1dm-crypt 2========= 3 4Device-Mapper's "crypt" target provides transparent encryption of block devices 5using the kernel crypto API. 6 7Parameters: <cipher> <key> <iv_offset> <device path> \ 8 <offset> [<#opt_params> <opt_params>] 9 10<cipher> 11 Encryption cipher and an optional IV generation mode. 12 (In format cipher[:keycount]-chainmode-ivopts:ivmode). 13 Examples: 14 des 15 aes-cbc-essiv:sha256 16 twofish-ecb 17 18 /proc/crypto contains supported crypto modes 19 20<key> 21 Key used for encryption. It is encoded as a hexadecimal number. 22 You can only use key sizes that are valid for the selected cipher. 23 24<keycount> 25 Multi-key compatibility mode. You can define <keycount> keys and 26 then sectors are encrypted according to their offsets (sector 0 uses key0; 27 sector 1 uses key1 etc.). <keycount> must be a power of two. 28 29<iv_offset> 30 The IV offset is a sector count that is added to the sector number 31 before creating the IV. 32 33<device path> 34 This is the device that is going to be used as backend and contains the 35 encrypted data. You can specify it as a path like /dev/xxx or a device 36 number <major>:<minor>. 37 38<offset> 39 Starting sector within the device where the encrypted data begins. 40 41<#opt_params> 42 Number of optional parameters. If there are no optional parameters, 43 the optional paramaters section can be skipped or #opt_params can be zero. 44 Otherwise #opt_params is the number of following arguments. 45 46 Example of optional parameters section: 47 1 allow_discards 48 49allow_discards 50 Block discard requests (a.k.a. TRIM) are passed through the crypt device. 51 The default is to ignore discard requests. 52 53 WARNING: Assess the specific security risks carefully before enabling this 54 option. For example, allowing discards on encrypted devices may lead to 55 the leak of information about the ciphertext device (filesystem type, 56 used space etc.) if the discarded blocks can be located easily on the 57 device later. 58 59Example scripts 60=============== 61LUKS (Linux Unified Key Setup) is now the preferred way to set up disk 62encryption with dm-crypt using the 'cryptsetup' utility, see 63http://code.google.com/p/cryptsetup/ 64 65[[ 66#!/bin/sh 67# Create a crypt device using dmsetup 68dmsetup create crypt1 --table "0 `blockdev --getsize $1` crypt aes-cbc-essiv:sha256 babebabebabebabebabebabebabebabe 0 $1 0" 69]] 70 71[[ 72#!/bin/sh 73# Create a crypt device using cryptsetup and LUKS header with default cipher 74cryptsetup luksFormat $1 75cryptsetup luksOpen $1 crypt1 76]] 77