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 ]]