[dm-crypt] Cryptesetup and dm-crypt

levent demir levent.demir at inria.fr
Wed Apr 27 14:56:50 CEST 2016


I am a PhD student and works and I am working with cryptsetup and a hw
crypto engine. 

I am using an ATMEL board which is able to do AES-ECB mode for example. 

We have implemented an XTS version with all operations (XOR, GF
Multiplication, 2 AES : one for IV one for data...)

My question is can we use the page size instead of the block size of 512
bytes ?

I have read some thread it is written that it could be a security issue
but I don't agree because : 

Now to implement XTS I do as follows basically : 

I take the IV encrypt it with the second part of the key, then I compute
the 32 tweaks values in -for example- an array tweak_buf[512]. 
Then I just have to compute the XOR with plaintext. 
I do a DMA encryption with ECB of the 512 bytes. 
Again I xor the ciphered data with tweak_buf.

Then I have ciphered a whole block of 512 bytes.

Now my question, the hardware is supporting a DMA operation till 4096, 
is it not interesting to encrypt 8 blocks at a time ? 
It is exactly the same operation as encrypting each block

Of course I need to pre-compute the 8 tweak_buf in a big
tweak_buf[4096].Then in only one DMA operation I will be able to encrypt
4096 bytes with ECB mode. 

I hope you understand what I mean.

Can you tell me if it is possible ? If yes can I use a previous patch, I
have seen that there are some works on it ?

Thank you. 

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