Commit de37457a authored by wagner's avatar wagner

sync with Wiki-Version

parent 057db3b3
......@@ -677,6 +677,18 @@ A. Contributors
longer as an FAQ can sustain. If in doubt, ask on the mailing list.
* 2.17 Is there a concern with 4k Sectors?
Not from dm-crypt itself. Encryption will be done in 512B blocks,
but if the partition and filesystem are aligned correctly and the
filesystem uses multiples of 4kiB as block size, the dm-crypt layer
will just process 8 x 512B = 4096B at a time with negligible
overhead. LUKS does place data at an offset, which is 2MiB per
default and will not break alignment. See also Item 6.12 of this
FAQ for more details. Note that if your partition or filesystem is
misaligned, dm-crypt can make the effect worse though.
3. Common Problems
......@@ -1380,7 +1392,9 @@ A. Contributors
XTS mode is potentially even more secure than cbc-essiv (but only if
cbc-essiv is insecure in your scenario). It is a NIST standard and
used, e.g. in Truecrypt. At the moment, if you want to use it, you
used, e.g. in Truecrypt. From version 1.6.0 of cryptsetup onwards,
aes-xts-plain64 is the default for LUKS. If you want to use it
with a cryptsetup before version 1.6.0 or with plain dm-crypt, you
have to specify it manually as "aes-xts-plain", i.e.
cryptsetup -c aes-xts-plain luksFormat <device>
......@@ -1516,8 +1530,8 @@ A. Contributors
try an ATA "secure erase" command for SSDs. That does not work for
USB keys though and may or may not be secure for a hybrid drive. If
it finishes on an SSD after a few seconds, it was possibly faked.
UNfortunately, for hybrid drives that indicator does not work, as
the drive may well take the time to dully erase the magnetic part,
Unfortunately, for hybrid drives that indicator does not work, as
the drive may well take the time to truly erase the magnetic part,
but only mark the SSD/Flash part as erased while data is still in
there.
......@@ -1526,7 +1540,7 @@ A. Contributors
one or several full overwrites!), you can use plain dm-crypt or
LUKS.
If you want or need the original LUKS security features to work,
If you want or need all the original LUKS security features to work,
you can use a detached LUKS header and put that on a conventional,
magnetic disk. That leaves potentially old encrypted data in the
pools on the disk, but otherwise you get LUKS with the same
......@@ -1564,6 +1578,59 @@ A. Contributors
completely secure for other uses and here it is.
* 5.21 Why is there no "Nuke-Option"?
A "Nuke-Option" or "Kill-switch" is a password that when entered
upon unlocking instead wipes the header and all passwords. So when
somebody forces you to enter your password, you can destroy the
data instead.
While this sounds attractive at first glance, it does not make sense
once a real security analysis is done. One problem is that you have
to have some kind of HSM (Hardware Security Module) in order to
implement it securely. In the movies, a HSM starts to smoke and
melt once the Nuke-Option has been activated. In reality, it just
wipes some battery-backed RAM cells. A proper HSM costs something
like 20'000...100'000 EUR/USD and there a Nuke-Option may make some
sense. BTW, a chipcard or a TPM is not a HSM, although some
vendors are promoting that myth.
Now, a proper HSMs will have a wipe option but not a Nuke-Option,
i.e. you can explicitly wipe the HSM, but by a different process
than unlocking it takes. Why is that? Simple: If somebody can force
you to reveal passwords, then they can also do bad things to you if
you do not or if you enter a nuke password instead. Think locking
you up for a few years for "destroying evidence" or for far longer
and without trial for being a "terrorist suspect". No HSM maker
will want to expose its customers to that risk.
Now think of the typical LUKS application scenario, i.e. disk
encryption. Usually the ones forcing you to hand over your password
will have access to the disk as well, and, if they have any real
suspicion, they will mirror your disk before entering anything
supplied by you. This neatly negates any Nuke-Option. If they have
no suspicion (just harassing people that cross some border for
example), the Nuke-Option would work, but see above about likely
negative consequences and remember that a Nuke-Option may not work
reliably on SSD and hybrid drives anyways.
Hence my advice is to never take data that you do not want to reveal
into any such situation in the first place. There is no need to
transfer data on physical carriers today. The Internet makes it
quite possible to transfer data between arbitrary places and modern
encryption makes it secure. If you do it right, nobody will even be
able to identify source or destination. (How to do that is out of
scope of this document. It does require advanced skills in this age
of pervasive surveillance.)
Hence, LUKS has not kill option because it would do much more harm
than good.
Still, if you have a good use-case (i.e. non-abstract real-world
situation) where a Nuke-Option would actually be beneficial, please
let me know.
6. Backup and Data Recovery
......@@ -1842,7 +1909,7 @@ http://code.google.com/p/cryptsetup/source/browse/misc/luks-header-from-active
change the password, you basically have to create a second
encrypted device with the new passphrase and copy your data over.
On the plus side, if you accidentally overwrite any part of a
dm-crypt device, the damage will be limited to the are you
dm-crypt device, the damage will be limited to the area you
overwrote.
......@@ -2114,6 +2181,16 @@ http://code.google.com/p/cryptsetup/source/browse/misc/luks-header-from-active
not be used anymore as well. My advice would be to drop SLED 10.
* 8.3 Gcrypt after 1.5.3 breaks Whirlpool
It is the other way round: In gcrypt 1.5.3 and before Whirlpool is
broken and it was fixed in the next version. If you selected
whirlpool as hash on creation of a LUKS container, it does not work
anymore with the fixed library. Currently, the only work-around is
to decrypt with the old version. This also shows one risk of using
rarely used settings.
9. References and Further Reading
......
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