[dm-crypt] inner workings of block mode encryption

U.Mutlu for-gmane at mutluit.com
Sun Feb 8 17:31:49 CET 2015

Ralf Ramsauer wrote, On 02/08/2015 03:22 PM:
> Hi,
> maybe you should start here:
> https://code.google.com/p/cryptsetup/wiki/FrequentlyAskedQuestions
> On 02/08/2015 03:06 PM, U.Mutlu wrote:
>> Hi,
>> I'm interessted in the inner workings of encrypting block devices
>> like the encrypted volumes created with the cryptsetup tool.
>> Let's say a file of size 200K is stored on an encrypted ext2 filesystem.
>> How is the enrcyption key applied to it?
> To sum it up:
> cryptsetup is independent of the overlying filesystem and does not care
> about it.
> It just acts as a "mapper" and creates a new block device which
> represents plain text data.
> You can use this device for any purpose of your choice: create a
> filesystem on it, use it as a LVM Volume, ....
>> I guess the password is used only for accessing/mounting the volume,
>> and the key is only a starting value (ie. a "seed") for the underlying
>> cipher.
> You can use a passphrase or a keyfile with cryptsetup LUKS.
> This key is applied to a key derivation function which derives an
> intermediate key which is used for decrypting a key slot which contains
> the actual master-key for decrypting your volume.
> This key slot or "lock box" concept opens the possibility that several
> key files or passphrases may unlock the volume.
> The material which is needed for decrypting the device is located in the
> LUKS header. See FAQ.
>> Since in these cases usually a blockwise operating cipher is used,
>> that means that the file will be encrypted of course blockwise,
>> for example in blocks of 32 bytes.
> Yes, almost.
> dm-crypt uses a sector size of (correct me if i'm wrong) 512 Byte which
> means that every sector of 512 Byte gets en/decrypted independently.
> The encryption of each sector is parameterized by an initialization
> vector which may be influenced by the logical number of the sector.
>> Now the question: are the blocks of that file all encrypted using the
>> same one key? Or does a kind of "streaming" or HMAC get used for the
>> subsequent blocks of the file?
> Well yes and no.
> Yes, the same key is used for all sectors of the volume and no, dm-crypt
> does not use HMAC to generate key streams.

Hmm. IMO this is the major weak point of such static/symmetric crypto solutions.
Knowing just one cleartext file, for example a well known static
system file from the /etc directory, and its encrpted data, could
easily lead to the master key (assuming the encrypted volume
contains such system files).

OTOH, a streaming crypto solution (I think also called 'asymmetric'),
ie. where each block gets encrypted with a new key derived from
the previous/initial key together with xoring with varying parts
of the user data in the block, would IMO make up a much more secure crypto 

> Just imagine: if you'd like to access the last sector of your volume
> you'd have to generate the whole key stream which would probably take a
> long time.

Yes, true, but I think this problem could be somehow solved.

> cheers
>    Ralf


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