Sections 1. General Questions 2. Setup 3. Common Problems 4. Security Aspects 5. Backup and Data Recovery 6. Issues with Specific Versions of cryptsetup A. Contributors 1. General Questions * What is this? This is the FAQ (Frequently Asked Questions) for cryptsetup. It covers Linux disk encryption with plain dm-crypt (one passphrase, no management, no descriptor on disk) and LUKS (multiple user keys with one master key, anti-forensics, descriptor block at start of device, ...). The latest version should usually be available at http://code.google.com/p/cryptsetup/wiki/FrequentlyAskedQuestions * Who wrote this? Current FAQ maintainer is Arno Wagner . Wherever contributions are from other people, their name should be included in brackets with the respective article. If you want to contribute, send your article, including a descriptive headline, to the maintainer, or the dm-crypt mailing list with something like "FAQ ..." in the subject. Please note that by contributing to this FAQ, you accept the license described below. This work is under the "Attribution-Share Alike 3.0 Unported" license, which means distribution is unlimited, you may create derived works, but attributions to original authors and this license statement must be retained and the derived work must be under the same license. See http://creativecommons.org/licenses/by-sa/3.0/ for more details of the license. Side note: I did text license research some time ago and I think this license is best suited for the purpose at hand and creates the least problems. 2. Setup * Can I encrypt an already existing, non-empty partition to use LUKS? There is no converter, and it is not really needed. The way to do this is to make a backup of the device in question, securely wipe the device (as LUKS device initialization does not clear away old data), do a luksFormat, optionally overwrite the encrypted device, create a new filesystem and restore your backup on the now encrypted device. Also refer to sections "Security Aspects" and "Backup and Data Recovery". For backup, plain GNU tar works well and backs up anything likely to be in a filesystem. * How do I use LUKS with a loop-device? Just the same as with any block device. If you want, for example, to use a 100MB file as LUKS container, do something like this: head -c 100M /dev/zero > luksfile # create empty file losetup /dev/loop0 luksfile # map luksfile to /dev/loop0 cryptsetup luksFormat /dev/loop0 # create LUKS on the loop device Afterwards just use /dev/loop0 as a you would use a LUKS partition. To unmap the file when done, use "losetup -d /dev/loop0". * How do I read a dm-crypt key from file? Note that the file will still be hashed first, just like keyboard input. Use the --key-file option, like this: cryptsetup create --key-file keyfile e1 /dev/loop0 * How do I read a LUKS slot key from file? What you really do here is to read a passphrase from file, just as you would with manual entry of a passphrase for a key-slot. You can add a new passphrase to a free key-slot, set the passphrase of an specific key-slot or put an already configured passphrase into a file. In the last case make sure no trailing newline (0x0a) is contained in the key file, or the passphrase will not work because the whole file is used as input. To add a new passphrase to a free key slot from file, use something like this: cryptsetup luksAddKey /dev/loop0 keyfile To add a new passphrase to a specific key-slot, use something like this: cryptsetup luksAddKey --key-slot 7 /dev/loop0 keyfile To supply a key from file to any LUKS command, use the --key-file option, e.g. like this: cryptsetup luksOpen --key-file keyfile /dev/loop0 e1 * How do I read the LUKS master key from file? The question you should ask yourself first, is why you would want to do this. The only legitimate reason I can think of is if you want to have two LUKS devices with the same master key. Even then, I think it would be preferable to just use key-slots with the same passphrase, or to use plain dm-crypt instead. If you really have a good reason, please tell me. If I am convinced, I will add how to do this here. * What are the security requirements for a key read from file? A file-stored key or passphrase has the same security requirements as one entered interactively, however you can use random bytes and thereby use bytes you cannot type on the keyboard. You can use any file you like as key file, for example a plain text file with a human readable passphrase. To generate a file with random bytes, use something like this: head -c 256 /dev/random > keyfile * If I map a journaled file system using dm-crypt/LUKS, does it still provide its usual transactional guarantees? As far as I know you do (but I may be wrong), but please note that these "guarantees" are far weaker than they appear to be. For example, you not not get a hard flush to disk surface even on a call to fsync. In addition, the HDD itself may do independent write reordering. Some other things can go wrong as well. The filesystem developers are aware of these problems and typically can make it work anyways. That said, dm-crypt/LUKS should not make things worse. Personally, I have several instances of ext3 on dm-crypt and have not noticed any specific issues so far. * Can I use LUKS or cryptsetup with a more secure (external) medium for key storage, e.g. TPM or a smartcard? Yes, see the answers on using a file-supplied key. You do have to write the glue-logic yourself though. Basically you can have cryptsetup read the key from STDIN and write it there with your own tool that in turn gets the key from the more secure key storage. * Can I resize a dm-crypt or LUKS partition? Yes, you can, as neither dm-crypt nor LUKS stores partition size. Whether you should is a different question. Personally I recommend backup, recreation of the encrypted partition with new size, recreation of the filesystem and restore. This gets around the tricky business of resizing the filesystem. The backup is really non-optional here, as a lot can go wrong, resulting in partial or complete data loss. Using something like gparted to resize an encrypted partition is slow, but pretty safe and should be fine. This will not change the size of the filesystem hidden under the encryption though. You also need to be aware of size-based limitations. The one currently relevant is that aes-xts-plain should not be used for encrypted container sizes larger than 2TB. 3. Common Problems * My dm-crypt/LUKS mapping does not work! What general steps are there to investigate the problem? If you get a specific error message, investigate what it claims first. If not, you may want to check the following things. - Check that "/dev", including "/dev/mapper/control" is there. If it is missing, you may have a problem with the "/dev" tree itself or you may have broken udev rules. - Check that you have the device mapper and the crypt target in your kernel. The output of "dmsetup targets" should list a "crypt" target. If it is not there or the command fails, add device mapper and crypt-target to the kernel. - Check that the hash-functions and ciphers you want to use are in the kernel. The output of "cat /proc/crypto" needs to list them. * My dm-crypt mapping suddenly stopped when upgrading cryptsetup. The default cipher, hash or mode may have changed (the mode changed from 1.0.x to 1.1.x). See under "6. Issues With Specific Versions of cryptsetup". * When I call cryptsetup from cron/CGI, I get errors about unknown features? If you get errors about unknown parameters or the like that are not present when cryptsetup is called from the shell, make sure you have no older version of cryptsetup on your system that then gets called by cron/CGI.For example some distributions install cryptsetup into /usr/sbin, while a manual install could go to /usr/local/sbin. As a debugging aid, call "cryptsetup --version" from cron/CGI or the non-shell mechanism to be sure you have the right version. * Unlocking a LUKS device takes very long. Why? The iteration time for every key-slot (iteration is needed to prevent dictionary attacks) is calculated during the luksFormat operation. By default it is 1 second on the machine where the format operation is done. If you format a device on a fast machine and then unlock it on a slow machine, the unlocking time can be much more longer. Also take into account that up to 8 key-slots have to be tried in order to find the right one. If this is problem, you can add another key-slot using the slow machine with the same passphrase and then remove the old key-slot. The new key-slot will have an iteration count adjusted to 1 second on the slow machine. Use luksKeyAdd and then luksKillSlot or luksRemoveKey. However, this operation will not change volume key iteration count. In order to change that, you will have to backup the data in the LUKS container, luksFormat on the slow machine and restore the data. * "blkid" sees a LUKS UUID and an ext2/swap UUID on the same device. What is wrong? Some old versions of cryptsetup have a bug where the header does not get completely wiped during LUKS format and an older ext2/swap signature remains on the device. This confuses blkid. Fix: Wipe the unused header areas by doing a backup and restore of the header with cryptsetup 1.1.x: cryptsetup luksHeaderBackup --header-backup-file cryptsetup luksHeaderRestore --header-backup-file If you cannot use a 1.1.x cryptsetup, you can also do a manual wipe of the area in question with the command below. Be very, VERY, careful and make sure to do a backup of the header before. If you get this wrong, your device may become permanently inaccessible. dd if=/dev/zero of= bs=512 seek=2 count=6 * cryptsetup segfaults on Gentoo amd64 hardened ... There seems to be some inteference between the hardening and and the way cryptsetup benchmarks PBKDF2. The solution to this is currently not quite clear for an encrypted root filesystem. For other uses, you can apparently specify USE="dynamic" as compile flag, see http://bugs.gentoo.org/show_bug.cgi?id=283470 4. Security Aspects * Should I initialize (overwrite) a new LUKS/dm-crypt partition? If you just create a filesystem on it, most of the old data will still be there. If the old data is sensitive, you should overwrite it before encrypting. In any case, not initializing will leave the old data there until the specific sector gets written. That may enable an attacker to determine how much and where on the partition data was written. If you think this is a risk, you can prevent this by overwriting the encrypted device (here assumed to be named "e1") with zeros like this: dd_rescue -w /dev/zero /dev/mapper/e1 or alternatively with one of the following more standard commands: cat /dev/zero > /dev/mapper/e1 dd if=/dev/zero of=/dev/mapper/e1 * How do I securely erase a LUKS (or other) partition? For LUKS, if you are in a desperate hurry, overwrite the first few kilobytes of the LUKS partition. This erases the salts and makes access impossible. However a LUKS header backup or full backup will still grant access to most or all data. To do this right, overwrite the whole LUKS partition with a single pass of zeros. This is enough for current HDDs. For SDDs you may want to erase the whole drive several times to be sure data is not retained by wear leveling. This is possibly insecure as SDD technology is not fully understood in this regard. Still, due to the anti-forensic properties of the LUKS key-slots, a single overwrite of an SSD could be enough. If in doubt, use physical destruction in addition. Keep in mind to also erase all backups. Example for a zero-overwrite erase of partition sda10 done with dd_rescue: dd_rescue -w /dev/zero /dev/sda10 * How do I securely erase a backup of a LUKS partition or header? That depends on the medium it is stored on. For HDD and SSD, use overwrite with zeros. For an SSD, you may want to overwrite the complete SSD several times and use physical destruction in addition, see last item. Treat USB flash drives the same as SSDs. For re-writable CD/DVD, a single overwrite should also be enough, due to the anti-forensic properties of the LUKS keyslots. For write-once media, use physical destruction. For low security requirements, just cut the CD/DVD into several parts. For high security needs, shred or burn the medium. If your backup is on magnetic tape, I advise physical destruction by shredding or burning. The problem with magnetic tape is that it has a higher dynamic range than HDDs and older data may well be recoverable after overwrites. Also write-head alignment issues can lead to data not actually being deleted at all during overwrites. * Why was the default aes-cbc-plain replaced with aes-cbc-essiv? The problem is that cbc-plain has a fingerprint vulnerability, where a specially crafted file placed into the crypto-container can be recognized from the outside. The issue here is that for cbc-plain the initialization vector (IV) is the sector number. The IV gets XORed to the first data chunk of the sector to be encrypted. If you make sure that the first data block to be stored in a sector contains the sector number as well, the first data block to be encrypted is all zeros and always encrypted to the same ciphertext. This also works if the first data chunk just has a constant XOR with the sector number. By having several shifted patterns you can take care of the case of a non-power-of-two start sector number of the file. This mechanism allows you to create a pattern of sectors that have the same first ciphertext block and signal one bit per sector to the outside, allowing you to e.g. mark media files that way for recognition without decryption. For large files this is a practical attack. For small ones, you do not have enough blocks to signal and take care of different file starting offsets. In order to prevent this attack, the default was changed to cbc-essiv. ESSIV uses a keyed hash of the sector number, with the encryption key as key. This makes the IV unpredictable without knowing the encryption key and the watermarking attack fails. * Are there any problems with "plain" IV? What is "plain64"? First, "plain" and "plain64" are both not safe to use with CBC, see previous FAQ item. However there are modes, like XTS, that are secure with "plain" IV. The next limit is that "plain" is 64 bit, with the upper 32 bit set to zero. This means that on volumes larger than 2TB, the IV repeats, creating a vulnerability that potentially leaks some data. To avoid this, use "plain64", which uses the full sector number up to 64 bit. Note that "plain64" requires a kernel >= 2.6.33. Also note that "plain64" is backwards compatible for volume sizes <= 2TB, but not for those > 2TB. Finally, "plain64" does not cause any performance penalty compared to "plain". * What about XTS mode? 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 have to specify it manually as "aes-xts-plain", i.e. cryptsetup -c aes-xts-plain luksFormat For volumes >2TB and kernels >= 2.6.33 use "plain64" (see FAQ item on "plain" and "plain64"): cryptsetup -c aes-xts-plain64 luksFormat 5. Backup and Data Recovery * *What happens if I overwrite the start of a LUKS partition or damage the LUKS header or key-slots?* There are two critical components for decryption: The salt values in the header itself and the key-slots. If the salt values are overwritten or changed, nothing (in the cryptographically strong sense) can be done to access the data, unless there is a backup of the LUKS header. If a key-slot is damaged, the data can still be read with a different keys-lot, if one is in use. * What does the on-disk structure of dm-crypt look like? There is none. dm-crypt takes a block device and gives encrypted access to each of its blocks with a key derived from the passphrase given. If you use a cipher different than the default, you have to specify that as a parameter to cryptsetup too. If you want to 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 overwrote. * What does the on-disk structure of LUKS look like? A LUKS partition consists of a header, followed by 8 key-slot descriptors, followed by 8 key slots, followed by the encrypted data area. Header and key-slot descriptors fill the first 592 bytes. The key-slot size depends on the creation parameters, namely on the number of anti-forensic stripes and on key block alignment. With 4000 stripes (the default), each key-slot is a bit less than 128kB in size. Due to sector alignment of the key-slot start, that means the key block 0 is at offset 0x1000-0x20400, key block 1 at offset 0x21000-0x40400, and key block 7 at offset 0xc1000-0xe0400. The space to the next full sector address is padded with zeros. Never used key-slots are filled with what the disk originally contained there, a key-slot removed with "luksRemoveKey" or "luksKillSlot" gets filled with 0xff. Start of bulk data (with the default 4000 stripes and 8 key-slots) is at 0x101000, i.e. at 1'052'672 bytes, i.e. at 1MiB + 4096 bytes from the start of the partition. This is also the value given by command "luksDump" with "Payload offset: 2056", just multiply by the sector size (512 bytes). Incidentally, "luksHeaderBackup" dumps exactly the first 1'052'672 bytes to file and "luksHeaderRestore" restores them. The exact specification of the format is here: http://code.google.com/p/cryptsetup/wiki/Specification * How do I backup a LUKS header? While you could just copy the appropriate number of bytes from the start of the LUKS partition, the best way is to use command option "luksHeaderBackup" of cryptsetup. This protects also against errors when non-standard parameters have been used in LUKS partition creation. Example: cryptsetup luksHeaderBackup --header-backup-file h_bak /dev/mapper/c1 * How do I backup a LUKS partition? You do a sector-image of the whole partition. This will contain the LUKS header, the keys-slots and the data ares. It can be done under Linux e.g. with dd_rescue (for a direct image copy) and with "cat" or "dd". Example: cat /dev/sda10 > sda10.img dd_rescue /dev/sda10 sda10.img You can also use any other backup software that is capable of making a sector image of a partition. Note that compression is ineffective for encrypted data, hence it does not sense to use it. * Do I need a backup of the full partition? Would the header and key-slots not be enough? Backup protects you against two things: Disk loss or corruption and user error. By far the most questions on the dm-crypt mailing list about how to recover a damaged LUKS partition are related to user error. For example, if you create a new filesystem on a LUKS partition, chances are good that all data is lost permanently. For this case, a header+key-slot backup would often be enough. But keep in mind that a HDD has roughly a failure risk of 5% per year. It is highly advisable to have a complete backup to protect against this case. * Are there security risks from a backup of the LUKS header or a whole LUKS partition? Yes. One risk is that if you remove access rights for specific key-slots by deleting their contents, the data can still be accessed with invalidated passphrase and the backup. The other risk is that if you erase a LUKS partition, a backup could still grant access, especially if you only erased the LUKS header and not the whole partition. * I think this is overly complicated. Is there an alternative? Yes, you can use plain dm-crypt. It does not allow multiple passphrases, but on the plus side, it has zero on disk description and if you overwrite some part of a plain dm-crypt partition, exactly the overwritten parts are lost (rounded up to sector borders). 6. Issues with Specific Versions of cryptsetup * When using the create command for plain dm-crypt with cryptsetup 1.1.x, the mapping is incompatible and my data is not accessible anymore! With cryptsetup 1.1.x, the distro maintainer can define different default encryption modes for LUKS and plain devices. You can check these compiled-in defaults using "cryptsetup --help". Moreover, the plain device default changed because the old IV mode was vulnerable to a watermarking attack. If you are using a plain device and you need a compatible mode, just specify cipher, key size and hash algorithm explicitly. For compatibility with cryptsetup 1.0.x defaults, simple use the following: cryptsetup create -c aes-cbc-plain -s 256 -h ripemd160 LUKS stores cipher and mode in the metadata on disk, avoiding this problem. * cryptsetup on SLED 10 has problems... SLED 10 is missing an essential kernel patch for dm-crypt, which is broken in its kernel as a result. There may be a very old version of cryptsetup (1.0.x) provided by SLED, which should also not be used anymore as well. My advice would be to drop SLED 10. A. Contributors In no particular order: - Arno Wagner - Milan Broz