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@@ -0,0 +1,2807 @@ +Frequently Asked Questions. + +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. 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 + + 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. + + DEBUG COMMANDS: While the --debug option does not leak 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 features 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 http://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 + + - Ubuntu as of 4/2011: It seems the installer offers to create LUKS + partitions in a way that several people mistook for an offer to + activate their existing LUKS partition. The installer gives no or an + inadequate warning and will destroy your old LUKS header, causing + permanent data loss. See also the section on Backup and Data + Recovery. + + This issue has been acknowledged by the Ubuntu dev team, see + here: http://launchpad.net/bugs/420080 + + Update 4/2013: I am still unsure whether this has been fixed by now, + best be careful. They also seem to have added even more LUKS killer + functionality to the Ubuntu installer. I can only strongly + recommended to not install Ubuntu on a system with existing LUKS + containers without complete backups. + + Update 11/2014: There seem to be other problems with existing LUKS + containers and Ubuntu as well, be extra careful when using LUKS + on Ubuntu in any way, but exactly as the Ubuntu installer does. + + + * 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 http://creativecommons.org/licenses/by-sa/3.0/ for + more details of the license. + + Side note: I did text license research some time ago and I think this + license is best suited for the purpose at hand and creates the least + problems. + + + * 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 at 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 though 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 have 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: http://www.tansi.org/tools/index.html) in the following + fashion: + + cat /dev/zero | wcs > <target device> + + + 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: + + cryptsetup luksFormat <target device> + + + Just follow the on-screen instructions. + + Note: Passphrase iteration is determined by cryptsetup depending on + CPU power. On a slow device, this may be lower than you want. I + recently benchmarked this on a Raspberry Pi and it came out at about + 1/15 of the iteration count for a typical PC. If security is + paramount, you may want to increase the time spent in iteration, at + the cost of a slower unlock later. For the Raspberry Pi, using + + cryptsetup luksFormat -i 15000 <target device> + + gives you an iteration count and security level equal to an average + PC for passphrase iteration and master-key iteration. If in doubt, + check the iteration counts with + + cryptsetup luksDump <target device> + + and adjust the iteration count accordingly by creating the container + again with a different iteration time (the number after '-i' is the + iteration time in milliseconds) until your requirements are met. + + 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 + + + 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. + + Note that at some time in the future, cryptsetup will do this for + you, but currently it is a TODO list item. + + 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 + + + Done. You can now use the encrypted file system to store data. Be + sure to read though 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? + + 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 do so 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 permanently 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. Be aware that you lose + autodetection with them and have to fall back to some user-space + script to do it. + + 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 move that to + additional encrypted partitions. If you really are concerned your + root partition may be sabotaged by somebody with physical access + (that 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 + 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. + + + * 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, if 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-disk RAID1 or by using LVM), so that it 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. 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/crypdisks 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. + + Advantages are a higher usability, automatic configuration of + non-default crypto parameters, defenses against low-entropy + passphrases like salting and iterated PBKDF2 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 (1MB+4096B for defaults + and 2MB for the most commonly used non-default XTS mode). + + + * 2.5 Can I encrypt an already existing, non-empty partition to use LUKS? + + There is no converter, and it is not really needed. The way to do + this is to make a backup of the device in question, securely wipe the + device (as LUKS device initialization does not clear away old data), + do a luksFormat, optionally overwrite the encrypted device, create a + new filesystem and restore your backup on the now encrypted device. + Also refer to sections "Security Aspects" and "Backup and Data + Recovery". + + For backup, plain GNU tar works well and backs up anything likely + to be in a filesystem. + + + * 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 luksfile to /dev/loop0 + cryptsetup luksFormat /dev/loop0 # create LUKS on loop device + + Afterwards just use /dev/loop0 as a you would use a LUKS partition. + To unmap the file when done, use "losetup -d /dev/loop0". + + + * 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? + + 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 autodetection 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 + | + RAID + | + Raw partitions + | + Raw disks <- bottom + + The big advantage is that you can manage the RAID container just like + any RAID container, it does not care that what is in it is encrypted. + + + * 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 still + 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. In the last case make sure no trailing newline (0x0a) is + contained in the key file, or the passphrase will not work because + the whole file is used as input. + + To add a new passphrase to a free key slot from file, use something + like this: + + cryptsetup luksAddKey /dev/loop0 keyfile + + + To add a new passphrase to a specific key-slot, use something + like this: + + cryptsetup luksAddKey --key-slot 7 /dev/loop0 keyfile + + + To supply a key from file to any LUKS command, use the --key-file + option, e.g. like this: + + cryptsetup luksOpen --key-file keyfile /dev/loop0 e1 + + + + * 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 onwards + by dm-crypt. A bit more information on the process by which + transactional guarantees are implemented can be found here: + + http://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. 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 though 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). It 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. + + For TPM support, you may want to have a look at tpm-luks at + https://github.com/shpedoikal/tpm-luks. Note that tpm-luks is not + related to the cryptsetup project. + + + * 2.15 Can I resize a dm-crypt or LUKS partition? + + Yes, you can, as neither dm-crypt nor LUKS stores partition size. + Whether you should is a different question. Personally I recommend + backup, recreation of the encrypted partition with new size, + recreation of the filesystem and restore. This gets around the + tricky business of resizing the filesystem. Resizing a dm-crypt or + LUKS container does not resize the filesystem in it. The backup is + really non-optional here, as a lot can go wrong, resulting in partial + or complete data loss. Using something like gparted to resize an + encrypted partition is slow, but typically works. This will not + change the size of the filesystem hidden under the encryption though. + + You also need to be aware of size-based limitations. The one + currently relevant is that aes-xts-plain should not be used for + encrypted container sizes larger than 2TiB. Use aes-xts-plain64 for + that. + + + * 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 + + It will output first iterations/second for the key-derivation + function PBKDF2 parameterized with different hash-functions, and then + the raw encryption speed of ciphers with different modes and + key-sizes. 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, 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. + + + * 2.19 How can I wipe a device with crypto-grade randomness? + + The conventional recommendation if you want to not just do a + zero-wipe is to use something like + + cat /dev/urandom > <taget-device> + + That is very slow and painful at 10-20MB/s on a fast computer. + Using cryptsetup and a plain dm-crypt device with a random key, + it is much faster and gives you 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/<block-device> to_be_wiped + + This maps the container as plain under /dev/mapper/to_be_wiped with a + random password. For the actual wipe you have several options. + 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 + http://www.tansi.org/tools/index.html ): + + cat /dev/zero | wcs > /dev/mapper/to_be_wiped + + + Remove the mapping at the end and you are done. + + * 2.20 How to I wipe only the LUKS header? + + This is not the emergency wipe procedure. That is in Item 5.4. This procedure + 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. + + Most safe way is this (backup is still a good idea): + + 01) Determine header size in 512 Byte sectors with "luksDump": + + cryptsetup luksDump <device with LUKS container> + +-> ... + Payload offset: <number> + ... + + 02) Take the result number and write number * 512 zeros to the start of the + device, e.g. like this: + + dd bs=512 count=<number> if=/dev/zero of=<device> + + That is it. + + +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 iteration 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 on the machine where the passphrase is set. + 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 have to be tried in order to find the + right one. + + If this is problem, you can add another key-slot using the slow + machine with the same passphrase and then remove the old key-slot. + The new key-slot will have an iteration count adjusted to 1 second on + the slow machine. Use luksKeyAdd and then luksKillSlot or + luksRemoveKey. + + However, this operation will not change volume key iteration count + (MK iterations in output of "cryptsetup luksDump"). 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 in the original LUKS specification this + value was fixed to 10, but it is now derived from the PBKDF2 + benchmark as well and set to iterations in 0.125 sec or 1000, + whichever is larger. Also note that MK iterations are not very + security relevant. But as each key-slot already takes 1 second, + spending the additional 0.125 seconds really does not matter. + + * 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: + + cryptsetup luksHeaderBackup --header-backup-file <file> <device> + cryptsetup luksHeaderRestore --header-backup-file <file> <device> + + + + * 3.6 cryptsetup segfaults on Gentoo amd64 hardened ... + + There seems to be some interference between the hardening and and the + way cryptsetup benchmarks PBKDF2. The solution to this is currently + not quite clear for an encrypted root filesystem. For other uses, + you can apparently specify USE="dynamic" as compile flag, see + http://bugs.gentoo.org/show_bug.cgi?id=283470 + + +4. Troubleshooting + + + * 4.1 I get the error "LUKS keyslot x is invalid." What does that mean? + + This means that the given keyslot has an offset that points outside + the valid keyslot area. Typically, the reason is a corrupted LUKS + header because something was written to the start of the device the + LUKS container is on. Refer to Section "Backup and Data Recovery" + and ask on the mailing list if you have trouble diagnosing and (if + still possible) repairing this. + + + * 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 if the passphrase is correct and the + respective key-slot 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 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 + + LUKS + XFS seems to uncover RAM problems that never caused obvious + problems before. Symptoms vary, but often the problem manifest + itself when copying large amounts of data, typically several times + larger than your main memory. + + Side note: One thing you should always do on large data + copy/movements is to run a verify, for example with the "-d" option + of "tar" or by doing a set of MD5 checksums on the source or target + with + + find . -type f -exec md5sum \{\} \; > checksum-file + + and then a "md5sum -c checksum-file" on the other side. If you get + mismatches here, RAM is the primary suspect. A lesser suspect is an + overclocked CPU. I have found countless hardware problems in verify + runs after copying or making backups. Bit errors are much more + common than most people think. + + Some RAM issues are even worse and corrupt structures in one of the + layers. This typically results in lockups, CPU state dumps in the + system logs, kernel panic or other things. It is quite possible to + have the problem with an encrypted device, but not with an otherwise + the same unencrypted device. The reason for that is that encryption + has an error amplification property: You flip one bit in an encrypted + data block, and the decrypted version has half of its bits flipped. + This is an important security property for modern ciphers. With the + usual modes in cryptsetup (CBC, ESSIV, XTS), you get up to a + completely changed 512 byte block per bit error. A corrupt block + causes a lot more havoc than the occasionally flipped single bit and + can result 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 some worth), run the tests with the + overclocking active. + + There are two good options. One is Memtest86+ and the other is + "memtester" by Charles Cazabon. Memtest86+ requires a reboot and + then takes over the machine, while memtester runs from a root-shell. + Both use different testing methods and I have found problems fast + with each one that the other needed long to find. I recommend + running the following procedure until the first error is found: + + - Run Memtest86+ for one cycle + + - Run memtester for one cycle (shut down as many other applications + as possible) + + - Run Memtest86+ for 24h or more + + - Run memtester for 24h or more + + If all that does not produce error messages, your RAM may be sound, + but I have had one weak bit that Memtest86+ needed around 60 hours to + find. If you can reproduce the original problem reliably, a good + additional test may be to remove half of the RAM (if you have more + than one module) and try whether the problem is still there and if + so, try with the other half. If you just have one module, get a + different one and try with that. If you do overclocking, reduce the + settings to the most conservative ones available and try with that. + + + * 4.5 Is there a risk using debugging tools like strace? + + There most definitely is. An 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. 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 + http://it.slashdot.org/story/12/12/05/0623215/new-25-gpu-monster-devours-strong-passwords-in-minutes + More references can be found a 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 + vastly higher cost than estimated here. + + LUKS uses SHA1 for hashing per default. The claim in the reference is + 63 billion tries/second for SHA1. We will leave aside the check + whether a try actually decrypts a key-slot. Now, the machine has 25 + GPUs, which I will estimate at an overall lifetime cost of USD/EUR + 1000 each, and an useful lifetime of 2 years. (This is on the low + side.) Disregarding downtime, the machine can then break + + N = 63*10^9 * 3600 * 24 * 365 * 2 ~ 4*10^18 + + passphrases for EUR/USD 25k. That is one 62 bit passphrase hashed + once with SHA1 for EUR/USD 25k. Note that as this can be + parallelized, it can be done faster than 2 years with several of + these machines. + + 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 LUKS, 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. + + LUKS: 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. However 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 that is just as + impossible as the other way round. + + 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? Though 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 (or other) partition? + + For LUKS, if you are in a desperate hurry, overwrite the LUKS header + and key-slot area. This means overwriting the first (keyslots x + stripes x keysize) + offset bytes. For the default parameters, this + is the 1'052'672 bytes, i.e. 1MiB + 4096 of the LUKS partition. For + 512 bit key length (e.g. for aes-xts-plain with 512 bit key) this is + 2MiB. (The different offset stems from differences in the sector + alignment of the key-slots.) If in doubt, just be generous and + overwrite the first 10MB or so, it will likely still be fast enough. + 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 1052672 /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. + + If you have time, overwrite the whole LUKS partition with a single + pass of zeros. This is enough for current HDDs. For SSDs or FLASH + (USB sticks) you may want to overwrite the whole drive several times + to be sure data is not retained by wear leveling. This is possibly + still insecure as SSD technology is not fully understood in this + regard. Still, due to the anti-forensic properties of the LUKS + key-slots, a single overwrite of an SSD or FLASH drive 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: http://www.usenix.org/events/fast11/tech/full_papers/Wei.pdf + + Keep in mind to also erase all backups. + + Example for a zero-overwrite erase of partition sde1 done with + dd_rescue: + + dd_rescue -w /dev/zero /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 zeros. For an SSD or FLASH drive (USB stick), you may + want to overwrite the complete SSD several times and use physical + destruction in addition, see last item. For re-writable CD/DVD, a + single overwrite should also be enough, due to the anti-forensic + properties of the LUKS keyslots. For write-once media, use physical + destruction. For low security requirements, just cut the CD/DVD into + several parts. For high security needs, shred or burn the medium. + If your backup is on magnetic tape, I advise physical destruction by + shredding or burning, 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 at all during + overwrites. + + + * 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 LUKS 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 LUKS 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 LUKS container?" in Section "Backup and Data Recovery". + + + * 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? + + Note: You should only use the 94 printable characters from 7 bit + ASCII code to prevent your passphrase from failing when the character + encoding changes, e.g. because of a system upgrade, see also the + note at the very start of this FAQ under "WARNINGS". + + 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 to 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 or switch some words around, + then this is good passphrase material. + + + * 5.10 What is "iteration count" and why is decreasing it a bad idea? + + 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 LUKS, 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 entropy low + passphrases are vulnerable. Lowering it even further increases this + danger significantly. + + + * 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. + + + * 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 is to supply the master key + yourself. If that key is low-entropy, then you get what you deserve. + The other known possibility 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). + + 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 large blocks. + LUKS and dm-crypt always use 512B 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 what that + means, so let me make clear that this refers to the situation where + the attackers can prove that there is data that 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 though 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 first + glance it seems possible to do it. And from a cryptographic point + of view, it actually is possible. + + So, does it 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 you + 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: http://www.schneier.com/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 and Hybrid Drives? + + The problem is that you cannot reliably erase parts of these devices, + mainly due to wear-leveling and possibly defect management. + + Basically, when overwriting a sector (of 512B), 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 for SSDs. That does not work for + USB keys though and may or may not be secure for a hybrid drive. If + it finishes on an SSD after a few seconds, it was possibly faked. + Unfortunately, for hybrid drives that indicator does not work, as the + drive may well take the time to truly erase the magnetic part, but + only mark the SSD/Flash part as erased while data is still in there. + + 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 or LUKS. + + If you want or need all the original LUKS security features to work, + you can use a detached LUKS header and put that on a conventional, + magnetic disk. That leaves potentially old encrypted data in the + pools on the disk, but otherwise you get LUKS with the same security + as on a magnetic disk. + + 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 LUKS 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 LUKS + 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. + + + * 5.21 Why is there no "Nuke-Option"? + + A "Nuke-Option" or "Kill-switch" is a password that when entered upon + unlocking instead wipes the header and all passwords. So when + somebody forces you to enter your password, you can destroy the data + instead. + + While this sounds attractive at first glance, it does not make sense + once a real security analysis is done. One problem is that you have + to have some kind of HSM (Hardware Security Module) in order to + implement it securely. In the movies, a HSM starts to smoke and melt + once the Nuke-Option has been activated. In reality, it just wipes + some battery-backed RAM cells. A proper HSM costs something like + 20'000...100'000 EUR/USD and there a Nuke-Option may make some sense. + BTW, a chipcard or a TPM is not a HSM, although some vendors are + promoting that myth. + + Now, a proper HSMs will have a wipe option but not a Nuke-Option, + i.e. you can explicitly wipe the HSM, but by a different process + than unlocking it takes. Why is that? Simple: If somebody can force + you to reveal passwords, then they can also do bad things to you if + you do not or if you enter a nuke password instead. Think locking + you up for a few years for "destroying evidence" or for far longer + and without trial for being a "terrorist suspect". No HSM maker will + want to expose its customers to that risk. + + Now think of the typical LUKS application scenario, i.e. disk + encryption. Usually the ones forcing you to hand over your password + will have access to the disk as well, and, if they have any real + suspicion, they will mirror your disk before entering anything + supplied by you. This neatly negates any Nuke-Option. If they have + no suspicion (just harassing people that cross some border for + example), the Nuke-Option would work, but see above about likely + negative consequences and remember that a Nuke-Option may not work + reliably on SSD and hybrid drives anyways. + + Hence my advice is to never take data that you do not want to reveal + into any such situation in the first place. There is no need to + transfer data on physical carriers today. The Internet makes it + quite possible to transfer data between arbitrary places and modern + encryption makes it secure. If you do it right, nobody will even be + able to identify source or destination. (How to do that is out of + scope of this document. It does require advanced skills in this age + of pervasive surveillance.) + + Hence, LUKS has not kill option because it would do much more harm + than good. + + Still, if you have a good use-case (i.e. non-abstract real-world + situation) where a Nuke-Option would actually be beneficial, please + let me know. + + + * 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 + LUKS 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 keys-lot, you are good. Do not forget to close + the device again. + + Under some circumstances (damaged header), this fails. Then use the + following steps: + + 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 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". Example: + + cat /dev/sda10 > sda10.img + dd_rescue /dev/sda10 sda10.img + + You can also use any other backup software that is capable of making + a sector image of a partition. Note that compression is ineffective + for encrypted data, hence it does not 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 LUKS + partition, chances are good that all data is lost permanently. + + For this case, a header+key-slot backup would often be enough. But + keep in mind that a 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. If the + salt values are overwritten or changed, nothing (in the + cryptographically strong sense) can be done to access the data, + unless there is a backup of the LUKS header. If a key-slot is + damaged, the data can still be read with a different key-slot, if + there is a remaining undamaged and used key-slot. Note that in order + to make a key-slot unrecoverable in a cryptographically strong sense, + changing about 4-6 bits in random locations of its 128kiB size is + quite enough. + + + * 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. 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 LUKS container? + + This is typically only needed if you managed to damage your LUKS + header, but the container is still mapped, i.e. "luksOpen"ed. It + also helps if you have a mapped container that you forgot or do not + know a passphrase for (e.g. on a long running server.) + + WARNING: Things go wrong, do a full backup before trying this! + + WARNING: This exposes the master key of the LUKS container. Note + that both ways to recreate a LUKS header with the old master key + described below will write the master key to disk. Unless you are + sure you have securely erased it afterwards, e.g. by writing it to + an encrypted partition, RAM disk or by erasing the filesystem you + wrote it to by a complete overwrite, you should change the master key + afterwards. Changing the master key requires a full data backup, + luksFormat and then restore of the backup. + + First, there is a script by Milan that automates the whole process, + except generating a new LUKS 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 LUKS 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 LUKS look like? + + A LUKS partition consists of a header, followed by 8 key-slot + descriptors, followed by 8 key slots, followed by the encrypted data + area. + + Header and key-slot descriptors fill the first 592 bytes. The + key-slot size depends on the creation parameters, namely on the + number of anti-forensic stripes, 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 LUKS 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 LUKS On-Disk Format Specification Version 1.2.1 + +LUKS 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 LUKS 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 LUKS 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 sector borders). + + * 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. You cannot change the + master key on a LUKS container, 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. + + 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 or does now work as I want it to + +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. Incidentally, at this time (1-Jan-17) CVE-2016-4484 still says +absolutely nothing, which means that the reporters could not be bothered +do actually describe the problem so far and hence it cannot be that bad. +If it were, you would expect that they would have a CVE description in +there more than 30 days (!) after reporting the problem to the press. + +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, than you have +bigger problems than this somewhat expected behavior. + +My take is this was much more driven by some big egos that wanted +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? + +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 an 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. + +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 *** (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. 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: + https://gitlab.com/cryptsetup/cryptsetup/wikis/Specification + + * 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 + + - http://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 + +___ |