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authorAndré Fabian Silva Delgado <emulatorman@parabola.nu>2015-08-05 17:04:01 -0300
committerAndré Fabian Silva Delgado <emulatorman@parabola.nu>2015-08-05 17:04:01 -0300
commit57f0f512b273f60d52568b8c6b77e17f5636edc0 (patch)
tree5e910f0e82173f4ef4f51111366a3f1299037a7b /Documentation/device-mapper/dm-crypt.txt
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+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: <cipher> <key> <iv_offset> <device path> \
+ <offset> [<#opt_params> <opt_params>]
+
+<cipher>
+ 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>
+ 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.
+
+<keycount>
+ Multi-key compatibility mode. You can define <keycount> keys and
+ then sectors are encrypted according to their offsets (sector 0 uses key0;
+ sector 1 uses key1 etc.). <keycount> must be a power of two.
+
+<iv_offset>
+ The IV offset is a sector count that is added to the sector number
+ before creating the IV.
+
+<device path>
+ 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 <major>:<minor>.
+
+<offset>
+ 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.
+
+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
+]]