linux/Documentation/device-mapper/dm-crypt.txt

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:
http://code.google.com/p/cryptsetup/wiki/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:
        1 allow_discards

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.

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://code.google.com/p/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
]]