Bumping up encryption to AES-256 by default

Hi. I noticed Gnome Disks uses AES-128 by default instead of AES-256 like Debian does out of the box. Having 256 bit symmetric keys is good practice for long term security especially in a coming era of quantum computers. (Whether they materialize or not is debatable but why not have a sufficient margin if it's easy enough?) It is also the recommended level by NIST.

This is verified by running:

cryptsetup luksDump --debug /device/

AES in XTS mode uses a keysize double its bit size (512 in this case) since with XTS the key is split in 2 so you actually get AES with 256-bit keys.

A partition created by Gnome Disks shows it's only using MK size 256 instead of the expected 512. Please modify the source [1] to pass 512 bit size to cryptsetup.

For more details and original research by me see:

[0] https://www.whonix.org/wiki/Full_Disk_Encryption_and_Encrypted_Images#Protection_Against_Powerful_Adversaries

[1] https://github.com/storaged-project/libblockdev/blob/master/src/plugins/crypto.h#L39