Based on kernel version 4.2. Page generated on 2015-09-09 12:08 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 and an optional IV generation mode. 15 (In format cipher[:keycount]-chainmode-ivmode[:ivopts]). 16 Examples: 17 des 18 aes-cbc-essiv:sha256 19 twofish-ecb 20 21 /proc/crypto contains supported crypto modes 22 23 <key> 24 Key used for encryption. It is encoded as a hexadecimal number. 25 You can only use key sizes that are valid for the selected cipher 26 in combination with the selected iv mode. 27 Note that for some iv modes the key string can contain additional 28 keys (for example IV seed) so the key contains more parts concatenated 29 into a single string. 30 31 <keycount> 32 Multi-key compatibility mode. You can define <keycount> keys and 33 then sectors are encrypted according to their offsets (sector 0 uses key0; 34 sector 1 uses key1 etc.). <keycount> must be a power of two. 35 36 <iv_offset> 37 The IV offset is a sector count that is added to the sector number 38 before creating the IV. 39 40 <device path> 41 This is the device that is going to be used as backend and contains the 42 encrypted data. You can specify it as a path like /dev/xxx or a device 43 number <major>:<minor>. 44 45 <offset> 46 Starting sector within the device where the encrypted data begins. 47 48 <#opt_params> 49 Number of optional parameters. If there are no optional parameters, 50 the optional paramaters section can be skipped or #opt_params can be zero. 51 Otherwise #opt_params is the number of following arguments. 52 53 Example of optional parameters section: 54 3 allow_discards same_cpu_crypt submit_from_crypt_cpus 55 56 allow_discards 57 Block discard requests (a.k.a. TRIM) are passed through the crypt device. 58 The default is to ignore discard requests. 59 60 WARNING: Assess the specific security risks carefully before enabling this 61 option. For example, allowing discards on encrypted devices may lead to 62 the leak of information about the ciphertext device (filesystem type, 63 used space etc.) if the discarded blocks can be located easily on the 64 device later. 65 66 same_cpu_crypt 67 Perform encryption using the same cpu that IO was submitted on. 68 The default is to use an unbound workqueue so that encryption work 69 is automatically balanced between available CPUs. 70 71 submit_from_crypt_cpus 72 Disable offloading writes to a separate thread after encryption. 73 There are some situations where offloading write bios from the 74 encryption threads to a single thread degrades performance 75 significantly. The default is to offload write bios to the same 76 thread because it benefits CFQ to have writes submitted using the 77 same context. 78 79 Example scripts 80 =============== 81 LUKS (Linux Unified Key Setup) is now the preferred way to set up disk 82 encryption with dm-crypt using the 'cryptsetup' utility, see 83 https://gitlab.com/cryptsetup/cryptsetup 84 85 [[ 86 #!/bin/sh 87 # Create a crypt device using dmsetup 88 dmsetup create crypt1 --table "0 `blockdev --getsize $1` crypt aes-cbc-essiv:sha256 babebabebabebabebabebabebabebabe 0 $1 0" 89 ]] 90 91 [[ 92 #!/bin/sh 93 # Create a crypt device using cryptsetup and LUKS header with default cipher 94 cryptsetup luksFormat $1 95 cryptsetup luksOpen $1 crypt1 96 ]]