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' or 'user' 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 124integrity:<bytes>:<type> 125 The device requires additional <bytes> metadata per-sector stored 126 in per-bio integrity structure. This metadata must by provided 127 by underlying dm-integrity target. 128 129 The <type> can be "none" if metadata is used only for persistent IV. 130 131 For Authenticated Encryption with Additional Data (AEAD) 132 the <type> is "aead". An AEAD mode additionally calculates and verifies 133 integrity for the encrypted device. The additional space is then 134 used for storing authentication tag (and persistent IV if needed). 135 136sector_size:<bytes> 137 Use <bytes> as the encryption unit instead of 512 bytes sectors. 138 This option can be in range 512 - 4096 bytes and must be power of two. 139 Virtual device will announce this size as a minimal IO and logical sector. 140 141iv_large_sectors 142 IV generators will use sector number counted in <sector_size> units 143 instead of default 512 bytes sectors. 144 145 For example, if <sector_size> is 4096 bytes, plain64 IV for the second 146 sector will be 8 (without flag) and 1 if iv_large_sectors is present. 147 The <iv_offset> must be multiple of <sector_size> (in 512 bytes units) 148 if this flag is specified. 149 150Example scripts 151=============== 152LUKS (Linux Unified Key Setup) is now the preferred way to set up disk 153encryption with dm-crypt using the 'cryptsetup' utility, see 154https://gitlab.com/cryptsetup/cryptsetup 155 156:: 157 158 #!/bin/sh 159 # Create a crypt device using dmsetup 160 dmsetup create crypt1 --table "0 `blockdev --getsz $1` crypt aes-cbc-essiv:sha256 babebabebabebabebabebabebabebabe 0 $1 0" 161 162:: 163 164 #!/bin/sh 165 # Create a crypt device using dmsetup when encryption key is stored in keyring service 166 dmsetup create crypt2 --table "0 `blockdev --getsize $1` crypt aes-cbc-essiv:sha256 :32:logon:my_prefix:my_key 0 $1 0" 167 168:: 169 170 #!/bin/sh 171 # Create a crypt device using cryptsetup and LUKS header with default cipher 172 cryptsetup luksFormat $1 173 cryptsetup luksOpen $1 crypt1 174