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97 lines
3.4 KiB
Plaintext
97 lines
3.4 KiB
Plaintext
dm-crypt
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=========
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Device-Mapper's "crypt" target provides transparent encryption of block devices
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using the kernel crypto API.
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For a more detailed description of supported parameters see:
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https://gitlab.com/cryptsetup/cryptsetup/wikis/DMCrypt
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Parameters: <cipher> <key> <iv_offset> <device path> \
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<offset> [<#opt_params> <opt_params>]
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<cipher>
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Encryption cipher and an optional IV generation mode.
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(In format cipher[:keycount]-chainmode-ivmode[:ivopts]).
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Examples:
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des
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aes-cbc-essiv:sha256
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twofish-ecb
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/proc/crypto contains supported crypto modes
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<key>
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Key used for encryption. It is encoded as a hexadecimal number.
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You can only use key sizes that are valid for the selected cipher
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in combination with the selected iv mode.
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Note that for some iv modes the key string can contain additional
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keys (for example IV seed) so the key contains more parts concatenated
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into a single string.
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<keycount>
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Multi-key compatibility mode. You can define <keycount> keys and
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then sectors are encrypted according to their offsets (sector 0 uses key0;
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sector 1 uses key1 etc.). <keycount> must be a power of two.
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<iv_offset>
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The IV offset is a sector count that is added to the sector number
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before creating the IV.
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<device path>
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This is the device that is going to be used as backend and contains the
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encrypted data. You can specify it as a path like /dev/xxx or a device
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number <major>:<minor>.
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<offset>
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Starting sector within the device where the encrypted data begins.
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<#opt_params>
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Number of optional parameters. If there are no optional parameters,
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the optional paramaters section can be skipped or #opt_params can be zero.
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Otherwise #opt_params is the number of following arguments.
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Example of optional parameters section:
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3 allow_discards same_cpu_crypt submit_from_crypt_cpus
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allow_discards
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Block discard requests (a.k.a. TRIM) are passed through the crypt device.
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The default is to ignore discard requests.
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WARNING: Assess the specific security risks carefully before enabling this
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option. For example, allowing discards on encrypted devices may lead to
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the leak of information about the ciphertext device (filesystem type,
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used space etc.) if the discarded blocks can be located easily on the
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device later.
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same_cpu_crypt
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Perform encryption using the same cpu that IO was submitted on.
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The default is to use an unbound workqueue so that encryption work
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is automatically balanced between available CPUs.
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submit_from_crypt_cpus
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Disable offloading writes to a separate thread after encryption.
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There are some situations where offloading write bios from the
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encryption threads to a single thread degrades performance
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significantly. The default is to offload write bios to the same
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thread because it benefits CFQ to have writes submitted using the
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same context.
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Example scripts
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===============
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LUKS (Linux Unified Key Setup) is now the preferred way to set up disk
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encryption with dm-crypt using the 'cryptsetup' utility, see
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https://gitlab.com/cryptsetup/cryptsetup
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[[
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#!/bin/sh
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# Create a crypt device using dmsetup
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dmsetup create crypt1 --table "0 `blockdev --getsize $1` crypt aes-cbc-essiv:sha256 babebabebabebabebabebabebabebabe 0 $1 0"
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]]
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[[
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#!/bin/sh
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# Create a crypt device using cryptsetup and LUKS header with default cipher
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cryptsetup luksFormat $1
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cryptsetup luksOpen $1 crypt1
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]]
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