Re: XTS cipher mode for cluster file encryption

On 10/15/21 23:02, Robert Haas wrote:
> On Fri, Oct 15, 2021 at 3:22 PM Stephen Frost <sfrost@snowman.net> wrote:
>> Specifically: The default cipher for LUKS is nowadays aes-xts-plain64
>>
>> and then this:
>>
>> https://gitlab.com/cryptsetup/cryptsetup/-/wikis/DMCrypt
>>
>> where plain64 is defined as:
>>
>> plain64: the initial vector is the 64-bit little-endian version of the
>> sector number, padded with zeros if necessary
>>
>> That is, the default for LUKS is AES, XTS, with a simple IV.  That
>> strikes me as a pretty ringing endorsement.
> 
> Yes, that sounds promising. It might not hurt to check for other
> precedents as well, but that seems like a pretty good one.
> 

TrueCrypt/VeraCrypt uses XTS too, I think. There's an overview of other 
FDE products at [1], and some of them use XTS, but I would take that 
with a grain of salt - some of the products are somewhat obscure, very 
old, or both.

What is probably more interesting is that there's an IEEE standard [2] 
dealing with encrypted shared storage, and that uses XTS too. I'd bet 
there's a bunch of smart cryptographers involved.


[1] https://en.wikipedia.org/wiki/Comparison_of_disk_encryption_software

[2] https://en.wikipedia.org/wiki/IEEE_P1619

> I'm not very convinced that using the LSN for any of this is a good
> idea. Something that changes most of the time but not all the time
> seems more like it could hurt by masking fuzzy thinking more than it
> helps anything.
> 

I haven't been following the discussion about using LSN, but I agree 
that while using it seems convenient, the consequences of some changes 
not incrementing LSN seem potentially disastrous, depending on the 
encryption mode.


regards

-- 
Tomas Vondra
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