Sections 1. General Questions 2. Setup 3. Common Problems 4. Troubleshooting 5. Security Aspects 6. Backup and Data Recovery 7. 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 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. Usually, 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! * Who wrote this? Current FAQ maintainer is Arno Wagner . Other contributors are listed at the end. 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 100MiB 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". * When I add a new key-slot to LUKS, it asks for a passphrase but then complains about there not being a key-slot with that passphrase? That is as intended. You are asked a passphrase of an existing key-slot first, before you can enter the passphrase for the new key-slot. Otherwise you could break the encryption by just adding a new key-slot. This way, you have to know the passphrase of one of the already configured key-slots in order to be able to configure a new key-slot. * 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. Update: I did run into frequent small freezes (1-2 sec) when putting a vmware image on ext3 over dm-crypt. This does indicate that the transactional guarantees are in place, but at a cost. When I went back to ext2, the problem went away. * 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 2TiB. Use aes-xts-plain64 for that. 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 "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. Troubleshooting * Can a bad RAM module cause problems? LUKS and dm-crypt can give the RAM quite a workout, especially when combined with software RAID. In particular the combination RAID5 + LUKS + XFS seems to uncover RAM problems that never caused obvious problems before. Symptoms vary, but often the problem manifest itself when copying large amounts of data, typically several times larger than your main memory. Side note: One thing you should always do on large data movements is to run a verify, for example with the "-d" option of "tar" or by doing a set of MD5 checksums on the source or target with find . -type f -exec md5sum \{\} \; > checksum-file and then a "md5sum -c checksum-file" on the other side. If you get mismatches here, RAM is the primary suspect. A lesser suspect is an overclocked CPU. I have found countless hardware problems in verify runs after copying or making backups. Bit errors are much more common than most people think. Some RAM issues are even worse and corrupt structures in one of the layers. This typically results in lockups, CPU state dumps in the system logs, kernel panic or other things. It is quite possible to have the problem with an encrypted device, but not with an otherwise the same unencrypted device. The reason for that is that encryption has an error amplification property: You flip one bit in an encrypted data block, and the decrypted version has half of its bits flipped. This is an important security property for modern ciphers. With the usual modes in cryptsetup (CBC, ESSIV, XTS), you get up to a completely changed 512 byte block per bit error. A corrupt block causes a lot more havoc than the occasionally flipped single bit and can result various obscure errors. Note however that a verify run on copying between encrypted or unencrypted devices can also show you corruption when the copying itself did not report any problems. If you find defect RAM, assume all backups and copied data to be suspect, unless you did a verify. * How do I test RAM? First you should know that overclocking often makes memory problems worse. So if you overclock (which I strongly recommend against in a system holding data that has some worth), run the tests with the overclocking active. There are two good options. One is Memtest86+ and the other is "memtester" by Charles Cazabon. Memtest86+ requires a reboot and then takes over the machine, while memtester runs from a root-shell. Both use different testing methods and I have found problems fast with each one that the other needed long to find. I recommend running the following procedure until the first error is found: - Run Memtest86+ for one cycle - Run memterster for one cycle (shut down as many other applications as possible) - Run Memtest86+ for 24h or more - Run memtester for 24h or more If all that does not produce error messages, your RAM may be sound, but I have had one weak bit that Memtest86+ needed around 60 hours to find. If you can reproduce the original problem reliably, a good additional test may be to remove half of the RAM (if you have more than one module) and try whether the problem is still there and if so, try with the other half. If you just have one module, get a different one and try with that. If you do overclocking, reduce the settings to the most conservative ones available and try with that. 5. 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. * What about backup? Does it compromise security? That depends. See next section. * 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 2TiB, 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 <= 2TiB, but not for those > 2TiB. 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 >2TiB and kernels >= 2.6.33 use "plain64" (see FAQ item on "plain" and "plain64"): cryptsetup -c aes-xts-plain64 luksFormat There is a potential security issue with XTS mode and large blocks. LUKS and dm-crypt always use 512B blocks and the issue does not apply. 6. Backup and Data Recovery * Does a backup compromise security? Depends on how you do it. First, a backup is non-optional with encrypted data just the same way it is with non-encrypted data. Disks do break and they do not care whether they make plain or encrypted data inaccessible. However there are risks introduced by backups. For example if you change/disable a key-slot in LUKS, a binary backup of the partition will still have the old key-slot. To deal with this, you have to be able to change the key-slot on the backup as well, or use a different set-up. One option is to have a different passphrase on the backup and to make the backup with both containers open. Another one is to make a backup of the original, opened container to a single file, e.g. with tar, and to encrypt that file with public-key-cryptography, e.g. with GnuPG. You can then keep the secret key in a safe place, because it is only used to decrypt a backup. The key the backup is encrypted with can be stored without special security measures, as long as an attacker cannot replace it with his own key. If you use dm-crypt, backup is simpler: As there is no key management, the main risk is that you cannot wipe the backup when wiping the original. However wiping the original for dm-crypt should consist of forgetting the passphrase and that you can do without actual access to the backup. In both cases, there is an additional (usually small) risk: An attacker can see how many sectors and which ones have been changed since the backup. This is not possible with the public-key method though. My personal advice is to use one USB disk (low value date) or three disks (high value data) in rotating order for backups, and either use different passphrases or keep them easily accessible in case you need to disable a key-slot. If you do network-backup or tape-backup, I strongly recommend to go the public-key path, especially as you typically cannot reliably delete data in these scenarios. (Well, you can burn the tape if it is under your control...) * 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 key-slot, if there is a remaining undamaged and used key-slot. Note that in order to make a key-slot unrecoverable in a cryptographically strong sense, changing about 4-6 bits in random locations of its 128kiB size is quite enough. * What happens if I (quick) format a LUKS partition? I have not tried the different ways to do this, but very likely you will have written a new boot-sector, which in turn overwrites the LUKS header, including the salts. You may also damage the key-slots in part or in full. See also last item. * 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 128kiB 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). 7. 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