Kernel-3.10.0-957.el7_dm-crypt

dm-crypt

Device-Mapper’s “crypt” target provides transparent encryption of block devices
using the kernel crypto API.

For a more detailed description of supported parameters see:
https://gitlab.com/cryptsetup/cryptsetup/wikis/DMCrypt

Parameters:
[<#opt_params> ]

Encryption cipher and an optional IV generation mode. (In format cipher[:keycount]-chainmode-ivmode[:ivopts]). Examples: des aes-cbc-essiv:sha256 twofish-ecb

/proc/crypto contains supported crypto modes

Key used for encryption. It is encoded as a hexadecimal number. You can only use key sizes that are valid for the selected cipher in combination with the selected iv mode. Note that for some iv modes the key string can contain additional keys (for example IV seed) so the key contains more parts concatenated into a single string. Multi-key compatibility mode. You can define keys and then sectors are encrypted according to their offsets (sector 0 uses key0; sector 1 uses key1 etc.). must be a power of two. The IV offset is a sector count that is added to the sector number before creating the IV. This is the device that is going to be used as backend and contains the encrypted data. You can specify it as a path like /dev/xxx or a device number :. Starting sector within the device where the encrypted data begins.

<#opt_params>
Number of optional parameters. If there are no optional parameters,
the optional paramaters section can be skipped or #opt_params can be zero.
Otherwise #opt_params is the number of following arguments.

Example of optional parameters section:
    3 allow_discards same_cpu_crypt submit_from_crypt_cpus

allow_discards
Block discard requests (a.k.a. TRIM) are passed through the crypt device.
The default is to ignore discard requests.

WARNING: Assess the specific security risks carefully before enabling this
option.  For example, allowing discards on encrypted devices may lead to
the leak of information about the ciphertext device (filesystem type,
used space etc.) if the discarded blocks can be located easily on the
device later.

same_cpu_crypt
Perform encryption using the same cpu that IO was submitted on.
The default is to use an unbound workqueue so that encryption work
is automatically balanced between available CPUs.

submit_from_crypt_cpus
Disable offloading writes to a separate thread after encryption.
There are some situations where offloading write bios from the
encryption threads to a single thread degrades performance
significantly. The default is to offload write bios to the same
thread because it benefits CFQ to have writes submitted using the
same context.

sector_size:
Use as the encryption unit instead of 512 bytes sectors.
This option can be in range 512 - 4096 bytes and must be power of two.
Virtual device will announce this size as a minimal IO and logical sector.

iv_large_sectors
IV generators will use sector number counted in units
instead of default 512 bytes sectors.

For example, if is 4096 bytes, plain64 IV for the second
sector will be 8 (without flag) and 1 if iv_large_sectors is present.
The must be multiple of (in 512 bytes units)
if this flag is specified.

Example scripts

LUKS (Linux Unified Key Setup) is now the preferred way to set up disk
encryption with dm-crypt using the ‘cryptsetup’ utility, see
https://gitlab.com/cryptsetup/cryptsetup

[[
#!/bin/sh

Create a crypt device using dmsetup

dmsetup create crypt1 –table “0 blockdev --getsize $1 crypt aes-cbc-essiv:sha256 babebabebabebabebabebabebabebabe 0 $1 0”
]]

[[
#!/bin/sh

Create a crypt device using cryptsetup and LUKS header with default cipher

cryptsetup luksFormat $1
cryptsetup luksOpen $1 crypt1
]]