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Re: [dm-devel] [PATCH 2/2] dm-crypt: Add TCW IV mode for old CBC TCRYPT containers.



The following patch header is confusing given the mix of legacy and
new concepts.  I understand you're trying to establish context for
what's new but to this reader I'm a bit lost in the jargon.  Specifics
below (and please forgive my naive questions).

On Sun, Oct 20 2013 at  9:16am -0400,
Milan Broz <gmazyland gmail com> wrote:

> The dmcrypt already can activate TCRYPT (TrueCrypt compatible) containers
> in LRW or XTS block encryption mode.
> 
> TCRYPT containers prior to version 4.1 used CBC mode with some
> additional tweaks.
> 
> This patch adds support for these containers.
> 
> For now, there is no support for chained ciphers, this TCW mode support
> only containers encrypted with one cipher
> (Tested with AES, Twofish, Serpentm CAST5 and TripleDES).

What does TCW mean?  How does it relate to CBC?
- Is TCW mode: "CBC mode with some additional tweaks"?

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

Now you're switching back to referring to "TCRYPT CBC mode".  What
happened to "TCW mode"?

> (Both userspace and kernel code is independent implementation
> based on format documentation and completely avoids use of original
> source code.)
> 
> Encryption uses CBC mode with special IV generated from
> additional key, xored with sector number.
> 
> There is also second key used for "whitening" of sectors.
> Whitening key is xored with sector number and mixed using
> CRC32 and resulting value is applied to whole sector.
> (Detailed calculation is in Truecrypt documentation for version < 4.1
> and will be also described on dmcrypt site.)

Can you add a pointer to the Truecrypt documentation for < 4.1?  Or a
pointer to the dmcrypt site documentation?

> IV and whitening key is concatenated with encryption key,
> so kernel receives all these keys as K|IV_key|Whitening_key
> in one string.
> Length of IV key is always the same as IV of selected cipher
> and length of whitening key is fixed to TCW_WHITENING_SIZE,
> so key string can be split properly.
> 
> The experimental support for activation of these containers
> is already present in git devel brach of cryptsetup.

Again, an example that documents a theoretical ctr line (before and
after patch?) would probably go a long way to help clarify what is new
here.


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