linux/Documentation/device-mapper/dm-crypt.txt
Milan Broz d1f9642381 dm crypt: add multi key capability
This patch adds generic multikey handling to be used
in following patch for Loop-AES mode compatibility.

This patch extends mapping table to optional keycount and
implements generic multi-key capability.

With more keys defined the <key> string is divided into
several <keycount> sections and these are used for tfms.

The tfm is used according to sector offset
(sector 0->tfm[0], sector 1->tfm[1], sector N->tfm[N modulo keycount])
(only power of two values supported for keycount here).

Because of tfms per-cpu allocation, this mode can be take
a lot of memory on large smp systems.

Signed-off-by: Milan Broz <mbroz@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>
Cc: Max Vozeler <max@hinterhof.net>
2011-01-13 19:59:54 +00:00

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dm-crypt
=========
Device-Mapper's "crypt" target provides transparent encryption of block devices
using the kernel crypto API.
Parameters: <cipher> <key> <iv_offset> <device path> <offset>
<cipher>
Encryption cipher and an optional IV generation mode.
(In format cipher[:keycount]-chainmode-ivopts:ivmode).
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.
<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.
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
http://clemens.endorphin.org/cryptography
[[
#!/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
]]