dm crypt: add documentation

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-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.

<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://luks.endorphin.org/

[[

#!/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

]]