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double algorithm question

dm-crypt Sam 2 participants 3 messages Aug 1, 2009 – Aug 2, 2009

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Sam

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#0 lore raw

Thanks Moji,

That will obviously provide a nice boost in performance over what I was trying! I appreciate your help.

Regards,
Sam

Quoted 72 lines

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

On Sat, 1 Aug 2009 07:39:42 -0400

Quoted 33 lines · Sam

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.

Quoted 6 lines

Regards,
Sam
_______________________________________________
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http://www.saout.de/mailman/listinfo/dm-crypt

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dm-crypt mailing list
[email protected]
http://www.saout.de/mailman/listinfo/dm-crypt

#1 lore raw

FWIW I wouldn't forget about Serpent, which came 2nd in the AES selection process.

From Practical Cryptography, by Niels Ferguson and Bruce Schneier:
"If you are paranoid about the security of your data, and speed is not important, then you should choose Seprent. During the AES process, every serious cryptographer agreed that Serpent was the most secure (or most conservative) of the submissions"

-- Roscoe

Quoted 85 lines · Sam

Thanks Moji,

That will obviously provide a nice boost in performance over what I was trying! I appreciate your help.

Regards,
Sam

Quoted 68 lines

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

On Sat, 1 Aug 2009 07:39:42 -0400

Quoted 33 lines · Sam

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.

Quoted 6 lines

Regards,
Sam
_______________________________________________
dm-crypt mailing list
[email protected]
http://www.saout.de/mailman/listinfo/dm-crypt

Quoted 4 lines

_______________________________________________
dm-crypt mailing list
[email protected]
http://www.saout.de/mailman/listinfo/dm-crypt

_______________________________________________
dm-crypt mailing list
[email protected]
http://www.saout.de/mailman/listinfo/dm-crypt

#2 lore raw

[Post attempt two, first one got blocked]

FWIW I wouldn't forget about Serpent, which came 2nd in the AES selection process.

From Practical Cryptography, by Niels Ferguson and Bruce Schneier:
"If  you are paranoid about the security of your data, and speed is not important, then you should choose Seprent. During the AES process, every serious cryptographer agreed that Serpent was the most secure (or most conservative) of the submissions"

-- Roscoe