[dm-crypt] Some questions about cryptsetup 1.6.x

Cpp tzornik at gmail.com
Wed Feb 12 10:49:54 CET 2014


Greetings.

I have a few questions about the use of cryptsetup and its security.
First I'd like to know something about the command line options. I've
seen people specify the digest (hash) in two different places in
cryptsetup. Consider the following line:

# cryptsetup -c aes-xts-plain64:sha512 -h sha512 -s 512 -y -i 5000
--use-random -y -v luksFormat /dev/sda1

What is the difference between specifying the hash in the -c parameter
i.e. aes-xts-plain64:sha512 or by using the -h parameter? Do they both
do the same thing meaning that the following two are equivalent?

# cryptsetup -c aes-xts-plain64:sha512 -s 512 -y -i 5000 --use-random
-y -v luksFormat /dev/sda1
# cryptsetup -c aes-xts-plain64 -h sha512 -s 512 -y -i 5000
--use-random -y -v luksFormat /dev/sda1


Next I'd like to ask about the memory management of the master key.
Suppose I mounted a volume using luksOpen (or --type luks open). What
happens when I invoke luksClose (close) on that container? Does the
master key get securely erased from memory (several overwrites with
random data) or is it simply blanked out (single overwrite with
zeros)?

How is the master key stored in memory? I read somewhere that having
the same data in the exact same location in RAM for an extended period
of time (like a 24/7 server) can "burn in" the data into the RAM
module, which can be later recovered. Is this of any concern with
current cryptsetup i.e. for attacks like cold boot?

Finally I'm interested to know about removing all the keyslots.
Suppose I mounted a container and erased every available keyslot
(please don't ask why). I know this would in theory make the data
irrecoverble, but the container is still mounted for the time being.
Assuming that the power doesn't disappear, is there a way to
reintroduce a new key slot into the LUKS container after all slots
have been erased, provided that the container is mounted and I can
read the master key from memory?


Best regards!


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