[dm-crypt] Two questions about LUKS2 format

Milan Broz gmazyland at gmail.com
Fri Dec 29 18:10:53 CET 2017


On 12/29/2017 05:41 PM, Geo Kozey wrote:
> 1. When creating new container with experimental ciphers, i.e. chacha20, the output of luksDump shows:
> 
> Data segments:
>   0: crypt
>         offset: 4194304 [bytes]
>         length: (whole device)
>         cipher: chacha20-random
>         sector: 512 [bytes]
>         integrity: poly1305
> 
> Keyslots:
>   0: luks2
>         Key:        256 bits
>         Priority:   normal
>         Cipher:     aes-xts-plain64
>         PBKDF:      argon2i
>         Time cost:  4
> 
> Why "Cipher: aes-xts-plain64" is shown under Keyslots metadata and is different than "cipher: chacha20-random" from Data segments?

The keyslot encryption cannot use AEAD directly (in fact keyslots are
already authenticated through key digest check).

For now I just hardcoded aes-xts-plain64 algorithm there, forgot to
mention it in release notes, sorry.
So if you use AEAD, keyslot will use aes-xts, if you use
length-preserving encryption (as in LUKS1), keyslot will use the same
algorithm as for data.

(The on-disk format allows per slot encryption setting but commandline
would bee too complicated - we can add options for it it later though.)

> 2. What happens when we create new luks container with argon2 as PBKDF under system with huge amount of RAM then try opening it under system with much lower amount (so memory cost will be higher than physical memory available)? Will it open but slower or will it fail?

It will unfortunately fail (it is behavior of libargon2 internals, but
even if is able to use swap, it would slow down unlocking drastically.
It is memory-hard function by definition so it behaves this way...).
Actually it can even trigger OOM killer and kill cryptsetup itself.

This is something we will need to tune-in in practise - nobody actually
started to use Argon2 this way and in academic papers it always work and
usually ignored because it is implementation detail ;-)

So, if you plan to use LUKS2 on device with very low memory later, you
have to add some slot with adequate low setting.
(If you format it there, it should decrease memory according to physical
available memory automatically.)
You can also have now one slot using Argon2 and another PBKDF2, but this
will obviously degrade resistance to brute force on GPUs etc.

Anyway, thanks for testing and questions!

Milan


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