ATTENTION: If you are going to read just one thing, make it the
section on Backup and Data Recovery. By far the most questions on
- the cryptsetup mailing list are from people that just managed to
- somehow format or overwrite the start of their LUKS partitions. In
+ the cryptsetup mailing list are from people that managed to damage
+ the start of their LUKS partitions, i.e. the LUKS header. In
most cases, there is nothing that can be done to help these poor
souls recover their data. Make sure you understand the problem and
- limitations imposed by the LUKS security model BEFORE you face such
- a disaster!
-
- PASSPHRASES: Some people have had difficulties when upgrading
- distributions. It is highly advisable to only use the 94 printable
- characters from the first 128 characters of the ASCII table, as
- they will always have the same binary representation. Other
- characters may have different encoding depending on system
- configuration and your passphrase will not work with a different
- encoding. A table of the standardized first 128 ASCII caracters
- can, e.g. be found on http://en.wikipedia.org/wiki/ASCII
+ limitations imposed by the LUKS security model BEFORE you face
+ such a disaster! In particular, make sure you have a current header
+ backup before doing any potentially dangerous operations.
+
+ BACKUP: Yes, encrypted disks die, just as normal ones do. A full
+ backup is mandatory, see Section "6. Backup and Data Recovery" on
+ options for doing encrypted backup.
+
+ DISTRIBUTION INSTALLERS: Some distribution installers offer to
+ create LUKS containers in a way that can be mistaken as activation
+ of an existing container. Creating a new LUKS container on top of
+ an existing one leads to permanent, complete and irreversible data
+ loss. It is strongly recommended to only use distribution
+ installers after a complete backup of all LUKS containers has been
+ made.
+
+ LUKS PASSPHRASE IS NOT THE MASTER KEY: The LUKS passphrase is not
+ used in deriving the master key. It is used in decrypting a master
+ key that is randomly selected on header creation. This means that
+ if you create a new LUKS header on top of an old one with
+ exactly the same parameters and exactly the same passphrase as the
+ old one, it will still have a different master key and your data
+ will be permanently lost.
+
+ PASSPHRASE CHARACTER SET: Some people have had difficulties with
+ this when upgrading distributions. It is highly advisable to only
+ use the 94 printable characters from the first 128 characters of
+ the ASCII table, as they will always have the same binary
+ representation. Other characters may have different encoding
+ depending on system configuration and your passphrase will not
+ work with a different encoding. A table of the standardized first
+ 128 ASCII caracters can, e.g. be found on
+ http://en.wikipedia.org/wiki/ASCII
* System Specific warnings
4. Troubleshooting
+ * I get the error "LUKS keyslot x is invalid." What does that mean?
+
+ This means that the given keyslot has an offset that points
+ outside the valid keyslot area. Typically, the reason is a
+ corrupted LUKS header because something was written to the start of
+ the device the LUKS contaner is on. Refer to Section "Backup and
+ Data Recovery" and ask on the mailing list if you have trouble
+ diagnosing and (if still possible) repairing this.
+
+
* Can a bad RAM module cause problems?
LUKS and dm-crypt can give the RAM quite a workout, especially when
to the last stage ("Acceptance") and think about what to do now.
There is one exception that I know of: If your LUKS container is
still open, then it may be possible to extract the master key from
- the running system. Ask on the mailing-list on how to do that and
- make sure nobody switches off the machine.
+ the running system. See Item "How do I recover the master key from
+ a mapped LUKS container?" in Section "Backup and Data Recovery".
* What is a "salt"?
* Does a backup compromise security?
Depends on how you do it. However if you do not have one, you are
- going to eventually loseyour encrypted data.
+ going to eventually lose your encrypted data.
There are risks introduced by backups. For example if you
change/disable a key-slot in LUKS, a binary backup of the partition
damage the key-slots in part or in full. See also last item.
+ * How do I recover the master key from a mapped LUKS container?
+
+ This is typically only needed if you managed to damage your LUKS
+ header, but the container is still mapped, i.e. "luksOpen"ed. It
+ also helps if you have a mapped container that you forgot or do not
+ know a passphrase for (e.g. on a long running server.)
+
+ WARNING: Things go wrong, do a full backup before trying this!
+
+ WARNING: This exposes the master key of the LUKS container. Note
+ that both ways to recreate a LUKS header with the old master key
+ described below will write the master key to disk. Unless you are
+ sure you have securely erased it afterwards, e.g. by writing it to
+ an encrypted partition, RAM disk or by erasing the filesystem you
+ wrote it to by a complete overwrite, you should change the master
+ key afterwards. Changing the master key requires a full data
+ backup, luksFormat and then restore of the backup.
+
+ First, there is a script by Milan that automatizes the whole
+ process, except generating a new LUKS header with the old master
+ key (it prints the command for that though):
+
+http://code.google.com/p/cryptsetup/source/browse/trunk/misc/luks-header-from-active
+
+ You can also do this manually. Here is how:
+
+ - Get the master key from the device mapper. This is done by the
+ following command. Substitute c5 for whatever you mapped to:
+
+ # dmsetup table --target crypt --showkey /dev/mapper/c5
+ Result:
+ 0 200704 crypt aes-cbc-essiv:sha256
+ a1704d9715f73a1bb4db581dcacadaf405e700d591e93e2eaade13ba653d0d09
+ 0 7:0 4096
+
+ The result is actually one line, wrapped here for clarity. The long
+ hex string is the master key.
+
+ - Convert the master key to a binary file representation. You can
+ do this manually, e.g. with hexedit. You can also use the tool
+ "xxd" from vim like this:
+
+ echo "a1704d9....53d0d09" | xxd -r -p > <master-key-file>
+
+ - Do a luksFormat to create a new LUKS header.
+
+ NOTE: If your header is intact and you just forgot the
+ passphrase, you can just set a new passphrase, see next subitem.
+
+ Unmap the device before you do that (luksClose). Then do
+
+ cryptsetup luksFormat --master-key-file=<master-key-file> <luks device>
+
+ Note that if the container was created with other than the default
+ settings of the cryptsetup version you are using, you need to give
+ additional parameters specifying the deviations. If in doubt, try
+ the script by Milan. It does recover the other parameters as well.
+
+ Side note: This is the way the decrypt_derived script gets at the
+ master key. It just omits the conversion and hashes the master key
+ string.
+
+ - If the header is intact and you just forgot the passphrase, just
+ set a new passphrase like this:
+
+ cryptsetup luksAddKey --master-key-file=<master-key-file> <luks device>
+
+ You may want to disable the old one afterwards.
+
+
* What does the on-disk structure of dm-crypt look like?
There is none. dm-crypt takes a block device and gives encrypted
projects / platform / upstream / cryptsetup.git / blobdiff