double algorithm question

Hi All,

I am wondering if this is a good idea:

encrypt a partition normally with cryptsetup luksFormat (using aes-xts-plain), then luksOpen,
mkfs.ext2 format the device mapper device that appears,
mount it.
Then, create a giant file that fills up the partition.
losetup it that file,
luksFormat the loop device (using twofish-xts-plain)
luksOpen it,
mkfs.ext2 format the device mapper device that appears,
mount it,
and use it...

My purpose is that I don't trust AES, but I don't trust twofish enough to be sure it is better than AES.

I am paranoid enough that the speed hit is acceptable.

Questions:

1) is this the best way to achieve my goal with dm-crypt?
2) is it secure? Or will somehow it cause my data to be less secure than just using one cipher? Or will it somehow defeat the security provided by XTS? (i would assume it becoming less secure in any way is impossible, but i am not a cryptoanalyst, so i don't want to be assuming such things).

I know truecrypt has a feature where you specify the cipher as aes-twofish. This is what I wish to achieve, but using dm-crypt.

Regards,
Sam

You do not need to make a filesystem on the intermediate device, because you treat the devices in /dev/mapper as block devices you can luksFormat any device that shows up in order to do cascade encryption. You just have to remember to close them first in last out.

cryptsetup luksFormat -c aes-xts-plain /dev/sdc cryptsetup luksOpen /dev/sdc first_layer
cryptsetup luksFormat -c aes-xts-plain /dev/mapper/first_layer cryptsetup luksOpen /dev/mapper/first second_layer mkfs.ext2 /dev/mapper/second_layer -m 0 -L "Test" mount /dev/mapper/second_layer /mnt/usb
umount /mnt/cdrom
cryptsetup luksClose second_layer
cryptsetup luksClose first_layer

[Of course omit the luksFormat/mkfs lines after the device is created to open/close the device.]

I do not know of any vulnerabilities with cascade encryption, it is normally just excessive, but someone else might.

I hope that helps you,

-MJ