All tomb keys forged using Tomb version 2.3 or preceeding are 256 bits
large, which is insufficient to trigger usage of AES-256 encryption in
XTS mode, which is the default. Therefore all tombs locked using
smaller keys are silently encrypted using AES-128, according to the
cryptsetup manual:
> "By default a 256 bit key-size is used. Note however that XTS splits the supplied key in half, so to use AES-256 instead of AES-128 you have to set the XTS key-size to 512."
This problem has been noticed and corrected in Tomb version 2.4 where
now the 'forge' command will automatically generate 512 bits keys. To
switch to AES-256 encrypted tombs the only possibility is to create
new keys, new tombs and copy the contents across, since the LUKS
formatting occurs when the 'lock' command is issued using a new
key. Using 'setkey' to switch key does not suffice to switch to
AES-256.
This problem is minor and doesn't seem to heavily affect the security
of Tombs created before 2.4 as the cryptographic strenght of AES-128
and AES-256 is comparable; yet it is reasonable to think that larger
key sizes resist better to Quantum computing attacks.
