Re: [dm-crypt] XTS performance

[Arno Wagner <arno@wagner.name>]

Hi Yaron,

effort for XTS per cipher block is a bit higher than ESSIV, 
as you need one additional GF(2^128) multiplication per 
crypto-block. But you are right, there is only one encryption 
for the "IV" per sector, so performance is often compareble 
to ESSIV. 

I think XTS mode does have a potentially larger cache footprint 
(the multiplication). Crypto hardware typically supports CBC 
mode but may not support XTS mode. In the standard case
these aspects do not really matter.

There may also have been some confusion with EME mode. EME 
mode does CBC twice, with some masking step in between.

Arno



On Tue, May 01, 2012 at 09:26:39AM +0300, Yaron Sheffer wrote:
> Hi,
> 
> Looking back at the archives, it appears that aes-xts-plain in dm-crypt
> runs at half the speed of aes-cbc-essiv:
> http://www.saout.de/pipermail/dm-crypt/2010-November/001348.html. Milan
> explained it at the time by XTS doing 2 AES operations for each
> plaintext block. But my understanding of XTS is that it is similar to
> ESSIV: 1 AES op per 16-byte block, plus 1 op for the sector.
> 
> - Did I misread the definition of XTS, and it's really 2 AES ops per
> 16-byte block?
> 
> - Does anybody have more recent performance comparisons, confirming (or
> not) the performance difference?
> 
> Thanks,
> 
>     Yaron
> 
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> 

-- 
Arno Wagner, Dr. sc. techn., Dipl. Inform., CISSP -- Email: arno@wagner.name 
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