Re: GRUB LUKS support is slow?

[Ludovic Courtès]

Hi Tobias,

Tobias Geerinckx-Rice <address@hidden> skribis:

> The whole point of the key derivation function (PBKDF2 in this case) is
> to take a long time — i.e., generate *a lot* of data — before deriving
> the actual encryption key from your passphrase. It's basically a slow
> hash, run many times over. That's the delay we're talking about here.
>
> PBKDF2 is designed to be ‘impossible’ to optimise, parallelise, or cut
> short[0], so there's no way around running several seconds of busy work
> before you can even check a passphrase.
>
> On GuixSD, the default run time is:
>
>   $ cryptsetup --help | grep iteration
>   -i, --iter-time=msecs        PBKDF2 iteration time for LUKS (in ms)
>   Default PBKDF2 iteration time for LUKS: 2000 (ms)
>
> However, this run time is measured at volume creation time, with all
> fancy CPU features available. It wouldn't surprise me if real-mode[1]
> GRUB and the early kernel code are badly optimised and take longer to
> complete the same number of iterations to obtain the key.

I see, thanks for the explanation!

Apparently GRUB embeds a copy of libgcrypt, which is supposed to be
somewhat optimized.  Maybe there’s really nothing we can do.

Ludo’.