getsize $1` crypt aes-cbc-essiv:sha256 :32:logon:my_prefix:my_key 0 $1 0" 206 207:: 208 209 #!/bin/sh 210 # Create a crypt device using cryptsetup and LUKS header with default cipher 211 cryptsetup luksFormat $1 212 cryptsetup luksOpen $1 crypt1 213