dm-crypt with LUKS (Italiano)
Articoli correlati
Questo articolo si concentra su come configurare un sistema completamente crittografato su Arch Linux, usando dm-crypt con LUKS.
dm-crypt è il device-mapper standard per le funzionalità di crittografia fornito dal kernel Linux. Può essere utilizzato direttamente da chi desidera avere un completo controllo degli aspetti del partizionamento e della gestione delle chiavi.
LUKS è un comodo strato aggiuntivo il quale archivia tutte le informazioni necessarie alla configurazione di dm-crypt sul disco stesso e si occupa anche della gestione delle partizioni e delle chiavi nel tentativo di facilitarne l’uso.
Per maggiori dettagli sui paragoni tra dm-crypt+LUKS e gli altri metodi di crittografia, consultare la tavola comparativa.
Contents
- 1 Avvertenze
- 2 Configurazione iniziale
-
3 Configurare LUKS
- 3.1 Mappare le partizioni fisiche con LUKS
- 3.2 Storing the Key File
- 3.3 Encrypting the Swap partition
- 4 Installing the system
- 5 Remote unlocking of the root (or other) partition
- 6 Backup the cryptheader
- 7 Encrypting a loopback filesystem
- 8 Encrypting a LVM setup
- 9 Using GPG or OpenSSL Encrypted Keyfiles
- 10 Securing the unencrypted boot partition
- 11 Automount user homes on login
- 12 Resources
Avvertenze
Configurazione iniziale
Panoramica e preparazione
Il supporto di installazione di Arch comprende gli strumenti necessari per la crittografia del sistema. L’installazione di un sistema crittografato è in gran parte simile ad una normale installazione, sarà quindi possibile seguire la Guida all’installazione di Arch Linux oppure la Beginners' Guide dopo aver configurato le partizioni crittografate. Sarà necessario sistemare la configurazione del sistema per far si che possa avviarsi dai volumi Luks.
La routine di creazione di un sistema criptato segue questi passaggi generali:
- Cancellazione sicura del/degli hard disk
- Partizionamento e configurazione della crittografia (LVM è opzionale)
- Selezione ed installazione dei pacchetti
- Configurazione del sistema
Cancellazione sicura del/degli hard disk
Si consiglia di cancellare in modo sicuro i dati accessibili nelle partizioni o dischi utilizzando dati casuali. I dati casuali saranno scritti per motivi di sicurezza perché sono indistinguibili dai dati successivamente scritti da dm-crypt. La cancellazione sicura dei dischi implica la sovrascrittura dell’intero disco con dati casuali.
Entrambi i dischi sia nuovi che usati dovrebbero essere sovrascritti in maniera sicura. Questo aiuta ad assicurare la privacy dei dati che si trovano sulle partizioni criptate. Il contenuto dei dischi comprati dal produttore non è assicurato. Se il disco è stato riempito con bit di valore zero e successivamente usato per dati criptati, allora sarà relativamente semplice identificare dove finiscono i dati criptati e dove iniziano gli zeri. Dato che una partizione criptata si suppone essere indistinguibile da dati casuali, la mancanza di dati casuali su di un disco sovrascritto con degli zeri rende il disco criptato un facile bersaglio per la cripto analisi.
Ripartizionare o riformattare un disco usato rimuove solamente la struttura del file system che serve per identificare dove fisicamente sono collocati i dati, ma lascia i dati intatti sul disco. Sarà relativamente semplice utilizzando programmi di recupero file accedere ai dati rimasti. Quindi i dischi dovrebbero essere sovrascritti in modo sicuro con dati casuali prima di effettuare la cifratura per evitare il recupero dei dati.
Nel decidere quale metodo utilizzare per la cancellazione sicura del disco, notare che non sarà necessario eseguirlo più di una volta fintanto che il disco viene utilizzato come disco criptato.
Usare un contenitore LUKS come generatore di numeri pseudo-casuali (alternativa)
Le FAQ di cryptsetup menzionano una procedura molto semplice che consiste nell'utilizzare un volume dm-crypt esistente per cancellare tutto lo spazio libero accessibile sul disco sottostante. Questa procedura inoltre mira a proteggere contro la rilevazione dei pattern ripetuti utilizzati per la cancellazione.
# dd if=/dev/zero of=/dev/mapper/luks-container
Cancellare lo spazio libero con un file criptato dopo l'installazione
Il solito effetto può essere ottenuto creando su tutte le partizioni un file che le riempia completamente dopo che il sistema è stato installato, avviato e con tutte le partizioni criptate montate. Questo perché i dati criptati sono indistinguibili dai dati casuali.
# dd if=/dev/zero of=/file/su/contenitore-luks # rm /file/su/contenitore-luks
Ovviamente sarà necessario ripetere il precedente processo per ogni contenitore criptato creato.
Cancellare i keyslot LUKS
#cryptsetup luksKillSlot device numero_key_slot
Questo cancellerà solamente un singolo keyslot.
Cancellare i LUKS header
Le partizioni formattate con dm-crypt/LUKS contengono un header contenete la cifratura e le opzioni utilizzate da dm-mod
quando viene aperta la periferica. Dopo l'header iniziano i dati criptati. Quindi, quando si dismette un disco (ad esempio se si vende il PC, sostituzione del disco, eccetera) potrebbe essere sufficiente cancellare l'header della partizione, anziché sovrascrivere l'intero disco - che potrebbe essere un processo molto lungo.
Cancellando l'header LUKS verrà cancellata anche la chiave principale PBKDF2-criptata (AES), salts ed altre.
Un header con un singolo keyslot di dimensione 256 bit di sarà grande 1024KB. E' consigliato di sovrascrivere inoltre i primi 4KB scritti da dm-crypt, quindi sarà necessario cancellare 1028KB. Che sono 1052672
Byte.
Per sovrascrivere con zero usare:
#head -c 1052672 /dev/zero > /dev/sda1; sync
Per chiavi lunghe 512 bit (ad esempio per aes-xts-plain con chiavi da 512 bit) l'header è grande 2MB.
In caso di dubbi, è meglio essere generosi e sovrascrivere i primi 10MB o all'incirca.
#dd if=/dev/zero of=/dev/sda1 bs=512 count=20480
Quando si cancella l'header con dati casuali e l'header è seguito da dati criptati scritti sopra dati casuali, tutto ciò che resta saranno dati casuali.
Supporto per discard/TRIM per i dischi a stato solido (SSD)
Gli utenti con dischi a stato solido dovrebbero essere consapevoli che per default, il meccanismo di cifratura del kernel Linux non invierà i comandi TRIM dal filesystem al disco sottostante. Gli sviluppatori del device-mapper hanno chiarito che il supporto per TRIM non sarà mai abilitato per default sul device dm-crypt a causa di potenziali problemi di sicurezza.
Molti utenti vorranno comunque utilizzare TRIM sui propri dischi SSD criptati. Questo crea un minimo di dispersione di informazioni riguardo ai blocchi liberati, comunque sufficienti a determinare il filesystem utilizzato, in caso TRIM sia abilitato. Una immagine ed una discussione del problema derivante dall'attivazione di TRIM è disponibile nel blog di uno sviluppatore di cryptsetup
.
As a semi-tangential caveat, it is worth noting that because TRIM provides information to the disk firmware about which blocks contain data, encryption schemes that rely on plausible deniability, like TrueCrypt's hidden volumes, should never be used on a device that utilizes TRIM. This is probably also valid for TC containers within a LUKS encrypted device that uses TRIM.
Gli sviluppatori di TrueCrypt sconsigliano l'uso di qualsiasi volume TC su un volume che effettua tecniche wear-leveling per estendere la vita del disco; molte periferiche flash inclusi i dischi SSD e le USB flash, utilizzano obbligatoriamente il wear-leveling a livello firmware. I device LUKS sono probabilmente non vulnerabili ai problemi derivanti dal wear-leveling se l'intero device viene cancellato prima dell'inizializzazione della partizione LUKS. Consultare questo articolo su trim e questo articolo sul wear-leveling per maggiori informazioni.
A partire dalla versione 3.1 di linux, il supporto di dm-crypt per l'inoltro dei comandi TRIM può essere attivata sul device alla creazione oppure durante il mount tramite dmsetup. Il supporto per questa opzione è presente anche in cryptsetup a partire dalla versione 1.4.0. Per attivare il supporto durante l'avvio, sarà necessario aggiungere :allow-discards
come opzione al cryptdevice
. Un esempio di device con abilitato TRIM sarà simile a questa:
cryptdevice=/dev/mapper/root:root:allow-discards
Per le principali configurazioni del cryptdevice
prima di :allow-discards
consultare i paragrafi che seguono.
Partizionamento
Dopo che il disco è stato sovrascritto in modo sicuro, è il momento di creare le partizioni e iniziare la configurazione del sistema criptato.
Ci sono diversi modi di creare partizioni sul disco:
LUKS è compatibile con sistemi che necessitano di LVM e/o RAID come con le partizioni standard che siano primarie, estese, e logiche.
Partizioni standard
Queste sono le partizioni con cui la maggior parte degli utenti avrà maggiore familiarità. Esse possono essere di tre tipi: partizioni primarie, partizioni estese e partizioni logiche.
- Partizioni primarie
- Sono le normali partizioni riconosciute dal BIOS. Possono esserne presenti fino a quattro in un partizionamento MBR.
- Partizioni estese
- Sono partizioni primarie che definiscono al loro interno altre partizioni. Le partizioni estese sono state create per ovviare il limite delle quattro partizioni primarie.
- Partizioni logiche
- Sono le partizioni definite nelle partizioni estese.
LVM: Logical Volume Manager
Il gestore di volumi logici(LVM) permette la creazione di gruppi di volumi per sistemi che richiedono complesse combinazioni di dischi multipli e partizioni che non si possono ottenere con le partizioni standard. LVM è spiegato più dettagliatamente nell'articolo del Wiki di Arch Linux che si consiglia di leggere prima di procedere con le istruzioni che seguiranno riguardanti la configurazione di LUKS con LVM.
LVM on LUKS
There is a growing preference towards logical volume management of LUKS encrypted physical media (LVM on LUKS). The deployment of LVM on LUKS is considered much more generalizable. In a LVM on LUKS scenario, the LUKS-partition has to be opened and mapped before LVM can access the underlaying setup volumes.
One reason for this is that using LUKS as the lowest level of infrastructure most closely approximates the deployment of physical disks with built-in hardware encryption. In that case, logical volume management would be layered on top of the hardware encryption - usage of LUKS would be superfluous.
LUKS on LVM
It is possible there may exist usage scenarios where encrypting logical volumes rather than physical disks is required (LUKS on LVM). A usage scenario for LUKS on LVM exists where utmost flexibility for assigning available diskspace or a mix of unencrypted and encrypted volumes is desired. Upon boot the LVM is setup and assigned before the LUKS-encrypted volumes are opened. In order to manage changes of volumes in a LUKS on LVM setup, both module layers' setup have to be taken into account, i.e. shrinking or expanding an encrypted volume has to include the resizing of the encrypted LUKS blockdevice to ensure integrity of it and the filesystem. That said, in simpler scenarios the usage of LVM may be superfluous.
Creare le partizioni sul disco
Le partizioni del disco vengono create utilizzando:
# cfdisk
Questo comando avvierà una interfaccia grafica per la creazione delle partizioni.
Ci sono due partizioni necessarie ad un sistema criptato:
- Il filesystem di root
-
/
sarà criptata e conterrà tutti i dati utente e di sistema (/usr
,/bin
,/var
,/home
, etc.)
- Una partizione di boot per l'avvio
- la partizione
/boot
non sarà criptata; il bootloader necessita di accedere alla cartella/boot
da cui caricherà il ramdisk iniziale i moduli per la crittografia e quelli necessari per l'avvio del sistema che sarà criptato (consultare mkinitcpio per maggiori informazioni). Per questa ragione, è necessario che/boot
risieda su di una propria partizione non criptata.
Sistemi con singolo disco
A seconda delle necessità del sistema, possono essere create altre partizioni. Queste partizioni possono essere create individualmente a questo livello definendo partizioni primarie oppure estese/logiche. Comunque, se si intende utilizzare LVM, lo spazio rimasto dopo la creazione della partizione di /boot
e quella di swap dovrebbe essere definito come una singola grande partizione che sarà suddivisa successivamente al livello LVM.
Sistemi con dischi multipli
Nei sistemi che utilizzano molteplici dischi, sono valide le opzioni elencate per un singolo disco. Dopo la creazione della partizione di /boot
e la partizione di swap, il rimanente spazio libero può essere diviso in partizioni a questo livello oppure grandi partizioni possono definire tutto lo spazio libero per ogni singolo disco con lo scopo di suddividerle mediante LVM.
Configurare LUKS
Questa sezione del Wiki l'uso della linea di comando per criptare manualmente il sistema utilizzando LUKS.
I passaggi necessari utilizzando l'installer grafico sono molto simili e possono essere trovati nella configurazione manuale del disco.
Mappare le partizioni fisiche con LUKS
Una volta create le partizioni necessarie, sarà il momento di formattarle come partizioni LUKS e successivamente effettuarne il mount mediante il device mapper.
Quando viene creata una partizione LUKS deve essere associata ad una chiave. La chiave verrà utilizzata per sbloccare l'header della partizione criptata con LUKS.
Una chiave può essere:
- Una parola d'ordine(Passphrase)
- Un file chiave(Keyfile)
Sarà possibile definire fino ad 8 differenti chiavi per ogni partizione LUKS. Questo permette all'utente di creare chiavi di accesso per effettuare i backup. Inoltre una chiave differente può essere creata per garantire l'accesso alla partizione ad un utente e successivamente revocata senza bisogno di criptare nuovamente la partizione.
Usare LUKS per formattare partizioni con la parola d'ordine
Cryptsetup viene utilizzato per formattare, effettuare il mount e l'umount delle partizioni criptate.
Usage:
# cryptsetup [OPTION...] <action> <action-specific>
Example:
# cryptsetup --cipher aes-xts-plain64 --key-size 512 --hash sha512 --iter-time 5000 --use-random --verify-passphrase luksFormat <device>
Common options used with luksFormat:
Available options | Cryptsetup defaults | Comment | Example | Comment |
---|---|---|---|---|
--cipher, -c | aes-cbc-essiv:sha256
|
Use the AES-cipher with CBC/ESSIV. | aes-xts-plain64
|
XTS. For volumes >2TiB use aes-xts-plain64 (requires kernel >= 2.6.33).
|
--key-size, -s | 256
|
The cipher is used with 256 bit key-size. | 512
|
XTS splits the supplied key into fraternal twins. For an effective AES-256 the XTS key-size must be 512 .
|
--hash, -h | sha1
|
Hash algorithm used for PBKDF2. | sha512
|
|
--iter-time, -i | 1000
|
Number of milliseconds to spend with PBKDF2 passphrase processing. | 5000
|
Using a hash stronger than sha1 results in less iterations if iter-time is not increased. |
--use-random | --use-urandom
|
/dev/urandom is used as randomness source for the (long-term) volume master key. | --use-random
|
Avoid generating an insecure master key if low on entropy. Will block if the entropy pool is used up. |
--verify-passphrase, -y | Yes | Default only for luksFormat and luksAddKey. | - | No need to type for archlinux at the moment. |
When comparing the defaults with the example, it has to be taken into account that a number of the options stated affect system performance just for creating the initial crypt-blockdevice or opening it, but not the disk-io operations when the system is running. A full list of options cryptsetup
accepts can be found in the manpage.
In the following examples for creating LUKS partitions, we will use the AES cipher in XTS mode; at present this is most generally used preferred cipher. Other ciphers can be used with cryptsetup, and details about them can be found here: Wikipedia:Block_cipher
Formatting LUKS Partitions
First of all make sure the device mapper kernel module is loaded by executing the following: # modprobe dm_mod
In order to format a desired partition as an encrypted LUKS partition execute:
# cryptsetup -c cipher -y -s key_size luksFormat /dev/partition_name
Enter passphrase: password Verify passphrase: password
Check results:
# cryptsetup luksDump /dev/drive
This should be repeated for all partitions except for /boot
and possibly swap.
The example below will create an encrypted root partition using the AES cipher in XTS mode (generally referred to as XTS-AES).
# cryptsetup -c aes-xts-plain -y -s 512 luksFormat /dev/sda2
Unlocking/Mapping LUKS Partitions with the Device Mapper
Once the LUKS partitions have been created it is time to unlock them.
The unlocking process will map the partitions to a new device name using the device mapper. This alerts the kernel that /dev/partition
is actually an encrypted device and should be addressed through LUKS using the /dev/mapper/name
so as not to overwrite the encrypted data. To guard against accidental overwriting, read about the possibilities to backup the cryptheader[broken link: invalid section] after finishing setup.
In order to open an encrypted LUKS partition execute:
# cryptsetup luksOpen /dev/partition_name device-mapper_name
Enter any LUKS passphrase: password key slot 0 unlocked. Command successful.
Usually the device mapped name is descriptive of the function of the partition that is mapped, example:
- cryptsetup luksOpen /dev/sda2 swap
- Once opened, the swap partition device address would be
/dev/mapper/swap
instead of/dev/sda2
.
- cryptsetup luksOpen /dev/sda3 root
- Once opened, the root partition device address would be
/dev/mapper/root
instead of/dev/sda3
.
- cryptsetup luksOpen /dev/sda3 lvmpool (alternate)
- For setting up LVM ontop the encryption layer the device file for the decrypted volume group would be anything like
/dev/mapper/lvmpool
instead of/dev/sda3
. LVM will then give additional names to all logical volumes created, e.g./dev/mapper/lvmpool-root
and/dev/mapper/lvmpool-swap
.
In order to write encrypted data into the partition it must be accessed through the device mapped name.
Using LUKS to Format Partitions with a Keyfile
What is a Keyfile?
A keyfile is any file in which the data contained within it is used as the passphrase to unlock an encrypted volume. Therefore if these files are lost or changed, decrypting the volume will no longer be possible.
Why use a Keyfile?
There are many kinds of keyfile. Each type of keyfile used has benefits and disadvantages summarized below:
-
keyfile.passphrase:
- this is my passphrase I would have typed during boot but I have placed it in a file instead
This is a keyfile containing a simple passphrase. The benefit of this type of keyfile is that if the file is lost the data it contained is known and hopefully easily remembered by the owner of the encrypted volume. However the disadvantage is that this does not add any security over entering a passphrase during the initial system start.
-
keyfile.randomtext:
- fjqweifj830149-57 819y4my1- 38t1934yt8-91m 34co3;t8y;9p3y-
This is a keyfile containing a block of random characters. The benefit of this type of keyfile is that it is much more resistant to dictionary attacks than a simple passphrase. An additional strength of keyfiles can be utilized in this situation which is the length of data used. Since this is not a string meant to be memorized by a person for entry, it is trivial to create files containing thousands of random characters as the key. The disadvantage is that if this file is lost or changed, it will most likely not be possible to access the encrypted volume without a backup passphrase.
-
keyfile.binary:
- where any binary file, images, text, video could be chosen as the keyfile
This is a binary file that has been defined as a keyfile. When identifying files as candidates for a keyfile, it is recommended to choose files that are relatively static such as photos, music, video clips. The benefit of these files is that they serve a dual function which can make them harder to identify as keyfiles. Instead of having a text file with a large amount of random text, the keyfile would look like a regular image file or music clip to the casual observer. The disadvantage is that if this file is lost or changed, it will most likely not be possible to access the encrypted volume without a backup passphrase. Additionally, there is a theoretical loss of randomness when compared to a randomly generated text file. This is due to the fact that images, videos and music have some intrinsic relationship between neighboring bits of data that does not exist for a text file. However this is controversial and has never been exploited publicly.
Creating a Keyfile with Random Characters
Here dd
is used to generate a keyfile of 2048 bits of random characters.
# dd if=/dev/urandom of=mykeyfile bs=512 count=4
The usage of dd
is similar to initially wiping the volume with random data prior to encryption.
Creating a new LUKS encrypted partition with a Keyfile
When creating a new LUKS encrypted partition, a keyfile may be associated with the partition on its creation using:
# cryptsetup -c <desired cipher> -s <key size> luksFormat /dev/<volume to encrypt> /path/to/mykeyfile
This is accomplished by appending the bold area to the standard cryptsetup command which defines where the keyfile is located.
Adding Additional Passphrases or Keyfiles to a LUKS Encrypted Partition
LUKS supports the association of up to 8 keyslots with any single encrypted volume. Keyslots can be either keyfiles or passphrases.
Once an encrypted partition has been created, the initial keyslot 0 is created. Additional keyslots are numbered from 1 to 7.
Adding new keyslots is accomplished using cryptsetup with the luksAddKey
action.
Don't forget wiping unused keyslots with luksKillSlot
as described in #Wipe LUKS keyslots[broken link: invalid section].)
# cryptsetup luksAddKey /dev/mapper/device (/path/to/additionalkeyfile)
Enter any passphrase: Enter new passphrase for key slot: Verify passphrase:
Where device
is the volume containing the LUKS header to wich the new keyslot is added. This works on header backup files as well.
If /path/to/additionalkeyfile
is given, cryptsetup will add a new keyslot for additionalkeyfile. Otherwise a new passphrase will be prompted for twice.
For adding keyslots cryptsetup has to decrypt the master key from an existing keyslot so it first asks for "any passphrase" (of an existing keyslot).
For getting the master key from an existing keyfile keyslot the --key-file
or -d
option followed by the "old" keyfile will try to unlock all available keyfile keyslots.
# cryptsetup luksAddKey /dev/mapper/device (/path/to/additionalkeyfile) -d /path/to/keyfile
Storing the Key File
External Storage on a USB Drive
Preparation for Persistent block device naming
For reading the file from an external storage device it is very convenient to access it through udev's Persistent block device naming features and not by ordinary device nodes like /dev/sdb1
whose naming depends on the order in which devices are plugged in. So in order to assure that the encrypt
HOOK in the initcpio finds your keyfile, you must use a permanent device name.
Persistent symlinks
A quick method (as opposed to setting up a udev rule) for doing so involves referencing the right partition by its UUID, id (based on hardware info and serial number) or filesystem label.
Plug the device in and print every file name under /dev/disk:
#ls -lR /dev/disk/
/dev/disk/: total 0 drwxr-xr-x 2 root root 180 Feb 12 10:11 by-id drwxr-xr-x 2 root root 60 Feb 12 10:11 by-label drwxr-xr-x 2 root root 100 Feb 12 10:11 by-path drwxr-xr-x 2 root root 180 Feb 12 10:11 by-uuid /dev/disk/by-id: total 0 lrwxrwxrwx 1 root root 9 Feb 12 10:11 usb-Generic_STORAGE_DEVICE_000000014583-0:0 -> ../../sdb lrwxrwxrwx 1 root root 10 Feb 12 10:11 usb-Generic_STORAGE_DEVICE_000000014583-0:0-part1 -> ../../sdb1 /dev/disk/by-label: total 0 lrwxrwxrwx 1 root root 10 Feb 12 10:11 Keys -> ../../sdb1 /dev/disk/by-path: total 0 lrwxrwxrwx 1 root root 9 Feb 12 10:11 pci-0000:00:1d.7-usb-0:1:1.0-scsi-0:0:0:0 -> ../../sdb lrwxrwxrwx 1 root root 10 Feb 12 10:11 pci-0000:00:1d.7-usb-0:1:1.0-scsi-0:0:0:0-part1 -> ../../sdb1 /dev/disk/by-uuid: total 0 lrwxrwxrwx 1 root root 10 Feb 12 10:11 baa07781-2a10-43a7-b876-c1715aba9d54 -> ../../sdb1
UUID
Using the filesystem UUID for persistent block device naming is considered very reliable. Filesystem UUIDs are stored in the filesystem itself, meaning that the UUID will be the same if you plug it into any other computer, and that a dd backup of it will always have the same UUID since dd does a bitwise copy.
The right device node for what is now /dev/sdb1
will always get symlinked by /dev/disk/by-uuid/baa07781-2a10-43a7-b876-c1715aba9d54
. Symlinks can be used in the bootloaders "cryptkey" kernel option or anywhere else.
For legacy filesystems like FAT the UUID will be much shorter but collision is still unlikely to happen if not mounting many different FAT filesystems at once.
Label
In the following example a FAT partition is labeled as "Keys" and will always get symlinked by /dev/disk/by-label/Keys
:
#mkdosfs -n >volume-name< /dev/sdb1
#blkid -o list
device fs_type label mount point UUID ------------------------------------------------------- /dev/sdb1 vfat Keys (not mounted) 221E-09C0
Persistent udev rule
Optionally you may choose to set up your flash drive with a udev rule. There is some documentation in the Arch wiki about that already; if you want more in-depth, structural info, read this guide. Here is quickly how it goes.
Get the serial number from your USB flash drive:
lsusb -v | grep -A 5 Vendor
Create a udev rule for it by adding the following to a file in /etc/udev/rules.d/
, such as 8-usbstick.rules
:
KERNEL=="sd*", ATTRS{serial}=="$SERIAL", SYMLINK+="$SYMLINK%n"
Replace $SYMLINK
and $SERIAL
with their respective values. %n
will expand to the partition (just like sda is subdivided into sda1, sda2, ...). You do not need to go with the 'serial' attribute. If you have a custom rule of your own, you can put it in as well (e.g. using the vendor name).
Rescan your sysfs:
udevadm trigger
Now check the contents of /dev
:
ls /dev
It should show your device with your desired name.
Generating the keyfile
Optionally you can mount a tmpfs for storing the temporary keyfile.
# mkdir ./mytmpfs # mount tmpfs ./mytmpfs -t tmpfs -o size=32m # cd ./mytmpfs
The advantage is that it resides in RAM and not on a physical disk, so after unmounting your keyfile is securly gone.
So copy your keyfile to some place you consider as secure before unmounting.
If you are planning to store the keyfile as a plain file on your USB device, you can also simply execute the following command in the corresponding directory, e.g. /media/sdb1
The keyfile can be of arbitrary content and size. We will generate a random temporary keyfile of 2048 bytes:
# dd if=/dev/urandom of=secretkey bs=512 count=4
If you stored your temporary keyfile on a physical storage device, remember to not just (re)move the keyfile later on, but use something like
cp secretkey /destination/path shred --remove --zero secretkey
to securely overwrite it. For overaged filesystems like FAT or ext2 this will suffice while in the case of journaling filesystems, flash memory hardware and other cases it is highly recommended to wipe the entire device or at least the keyfiles partition.
Add a keyslot for the temporary keyfile to the LUKS header:
# cryptsetup luksAddKey /dev/sda2 secretkey
Enter any LUKS passphrase: key slot 0 unlocked. Command successful.
Storing the keyfile
To store the key file, you have two options. The first is less risky than the other, but perhaps a bit more secure (if you consider security by obscurity as more secure). In any case you have to do some further configuration, if not already done above.
Configuration of initcpio
You have to add two extra modules in your /etc/mkinitcpio.conf
, one for the drive's file system and one for the codepage. Further if you created a udev rule, you should tell mkinitcpio
about it:
MODULES="ata_generic ata_piix nls_cp437 vfat" FILES="/etc/udev/rules.d/8-usbstick.rules"
In this example it is assumed that you use a FAT formatted USB drive. Replace those module names if you use another file system on your USB stick (e.g. ext2) or another codepage. Users running the stock Arch kernel should stick to the codepage mentioned here.
Additionally, insert the usb
hook somewhere before the encrypt
hook.
HOOKS="... usb encrypt ... filesystems ..."
If you have a non-US keyboard, it might prove useful to load your keyboard layout before you are prompted to enter the password to unlock the root partition at boot. For this, you will need the keymap
hook before encrypt
.
Generate a new image (maybe you should backup a copy of your old /boot/initramfs-linux.img
first):
# mkinitcpio -p linux
Storing the key as a plain (visible) file
Be sure to choose a plain name for your key – a bit of 'security through obscurity' is always nice ;-). Avoid using dotfiles (hidden files) – the encrypt
hook will fail to find the keyfile during the boot process.
You have to add a kernel parameter in your /boot/grub/menu.lst
(GRUB). It should look something like this:
kernel /vmlinuz-linux cryptdevice=/dev/sda3:root root=/dev/mapper/root ro cryptkey=/dev/usbstick:vfat:/secretkey
This assumes /dev/usbstick
is the FAT partition of your choice. Replace it with /dev/disk/by-...
or whatever your device is.
That is all, reboot and have fun!
Storing the key between MBR and 1st partition
We will write the key directly between the Master Boot Record (MBR) and the first partition.
If you have a bootloader installed on your drive you have to adjust the values. E.g. GRUB needs the first 16 sectors (actually, it depends on the type of the file system, so do not rely on this too much), so you would have to replace seek=4
with seek=16
; otherwise you would overwrite parts of your GRUB installation. When in doubt, take a look at the first 64 sectors of your drive and decide on your own where to place your key.
Optional If you do not know if you have enough free space before the first partition, you can do
dd if=/dev/usbstick of=64sectors bs=512 count=64 # gives you copy of your first 64 sectors hexcurse 64sectors # determine free space xxd 64sectors | less # alternative hex viewer
Write your key to the disk:
dd if=secretkey of=/dev/usbstick bs=512 seek=4
If everything went fine you can now overwrite and delete your temporary secretkey as noted above.
You should not simply use rm
as the keyfile would only be unlinked from your filesystem and be left physically intact.
Now you have to add a kernel parameter in your /boot/grub/menu.lst
file (GRUB); it should look something like this:
kernel /vmlinuz-linux cryptdevice=/dev/sda3:root root=/dev/mapper/root ro cryptkey=/dev/usb:2048:2048
Format for the cryptkey
option:
cryptkey=BLOCKDEVICE:OFFSET:SIZE
OFFSET
and SIZE
match in this example, but this is just coincidence - they can differ (and often will). An other possible example could be
kernel /vmlinuz-linux cryptdevice=/dev/sda3:root root=/dev/mapper/root ro cryptkey=/dev/usb:8192:2048
That is all, reboot and have fun! And look if your partitions still work after that ;-).
Encrypting the Swap partition
Without suspend-to-disk support
In systems where suspend to disk is not a desired feature, it is possible to create a swap file that will have a random master key with each boot. This is accomplished by using dm-crypt directly without LUKS extensions.
The /etc/crypttab
is well commented and you can basically just uncomment the swap line and change device to a persistent symlink.
/etc/crypttab
# name device password options # swap /dev/hdx4 /dev/urandom swap,cipher=aes-cbc-essiv:sha256,size=256
Where:
- name
- Represents the name (
/dev/mapper/name
) to list in /etc/fstab. - device
- Should be the symlink to the actual partition's device file.
- password
-
/dev/urandom
sets the dm-crypt master key to be randomized on every volume recreation. - options
- The
swap
option runs mkswap after cryptographic's are setup.
Persistent block device naming is implemented with simple symlinks. Using UUID's or filesystem-labels is not possible as plain dm-crypt writes only encrypted data without a persistent header like LUKS. If you are not familar with one of the directories under /dev/disk/
read on in the section on #Preparation for Persistent block device naming
#ls -l /dev/disk/*/* | grep sda2
lrwxrwxrwx 1 root root 10 Oct 12 16:54 /dev/disk/by-id/ata-WDC_WD2500BEVT-22ZCT0_WD-WXE908VF0470-part2 -> ../../sda2
Example line for the /dev/sda2
symlink from above:
/etc/crypttab
# name device password options swap /dev/disk/by-id/ata-WDC_WD2500BEVT-22ZCT0_WD-WXE908VF0470-part2 /dev/urandom swap,cipher=aes-cbc-essiv:sha256,size=256
This will map /dev/sda2
to /dev/mapper/swap
as a swap partition that can be added in /etc/fstab
like a normal swap.
If the partition chosen for swap was previously a LUKS partition, crypttab will not overwrite the partition to create a swap partition. This is a safety measure to prevent data loss from accidental mis-identification of the swap partition in crypttab. In order to use such a partition the LUKS header must be overwritten[broken link: invalid section] once.
With suspend-to-disk support
To be able to resume after suspending the computer to disk (hibernate), it is required to keep the swap filesystem intact. Therefore, it is required to have a pre-existent LUKS swap partition, which can be stored on the disk or input manually at startup. Because the resume takes place before /etc/crypttab
can be used, it is required to create a hook in /etc/mkinitcpio.conf
to open the swap LUKS device before resuming.
If you want to use a partition which is currently used by the system, you have to disable it first:
# swapoff /dev/device
Also make sure you remove any line in /etc/crypttab
pointing to this device.
A simple way to realize encrypted swap with suspend-to-disk support is by using LVM ontop the encryption layer, so one encrypted partition can contain infinite filesystems (root, swap, home, ...). Follow the instructions on #Encrypting a LVM setup.
The following setup has the disadvantage of having to insert an additional passphrase for the swap partition manually on every boot.
To format the encrypted container for the swap partition, follow steps similar to those described in #Configurare LUKS above and create keyslot for a user-memorizable passphrase.
Open the partition in /dev/mapper
:
# cryptsetup luksOpen /dev/device swapDevice
Create a swap filesystem inside the mapped partition:
# mkswap /dev/mapper/swapDevice
Now you have to create a hook to open the swap at boot time.
- Create a hook file containing the open command:
/lib/initcpio/hooks/openswap
# vim: set ft=sh: run_hook () { cryptsetup luksOpen /dev/<device> swapDevice }
- Then create and edit the hook setup file:
/lib/initcpio/install/openswap
# vim: set ft=sh: build () { add_runscript } help () { cat<<HELPEOF This opens the swap encrypted partition /dev/<device> in /dev/mapper/swapDevice HELPEOF }
- Add the hook
openswap
in theHOOKS
array in/etc/mkinitcpio.conf
, beforefilesystem
but afterencrypt
. Do not forget to add theresume
hook afteropenswap
.
HOOKS="... encrypt openswap resume filesystems ..."
- Regenerate the boot image:
# mkinitcpio -p linux
- Add the mapped partition to
/etc/fstab
by adding the following line:
/dev/mapper/swapDevice swap swap defaults 0 0
- Set up your system to resume from
/dev/mapper/swapDevice
. For example, if you use GRUB with kernel hibernation support, addresume=/dev/mapper/swapDevice
to the kernel line in/boot/grub/menu.lst
. A line with encrypted root and swap partitions can look like this:
kernel /vmlinuz-linux cryptdevice=/dev/sda2:rootDevice root=/dev/mapper/rootDevice resume=/dev/mapper/swapDevice ro
At boot time, the openswap
hook will open the swap partition so the kernel resume may use it. If you use special hooks for resuming from hibernation, make sure they are placed after openswap
in the HOOKS
array. Please note that because of initrd opening swap, there is no entry for swapDevice in /etc/crypttab
needed in this case.
Using a swap file for suspend-to-disk support
- Choose a mapped partition (e.g.
/dev/mapper/rootDevice
) whose mounted filesystem (e.g./
) contains enough free space to hold the entire contents of your system's RAM. For example, if your system has 4 GiB RAM, then you need at least that much free space on the mounted filesystem of your chosen mapped partition for the swap file.
- Create the swap file (e.g.
/swapfile
) inside the mounted filesystem of your chosen mapped partition. Be sure to activate it withswapon
and also add it to your/etc/fstab
file afterward.
- Set up your system to resume from your chosen mapped partition. For example, if you use GRUB with kernel hibernation support, add
resume=
your chosen mapped partition andresume_offset=
see calculation command below to the kernel line in/boot/grub/menu.lst
. A line with encrypted root partition can look like this:
kernel /vmlinuz-linux cryptdevice=/dev/sda2:rootDevice root=/dev/mapper/rootDevice resume=/dev/mapper/rootDevice resume_offset=123456789 ro
You can calculate the resume_offset
of your swap file like this:
# filefrag -v /swapfile | awk '{if($1==0){print $3}}'
- Add the
resume
hook to youretc/mkinitcpio.conf
file and rebuild the image afterward:
HOOKS="... encrypt resume ... filesystems ..."
- If you use a USB keyboard to enter your decryption password, then the
usbinput
module must appear in front of theencrypt
hook, as shown below. Otherwise, you will not be able to boot your computer because you couldn't enter your decryption password to decrypt your Linux root partition!
HOOKS="... usbinput encrypt ..."
Installing the system
Prepare hard drive for Arch Install Scripts
This assumes you want to install an encrypted system with the Arch Install Scripts, have created partitions for /
(e.g. /dev/sdaX
) and /boot
(/dev/sdaY
) at least, following the Installation Guide and deciding against using LVM[broken link: invalid section]. Prior to creating the partitions you have done a preparation[broken link: invalid section] of the disk for encryption according to your necessities (the necessary tools are on the installation-ISO).
First check, if the blockdevice mapper dm_mod
is loaded with
# lsmod | grep mod
If one wants to use the default LUKS-cipher algorithm, there is no need to specify one for the luksFormat. At the time of writing the default is set to an AES variant with 256 bit key-length. With that a dm-crypt/LUKS blockdevice for the crypted root can be created
# cryptsetup -y -v luksFormat /dev/sdaX
opened
# cryptsetup luksOpen /dev/sdaX cryptroot
formatted with your desired filesystem
# mkfs -t ext4 /dev/mapper/cryptroot
and mounted
# mount -t ext4 /dev/mapper/cryptroot /mnt
At this point, just before installing the base system, it might be useful to check the mapping works as intended:
# umount /mnt # cryptsetup luksClose cryptroot
and mount it again to check.
If you created a separate /home
partition, the steps have to be adapted and repeated for that.
What you do have to setup is a non-encrypted /boot
partition, which is needed for a crypted root. For a standard MBR/non-EFI /boot
partition that may be achieved by formatting
# mkfs -t ext2 /dev/sdaY
creating a mount-point for installation
# mkdir /mnt/boot
and mounting it
# mount -t ext2 /dev/sdaY /mnt/boot
That is basically what is necessary at this point before installing the base system with the Arch Install Scripts. Take care to install the bootloader to /mnt/boot
with the pacstrap
script. Additional configuration steps must be followed before booting the installed system.
Configure initramfs
One important point is to add the hooks relevant for your particular install in the correct order to /etc/mkinitcpio.conf
. The one you have to add when encrypting the root filesystem is encrypt
. A recommended hook for LUKS encrypted blockdevices is shutdown
to ensure controlled unmounting during system shutdown. Others needed, e.g. keymap
, should be clear from other manual steps you follow during the installation and further details in the following. For detailed information about initramfs configuration and available Hooks refer to Mkinitcpio#HOOKS.
It is important that the encrypt
hook comes before the filesystems
hook (in case you are using LVM on LUKS, the order should be: encrypt lvm2 filesystems
), so make sure that your HOOKS
array looks something like this:
etc/mkinitcpio.conf
HOOKS="(base udev) ... encrypt ... filesystems ..."
If you need support for foreign keymaps for your encryption password, you have to specify the hook keymap
as well before encrypt
.
If you have a USB keyboard, you will need the usbinput
hook. Without it, no USB keyboard will work in early userspace.
If the encrypted root container has keyslots for keyfiles loaded from external usb devices or root itself is on usb storage the usb
hook is needed before encrypt.
HOOKS="... usb encrypt ... filesystems ..."
In the same file, you may want to add to "MODULES" dm_mod and the filesystem types used, e.g: MODULES="dm_mod ext4"
After you are done don't forget:
mkinitcpio -p linux
Kernel parameter configuration of the bootloader
GRUB2
The article on GRUB2 gives a brief introduction and features a small section on configuration for system encryption.
The recommended way for configuring GRUB is /etc/default/grub
where the kernel line has to be adjusted so GRUB can pass to the kernel how to map the (encrypted) root device.
The syntax for cryptdevice is:
cryptdevice=device:dmname
- device
- The path to the raw encrypted device. Usage of Persistent block device naming is advisable.
- dmname
- The name given to the device after decryption, will be available as
/dev/mapper/dmname
. (DO NOT set dmname to a name you already used for your LVM partitions!)
So if the encrypted root device is in example /dev/sda2
and the decrypted one should be mapped to /dev/mapper/cryptroot
the kernel line must look like:
/etc/default/grub
GRUB_CMDLINE_LINUX="cryptdevice=/dev/sda2:cryptroot"
Depending on the setup other parameters are required as well:
GRUB_CMDLINE_LINUX="cryptdevice=device:dmname root=device resume=device cryptkey=device:fstype:path"
- root=device
- The device file of the actual (decrypted) root filesystem. If the filesystem is formatted directly on the decrypted device file this will be
/dev/mapper/dmname
. If LVM is in between sth. like/dev/mapper/volgroup-pvol
or/dev/volgroup/pvol
does the trick. - resume=device
- The device file of the decrypted (swap) filesystem used for suspend2disk.
- cryptkey=device:fstype:path
- Required for reading a keyfile from a filesystem.
The syntax for the optional cryptkey parameter is:
cryptkey=device:fstype:path
- device
- The raw block device where the key exists.
- fstype
- The filesystem type of device (or auto).
- path
- The absolute path of the keyfile within the device.
Make sure to regenerate your configuration file after editing with:
grub-mkconfig -o /boot/grub/grub.cfg
The device name your harddrive should be mapped to depends on your setup and configuration. If you are directly decrypting your root partition, the parameter root=/dev/mapper/cryptroot
will indicate the expected name. In case of LVM this name can be any arbitrary name, as LVM will parse all available devices if specified as the next hook.
Syslinux
The Article on Syslinux describes in the basic config-section[broken link: invalid section] how to use LUKS for system encryption (added 2012):
APPEND root=/dev/mapper/name cryptdevice=/dev/sda2:name ro
GRUB Legacy
GRUB Legacy: You have to make some small changes to the entries generated by the installer by replacing /dev/mapper/root
with /dev/sda3
. The important point to remember here is to use the same cryptdevice
name you assigned when you initially unlocked your device. In this example, the device name is cryptroot
; customize yours accordingly:
# (0) Arch Linux title Arch Linux root (hd0,0) kernel /vmlinuz-linux cryptdevice=/dev/sda3:cryptroot root=/dev/mapper/cryptroot ro initrd /initramfs-linux.img
For kernels older than 2.6.37, the syntax is:
# (0) Arch Linux title Arch Linux root (hd0,0) kernel /vmlinuz26 root=/dev/sda3 ro initrd /kernel26.img
LILO
LILO: Edit the Arch Linux section in /etc/lilo.conf
and include a line for the append
option, over the initrd, with the root=/dev/sda3
parameter. The append
section makes the same kernel line as in GRUB. Also, you can omit the root
option above the image
option. The section looks like this:
# Arch Linux lilo section image = /vmlinuz-linux # root = /dev/sda3 label = Arch initrd = /initramfs-linux.img append = "root=/dev/sda3" read-only
Fstab
Further, double-check the genfstab
scripts result for your /dev/mapper/cryptroot
and other mounts.
Remote unlocking of the root (or other) partition
If you want to be able to reboot a fully LUKS-encrypted system remotely, or start it with a Wake-on-LAN service, you will need a way to enter a passphrase for the root partition/volume at startup. This can be achieved by running the net
hook along with an SSH server in initrd. Install the dropbear_initrd_encryptAUR[broken link: archived in aur-mirror] package from the AUR and follow the post-installation instructions. Replace the encrypt
hook with dropbear encryptssh
in /etc/mkinitcpio.conf
. Put the net
hook early in the HOOKS array if your DHCP server takes a long time to lease IP addresses.
If you would simply like a nice solution to mount other encrypted partitions (such as /home
)remotely, you may want to look at this forum thread.
Backup the cryptheader
If the header of your encrypted partition gets destroyed, you will not be able to decrypt your data. It is just as much as a dilemma as forgetting the passphrase or damaging a key-file used to unlock the partition. A damage may occur by your own fault while re-partitioning the disk later or by third-party programs misinterpreting the partition table.
Therefore, having a backup of the headers and storing them on another disk might be a good idea.
Attention: Many people recommend NOT backing up the cryptheader, but even so it's a single point of failure! In short, the problem is that LUKS is not aware of the duplicated cryptheader, which contains the master key which is used to encrypt all files on your partition. Of course this master key is encrypted with your passphrases or keyfiles. But if one of those gets compromised and you want to revoke it you have to do this on all copies of the cryptheader! I.e. if someone has got your cryptheader and one of your keys he can decrypt the master key and access all your data. Of course the same is true for all backups you create of your partions. So you decide if you are one of those paranoids brave enough to go without a backup for the sake of security or not. See also the LUKS FAQ for further details on this.
Backup
Using cryptsetup
Cryptsetups luksHeaderBackup
action stores a binary backup of the LUKS header and keyslot area:
# cryptsetup luksHeaderBackup /dev/device --header-backup-file /mnt/backup/file.img
where device is the partition containing the LUKS volume.
# mkdir /root/tmp/ # mount ramfs /root/tmp/ -t ramfs # cryptsetup luksHeaderBackup /dev/device --header-backup-file /root/tmp/file.img # gpg2 --recipient <User ID> --encrypt /root/tmp/file.img # cp /root/tmp/file.img.gpg /mnt/backup/ # umount /root/tmp
Manually
First you have to find out the payload offset of the crypted partition:
# cryptsetup luksDump /dev/device | grep "Payload offset"
Payload offset: 4040
Now that you know the value, you can backup the header with a simple dd command:
dd if=/dev/device of=/path/to/file.img bs=512 count=4040
Restore
Be careful before restore: make sure that you chose the right partition (again replace sdaX with the corresponding partition). Restoring the wrong header or restoring to an unencrypted partition will cause data loss.
Using cryptsetup
Or you can use the luksHeaderRestore command:
cryptsetup luksHeaderRestore /dev/sdaX --header-backup-file ./backup.img
Note: All the keyslot areas are overwritten; only active keyslots from the backup file are available after issuing this command.
Manually
Again, you will need to the same values as when backing up:
dd if=./backup.img of=/dev/sdX bs=512 count=4040
Encrypting a loopback filesystem
[This paragraph has been merged from another page; its consistency with the other paragraphs should be improved]
Preparation and mapping
First, start by creating an encrypted container!
dd if=/dev/urandom of=/bigsecret bs=1M count=10
This will create the file bigsecret
with a size of 10 megabytes.
losetup /dev/loop0 /bigsecret
This will create the device node /dev/loop0
, so that we can mount/use our container.
cryptsetup luksFormat /dev/loop0
This will ask you for a password for your new container file.
cryptsetup luksOpen /dev/loop0 secret
The encrypted container is now available through the device file /dev/mapper/secret
.
Now we are able to create a partition in the container:
mkfs.ext2 /dev/mapper/secret
and mount it...
mkdir /mnt/secret mount -t ext2 /dev/mapper/secret /mnt/secret
We can now use the container as if it was a normal partition! To unmount the container:
umount /mnt/secret cryptsetup luksClose secret losetup -d /dev/loop0 # free the loopdevice.
so, if you want to mount the container again, you just apply the following commands:
losetup /dev/loop0 /bigsecret cryptsetup luksOpen /dev/loop0 secret mount -t ext2 /dev/mapper/secret /mnt/secret
Encrypt using a key-file
Let us first generate a 2048 byte random keyfile:
dd if=/dev/urandom of=keyfile bs=1k count=2
We can now format our container using this key
cryptsetup luksFormat /dev/loop0 keyfile
or our partition :
cryptsetup luksFormat /dev/hda2 keyfile
Once formatted, we can now open the LUKS device using the key:
cryptsetup -d keyfile luksOpen /dev/loop0 container
You can now like before format the device /dev/mapper/container
with your favorite filesystem and then mount it just as easily.
The keyfile is now the only key to your file. I personally advise encrypting your keyfile using your private GPG key and storing an off-site secured copy of the file.
Resizing the loopback filesystem
First we should unmount the encrypted container:
umount /mnt/secret cryptsetup luksClose secret losetup -d /dev/loop0 # free the loopdevice.
After this we need to expand our container file with the size of the data we want to add:
dd if=/dev/urandom bs=1M count=1024 | cat - >> /bigsecret
Be careful to really use TWO >
, or you will override your current container!
You could use /dev/zero
instead of /dev/urandom
to significantly speed up the process, but with /dev/zero
your encrypted filesystems will not be as secure. (A better option to create random data quicker than /dev/urandom
is frandom
[1], available from the AUR).
A faster (almost instant) method than dd is truncate
, but its use has the same security implications as using /dev/zero. The size passed to truncate is the final size to make the file, so don't use a value less than that of the current file or you will lose data. e.g. to increase a 20G file by 10G: truncate -s 30G filename.
Now we have to map the container to the loop device:
losetup /dev/loop0 /bigsecret cryptsetup luksOpen /dev/loop0 secret
After this we will resize the encrypted part of the container to the maximum size of the container file:
cryptsetup resize secret
Finally, we can resize the filesystem. Here is an example for ext2/3/4:
e2fsck -f /dev/mapper/secret # Just doing a filesystem check, because it's a bad idea to resize a broken fs resize2fs /dev/mapper/secret
You can now mount your container again:
mount /dev/mapper/secret /mnt/secret
Encrypting a LVM setup
It's really easy to use encryption with LVM. If you do not know how to set up LVM, then read Installing with Software RAID or LVM.
LVM on LUKS
The easiest and best method is to set up LVM on top of the encrypted partition instead of the other way round. This link here is easy to follow and explains everything: Arch Linux: LVM on top of an encrypted partition
The most important thing in setting LVM on top of encryption is to configure the initramfs for running the encrypt
hook before the lvm2
hook (and those two before the filesystems
hook).
LUKS on LVM
To use encryption on top of LVM, you have to first set up your LVM volumes and then use them as the base for the encrypted partitions. That means, in short, that you have to set up LVM first. Then follow this guide, but replace all occurrences of /dev/sdXy
in the guide with its LVM counterpart. (E.g.: /dev/sda5
-> /dev/<volume group name>/home
). This is used to setup partitions (inside the LVM) which can be unlocked separately or a mixture of encrypted and non-encrypted partitions.
For encrypted partitions inside an LVM, the LVM-hook has to run first, before the respective encrypted logical volumes can be unlocked. So for this add the encrypt
hook in /etc/mkinitcpio.conf
after the lvm2
hook, if you chose to set up encrypted partitions on top of LVM. Also remember to change USELVM
in /etc/rc.conf
to "yes"
.
LVM with Arch Linux Installer (>2009.08 <2012.07.15)
In between Arch Linux installation media release 2009.08 and 2012.07.15 LVM and dm_crypt had been supported by the installer out of the box. This made it very easy to configure a system for LVM on dm-crypt or vice versa. Actually the configuration is done exactly as without LVM: see the corresponding section above. It differs only in two aspects.
The partition and filesystem choice
Create a small, unencrypted boot partition and use the remaining space for a single partition which can later be split up into multiple logic volumes by LVM.
For a LVM-on-dm-crypt system set up the filesystems and mounting points for example like this:
/dev/sda1 raw->ext2;yes;/boot;no_opts;no_label;no_params /dev/sda2 raw->dm_crypt;yes;no_mountpoint;no_opts;sda2crypt;-c_aes-xts-plain_-y_-s_512 /dev/mapper/sda2crypt dm_crypt->lvm-vg;yes;no_mountpoint;no_opts;no_label;no_params /dev/mapper/sda2crypt+ lvm-pv->lvm-vg;yes;no_mountpoint;no_opts;cryptpool;no_params /dev/mapper/cryptpool lvm-vg(cryptpool)->lvm-lv;yes;no_mountpoint;no_opts;cryptroot;10000M|lvm-lv;yes;no_mountpoint;no_opts;crypthome;20000M /dev/mapper/cryptpool-cryptroot lvm-lv(cryptroot)->ext3;yes;/;no_opts;cryptroot;no_params /dev/mapper/cryptpool-crypthome lvm-lv(crypthome)->ext3;yes;/home;no_opts;cryptroot;no_params
The configuration stage
- In
/etc/mkinitcpio.conf
add theencrypt
hook before thelvm2
hook in theHOOKS
array, if you set up LVM on top of the encrypted partition.
That is it for the LVM & dm_crypt specific part. The rest is done as usual.
- In
/etc/rc.conf
setUSELVM
to"yes"
.
Applying this to a non-root partition
You might get tempted to apply all this fancy stuff to a non-root partition. Arch does not support this out of the box, however, you can easily change the cryptdev and cryptname values in /lib/initcpio/hooks/encrypt
(the first one to your /dev/sd*
partition, the second to the name you want to attribute). That should be enough.
The big advantage is you can have everything automated, while setting up /etc/crypttab
with an external key file (i.e. the keyfile is not on any internal hard drive partition) can be a pain - you need to make sure the USB/FireWire/... device gets mounted before the encrypted partition, which means you have to change the order of /etc/fstab
(at least).
Of course, if the cryptsetup package gets upgraded, you will have to change this script again. However, this solution is to be preferred over hacking /etc/rc.sysinit
or similar files. Unlike /etc/crypttab
, only one partition is supported, but with some further hacking one should be able to have multiple partitions unlocked.
If you want to do this on a software RAID partition, there is one more thing you need to do. Just setting the /dev/mdX
device in /lib/initcpio/hooks/encrypt
is not enough; the encrypt
hook will fail to find the key for some reason, and not prompt for a passphrase either. It looks like the RAID devices are not brought up until after the encrypt
hook is run. You can solve this by putting the RAID array in /boot/grub/menu.lst
, like
kernel /boot/vmlinuz-linux md=1,/dev/hda5,/dev/hdb5
If you set up your root partition as a RAID, you will notice the similarities with that setup ;-). GRUB can handle multiple array definitions just fine:
kernel /boot/vmlinuz-linux root=/dev/md0 ro md=0,/dev/sda1,/dev/sdb1 md=1,/dev/sda5,/dev/sdb5,/dev/sdc5
LVM and dm-crypt manually (short version)
Notes
If you are smart enough for this, you will be smart enough to ignore/replace LVM-specific things if you do not want to use LVM.
Partitioning scheme
/dev/sda1
->/boot
/dev/sda2
-> LVM
The commands
cryptsetup -d /dev/random -c aes-xts-plain -s 512 create lvm /dev/sda2 dd if=/dev/urandom of=/dev/mapper/lvm cryptsetup remove lvm lvm pvcreate /dev/sda2 lvm vgcreate lvm /dev/sda2 lvm lvcreate -L 10G -n root lvm lvm lvcreate -L 500M -n swap lvm lvm lvcreate -L 500M -n tmp lvm lvm lvcreate -l 100%FREE -n home lvm cryptsetup luksFormat -c aes-xts-plain -s 512 /dev/lvm/root cryptsetup luksOpen /dev/lvm/root root mkreiserfs /dev/mapper/root mount /dev/mapper/root /mnt dd if=/dev/zero of=/dev/sda1 bs=1M mkreiserfs /dev/sda1 mkdir /mnt/boot mount /dev/sda1 /mnt/boot mkdir -p -m 700 /mnt/etc/luks-keys dd if=/dev/random of=/mnt/etc/luks-keys/home bs=1 count=256
Install Arch Linux
Run /arch/setup
Configuration
/etc/rc.conf
Change USELVM="no"
to USELVM="yes"
.
/etc/mkinitcpio.conf
Put lvm2
and encrypt
(in that order) before filesystems
in the HOOKS
array. Again, note that you are setting encryption on top of LVM.)
if you want install the system on a usb stick, you need to put usb
just after udev
.
Change root=/dev/hda3
to root=/dev/lvm/root
.
For kernel >= 2.6.30, you should change root=/dev/hda3
to the following:
cryptdevice=/dev/lvm/root:root root=/dev/mapper/root
if you want install the system on a usb stick, you need to add lvmdelay=/dev/mapper/lvm-root
/etc/fstab
/dev/mapper/root / reiserfs defaults 0 1 /dev/sda1 /boot reiserfs defaults 0 2 /dev/mapper/tmp /tmp tmpfs defaults 0 0 /dev/mapper/swap none swap sw 0 0
/etc/crypttab
swap /dev/lvm/swap SWAP -c aes-xts-plain -h whirlpool -s 512 tmp /dev/lvm/tmp /dev/urandom -c aes-xts-plain -s 512
After rebooting
The commands
cryptsetup luksFormat -c aes-xts-plain -s 512 /dev/lvm/home /etc/luks-keys/home cryptsetup luksOpen -d /etc/luks-keys/home /dev/lvm/home home mkreiserfs /dev/mapper/home mount /dev/mapper/home /home
/etc/crypttab
home /dev/lvm/home /etc/luks-keys/home
/etc/fstab
/dev/mapper/home /home reiserfs defaults 0 0
/ on LVM on LUKS
Make sure your kernel command line looks like this:
root=/dev/mapper/<volume-group>-<logical-volume> cryptdevice=/dev/<luks-part>:<volume-group>
For example:
root=/dev/mapper/vg-arch cryptdevice=/dev/sda4:vg
Or like this:
cryptdevice=/dev/<volume-group>/<logical-volume>:root root=/dev/mapper/root
Using GPG or OpenSSL Encrypted Keyfiles
The following forum posts give instructions to use two factor authentication, gpg or openssl encrypted keyfiles, instead of a plaintext keyfile described earlier in this wiki article System Encryption using LUKS with GPG encrypted keys:
- GnuPG: Post regarding GPG encrypted keys This post has the generic instructions.
- OpenSSL: Post regarding OpenSSL encrypted keys This post only has the
ssldec
hooks. - OpenSSL: Post regarding OpenSSL salted bf-cbc encrypted keys This post has the
bfkf
initcpio hooks, install, and encrypted keyfile generator scripts.
Note that:
- You can follow the above instructions with only two primary partitions one boot partition
(required because of LVM), and one primary LVM partition. Within the LVM partition you can have
as many partitions as you need, but most importantly it should contain at least root, swap, and
home logical volume partitions. This has the added benefit of having only one keyfile for all
your partitions, and having the ability to hibernate your computer (suspend to disk) where the
swap partition is encrypted. If you decide to do so your hooks in /etc/mkinitcpio.conf
should look like
HOOKS=" ... usb usbinput (etwo or ssldec) encrypt(if using openssl) lvm2 resume ... "
and you should add to your kernel
line(if using grub, /boot/grub/menu.lst
) or
GRUB_CMD_LINE
(if using grub2, /etc/default/grub
):
"resume=/dev/mapper/VolumeGroupName-LVNameOfSwap"
- If you need to temporarily store the unecrypted keyfile somewhere, do not store them on an
unencrytped disk. Even better make sure to store them to RAM such as /dev/shm
.
- If you want to use a GPG encrypted keyfile, you need to use a statically compiled GnuPG version 1.4 or you could edit the hooks and use this AUR package gnupg1
- It is possible that an update to OpenSSL could break the custom
ssldec
mentioned in the second forum post.
Securing the unencrypted boot partition
Referring to an article from the ct-magazine (Issue 3/12, page 146, 01.16.2012 http://www.heise.de/ct/inhalt/2012/03/6/) the following script checks all files under /boot
for changes of SHA-1 hash, inode and occupied blocks on the hard drive. It also checks the MBR.
The script with installation instructions is available here: ftp://ftp.heise.de/pub/ct/listings/1203-146.zip (Author: Juergen Schmidt, ju at heisec.de; License: GPLv2). There is also an AUR package: chkbootAUR
After installation:
- For classical sysvinit: add
/usr/local/bin/chkboot.sh &
to your/etc/rc.local
- For systemd: add a service file and enable the service: systemd. The service file might look like:
[Unit] Description=Check that boot is what we want Requires=systemd-remount-fs.service After=systemd-remount-fs.service [Service] Type=oneshot ExecStart=/usr/local/bin/chkboot.sh [Install] WantedBy=multi-user.target
There is a small caveat for systemd: At the time of writing the original chkboot.sh
script provided contains an empty space at the beginning of #!/bin/bash
which has to be removed for the service to start successfully.
As /usr/local/bin/chkboot_user.sh
need to be excuted after login, add it to the autostart (e.g. under KDE -> System Settings -> Startup and Shutdown -> Autostart; Gnome3: gnome-session-properties).
With Arch Linux changes to /boot
are pretty frequent, for example by new kernels rolling-in. Therefore it may be helpful to use the scripts with every full system update. One way to do so:
#!/bin/bash # # Note: Insert your <user> and execute it with sudo for pacman & chkboot to work automagically # echo "Pacman update [1] Quickcheck before updating" & sudo -u <user> /usr/local/bin/chkboot_user.sh # insert your logged on <user> /usr/local/bin/chkboot.sh sync # sync disks with any results sudo -u <user> /usr/local/bin/chkboot_user.sh # insert your logged on <user> echo "Pacman update [2] Syncing repos for pacman" pacman -Syu /usr/local/bin/chkboot.sh sync sudo -u <user> /usr/local/bin/chkboot_user.sh # insert your logged on <user> echo "Pacman update [3] All done, let's roll on ..."
Automount user homes on login
See pam_mount.
Resources
- cryptsetup FAQ - The main and foremost help resource, directly from the developers.
- FreeOTFE - Supports unlocking LUKS encrypted volumes in Microsoft Windows.
- Arch cryptsetup example video - A HowTo video on setting up an encrypted Arch system from scratch. The video still shows an installation with AIF, which is at the time of writing deprecated / not developed further. The important partitioning and cryptsetup are shown outside AIF though.
- Install Arch Linux on top of RAID1, LVM2, and encrypted partitions - The HowTo instructs to make a full Fedora 13 Install only to set up the mapping. Then legacy AIF is used.