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@@ -0,0 +1,3019 @@ +Frequently Asked Questions Cryptsetup/LUKS + +Sections +1. General Questions +2. Setup +3. Common Problems +4. Troubleshooting +5. Security Aspects +6. Backup and Data Recovery +7. Interoperability with other Disk Encryption Tools +8. Issues with Specific Versions of cryptsetup +9. The Initrd question +10. LUKS2 Questions +11. References and Further Reading +A. Contributors + +1. General Questions + + + * 1.1 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 metadata on disk) and LUKS (multiple user keys with one + master key, anti-forensic features, metadata block at start of device, + ...). The latest version of this FAQ should usually be available at + https://gitlab.com/cryptsetup/cryptsetup/wikis/FrequentlyAskedQuestions + + + * 1.2 WARNINGS + + LUKS2 COMPATIBILITY: This FAQ was originally written for LUKS1, not + LUKS2. Hence regarding LUKS2, some of the answers found here may not + apply. Updates for LUKS2 have been done and anything not applying to + LUKS2 should clearly say LUKS1. However, this is a Frequently Asked + Questions, and questions for LUKS2 are limited at this time or at least + those that have reached me are. In the following, "LUKS" refers to both + LUKS1 and LUKS2. + + The LUKS1 on-disk format specification is at + https://www.kernel.org/pub/linux/utils/cryptsetup/LUKS_docs/on-disk-format.pdf + The LUKS2 on-disk format specification is at + https://gitlab.com/cryptsetup/LUKS2-docs + + 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 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! In particular, make + sure you have a current header backup before doing any potentially + dangerous operations. The LUKS2 header should be a bit more resilient + as critical data starts later and is stored twice, but you can decidedly + still destroy it or a keyslot permanently by accident. + + DEBUG COMMANDS: While the --debug and --debug-json options should not + leak secret data, "strace" and the like can leak your full passphrase. + Do not post an strace output with the correct passphrase to a + mailing-list or online! See Item 4.5 for more explanation. + + SSDs/FLASH DRIVES: SSDs and Flash are different. Currently it is + unclear how to get LUKS or plain dm-crypt to run on them with the full + set of security assurances intact. This may or may not be a problem, + depending on the attacker model. See Section 5.19. + + 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. + + CLONING/IMAGING: If you clone or image a LUKS container, you make a copy + of the LUKS header and the master key will stay the same! That means + that if you distribute an image to several machines, the same master key + will be used on all of them, regardless of whether you change the + passphrases. Do NOT do this! If you do, a root-user on any of the + machines with a mapped (decrypted) container or a passphrase on that + machine can decrypt all other copies, breaking security. See also Item + 6.15. + + 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. + + UBUNTU INSTALLER: In particular the Ubuntu installer seems to be quite + willing to kill LUKS containers in several different ways. Those + responsible at Ubuntu seem not to care very much (it is very easy to + recognize a LUKS container), so treat the process of installing Ubuntu + as a severe hazard to any LUKS container you may have. + + NO WARNING ON NON-INTERACTIVE FORMAT: If you feed cryptsetup from STDIN + (e.g. via GnuPG) on LUKS format, it does not give you the warning that + you are about to format (and e.g. will lose any pre-existing LUKS + container on the target), as it assumes it is used from a script. In + this scenario, the responsibility for warning the user and possibly + checking for an existing LUKS header is shifted to the script. This is + a more general form of the previous item. + + 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 95 + 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 characters can, e.g. be found on + https://en.wikipedia.org/wiki/ASCII + + KEYBOARD NUM-PAD: Apparently some pre-boot authentication environments + (these are done by the distro, not by cryptsetup, so complain there) + treat digits entered on the num-pad and ones entered regularly + different. This may be because the BIOS USB keyboard driver is used and + that one may have bugs on some computers. If you cannot open your + device in pre-boot, try entering the digits over the regular digit keys. + + + * 1.3 System specific warnings + + - The Ubuntu Natty uinstaller has a "won't fix" defect that may destroy + LUKS containers. This is quite old an not relevant for most people. + Reference: + https://bugs.launchpad.net/ubuntu/+source/partman-crypto/+bug/420080 + + + * 1.4 My LUKS-device is broken! Help! + + First: Do not panic! In many cases the data is still recoverable. + Do not do anything hasty! Steps: + + - Take some deep breaths. Maybe add some relaxing music. This may + sound funny, but I am completely serious. Often, critical damage is + done only after the initial problem. + + - Do not reboot. The keys may still be in the kernel if the device is + mapped. + + - Make sure others do not reboot the system. + + - Do not write to your disk without a clear understanding why this will + not make matters worse. Do a sector-level backup before any writes. + Often you do not need to write at all to get enough access to make a + backup of the data. + + - Relax some more. + + - Read section 6 of this FAQ. + + - Ask on the mailing-list if you need more help. + + + * 1.5 Who wrote this? + + Current FAQ maintainer is Arno Wagner <arno@wagner.name>. If you want + to send me encrypted email, my current PGP key is DSA key CB5D9718, + fingerprint 12D6 C03B 1B30 33BB 13CF B774 E35C 5FA1 CB5D 9718. + + 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. You can also send more raw information and have + me write the section. 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 + https://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. + + + * 1.6 Where is the project website? + + There is the project website at + https://gitlab.com/cryptsetup/cryptsetup/ Please do not post + questions there, nobody will read them. Use the mailing-list + instead. + + + * 1.7 Is there a mailing-list? + + Instructions on how to subscribe to the mailing-list are on the + project website. People are generally helpful and friendly on the + list. + + The question of how to unsubscribe from the list does crop up sometimes. + For this you need your list management URL, which is sent to you + initially and once at the start of each month. Go to the URL mentioned + in the email and select "unsubscribe". This page also allows you to + request a password reminder. + + Alternatively, you can send an Email to dm-crypt-request@saout.de with + just the word "help" in the subject or message body. Make sure to send + it from your list address. + + The mailing list archive is here: + https://marc.info/?l=dm-crypt + + + * 1.8 Unsubscribe from the mailing-list + + Send mail to dm-crypt-unsubscribe@saout.de from the subscribed account. + You will get an email with instructions. + + Basically, you just have to respond to it unmodified to get + unsubscribed. The listserver admin functions are not very fast. It can + take 15 minutes or longer for a reply to arrive (I suspect greylisting + is in use), so be patient. + + Also note that nobody on the list can unsubscribe you, sending demands + to be unsubscribed to the list just annoys people that are entirely + blameless for you being subscribed. + + If you are subscribed, a subscription confirmation email was sent to + your email account and it had to be answered before the subscription + went active. The confirmation emails from the listserver have subjects + like these (with other numbers): + + Subject: confirm 9964cf10..... + + and are sent from dm-crypt-request@saout.de. You should check whether + you have anything like it in your sent email folder. If you find + nothing and are sure you did not confirm, then you should look into a + possible compromise of your email account. + + +2. Setup + + * 2.1 LUKS Container Setup mini-HOWTO + + This item tries to give you a very brief list of all the steps you + should go through when creating a new LUKS encrypted container, i.e. + encrypted disk, partition or loop-file. + + 01) All data will be lost, if there is data on the target, make a + backup. + + 02) Make very sure you use the right target disk, partition or + loop-file. + + 03) If the target was in use previously, it is a good idea to wipe it + before creating the LUKS container in order to remove any trace of old + file systems and data. For example, some users have managed to run + e2fsck on a partition containing a LUKS container, possibly because of + residual ext2 superblocks from an earlier use. This can do arbitrary + damage up to complete and permanent loss of all data in the LUKS + container. + + To just quickly wipe file systems (old data may remain), use + + wipefs -a <target device> + + To wipe file system and data, use something like + + cat /dev/zero > <target device> + + This can take a while. To get a progress indicator, you can use the + tool dd_rescue (->google) instead or use my stream meter "wcs" (source + here: https://www.tansi.org/tools/index.html) in the following fashion: + + cat /dev/zero | wcs > <target device> + + Plain "dd" also gives you the progress on a SIGUSR1, see its man-page. + + Be very sure you have the right target, all data will be lost! + + Note that automatic wiping is on the TODO list for cryptsetup, so at + some time in the future this will become unnecessary. + + Alternatively, plain dm-crypt can be used for a very fast wipe with + crypto-grade randomness, see Item 2.19 + + 04) Create the LUKS container. + + LUKS1: + + cryptsetup luksFormat --type luks1 <target device> + + LUKS2: + + cryptsetup luksFormat --type luks2 <target device> + + + Just follow the on-screen instructions. + + Note: Passphrase iteration count is based on time and hence security + level depends on CPU power of the system the LUKS container is created + on. For example on a Raspberry Pi and LUKS1, I found some time ago that + the iteration count is 15 times lower than for a regular PC (well, for + my old one). Depending on security requirements, this may need + adjustment. For LUKS1, you can just look at the iteration count on + different systems and select one you like. You can also change the + benchmark time with the -i parameter to create a header for a slower + system. + + For LUKS2, the parameters are more complex. ARGON2 has iteration, + parallelism and memory parameter. cryptsetup actually may adjust the + memory parameter for time scaling. Hence to use -i is the easiest way + to get slower or faster opening (default: 2000 = 2sec). Just make sure + to not drop this too low or you may get a memory parameter that is to + small to be secure. The luksDump command lists the memory parameter of + a created LUKS2 keyslot in kB. That parameter should probably be not + much lower than 100000, i.e. 100MB, but don't take my word for it. + + 05) Map the container. Here it will be mapped to /dev/mapper/c1: + + cryptsetup luksOpen <target device> c1 + + 06) (Optionally) wipe the container (make sure you have the right + target!): + + cat /dev/zero > /dev/mapper/c1 + + This will take a while. Note that this creates a small information + leak, as an attacker can determine whether a 512 byte block is zero if + the attacker has access to the encrypted container multiple times. + Typically a competent attacker that has access multiple times can + install a passphrase sniffer anyways, so this leakage is not very + significant. For getting a progress indicator, see step 03. + + 07) Create a file system in the mapped container, for example an + ext3 file system (any other file system is possible): + + mke2fs -j /dev/mapper/c1 + + 08) Mount your encrypted file system, here on /mnt: + + mount /dev/mapper/c1 /mnt + + 09) Make a LUKS header backup and plan for a container backup. + See Section 6 for details. + + Done. You can now use the encrypted file system to store data. Be sure + to read through the rest of the FAQ, these are just the very basics. In + particular, there are a number of mistakes that are easy to make, but + will compromise your security. + + + * 2.2 LUKS on partitions or raw disks? What about RAID? + + Also see Item 2.8. + This is a complicated question, and made more so by the availability of + RAID and LVM. I will try to give some scenarios and discuss advantages + and disadvantages. Note that I say LUKS for simplicity, but you can do + all the things described with plain dm-crypt as well. Also note that + your specific scenario may be so special that most or even all things I + say below do not apply. + + Be aware that if you add LVM into the mix, things can get very + complicated. Same with RAID but less so. In particular, data recovery + can get exceedingly difficult. Only add LVM if you have a really good + reason and always remember KISS is what separates an engineer from an + amateur. Of course, if you really need the added complexity, KISS is + satisfied. But be very sure as there is a price to pay for it. In + engineering, complexity is always the enemy and needs to be fought + without mercy when encountered. + + Also consider using RAID instead of LVM, as at least with the old + superblock format 0.90, the RAID superblock is in the place (end of + disk) where the risk of it damaging the LUKS header is smallest and you + can have your array assembled by the RAID controller (i.e. the kernel), + as it should be. Use partition type 0xfd for that. I recommend staying + away from superblock formats 1.0, 1.1 and 1.2 unless you really need + them. + + Scenarios: + + (1) Encrypted partition: Just make a partition to your liking, and put + LUKS on top of it and a filesystem into the LUKS container. This gives + you isolation of differently-tasked data areas, just as ordinary + partitioning does. You can have confidential data, non-confidential + data, data for some specific applications, user-homes, root, etc. + Advantages are simplicity as there is a 1:1 mapping between partitions + and filesystems, clear security functionality and the ability to + separate data into different, independent (!) containers. + + Note that you cannot do this for encrypted root, that requires an + initrd. On the other hand, an initrd is about as vulnerable to a + competent attacker as a non-encrypted root, so there really is no + security advantage to doing it that way. An attacker that wants to + compromise your system will just compromise the initrd or the kernel + itself. The better way to deal with this is to make sure the root + partition does not store any critical data and to move that to + additional encrypted partitions. If you really are concerned your root + partition may be sabotaged by somebody with physical access (who would + however strangely not, say, sabotage your BIOS, keyboard, etc.), protect + it in some other way. The PC is just not set-up for a really secure + boot-chain (whatever some people may claim). + + (2) Fully encrypted raw block device: For this, put LUKS on the raw + device (e.g. /dev/sdb) and put a filesystem into the LUKS container, no + partitioning whatsoever involved. This is very suitable for things like + external USB disks used for backups or offline data-storage. + + (3) Encrypted RAID: Create your RAID from partitions and/or full + devices. Put LUKS on top of the RAID device, just if it were an + ordinary block device. Applications are just the same as above, but you + get redundancy. (Side note as many people seem to be unaware of it: You + can do RAID1 with an arbitrary number of components in Linux.) See also + Item 2.8. + + (4) Now, some people advocate doing the encryption below the RAID layer. + That has several serious problems. One is that suddenly debugging RAID + issues becomes much harder. You cannot do automatic RAID assembly + anymore. You need to keep the encryption keys for the different RAID + components in sync or manage them somehow. The only possible advantage + is that things may run a little faster as more CPUs do the encryption, + but if speed is a priority over security and simplicity, you are doing + this wrong anyways. A good way to mitigate a speed issue is to get a + CPU that does hardware AES as most do today. + + + * 2.3 How do I set up encrypted swap? + + As things that are confidential can end up in swap (keys, passphrases, + etc. are usually protected against being swapped to disk, but other + things may not be), it may be advisable to do something about the issue. + One option is to run without swap, which generally works well in a + desktop-context. It may cause problems in a server-setting or under + special circumstances. The solution to that is to encrypt swap with a + random key at boot-time. + + NOTE: This is for Debian, and should work for Debian-derived + distributions. For others you may have to write your own startup script + or use other mechanisms. + + 01) Add the swap partition to /etc/crypttab. A line like the + following should do it: + + swap /dev/<partition> /dev/urandom swap,noearly + + Warning: While Debian refuses to overwrite partitions with a filesystem + or RAID signature on it, as your disk IDs may change (adding or removing + disks, failure of disk during boot, etc.), you may want to take + additional precautions. Yes, this means that your kernel device names + like sda, sdb, ... can change between reboots! This is not a concern + if you have only one disk. One possibility is to make sure the + partition number is not present on additional disks or also swap there. + Another is to encapsulate the swap partition (by making it a 1-partition + RAID1 or by using LVM), as that gets a persistent identifier. + Specifying it directly by UUID does not work, unfortunately, as the UUID + is part of the swap signature and that is not visible from the outside + due to the encryption and in addition changes on each reboot with this + setup. + + Note: Use /dev/random if you are paranoid or in a potential low-entropy + situation (embedded system, etc.). This may cause the operation to take + a long time during boot however. If you are in a "no entropy" + situation, you cannot encrypt swap securely. In this situation you + should find some entropy, also because nothing else using crypto will be + secure, like ssh, ssl or GnuPG. + + Note: The "noearly" option makes sure things like LVM, RAID, etc. are + running. As swap is non-critical for boot, it is fine to start it late. + + 02) Add the swap partition to /etc/fstab. A line like the following + should do it: + + /dev/mapper/swap none swap sw 0 0 + + That is it. Reboot or start it manually to activate encrypted swap. + Manual start would look like this: + + /etc/init.d/cryptdisks start + swapon /dev/mapper/swap + + + * 2.4 What is the difference between "plain" and LUKS format? + + First, unless you happen to understand the cryptographic background + well, you should use LUKS. It does protect the user from a lot of + common mistakes. Plain dm-crypt is for experts. + + Plain format is just that: It has no metadata on disk, reads all + parameters from the commandline (or the defaults), derives a master-key + from the passphrase and then uses that to de-/encrypt the sectors of the + device, with a direct 1:1 mapping between encrypted and decrypted + sectors. + + Primary advantage is high resilience to damage, as one damaged encrypted + sector results in exactly one damaged decrypted sector. Also, it is not + readily apparent that there even is encrypted data on the device, as an + overwrite with crypto-grade randomness (e.g. from + /dev/urandom) looks exactly the same on disk. + + Side-note: That has limited value against the authorities. In civilized + countries, they cannot force you to give up a crypto-key anyways. In + quite a few countries around the world, they can force you to give up + the keys (using imprisonment or worse to pressure you, sometimes without + due process), and in the worst case, they only need a nebulous + "suspicion" about the presence of encrypted data. Sometimes this + applies to everybody, sometimes only when you are suspected of having + "illicit data" (definition subject to change) and sometimes specifically + when crossing a border. Note that this is going on in countries like + the US and the UK to different degrees and sometimes with courts + restricting what the authorities can actually demand. + + My advice is to either be ready to give up the keys or to not have + encrypted data when traveling to those countries, especially when + crossing the borders. The latter also means not having any high-entropy + (random) data areas on your disk, unless you can explain them and + demonstrate that explanation. Hence doing a zero-wipe of all free + space, including unused space, may be a good idea. + + Disadvantages are that you do not have all the nice features that the + LUKS metadata offers, like multiple passphrases that can be changed, the + cipher being stored in the metadata, anti-forensic properties like + key-slot diffusion and salts, etc.. + + LUKS format uses a metadata header and 8 key-slot areas that are being + placed at the beginning of the disk, see below under "What does the LUKS + on-disk format looks like?". The passphrases are used to decrypt a + single master key that is stored in the anti-forensic stripes. LUKS2 + adds some more flexibility. + + Advantages are a higher usability, automatic configuration of + non-default crypto parameters, defenses against low-entropy passphrases + like salting and iterated PBKDF2 or ARGON 2 passphrase hashing, the + ability to change passphrases, and others. + + Disadvantages are that it is readily obvious there is encrypted data on + disk (but see side note above) and that damage to the header or + key-slots usually results in permanent data-loss. See below under "6. + Backup and Data Recovery" on how to reduce that risk. Also the sector + numbers get shifted by the length of the header and key-slots and there + is a loss of that size in capacity. Unless you have a specific need, + use LUKS2. + + + * 2.5 Can I encrypt an 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. + + + * 2.6 How do I use LUKS with a loop-device? + + This can be very handy for experiments. Setup is 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 file to /dev/loop0 + cryptsetup luksFormat --type luks2 /dev/loop0 # create LUKS2 container + + Afterwards just use /dev/loop0 as a you would use a LUKS partition. + To unmap the file when done, use "losetup -d /dev/loop0". + + + * 2.7 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. + + + * 2.8 Encryption on top of RAID or the other way round? + + Also see Item 2.2. + Unless you have special needs, place encryption between RAID and + filesystem, i.e. encryption on top of RAID. You can do it the other + way round, but you have to be aware that you then need to give the + passphrase for each individual disk and RAID auto-detection will not + work anymore. Therefore it is better to encrypt the RAID device, e.g. + /dev/dm0 . + + This means that the typical layering looks like this: + + Filesystem <- top + | + Encryption (LUKS) + | + RAID + | + Raw partitions (optional) + | + Raw disks <- bottom + + The big advantage of this is that you can manage the RAID container just + like any other regular RAID container, it does not care that its content + is encrypted. This strongly cuts down on complexity, something very + valuable with storage encryption. + + + * 2.9 How do I read a dm-crypt key from file? + + Use the --key-file option, like this: + + cryptsetup create --key-file keyfile e1 /dev/loop0 + + This will read the binary key from file, i.e. no hashing or + transformation will be applied to the keyfile before its bits are used + as key. Extra bits (beyond the length of the key) at the end are + ignored. Note that if you read from STDIN, the data will be hashed, + just as a key read interactively from the terminal. See the man-page + sections "NOTES ON PASSPHRASE PROCESSING..." for more detail. + + + * 2.10 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. Make sure + no trailing newline (0x0a) is contained in the input 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 + + + + * 2.11 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. + + + * 2.12 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 + + + + * 2.13 If I map a journaled file system using dm-crypt/LUKS, does + it still provide its usual transactional guarantees? + + Yes, it does, unless a very old kernel is used. The required flags come + from the filesystem layer and are processed and passed onward by + dm-crypt (regardless of direct key management or LUKS key management). + A bit more information on the process by which transactional guarantees + are implemented can be found here: + + https://lwn.net/Articles/400541/ + + Please note that these "guarantees" are weaker than they appear to be. + One problem is that quite a few disks lie to the OS about having flushed + their buffers. This is likely still true with SSDs. 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 will not make things worse. + + One specific problem you can run into is that you can get short freezes + and other slowdowns due to the encryption layer. Encryption takes time + and forced flushes will block for that time. For example, I did run + into frequent small freezes (1-2 sec) when putting a vmware image on + ext3 over dm-crypt. When I went back to ext2, the problem went away. + This seems to have gotten better with kernel 2.6.36 and the reworking of + filesystem flush locking mechanism (less blocking of CPU activity during + flushes). This should improve further and eventually the problem should + go away. + + + * 2.14 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. + + + * 2.15 Can I resize a dm-crypt or LUKS container? + + Yes, you can, as neither dm-crypt nor LUKS1 stores partition size and + LUKS2 uses a generic "whole device" size as default. Note that LUKS2 + can use specified data-area sizes as a non-standard case and that these + may cause issues when resizing a LUKS2 container if set to a specific + value. + + Whether you should do this is a different question. Personally I + recommend backup, recreation of the dm-crypt or LUKS container with new + size, recreation of the filesystem and restore. This gets around the + tricky business of resizing the filesystem. Resizing a dm-crypt or LUKS + container does not resize the filesystem in it. A backup is really + non-optional here, as a lot can go wrong, resulting in partial or + complete data loss. But if you have that backup, you can also just + recreate everything. + + 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. + + + * 2.16 How do I Benchmark the Ciphers, Hashes and Modes? + + Since version 1.60 cryptsetup supports the "benchmark" command. + Simply run as root: + + cryptsetup benchmark + + You can get more than the default benchmarks, see the man-page for the + relevant parameters. Note that XTS mode takes two keys, hence the + listed key sizes are double that for other modes and half of it is the + cipher key, the other half is the XTS key. + + + * 2.17 How do I Verify I have an Authentic cryptsetup Source Package? + + Current maintainer is Milan Broz and he signs the release packages with + his PGP key. The key he currently uses is the "RSA key ID D93E98FC", + fingerprint 2A29 1824 3FDE 4664 8D06 86F9 D9B0 577B D93E 98FC. While I + have every confidence this really is his key and that he is who he + claims to be, don't depend on it if your life is at stake. For that + matter, if your life is at stake, don't depend on me being who I claim + to be either. + + That said, as cryptsetup is under good version control and a malicious + change should be noticed sooner or later, but it may take a while. + Also, the attacker model makes compromising the sources in a non-obvious + way pretty hard. Sure, you could put the master-key somewhere on disk, + but that is rather obvious as soon as somebody looks as there would be + data in an empty LUKS container in a place it should not be. Doing this + in a more nefarious way, for example hiding the master-key in the salts, + would need a look at the sources to be discovered, but I think that + somebody would find that sooner or later as well. + + That said, this discussion is really a lot more complicated and longer + as an FAQ can sustain. If in doubt, ask on the mailing list. + + + * 2.18 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. Also note that SSDs typically have much larger + blocks internally (e.g. 128kB or even larger). + + + * 2.19 How can I wipe a device with crypto-grade randomness? + + The conventional recommendation if you want to do more than just a + zero-wipe is to use something like + + cat /dev/urandom > <target-device> + + That used to very slow and painful at 10-20MB/s on a fast computer, but + newer kernels can give you > 200MB/s (depending on hardware). An + alternative is using cryptsetup and a plain dm-crypt device with a + random key, which is fast and on the same level of security. The + defaults are quite enough. + + For device set-up, do the following: + + cryptsetup open --type plain -d /dev/urandom /dev/<device> target + + This maps the container as plain under /dev/mapper/target with a random + password. For the actual wipe you have several options. Basically, you + pipe zeroes into the opened container that then get encrypted. Simple + wipe without progress-indicator: + + cat /dev/zero > /dev/mapper/to_be_wiped + + Progress-indicator by dd_rescue: + + dd_rescue -w /dev/zero /dev/mapper/to_be_wiped + + Progress-indicator by my "wcs" stream meter (available from + https://www.tansi.org/tools/index.html ): + + cat /dev/zero | wcs > /dev/mapper/to_be_wiped + + Or use plain "dd", which gives you the progress when sent a SIGUSR1, see + the dd man page. + + Remove the mapping at the end and you are done. + + + * 2.20 How do I wipe only the LUKS header? + + This does _not_ describe an emergency wipe procedure, see Item 5.4 for + that. This procedure here is intended to be used when the data should + stay intact, e.g. when you change your LUKS container to use a detached + header and want to remove the old one. Please only do this if you have + a current backup. + + LUKS1: + 01) Determine header size in 512 Byte sectors with luksDump: + + cryptsetup luksDump <device with LUKS container> + +-> ... + Payload offset: <number> + ... + + 02) Take the result number, multiply by 512 zeros and write to + the start of the device, e.g. like this: + + dd bs=512 count=<number> if=/dev/zero of=<device> + + + LUKS2: (warning, untested! Remember that backup?) This assumes the + LUKS2 container uses the defaults, in particular there is only one data + segment. 01) Determine the data-segment offset using luksDump, same + as above for LUKS1: + +-> ... + Data segments: + 0: crypt + offset: <number> [bytes] + ... + + 02) Overwrite the stated number of bytes from the start of the device. + Just to give yet another way to get a defined number of zeros: + + head -c /dev/zero > /dev/<device> + + +3. Common Problems + + + * 3.1 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. + + + * 3.2 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". + + + * 3.3 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 the right version gets called. + + + * 3.4 Unlocking a LUKS device takes very long. Why? + + The unlock time for a key-slot (see Section 5 for an explanation what + iteration does) is calculated when setting a passphrase. By default it + is 1 second (2 seconds for LUKS2). If you set a passphrase on a fast + machine and then unlock it on a slow machine, the unlocking time can be + much longer. Also take into account that up to 8 key-slots (LUKS2: up + to 32 key-slots) have to be tried in order to find the right one. + + If this is the 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 the unlock time adjusted to the slow machine. + Use luksKeyAdd and then luksKillSlot or luksRemoveKey. You can also use + the -i option to reduce iteration time (and security level) when setting + a passphrase. Default is 1000 (1 sec) for LUKS1 and 2000 (2sec) for + LUKS2. + + However, this operation will not change volume key iteration count ("MK + iterations" for LUKS1, "Iterations" under "Digests" for LUKS2). In + order to change that, you will have to backup the data in the LUKS + container (i.e. your encrypted data), luksFormat on the slow machine + and restore the data. Note that MK iterations are not very security + relevant. + + + * 3.5 "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 or later: + + cryptsetup luksHeaderBackup --header-backup-file <file> <device> + cryptsetup luksHeaderRestore --header-backup-file <file> <device> + + + +4. Troubleshooting + + + * 4.1 I get the error "LUKS keyslot x is invalid." What does that mean? + + For LUKS1, this means that the given keyslot has an offset that points + outside the valid keyslot area. Typically, the reason is a corrupted + LUKS1 header because something was written to the start of the device + the LUKS1 container is on. For LUKS2, I do not know when this error can + happen, but I expect it will be something similar. Refer to Section + "Backup and Data Recovery" and ask on the mailing list if you have + trouble diagnosing and (if still possible) repairing this. + + + * 4.2 I cannot unlock my LUKS container! What could be the problem? + + First, make sure you have a correct passphrase. Then make sure you have + the correct key-map and correct keyboard. And then make sure you have + the correct character set and encoding, see also "PASSPHRASE CHARACTER + SET" under Section 1.2. + + If you are sure you are entering the passphrase right, there is the + possibility that the respective key-slot has been damaged. There is no + way to recover a damaged key-slot, except from a header backup (see + Section 6). For security reasons, there is also no checksum in the + key-slots that could tell you whether a key-slot has been damaged. The + only checksum present allows recognition of a correct passphrase, but + that only works with that correct passphrase and a respective key-slot + that is intact. + + In order to find out whether a key-slot is damaged one has to look for + "non-random looking" data in it. There is a tool that automates this + for LUKS1 in the cryptsetup distribution from version 1.6.0 onwards. It + is located in misc/keyslot_checker/. Instructions how to use and how to + interpret results are in the README file. Note that this tool requires + a libcryptsetup from cryptsetup 1.6.0 or later (which means + libcryptsetup.so.4.5.0 or later). If the tool complains about missing + functions in libcryptsetup, you likely have an earlier version from your + distribution still installed. You can either point the symbolic link(s) + from libcryptsetup.so.4 to the new version manually, or you can + uninstall the distribution version of cryptsetup and re-install that + from cryptsetup >= 1.6.0 again to fix this. + + + * 4.3 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 + + LUKS1 + XFS seems to uncover RAM problems that do not cause obvious + problems otherwise. Symptoms vary, but often the problem manifests + itself when copying large amounts of data, typically several times + larger than your main memory. + + Note: One thing you should always do on large data copying or 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 data 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 + a 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: If you flip one bit in an encrypted data block, + the decrypted version has half of its bits flipped. This is actually an + important security property for modern ciphers. With the usual modes in + cryptsetup (CBC, ESSIV, XTS), you can get a completely changed 512 byte + block for a bit error. A corrupt block causes a lot more havoc than the + occasionally flipped single bit and can result in various obscure + errors. + + Note that a verify run on copying between encrypted or unencrypted + devices will reliably detect corruption, even 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. + + + * 4.4 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 any 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 either + 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 memtester for one cycle (shut down as many other applications + as possible and use the largest memory area you can get) + + - 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 in the past 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. + + + * 4.5 Is there a risk using debugging tools like strace? + + There most definitely is. A dump from strace and friends can contain + all data entered, including the full passphrase. Example with strace + and passphrase "test": + + > strace cryptsetup luksOpen /dev/sda10 c1 + ... + read(6, "test\n", 512) = 5 + ... + + Depending on different factors and the tool used, the passphrase may + also be encoded and not plainly visible. Hence it is never a good idea + to give such a trace from a live container to anybody. Recreate the + problem with a test container or set a temporary passphrase like "test" + and use that for the trace generation. Item 2.6 explains how to create + a loop-file backed LUKS container that may come in handy for this + purpose. + + See also Item 6.10 for another set of data you should not give to + others. + + +5. Security Aspects + + + * 5.1 How long is a secure passphrase? + + This is just the short answer. For more info and explanation of some of + the terms used in this item, read the rest of Section 5. The actual + recommendation is at the end of this item. + + First, passphrase length is not really the right measure, passphrase + entropy is. If your passphrase is 200 times the letter "a", it is long + but has very low entropy and is pretty insecure. + + For example, a random lowercase letter (a-z) gives you 4.7 bit of + entropy, one element of a-z0-9 gives you 5.2 bits of entropy, an element + of a-zA-Z0-9 gives you 5.9 bits and a-zA-Z0-9!@#$%\^&:-+ gives you 6.2 + bits. On the other hand, a random English word only gives you 0.6...1.3 + bits of entropy per character. Using sentences that make sense gives + lower entropy, series of random words gives higher entropy. Do not use + sentences that can be tied to you or found on your computer. This type + of attack is done routinely today. + + That said, it does not matter too much what scheme you use, but it does + matter how much entropy your passphrase contains, because an attacker + has to try on average + + 1/2 * 2^(bits of entropy in passphrase) + + different passphrases to guess correctly. + + Historically, estimations tended to use computing time estimates, but + more modern approaches try to estimate cost of guessing a passphrase. + + As an example, I will try to get an estimate from the numbers in + https://gist.github.com/epixoip/a83d38f412b4737e99bbef804a270c40 This + thing costs 23kUSD and does 68Ghashes/sec for SHA1. This is in 2017. + + Incidentally, my older calculation for a machine around 1000 times + slower was off by a factor of about 1000, but in the right direction, + i.e. I estimated the attack to be too easy. Nobody noticed ;-) On the + plus side, the tables are now (2017) pretty much accurate. + + More references can be found at the end of this document. Note that + these are estimates from the defender side, so assuming something is + easier than it actually is is fine. An attacker may still have + significantly higher cost than estimated here. + + LUKS1 used SHA1 (since version 1.7.0 it uses SHA256) for hashing per + default. We will leave aside the check whether a try actually decrypts + a key-slot. I will assume a useful lifetime of the hardware of 2 years. + (This is on the low side.) Disregarding downtime, the machine can then + break + + N = 68*10^9 * 3600 * 24 * 365 * 2 ~ 4*10^18 + + passphrases for EUR/USD 23k. That is one 62 bit passphrase hashed once + with SHA1 for EUR/USD 23k. This can be parallelized, it can be done + faster than 2 years with several of these machines. + + For LUKS2, things look a bit better, as the advantage of using graphics + cards is massively reduced. Using the recommendations below should + hence be fine for LUKS2 as well and give a better security margin. + + For plain dm-crypt (no hash iteration) this is it. This gives (with + SHA1, plain dm-crypt default is ripemd160 which seems to be slightly + slower than SHA1): + + Passphrase entropy Cost to break + 60 bit EUR/USD 6k + 65 bit EUR/USD 200K + 70 bit EUR/USD 6M + 75 bit EUR/USD 200M + 80 bit EUR/USD 6B + 85 bit EUR/USD 200B + ... ... + + + For LUKS1, you have to take into account hash iteration in PBKDF2. + For a current CPU, there are about 100k iterations (as can be queried + with ''cryptsetup luksDump''. + + The table above then becomes: + + Passphrase entropy Cost to break + 50 bit EUR/USD 600k + 55 bit EUR/USD 20M + 60 bit EUR/USD 600M + 65 bit EUR/USD 20B + 70 bit EUR/USD 600B + 75 bit EUR/USD 20T + ... ... + + + Recommendation: + + To get reasonable security for the next 10 years, it is a good idea + to overestimate by a factor of at least 1000. + + Then there is the question of how much the attacker is willing to spend. + That is up to your own security evaluation. For general use, I will + assume the attacker is willing to spend up to 1 million EUR/USD. Then + we get the following recommendations: + + Plain dm-crypt: Use > 80 bit. That is e.g. 17 random chars from a-z + or a random English sentence of > 135 characters length. + + LUKS1 and LUKS2: Use > 65 bit. That is e.g. 14 random chars from a-z + or a random English sentence of > 108 characters length. + + If paranoid, add at least 20 bit. That is roughly four additional + characters for random passphrases and roughly 32 characters for a + random English sentence. + + + * 5.2 Is LUKS insecure? Everybody can see I have encrypted data! + + In practice it does not really matter. In most civilized countries you + can just refuse to hand over the keys, no harm done. In some countries + they can force you to hand over the keys if they suspect encryption. + The suspicion is enough, they do not have to prove anything. This is + for practical reasons, as even the presence of a header (like the LUKS + header) is not enough to prove that you have any keys. It might have + been an experiment, for example. Or it was used as encrypted swap with + a key from /dev/random. So they make you prove you do not have + encrypted data. Of course, if true, that is impossible and hence the + whole idea is not compatible with fair laws. Note that in this context, + countries like the US or the UK are not civilized and do not have fair + laws. + + This means that if you have a large set of random-looking data, they can + already lock you up. Hidden containers (encryption hidden within + encryption), as possible with Truecrypt, do not help either. They will + just assume the hidden container is there and unless you hand over the + key, you will stay locked up. Don't have a hidden container? Tough + luck. Anybody could claim that. + + Still, if you are concerned about the LUKS header, use plain dm-crypt + with a good passphrase. See also Section 2, "What is the difference + between "plain" and LUKS format?" + + + * 5.3 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 + + + + * 5.4 How do I securely erase a LUKS container? + + For LUKS, if you are in a desperate hurry, overwrite the LUKS header and + key-slot area. For LUKS1 and LUKS2, just be generous and overwrite the + first 100MB. A single overwrite with zeros should be enough. If you + anticipate being in a desperate hurry, prepare the command beforehand. + Example with /dev/sde1 as the LUKS partition and default parameters: + + head -c 100000000 /dev/zero > /dev/sde1; sync + + A LUKS header backup or full backup will still grant access to most or + all data, so make sure that an attacker does not have access to backups + or destroy them as well. + + Also note that SSDs and also some HDDs (SMR and hybrid HDDs, for + example) may not actually overwrite the header and only do that an + unspecified and possibly very long time later. The only way to be sure + there is physical destruction. If the situation permits, do both + overwrite and physical destruction. + + If you have time, overwrite the whole drive with a single pass of random + data. This is enough for most HDDs. For SSDs or FLASH (USB sticks) or + SMR or hybrid drives, you may want to overwrite the whole drive several + times to be sure data is not retained. This is possibly still insecure + as the respective technologies are not fully understood in this regard. + Still, due to the anti-forensic properties of the LUKS key-slots, a + single overwrite could be enough. If in doubt, use physical destruction + in addition. Here is a link to some current research results on erasing + SSDs and FLASH drives: + https://www.usenix.org/events/fast11/tech/full_papers/Wei.pdf + + Keep in mind to also erase all backups. + + Example for a random-overwrite erase of partition sde1 done with + dd_rescue: + + dd_rescue -w /dev/urandom /dev/sde1 + + + + * 5.5 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 random data. For an SSD, FLASH drive (USB stick) hybrid + HDD or SMR HDD, you may want to overwrite the complete drive several + times and use physical destruction in addition, see last item. For + re-writable CD/DVD, a single overwrite should 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, after (!) overwriting. 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 during overwrites. + + The best option is to actually encrypt the backup, for example with + PGP/GnuPG and then just destroy all copies of the encryption key if + needed. Best keep them on paper, as that has excellent durability and + secure destruction is easy, for example by burning and then crushing the + ashes to a fine powder. A blender and water also works nicely. + + + * 5.6 What about backup? Does it compromise security? + + That depends. See item 6.7. + + + * 5.7 Why is all my data permanently gone if I overwrite the LUKS header? + + Overwriting the LUKS header in part or in full is the most common reason + why access to LUKS containers is lost permanently. Overwriting can be + done in a number of fashions, like creating a new filesystem on the raw + LUKS partition, making the raw partition part of a RAID array and just + writing to the raw partition. + + The LUKS1 header contains a 256 bit "salt" per key-slot and without that + no decryption is possible. While the salts are not secret, they are + key-grade material and cannot be reconstructed. This is a + cryptographically strong "cannot". From observations on the cryptsetup + mailing-list, people typically go though the usual stages of grief + (Denial, Anger, Bargaining, Depression, Acceptance) when this happens to + them. Observed times vary between 1 day and 2 weeks to complete the + cycle. Seeking help on the mailing-list is fine. Even if we usually + cannot help with getting back your data, most people found the feedback + comforting. + + If your header does not contain an intact key-slot salt, best go + directly to the last stage ("Acceptance") and think about what to do + now. There is one exception that I know of: If your LUKS1 container is + still open, then it may be possible to extract the master key from the + running system. See Item "How do I recover the master key from a mapped + LUKS1 container?" in Section "Backup and Data Recovery". + + For LUKS2, things are both better and worse. First, the salts are in a + less vulnerable position now. But, on the other hand, the keys of a + mapped (open) container are now stored in the kernel key-store, and + while there probably is some way to get them out of there, I am not sure + how much effort that needs. + + + * 5.8 What is a "salt"? + + A salt is a random key-grade value added to the passphrase before it is + processed. It is not kept secret. The reason for using salts is as + follows: If an attacker wants to crack the password for a single LUKS + container, then every possible passphrase has to be tried. Typically an + attacker will not try every binary value, but will try words and + sentences from a dictionary. + + If an attacker wants to attack several LUKS containers with the same + dictionary, then a different approach makes sense: Compute the resulting + slot-key for each dictionary element and store it on disk. Then the + test for each entry is just the slow unlocking with the slot key (say + 0.00001 sec) instead of calculating the slot-key first (1 sec). For a + single attack, this does not help. But if you have more than one + container to attack, this helps tremendously, also because you can + prepare your table before you even have the container to attack! The + calculation is also very simple to parallelize. You could, for example, + use the night-time unused CPU power of your desktop PCs for this. + + This is where the salt comes in. If the salt is combined with the + passphrase (in the simplest form, just appended to it), you suddenly + need a separate table for each salt value. With a reasonably-sized salt + value (256 bit, e.g.) this is quite infeasible. + + + * 5.9 Is LUKS secure with a low-entropy (bad) passphrase? + + Short answer: yes. Do not use a low-entropy passphrase. + + Note: For LUKS2, protection for bad passphrases is a bit better + due to the use of Argon2, but that is only a gradual improvement. + + Longer answer: + This needs a bit of theory. The quality of your passphrase is directly + related to its entropy (information theoretic, not thermodynamic). The + entropy says how many bits of "uncertainty" or "randomness" are in you + passphrase. In other words, that is how difficult guessing the + passphrase is. + + Example: A random English sentence has about 1 bit of entropy per + character. A random lowercase (or uppercase) character has about 4.7 + bit of entropy. + + Now, if n is the number of bits of entropy in your passphrase and t + is the time it takes to process a passphrase in order to open the + LUKS container, then an attacker has to spend at maximum + + attack_time_max = 2^n * t + + time for a successful attack and on average half that. There is no way + getting around that relationship. However, there is one thing that does + help, namely increasing t, the time it takes to use a passphrase, see + next FAQ item. + + Still, if you want good security, a high-entropy passphrase is the only + option. For example, a low-entropy passphrase can never be considered + secure against a TLA-level (Three Letter Agency level, i.e. + government-level) attacker, no matter what tricks are used in the + key-derivation function. Use at least 64 bits for secret stuff. That + is 64 characters of English text (but only if randomly chosen) or a + combination of 12 truly random letters and digits. + + For passphrase generation, do not use lines from very well-known texts + (religious texts, Harry Potter, etc.) as they are too easy to guess. + For example, the total Harry Potter has about 1'500'000 words (my + estimation). Trying every 64 character sequence starting and ending at + a word boundary would take only something like 20 days on a single CPU + and is entirely feasible. To put that into perspective, using a number + of Amazon EC2 High-CPU Extra Large instances (each gives about 8 real + cores), this test costs currently about 50USD/EUR, but can be made to + run arbitrarily fast. + + On the other hand, choosing 1.5 lines from, say, the Wheel of Time, is + in itself not more secure, but the book selection adds quite a bit of + entropy. (Now that I have mentioned it here, don't use tWoT either!) If + you add 2 or 3 typos and switch some words around, then this is good + passphrase material. + + + * 5.10 What is "iteration count" and why is decreasing it a bad idea? + + LUKS1: + Iteration count is the number of PBKDF2 iterations a passphrase is put + through before it is used to unlock a key-slot. Iterations are done + with the explicit purpose to increase the time that it takes to unlock a + key-slot. This provides some protection against use of low-entropy + passphrases. + + The idea is that an attacker has to try all possible passphrases. Even + if the attacker knows the passphrase is low-entropy (see last item), it + is possible to make each individual try take longer. The way to do this + is to repeatedly hash the passphrase for a certain time. The attacker + then has to spend the same time (given the same computing power) as the + user per try. With LUKS1, the default is 1 second of PBKDF2 hashing. + + Example 1: Lets assume we have a really bad passphrase (e.g. a + girlfriends name) with 10 bits of entropy. With the same CPU, an + attacker would need to spend around 500 seconds on average to break that + passphrase. Without iteration, it would be more like 0.0001 seconds on + a modern CPU. + + Example 2: The user did a bit better and has 32 chars of English text. + That would be about 32 bits of entropy. With 1 second iteration, that + means an attacker on the same CPU needs around 136 years. That is + pretty impressive for such a weak passphrase. Without the iterations, + it would be more like 50 days on a modern CPU, and possibly far less. + + In addition, the attacker can both parallelize and use special hardware + like GPUs or FPGAs to speed up the attack. The attack can also happen + quite some time after the luksFormat operation and CPUs can have become + faster and cheaper. For that reason you want a bit of extra security. + Anyways, in Example 1 your are screwed. In example 2, not necessarily. + Even if the attack is faster, it still has a certain cost associated + with it, say 10000 EUR/USD with iteration and 1 EUR/USD without + iteration. The first can be prohibitively expensive, while the second + is something you try even without solid proof that the decryption will + yield something useful. + + The numbers above are mostly made up, but show the idea. Of course the + best thing is to have a high-entropy passphrase. + + Would a 100 sec iteration time be even better? Yes and no. + Cryptographically it would be a lot better, namely 100 times better. + However, usability is a very important factor for security technology + and one that gets overlooked surprisingly often. For LUKS, if you have + to wait 2 minutes to unlock the LUKS container, most people will not + bother and use less secure storage instead. It is better to have less + protection against low-entropy passphrases and people actually use LUKS, + than having them do without encryption altogether. + + Now, what about decreasing the iteration time? This is generally a very + bad idea, unless you know and can enforce that the users only use + high-entropy passphrases. If you decrease the iteration time without + ensuring that, then you put your users at increased risk, and + considering how rarely LUKS containers are unlocked in a typical + work-flow, you do so without a good reason. Don't do it. The iteration + time is already low enough that users with low entropy passphrases are + vulnerable. Lowering it even further increases this danger + significantly. + + LUKS2: Pretty much the same reasoning applies. The advantages of using + GPUs or FPGAs in an attack have been significantly reduced, but that + is the only main difference. + + + * 5.11 Some people say PBKDF2 is insecure? + + There is some discussion that a hash-function should have a "large + memory" property, i.e. that it should require a lot of memory to be + computed. This serves to prevent attacks using special programmable + circuits, like FPGAs, and attacks using graphics cards. PBKDF2 does not + need a lot of memory and is vulnerable to these attacks. However, the + publication usually referred in these discussions is not very convincing + in proving that the presented hash really is "large memory" (that may + change, email the FAQ maintainer when it does) and it is of limited + usefulness anyways. Attackers that use clusters of normal PCs will not + be affected at all by a "large memory" property. For example the US + Secret Service is known to use the off-hour time of all the office PCs + of the Treasury for password breaking. The Treasury has about 110'000 + employees. Assuming every one has an office PC, that is significant + computing power, all of it with plenty of memory for computing "large + memory" hashes. Bot-net operators also have all the memory they want. + The only protection against a resourceful attacker is a high-entropy + passphrase, see items 5.9 and 5.10. + + That said, LUKS2 defaults to Argon2, which has a large-memory property + and massively reduces the advantages of GPUs and FPGAs. + + + * 5.12 What about iteration count with plain dm-crypt? + + Simple: There is none. There is also no salting. If you use plain + dm-crypt, the only way to be secure is to use a high entropy passphrase. + If in doubt, use LUKS instead. + + + * 5.13 Is LUKS with default parameters less secure on a slow CPU? + + Unfortunately, yes. However the only aspect affected is the protection + for low-entropy passphrase or master-key. All other security aspects + are independent of CPU speed. + + The master key is less critical, as you really have to work at it to + give it low entropy. One possibility to mess this up is to supply the + master key yourself. If that key is low-entropy, then you get what you + deserve. The other known possibility to create a LUKS container with a + bad master key is to use /dev/urandom for key generation in an + entropy-starved situation (e.g. automatic installation on an embedded + device without network and other entropy sources or installation in a VM + under certain circumstances). + + For the passphrase, don't use a low-entropy passphrase. If your + passphrase is good, then a slow CPU will not matter. If you insist on a + low-entropy passphrase on a slow CPU, use something like + "--iter-time=10000" or higher and wait a long time on each LUKS unlock + and pray that the attacker does not find out in which way exactly your + passphrase is low entropy. This also applies to low-entropy passphrases + on fast CPUs. Technology can do only so much to compensate for problems + in front of the keyboard. + + Also note that power-saving modes will make your CPU slower. This will + reduce iteration count on LUKS container creation. It will keep unlock + times at the expected values though at this CPU speed. + + + * 5.14 Why was the default aes-cbc-plain replaced with aes-cbc-essiv? + + Note: This item applies both to plain dm-crypt and to LUKS + + 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. + + + * 5.15 Are there any problems with "plain" IV? What is "plain64"? + + First, "plain" and "plain64" are both not secure 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 or more recent. Also note that + "plain64" is backwards compatible for volume sizes of maximum size 2TiB, + but not for those > 2TiB. Finally, "plain64" does not cause any + performance penalty compared to "plain". + + + * 5.16 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. 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> + + For volumes >2TiB and kernels >= 2.6.33 use "plain64" (see FAQ item + on "plain" and "plain64"): + + cryptsetup -c aes-xts-plain64 luksFormat <device> + + There is a potential security issue with XTS mode and blocks larger + than 2^20 bytes or so. LUKS and dm-crypt always use smaller blocks + and the issue does not apply. + + + * 5.17 Is LUKS FIPS-140-2 certified? + + No. But that is more a problem of FIPS-140-2 than of LUKS. From a + technical point-of-view, LUKS with the right parameters would be + FIPS-140-2 compliant, but in order to make it certified, somebody has to + pay real money for that. And then, whenever cryptsetup is changed or + extended, the certification lapses and has to be obtained again. + + From the aspect of actual security, LUKS with default parameters should + be as good as most things that are FIPS-140-2 certified, although you + may want to make sure to use /dev/random (by specifying --use-random on + luksFormat) as randomness source for the master key to avoid being + potentially insecure in an entropy-starved situation. + + + * 5.18 What about Plausible Deniability? + + First let me attempt a definition for the case of encrypted filesystems: + Plausible deniability is when you store data inside an encrypted + container and it is not possible to prove it is there without having a + special passphrase. And at the same time it must be "plausible" that + there actually is no hidden data there. + + As a simple entropy-analysis will show that here may be data there, the + second part is what makes it tricky. + + There seem to be a lot of misunderstandings about this idea, so let me + make it clear that this refers to the situation where the attackers can + prove that there is data that either may be random or may be part of a + plausible-deniability scheme, they just cannot prove which one it is. + Hence a plausible-deniability scheme must hold up when the attackers + know there is something potentially fishy. If you just hide data and + rely on it not being found, that is just simple deniability, not + "plausible" deniability and I am not talking about that in the + following. Simple deniability against a low-competence attacker may be + as simple as renaming a file or putting data into an unused part of a + disk. Simple deniability against a high-skill attacker with time to + invest is usually pointless unless you go for advanced steganographic + techniques, which have their own drawbacks, such as low data capacity. + + Now, the idea of plausible deniability is compelling and on a first + glance it seems possible to do it. And from a cryptographic point of + view, it actually is possible. + + So, does the idea work in practice? No, unfortunately. The reasoning + used by its proponents is fundamentally flawed in several ways and the + cryptographic properties fail fatally when colliding with the real + world. + + First, why should "I do not have a hidden partition" be any more + plausible than "I forgot my crypto key" or "I wiped that partition with + random data, nothing in there"? I do not see any reason. + + Second, there are two types of situations: Either they cannot force you + to give them the key (then you simply do not) or they can. In the + second case, they can always do bad things to you, because they cannot + prove that you have the key in the first place! This means they do not + have to prove you have the key, or that this random looking data on your + disk is actually encrypted data. So the situation will allow them to + waterboard/lock-up/deport you anyways, regardless of how "plausible" + your deniability is. Do not have a hidden partition you could show to + them, but there are indications you may? Too bad for you. + Unfortunately "plausible deniability" also means you cannot prove there + is no hidden data. + + Third, hidden partitions are not that hidden. There are basically just + two possibilities: a) Make a large crypto container, but put a smaller + filesystem in there and put the hidden partition into the free space. + Unfortunately this is glaringly obvious and can be detected in an + automated fashion. This means that the initial suspicion to put you + under duress in order to make you reveal your hidden data is given. b) + Make a filesystem that spans the whole encrypted partition, and put the + hidden partition into space not currently used by that filesystem. + Unfortunately that is also glaringly obvious, as you then cannot write + to the filesystem without a high risk of destroying data in the hidden + container. Have not written anything to the encrypted filesystem in a + while? Too bad, they have the suspicion they need to do unpleasant + things to you. + + To be fair, if you prepare option b) carefully and directly before going + into danger, it may work. But then, the mere presence of encrypted data + may already be enough to get you into trouble in those places were they + can demand encryption keys. + + Here is an additional reference for some problems with plausible + deniability: + https://www.schneier.com/academic/paperfiles/paper-truecrypt-dfs.pdf + I strongly suggest you read it. + + So, no, I will not provide any instructions on how to do it with plain + dm-crypt or LUKS. If you insist on shooting yourself in the foot, you + can figure out how to do it yourself. + + + * 5.19 What about SSDs, Flash, Hybrid and SMR Drives? + + The problem is that you cannot reliably erase parts of these devices, + mainly due to wear-leveling and possibly defect management and delayed + writes to the main data area. + + For example for SSDs, when overwriting a sector, what the device does is + to move an internal sector (may be 128kB or even larger) to some pool of + discarded, not-yet erased unused sectors, take a fresh empty sector from + the empty-sector pool and copy the old sector over with the changes to + the small part you wrote. This is done in some fashion so that larger + writes do not cause a lot of small internal updates. + + The thing is that the mappings between outside-addressable sectors and + inside sectors is arbitrary (and the vendors are not talking). Also the + discarded sectors are not necessarily erased immediately. They may + linger a long time. + + For plain dm-crypt, the consequences are that older encrypted data may + be lying around in some internal pools of the device. Thus may or may + not be a problem and depends on the application. Remember the same can + happen with a filesystem if consecutive writes to the same area of a + file can go to different sectors. + + However, for LUKS, the worst case is that key-slots and LUKS header may + end up in these internal pools. This means that password management + functionality is compromised (the old passwords may still be around, + potentially for a very long time) and that fast erase by overwriting the + header and key-slot area is insecure. + + Also keep in mind that the discarded/used pool may be large. For + example, a 240GB SSD has about 16GB of spare area in the chips that it + is free to do with as it likes. You would need to make each individual + key-slot larger than that to allow reliable overwriting. And that + assumes the disk thinks all other space is in use. Reading the internal + pools using forensic tools is not that hard, but may involve some + soldering. + + What to do? + + If you trust the device vendor (you probably should not...) you can try + an ATA "secure erase" command. That is not present in USB keys though + and may or may not be secure for a hybrid drive. + + If you can do without password management and are fine with doing + physical destruction for permanently deleting data (always after one or + several full overwrites!), you can use plain dm-crypt. + + 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 + main disk, but otherwise you get LUKS with the same security as on a + traditional magnetic disk. Note however that storage vendors are prone + to lying to their customers. For example, it recently came out that + HDDs sold without any warning or mentioning in the data-sheets were + actually using SMR and that will write data first to a faster area and + only overwrite the original data area some time later when things are + quiet. + + If you are concerned about your laptop being stolen, you are likely fine + using LUKS on an SSD or hybrid drive. An attacker would need to have + access to an old passphrase (and the key-slot for this old passphrase + would actually need to still be somewhere in the SSD) for your data to + be at risk. So unless you pasted your old passphrase all over the + Internet or the attacker has knowledge of it from some other source and + does a targeted laptop theft to get at your data, you should be fine. + + + * 5.20 LUKS1 is broken! It uses SHA-1! + + No, it is not. SHA-1 is (academically) broken for finding collisions, + but not for using it in a key-derivation function. And that collision + vulnerability is for non-iterated use only. And you need the hash-value + in verbatim. + + This basically means that if you already have a slot-key, and you have + set the PBKDF2 iteration count to 1 (it is > 10'000 normally), you could + (maybe) derive a different passphrase that gives you the the same + slot-key. But if you have the slot-key, you can already unlock the + key-slot and get the master key, breaking everything. So basically, + this SHA-1 vulnerability allows you to open a LUKS1 container with high + effort when you already have it open. + + The real problem here is people that do not understand crypto and claim + things are broken just because some mechanism is used that has been + broken for a specific different use. The way the mechanism is used + matters very much. A hash that is broken for one use can be completely + secure for other uses and here it is. + + Since version 1.7.0, cryptsetup uses SHA-256 as default to ensure that + it will be compatible in the future. There are already some systems + where SHA-1 is completely phased out or disabled by a security policy. + + + * 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 actual 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 no 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. + + + * 5.22 Does cryptsetup open network connections to websites, etc. ? + + This question seems not to make much sense at first glance, but here is + an example form the real world: The TrueCrypt GUI has a "Donation" + button. Press it, and a web-connection to the TrueCrypt website is + opened via the default browser, telling everybody that listens that you + use TrueCrypt. In the worst case, things like this can get people + tortured or killed. + + So: Cryptsetup will never open any network connections except the + local netlink socket it needs to talk to the kernel crypto API. + + In addition, the installation package should contain all documentation, + including this FAQ, so that you do not have to go to a web-site to read + it. (If your distro cuts the docu, please complain to them.) In + security software, any connection initiated to anywhere outside your + machine should always be the result of an explicit request for such a + connection by the user and cryptsetup will stay true to that principle. + + +6. Backup and Data Recovery + + + * 6.1 Why do I need Backup? + + First, disks die. The rate for well-treated (!) disk is about 5% per + year, which is high enough to worry about. There is some indication + that this may be even worse for some SSDs. This applies both to LUKS + and plain dm-crypt partitions. + + Second, for LUKS, if anything damages the LUKS header or the key-stripe + area then decrypting the LUKS device can become impossible. This is a + frequent occurrence. For example an accidental format as FAT or some + software overwriting the first sector where it suspects a partition boot + sector typically makes a LUKS1 partition permanently inaccessible. See + more below on LUKS header damage. + + So, data-backup in some form is non-optional. For LUKS, you may also + want to store a header backup in some secure location. This only needs + an update if you change passphrases. + + + * 6.2 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 <file> <device> + + To restore, use the inverse command, i.e. + + cryptsetup luksHeaderRestore --header-backup-file <file> <device> + + If you are unsure about a header to be restored, make a backup of the + current one first! You can also test the header-file without restoring + it by using the --header option for a detached header like this: + + cryptsetup --header <file> luksOpen <device> </dev/mapper/name> + + If that unlocks your key-slot, you are good. Do not forget to close + the device again. + + Under some circumstances (damaged header), this fails. Then use the + following steps in case it is LUKS1: + + First determine the master-key size: + + cryptsetup luksDump <device> + + gives a line of the form + + MK bits: <bits> + + with bits equal to 256 for the old defaults and 512 for the new + defaults. 256 bits equals a total header size of 1'052'672 Bytes and + 512 bits one of 2MiB. (See also Item 6.12) If luksDump fails, assume + 2MiB, but be aware that if you restore that, you may also restore the + first 1M or so of the filesystem. Do not change the filesystem if you + were unable to determine the header size! With that, restoring a + too-large header backup is still safe. + + Second, dump the header to file. There are many ways to do it, I + prefer the following: + + head -c 1052672 <device> > header_backup.dmp + + or + + head -c 2M <device> > header_backup.dmp + + for a 2MiB header. Verify the size of the dump-file to be sure. + + To restore such a backup, you can try luksHeaderRestore or do a more + basic + + cat header_backup.dmp > <device> + + + + * 6.3 How do I test for a LUKS header? + + Use + + cryptsetup -v isLuks <device> + + on the device. Without the "-v" it just signals its result via + exit-status. You can also use the more general test + + blkid -p <device> + + which will also detect other types and give some more info. Omit + "-p" for old versions of blkid that do not support it. + + + * 6.4 How do I backup a LUKS or dm-crypt partition? + + There are two options, a sector-image and a plain file or filesystem + backup of the contents of the partition. The sector image is already + encrypted, but cannot be compressed and contains all empty space. The + filesystem backup can be compressed, can contain only part of the + encrypted device, but needs to be encrypted separately if so desired. + + A sector-image will contain the whole partition in encrypted form, for + LUKS the LUKS header, the keys-slots and the data area. It can be done + under Linux e.g. with dd_rescue (for a direct image copy) and with + "cat" or "dd". Examples: + + 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 make sense to use it. + + For a filesystem backup, you decrypt and mount the encrypted partition + and back it up as you would a normal filesystem. In this case the + backup is not encrypted, unless your encryption method does that. For + example you can encrypt a backup with "tar" as follows with GnuPG: + + tar cjf - <path> | gpg --cipher-algo AES -c - > backup.tbz2.gpg + + And verify the backup like this if you are at "path": + + cat backup.tbz2.gpg | gpg - | tar djf - + + Note: Always verify backups, especially encrypted ones! + + There is one problem with verifying like this: The kernel may still have + some files cached and in fact verify them against RAM or may even verify + RAM against RAM, which defeats the purpose of the exercise. The + following command empties the kernel caches: + + echo 3 > /proc/sys/vm/drop_caches + + Run it after backup and before verify. + + In both cases GnuPG will ask you interactively for your symmetric key. + The verify will only output errors. Use "tar dvjf -" to get all + comparison results. To make sure no data is written to disk + unencrypted, turn off swap if it is not encrypted before doing the + backup. + + Restore works like certification with the 'd' ('difference') replaced + by 'x' ('eXtract'). Refer to the man-page of tar for more explanations + and instructions. Note that with default options tar will overwrite + already existing files without warning. If you are unsure about how + to use tar, experiment with it in a location where you cannot do damage. + + You can of course use different or no compression and you can use an + asymmetric key if you have one and have a backup of the secret key that + belongs to it. + + A second option for a filesystem-level backup that can be used when the + backup is also on local disk (e.g. an external USB drive) is to use a + LUKS container there and copy the files to be backed up between both + mounted containers. Also see next item. + + + * 6.5 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 non-mapped LUKS container, + 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 well-treated (!) HDD has roughly a failure risk of 5% per + year. It is highly advisable to have a complete backup to protect + against this case. + + + * 6.6 What do I need to backup if I use "decrypt_derived"? + + This is a script in Debian, intended for mounting /tmp or swap with a + key derived from the master key of an already decrypted device. If you + use this for an device with data that should be persistent, you need to + make sure you either do not lose access to that master key or have a + backup of the data. If you derive from a LUKS device, a header backup + of that device would cover backing up the master key. Keep in mind that + this does not protect against disk loss. + + Note: If you recreate the LUKS header of the device you derive from + (using luksFormat), the master key changes even if you use the same + passphrase(s) and you will not be able to decrypt the derived device + with the new LUKS header. + + + * 6.7 Does a backup compromise security? + + Depends on how you do it. However if you do not have one, you are 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 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, securely erase the backup or + do a filesystem-level backup instead of a binary one. + + 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 with binary + backups: An attacker can see how many sectors and which ones have been + changed since the backup. To prevent this, use a filesystem level + backup method that encrypts the whole backup in one go, e.g. as + described above with tar and GnuPG. + + My personal advice is to use one USB disk (low value data) or three + disks (high value data) in rotating order for backups, and either use + independent LUKS partitions on them, or use encrypted backup with tar + and GnuPG. + + If you do network-backup or tape-backup, I strongly recommend to go + the filesystem backup path with independent encryption, as you + typically cannot reliably delete data in these scenarios, especially + in a cloud setting. (Well, you can burn the tape if it is under your + control...) + + + * 6.8 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 + key-slot descriptors of the header and the key-slots. For LUKS2 they + are a bit better protected. but for LUKS1, these are right in the first + sector. 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 completely unrecoverable, changing about 4-6 bits in random + locations of its 128kiB size is quite enough. + + + * 6.9 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, making your data permanently irretrievable, + unless you have a LUKS header backup. For LUKS2 this may still be + recoverable without that header backup, for LUKS1 it is not. You may + also damage the key-slots in part or in full. See also last item. + + + * 6.10 How do I recover the master key from a mapped LUKS1 container? + + Note: LUKS2 uses the kernel keyring to store keys and hence this + procedure does not work unless you have explicitly disabled the use of + the keyring with "--disable-keyring" on opening. + + This is typically only needed if you managed to damage your LUKS1 + 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 LUKS1 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. Alternatively the tool cryptsetup-reencrypt from + the cryptsetup package can be used to change the master key (see its + man-page), but a full backup is still highly recommended. + + First, there is a script by Milan that automates the whole process, + except generating a new LUKS1 header with the old master key (it prints + the command for that though): + + https://gitlab.com/cryptsetup/cryptsetup/blob/master/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 LUKS1 header. + + NOTE: If your header is intact and you just forgot the passphrase, + you can just set a new passphrase, see next sub-item. + + 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. + + + * 6.11 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 area you overwrote. + + + * 6.12 What does the on-disk structure of LUKS1 look like? + + Note: For LUKS2, refer to the LUKS2 document referenced in Item 1.2 + + A LUKS1 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, key material offset and master key size. + + With the default parameters, 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. Due to 2MiB default alignment, start of the data area for + cryptsetup 1.3 and later is at 2MiB, i.e. at 0x200000. For older + versions, it is at 0x101000, i.e. at 1'052'672 bytes, i.e. at 1MiB + + 4096 bytes from the start of the partition. Incidentally, + "luksHeaderBackup" for a LUKS container created with default parameters + dumps exactly the first 2MiB (or 1'052'672 bytes for headers created + with cryptsetup versions < 1.3) to file and "luksHeaderRestore" restores + them. + + For non-default parameters, you have to figure out placement yourself. + "luksDump" helps. See also next item. For the most common non-default + settings, namely aes-xts-plain with 512 bit key, the offsets are: 1st + keyslot 0x1000-0x3f800, 2nd keyslot 0x40000-0x7e000, 3rd keyslot + 0x7e000-0xbd800, ..., and start of bulk data at 0x200000. + + The exact specification of the format is here: + https://gitlab.com/cryptsetup/cryptsetup/wikis/Specification + + For your convenience, here is the LUKS1 header with hex offsets. + NOTE: + The spec counts key-slots from 1 to 8, but the cryptsetup tool counts + from 0 to 7. The numbers here refer to the cryptsetup numbers. + + +Refers to LUKS1 On-Disk Format Specification Version 1.2.3 + +LUKS1 header: + +offset length name data type description +----------------------------------------------------------------------- +0x0000 0x06 magic byte[] 'L','U','K','S', 0xba, 0xbe + 0 6 +0x0006 0x02 version uint16_t LUKS version + 6 3 +0x0008 0x20 cipher-name char[] cipher name spec. + 8 32 +0x0028 0x20 cipher-mode char[] cipher mode spec. + 40 32 +0x0048 0x20 hash-spec char[] hash spec. + 72 32 +0x0068 0x04 payload-offset uint32_t bulk data offset in sectors + 104 4 (512 bytes per sector) +0x006c 0x04 key-bytes uint32_t number of bytes in key + 108 4 +0x0070 0x14 mk-digest byte[] master key checksum + 112 20 calculated with PBKDF2 +0x0084 0x20 mk-digest-salt byte[] salt for PBKDF2 when + 132 32 calculating mk-digest +0x00a4 0x04 mk-digest-iter uint32_t iteration count for PBKDF2 + 164 4 when calculating mk-digest +0x00a8 0x28 uuid char[] partition UUID + 168 40 +0x00d0 0x30 key-slot-0 key slot key slot 0 + 208 48 +0x0100 0x30 key-slot-1 key slot key slot 1 + 256 48 +0x0130 0x30 key-slot-2 key slot key slot 2 + 304 48 +0x0160 0x30 key-slot-3 key slot key slot 3 + 352 48 +0x0190 0x30 key-slot-4 key slot key slot 4 + 400 48 +0x01c0 0x30 key-slot-5 key slot key slot 5 + 448 48 +0x01f0 0x30 key-slot-6 key slot key slot 6 + 496 48 +0x0220 0x30 key-slot-7 key slot key slot 7 + 544 48 + + +Key slot: + +offset length name data type description +------------------------------------------------------------------------- +0x0000 0x04 active uint32_t key slot enabled/disabled + 0 4 +0x0004 0x04 iterations uint32_t PBKDF2 iteration count + 4 4 +0x0008 0x20 salt byte[] PBKDF2 salt + 8 32 +0x0028 0x04 key-material-offset uint32_t key start sector + 40 4 (512 bytes/sector) +0x002c 0x04 stripes uint32_t number of anti-forensic + 44 4 stripes + + + + * 6.13 What is the smallest possible LUKS1 container? + + Note: From cryptsetup 1.3 onwards, alignment is set to 1MB. With modern + Linux partitioning tools that also align to 1MB, this will result in + alignment to 2k sectors and typical Flash/SSD sectors, which is highly + desirable for a number of reasons. Changing the alignment is not + recommended. + + That said, with default parameters, the data area starts at exactly 2MB + offset (at 0x101000 for cryptsetup versions before 1.3). The smallest + data area you can have is one sector of 512 bytes. Data areas of 0 + bytes can be created, but fail on mapping. + + While you cannot put a filesystem into something this small, it may + still be used to contain, for example, key. Note that with current + formatting tools, a partition for a container this size will be 3MiB + anyways. If you put the LUKS container into a file (via losetup and a + loopback device), the file needs to be 2097664 bytes in size, i.e. 2MiB + + 512B. + + The two ways to influence the start of the data area are key-size and + alignment. + + For alignment, you can go down to 1 on the parameter. This will still + leave you with a data-area starting at 0x101000, i.e. 1MiB+4096B + (default parameters) as alignment will be rounded up to the next + multiple of 8 (i.e. 4096 bytes) If in doubt, do a dry-run on a larger + file and dump the LUKS header to get actual information. + + For key-size, you can use 128 bit (e.g. AES-128 with CBC), 256 bit + (e.g. AES-256 with CBC) or 512 bit (e.g. AES-256 with XTS mode). You + can do 64 bit (e.g. blowfish-64 with CBC), but anything below 128 bit + has to be considered insecure today. + + Example 1 - AES 128 bit with CBC: + + cryptsetup luksFormat -s 128 --align-payload=8 <device> + + This results in a data offset of 0x81000, i.e. 516KiB or 528384 + bytes. Add one 512 byte sector and the smallest LUKS container size + with these parameters is 516KiB + 512B or 528896 bytes. + + Example 2 - Blowfish 64 bit with CBC (WARNING: insecure): + + cryptsetup luksFormat -c blowfish -s 64 --align-payload=8 /dev/loop0 + + This results in a data offset of 0x41000, i.e. 260kiB or 266240 + bytes, with a minimal LUKS1 container size of 260kiB + 512B or 266752 + bytes. + + + * 6.14 I think this is overly complicated. Is there an alternative? + + Not really. Encryption comes at a price. You can use plain dm-crypt to + simplify things a bit. 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 full sectors). + + * 6.15 Can I clone a LUKS container? + + You can, but it breaks security, because the cloned container has the + same header and hence the same master key. Even if you change the + passphrase(s), the master key stays the same. That means whoever has + access to one of the clones can decrypt them all, completely bypassing + the passphrases. + + While you can use cryptsetup-reencrypt to change the master key, + this is probably more effort than to create separate LUKS containers + in the first place. + + The right way to do this is to first luksFormat the target container, + then to clone the contents of the source container, with both containers + mapped, i.e. decrypted. You can clone the decrypted contents of a LUKS + container in binary mode, although you may run into secondary issues + with GUIDs in filesystems, partition tables, RAID-components and the + like. These are just the normal problems binary cloning causes. + + Note that if you need to ship (e.g.) cloned LUKS containers with a + default passphrase, that is fine as long as each container was + individually created (and hence has its own master key). In this case, + changing the default passphrase will make it secure again. + + +7. Interoperability with other Disk Encryption Tools + + + * 7.1 What is this section about? + + Cryptsetup for plain dm-crypt can be used to access a number of on-disk + formats created by tools like loop-aes patched into losetup. This + sometimes works and sometimes does not. This section collects insights + into what works, what does not and where more information is required. + + Additional information may be found in the mailing-list archives, + mentioned at the start of this FAQ document. If you have a solution + working that is not yet documented here and think a wider audience may + be interested, please email the FAQ maintainer. + + + * 7.2 loop-aes: General observations. + + One problem is that there are different versions of losetup around. + loop-aes is a patch for losetup. Possible problems and deviations + from cryptsetup option syntax include: + + - Offsets specified in bytes (cryptsetup: 512 byte sectors) + + - The need to specify an IV offset + + - Encryption mode needs specifying (e.g. "-c twofish-cbc-plain") + + - Key size needs specifying (e.g. "-s 128" for 128 bit keys) + + - Passphrase hash algorithm needs specifying + + Also note that because plain dm-crypt and loop-aes format does not have + metadata, and while the loopAES extension for cryptsetup tries + autodetection (see command loopaesOpen), it may not always work. If you + still have the old set-up, using a verbosity option (-v) on mapping with + the old tool or having a look into the system logs after setup could + give you the information you need. Below, there are also some things + that worked for somebody. + + + * 7.3 loop-aes patched into losetup on Debian 5.x, kernel 2.6.32 + + In this case, the main problem seems to be that this variant of + losetup takes the offset (-o option) in bytes, while cryptsetup takes + it in sectors of 512 bytes each. + + Example: The losetup command + + losetup -e twofish -o 2560 /dev/loop0 /dev/sdb1 + mount /dev/loop0 mount-point + + translates to + + cryptsetup create -c twofish -o 5 --skip 5 e1 /dev/sdb1 + mount /dev/mapper/e1 mount-point + + + + * 7.4 loop-aes with 160 bit key + + This seems to be sometimes used with twofish and blowfish and represents + a 160 bit ripemed160 hash output padded to 196 bit key length. It seems + the corresponding options for cryptsetup are + + --cipher twofish-cbc-null -s 192 -h ripemd160:20 + + + + * 7.5 loop-aes v1 format OpenSUSE + + Apparently this is done by older OpenSUSE distros and stopped working + from OpenSUSE 12.1 to 12.2. One user had success with the following: + + cryptsetup create <target> <device> -c aes -s 128 -h sha256 + + + + * 7.6 Kernel encrypted loop device (cryptoloop) + + There are a number of different losetup implementations for using + encrypted loop devices so getting this to work may need a bit of + experimentation. + + NOTE: Do NOT use this for new containers! Some of the existing + implementations are insecure and future support is uncertain. + + Example for a compatible mapping: + + losetup -e twofish -N /dev/loop0 /image.img + + translates to + + cryptsetup create image_plain /image.img -c twofish-cbc-plain -H plain + + with the mapping being done to /dev/mapper/image_plain instead of + to /dev/loop0. + + More details: + + Cipher, mode and password hash (or no hash): + + -e cipher [-N] => -c cipher-cbc-plain -H plain [-s 256] + -e cipher => -c cipher-cbc-plain -H ripemd160 [-s 256] + + + Key size and offsets (losetup: bytes, cryptsetuop: sectors of 512 bytes): + + -k 128 => -s 128 + -o 2560 => -o 5 -p 5 # 2560/512 = 5 + + + There is no replacement for --pass-fd, it has to be emulated using + keyfiles, see the cryptsetup man-page. + + +8. Issues with Specific Versions of cryptsetup + + + * 8.1 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. You can check the 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 <name> <dev> + + LUKS stores cipher and mode in the metadata on disk, avoiding this + problem. + + + * 8.2 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. + + + * 8.3 Gcrypt 1.6.x and later break Whirlpool + + It is the other way round: In gcrypt 1.5.x, Whirlpool is broken and it + was fixed in 1.6.0 and later. If you selected whirlpool as hash on + creation of a LUKS container, it does not work anymore with the fixed + library. This shows one serious risk of using rarely used settings. + + Note that at the time this FAQ item was written, 1.5.4 was the latest + 1.5.x version and it has the flaw, i.e. works with the old Whirlpool + version. Possibly later 1.5.x versions will work as well. If not, + please let me know. + + The only two ways to access older LUKS containers created with Whirlpool + are to either decrypt with an old gcrypt version that has the flaw or to + use a compatibility feature introduced in cryptsetup 1.6.4 and gcrypt + 1.6.1 or later. Version 1.6.0 cannot be used. + + Steps: + + - Make at least a header backup or better, refresh your full backup. + (You have a full backup, right? See Item 6.1 and following.) + + - Make sure you have cryptsetup 1.6.4 or later and check the gcrypt + version: + + cryptsetup luksDump <your luks device> --debug | grep backend + + If gcrypt is at version 1.5.x or before: + + - Reencrypt the LUKS header with a different hash. (Requires entering + all keyslot passphrases. If you do not have all, remove the ones you + do not have before.): + + cryptsetup-reencrypt --keep-key --hash sha256 <your luks device> + + If gcrypt is at version 1.6.1 or later: + + - Patch the hash name in the LUKS header from "whirlpool" to + "whirlpool_gcryptbug". This activates the broken implementation. + The detailed header layout is in Item 6.12 of this FAQ and in the + LUKS on-disk format specification. One way to change the hash is + with the following command: + + echo -n -e 'whirlpool_gcryptbug\0' | dd of=<luks device> bs=1 seek=72 conv=notrunc + + - You can now open the device again. It is highly advisable to change + the hash now with cryptsetup-reencrypt as described above. While you + can reencrypt to use the fixed whirlpool, that may not be a good idea + as almost nobody seems to use it and hence the long time until the + bug was discovered. + + +9. The Initrd question + + + * 9.1 My initrd is broken with cryptsetup + + That is not nice! However the initrd is supplied by your distribution, + not by the cryptsetup project and hence you should complain to them. We + cannot really do anything about it. + + + * 9.2 CVE-2016-4484 says cryptsetup is broken! + + Not really. It says the initrd in some Debian versions have a behavior + that under some very special and unusual conditions may be considered + a vulnerability. + + What happens is that you can trick the initrd to go to a rescue-shell if + you enter the LUKS password wrongly in a specific way. But falling back + to a rescue shell on initrd errors is a sensible default behavior in the + first place. It gives you about as much access as booting a rescue + system from CD or USB-Stick or as removing the disk would give you. So + this only applies when an attacker has physical access, but cannot boot + anything else or remove the disk. These will be rare circumstances + indeed, and if you rely on the default distribution initrd to keep you + safe under these circumstances, then you have bigger problems than this + somewhat expected behavior. + + The CVE was exaggerated and should not be assigned to upstream + cryptsetup in the first place (it is a distro specific initrd issue). + It was driven more by a try to make a splash for self-aggrandizement, + than by any actual security concerns. Ignore it. + + + * 9.3 How do I do my own initrd with cryptsetup? + + Note: The instructions here apply to an initrd in initramfs format, not + to an initrd in initrd format. The latter is a filesystem image, not a + cpio-archive, and seems to not be widely used anymore. + + It depends on the distribution. Below, I give a very simple example and + step-by-step instructions for Debian. With a bit of work, it should be + possible to adapt this to other distributions. Note that the + description is pretty general, so if you want to do other things with an + initrd it provides a useful starting point for that too. + + 01) Unpacking an existing initrd to use as template + + A Linux initrd is in gzip'ed cpio format. To unpack it, use something + like this: + + md tmp; cd tmp; cat ../initrd | gunzip | cpio -id + + After this, you have the full initrd content in tmp/ + + 02) Inspecting the init-script + + The init-script is the only thing the kernel cares about. All activity + starts there. Its traditional location is /sbin/init on disk, but /init + in an initrd. In an initrd unpacked as above it is tmp/init. + + While init can be a binary despite usually being called "init script", + in Debian the main init on the root partition is a binary, but the init + in the initrd (and only that one is called by the kernel) is a script + and starts like this: + + #!/bin/sh + .... + + The "sh" used here is in tmp/bin/sh as just unpacked, and in Debian it + currently is a busybox. + + 03) Creating your own initrd + + The two examples below should give you most of what is needed. This is + tested with LUKS1 and should work with LUKS2 as well. If not, please + let me know. + + Here is a really minimal example. It does nothing but set up some + things and then drop to an interactive shell. It is perfect to try out + things that you want to go into the init-script. + + #!/bin/sh + export PATH=/sbin:/bin + [ -d /sys ] || mkdir /sys + [ -d /proc ] || mkdir /proc + [ -d /tmp ] || mkdir /tmp + mount -t sysfs -o nodev,noexec,nosuid sysfs /sys + mount -t proc -o nodev,noexec,nosuid proc /proc + echo "initrd is running, starting BusyBox..." + exec /bin/sh --login + + + Here is an example that opens the first LUKS-partition it finds with the + hard-coded password "test2" and then mounts it as root-filesystem. This + is intended to be used on an USB-stick that after boot goes into a safe, + as it contains the LUKS-passphrase in plain text and is not secure to be + left in the system. The script contains debug-output that should make it + easier to see what is going on. Note that the final hand-over to the init + on the encrypted root-partition is done by "exec switch_root /mnt/root + /sbin/init", after mounting the decrypted LUKS container with "mount + /dev/mapper/c1 /mnt/root". The second argument of switch_root is relative + to the first argument, i.e. the init started with this command is really + /mnt/sbin/init before switch_root runs. + + #!/bin/sh + export PATH=/sbin:/bin + [ -d /sys ] || mkdir /sys + [ -d /proc ] || mkdir /proc + [ -d /tmp ] || mkdir /tmp + mount -t sysfs -o nodev,noexec,nosuid sysfs /sys + mount -t proc -o nodev,noexec,nosuid proc /proc + echo "detecting LUKS containers in sda1-10, sdb1-10"; sleep 1 + for i in a b + do + for j in 1 2 3 4 5 6 7 8 9 10 + do + sleep 0.5 + d="/dev/sd"$i""$j + echo -n $d + cryptsetup isLuks $d >/dev/null 2>&1 + r=$? + echo -n " result: "$r"" + # 0 = is LUKS, 1 = is not LUKS, 4 = other error + if expr $r = 0 > /dev/null + then + echo " is LUKS, attempting unlock" + echo -n "test2" | cryptsetup luksOpen --key-file=- $d c1 + r=$? + echo " result of unlock attempt: "$r"" + sleep 2 + if expr $r = 0 > /dev/null + then + echo "*** LUKS partition unlocked, switching root *** + echo " (waiting 30 seconds before doing that)" + mount /dev/mapper/c1 /mnt/root + sleep 30 + exec switch_root /mnt/root /sbin/init + fi + else + echo " is not LUKS" + fi + done + done + echo "FAIL finding root on LUKS, loading BusyBox..."; sleep 5 + exec /bin/sh --login + + + 04) What if I want a binary in the initrd, but libraries are missing? + + That is a bit tricky. One option is to compile statically, but that + does not work for everything. Debian puts some libraries into lib/ and + lib64/ which are usually enough. If you need more, you can add the + libraries you need there. That may or may not need a configuration + change for the dynamic linker "ld" as well. Refer to standard Linux + documentation on how to add a library to a Linux system. A running + initrd is just a running Linux system after all, it is not special in + any way. + + 05) How do I repack the initrd? + + Simply repack the changed directory. While in tmp/, do + the following: + ``` + find . | cpio --create --format='newc' | gzip > ../new_initrd + ``` + Rename "new_initrd" to however you want it called (the name of + the initrd is a kernel-parameter) and move to /boot. That is it. + + +10. LUKS2 Questions + + + * 10.1 Is the cryptography of LUKS2 different? + + Mostly not. The header has changed in its structure, but the + crytpgraphy is the same. The one exception is that PBKDF2 has been + replaced by Argon2 to give better resilience against attacks by + graphics cards and other hardware with lots of computing power but + limited local memory per computing element. + + + * 10.2 What new features does LUKS2 have? + + There are quite a few. I recommend reading the man-page and the on-disk + format specification, see Item 1.2. + + To list just some: + - A lot of the metadata is JSON, allowing for easier extension + - Max 32 key-slots per default + - Better protection for bad passphrases now available with Argon2 + - Authenticated encryption + - The LUKS2 header is less vulnerable to corruption and has a 2nd copy + + + * 10.3 Why does LUKS2 need so much memory? + + LUKS2 uses Argon2 instead of PBKDF2. That causes the increase in memory. + See next item. + + + * 10.4 Why use Argon2 in LUKS 2 instead of PBKDF2? + + LUKS tries to be secure with not-so-good passwords. Bad passwords need to + be protected in some way against an attacker that just tries all possible + combinations. (For good passwords, you can just wait for the attacker to + die of old age...) The situation with LUKS is not quite the same as with a + password stored in a database, but there are similarities. + + LUKS does not store passwords on disk. Instead, the passwords are used to + decrypt the master-key with it and that one is stored on disk in encrypted + form. If you have a good password, with, say, more than 80 bits of + entropy, you could just put the password through a single crypto-hash (to + turn it into something that can be used as a key) and that would be secure. + This is what plain dm-crypt does. + + If the password has lower entropy, you want to make this process cost some + effort, so that each try takes time and resources and slows the attacker + down. LUKS1 uses PBKDF2 for that, adding an iteration count and a salt. + The iteration count is per default set to that it takes 1 second per try on + the CPU of the device where the respective passphrase was set. The salt is + there to prevent precomputation. + + The problem with that is that if you use a graphics card, you can massively + speed up these computations as PBKDF2 needs very little memory to compute + it. A graphics card is (grossly simplified) a mass of small CPUs with some + small very fast local memory per CPU and a large slow memory (the 4/6/8 GB + a current card may have). If you can keep a computation in the small, + CPU-local memory, you can gain a speed factor of 1000 or more when trying + passwords with PBKDF2. + + Argon2 was created to address this problem. It adds a "large memory + property" where computing the result with less memory than the memory + parameter requires is massively (exponentially) slowed down. That means, + if you set, for example, 4GB of memory, computing Argon2 on a graphics card + with around 100kB of memory per "CPU" makes no sense at all because it is + far too slow. An attacker has hence to use real CPUs and furthermore is + limited by main memory bandwidth. + + Hence the large amount of memory used is a security feature and should not + be turned off or reduced. If you really (!) understand what you are doing + and can assure good passwords, you can either go back to PBKDF2 or set a + low amount of memory used for Argon2 when creating the header. + + + * 10.5 LUKS2 is insecure! It uses less memory than the Argon2 RFC say! + + Well, not really. The RFC recommends 6GiB of memory for use with disk + encryption. That is a bit insane and something clearly went wrong in the + standardization process here. First, that makes Argon2 unusable on any 32 + bit Linux and that is clearly a bad thing. Second, there are many small + Linux devices around that do not have 6GiB of RAM in the first place. For + example, the current Raspberry Pi has 1GB, 2GB or 4GB of RAM, and with the + RFC recommendations, none of these could compute Argon2 hashes. + + Hence LUKS2 uses a more real-world approach. Iteration is set to a + minimum of 4 because there are some theoretical attacks that work up to an + iteration count of 3. The thread parameter is set to 4. To achieve 2 + second/slot unlock time, LUKS2 adjusts the memory parameter down if + needed. In the other direction, it will respect available memory and not + exceed it. On a current PC, the memory parameter will be somewhere around + 1GB, which should be quite generous. The minimum I was able to set in an + experiment with "-i 1" was 400kB of memory and that is too low to be + secure. A Raspberry Pi would probably end up somewhere around 50MB (have + not tried it) and that should still be plenty. + + That said, if you have a good, high-entropy passphrase, LUKS2 is secure + with any memory parameter. + + + * 10.6 How does re-encryption store data while it is running? + + All metadata necessary to perform a recovery of said segment (in case of + crash) is stored in the LUKS2 metadata area. No matter if the LUKS2 + reencryption was run in online or offline mode. + + + * 10.7 What do I do if re-encryption crashes? + + In case of a reencryption application crash, try to close the original + device via following command first: + + cryptsetup close <my_crypt_device>. + + Cryptsetup assesses if it's safe to teardown the reencryption device stack + or not. It will also cut off I/O (via dm-error mapping) to current + hotzone segment (to make later recovery possible). If it can't be torn + down, i.e. due to a mounted fs, you must unmount the filesystem first. + Never try to tear down reencryption dm devices manually using e.g. + dmsetup tool, at least not unless cryptsetup says it's safe to do so. It + could damage the data beyond repair. + + + * 10.8 Do I need to enter two passphrases to recover a crashed + re-encryption? + + Cryptsetup (command line utility) expects the passphrases to be identical + for the keyslot containing old volume key and for the keyslot containing + new one. So the recovery happens during normal the "cryptsetup open" + operation or the equivalent during boot. + + Re-encryption recovery can be also performed in offline mode by + the "cryptsetup repair" command. + + + * 10.9 What is an unbound keyslot and what is it used for? + + Quite simply, an 'unbound key' is an independent 'key' stored in a luks2 + keyslot that cannot be used to unlock a LUKS2 data device. More specifically, + an 'unbound key' or 'unbound luks2 keyslot' contains a secret that is not + currently associated with any data/crypt segment (encrypted area) in the + LUKS2 'Segments' section (displayed by luksDump). + + This is a bit of a more general idea. It basically allows to use a keyslot + as a container for a key to be used in other things than decrypting a + data segment. + + As of April 2020, the following uses are defined: + + 1) LUKS2 re-encryption. The new volume key is stored in an unbound keyslot + which becomes a regular LUKS2 keyslot later when re-encryption is + finished. + + 2) Somewhat similar is the use with a wrapped key scheme (e.g. with the + paes cipher). In this case, the VK (Volume Key) stored in a keyslot + is an encrypted binary binary blob. The KEK (Key Encryption Key) for + that blob may be refreshed (Note that this KEK is not managed by + cryptsetup!) and the binary blob gets changed. The KEK refresh process + uses an 'unbound keyslot'. First the future effective VK is placed + in the unbound keyslot and later it gets turned into the new real VK + (and bound to the respective crypt segment). + + + * 10.10 What about the size of the LUKS2 header? + + While the LUKS1 header has a fixed size that is determined by the cipher + spec (see Item 6.12), LUKS2 is more variable. The default size is 16MB, + but it can be adjusted on creation by using the --luks2-metadata-size + and --luks2-keyslots-size options. Refer to the man-page for details. + While adjusting the size in an existing LUKS2 container is possible, + it is somewhat complicated and risky. My advice is to do a backup, + recreate the container with changed parameters and restore that backup. + + + * 10.11 Does LUKS2 store metadata anywhere except in the header? + + It does not. But note that if you use the experimental integrity support, + there will be an integrity header as well at the start of the data area + and things get a bit more complicated. All metadata will still be at the + start of the device, nothing gets stored somewhere in the middle or at + the end. + + +11. References and Further Reading + + * Purpose of this Section + + The purpose of this section is to collect references to all materials + that do not fit the FAQ but are relevant in some fashion. This can be + core topics like the LUKS spec or disk encryption, but it can also be + more tangential, like secure storage management or cryptography used in + LUKS. It should still have relevance to cryptsetup and its + applications. + + If you want to see something added here, send email to the maintainer + (or the cryptsetup mailing list) giving an URL, a description (1-3 lines + preferred) and a section to put it in. You can also propose new + sections. + + At this time I would like to limit the references to things that are + available on the web. + + * Specifications + + - LUKS on-disk format spec: See Item 1.2 + + * Other Documentation + + - Arch Linux on LUKS, LVM and full-disk encryption: + https://wiki.archlinux.org/index.php/Dm-crypt/Encrypting_an_entire_system + + * Code Examples + + - Some code examples are in the source package under docs/examples + + - LUKS AF Splitter in Ruby by John Lane: https://rubygems.org/gems/afsplitter + + * Brute-forcing passphrases + + - http://news.electricalchemy.net/2009/10/password-cracking-in-cloud-part-5.html + + - https://it.slashdot.org/story/12/12/05/0623215/new-25-gpu-monster-devours-strong-passwords-in-minutes + + * Tools + + * SSD and Flash Disk Related + + * Disk Encryption + + * Attacks Against Disk Encryption + + * Risk Management as Relevant for Disk Encryption + + * Cryptography + + * Secure Storage + + +A. Contributors +In no particular order: + + - Arno Wagner + + - Milan Broz + +___ |