tree 0f1ebef7bef74d467c850ef676514847d6006d0b
parent da31a0787a2ac92dd219ce0d33322160b66d6a01
author Milan Broz <gmazyland@gmail.com> 1382998864 +0100
committer Mike Snitzer <snitzer@redhat.com> 1384039220 -0500

dm crypt: add TCW IV mode for old CBC TCRYPT containers

dm-crypt can already activate TCRYPT (TrueCrypt compatible) containers
in LRW or XTS block encryption mode.

TCRYPT containers prior to version 4.1 use CBC mode with some additional
tweaks, this patch adds support for these containers.

This new mode is implemented using special IV generator named TCW
(TrueCrypt IV with whitening).  TCW IV only supports containers that are
encrypted with one cipher (Tested with AES, Twofish, Serpent, CAST5 and
TripleDES).

While this mode is legacy and is known to be vulnerable to some
watermarking attacks (e.g. revealing of hidden disk existence) it can
still be useful to activate old containers without using 3rd party
software or for independent forensic analysis of such containers.

(Both the userspace and kernel code is an independent implementation
based on the format documentation and it completely avoids use of
original source code.)

The TCW IV generator uses two additional keys: Kw (whitening seed, size
is always 16 bytes - TCW_WHITENING_SIZE) and Kiv (IV seed, size is
always the IV size of the selected cipher).  These keys are concatenated
at the end of the main encryption key provided in mapping table.

While whitening is completely independent from IV, it is implemented
inside IV generator for simplification.

The whitening value is always 16 bytes long and is calculated per sector
from provided Kw as initial seed, xored with sector number and mixed
with CRC32 algorithm.  Resulting value is xored with ciphertext sector
content.

IV is calculated from the provided Kiv as initial IV seed and xored with
sector number.

Detailed calculation can be found in the Truecrypt documentation for
version < 4.1 and will also be described on dm-crypt site, see:
http://code.google.com/p/cryptsetup/wiki/DMCrypt

The experimental support for activation of these containers is already
present in git devel brach of cryptsetup.

Signed-off-by: Milan Broz <gmazyland@gmail.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>