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diff --git a/debian/README.Debian b/debian/README.Debian new file mode 100644 index 0000000..53e0789 --- /dev/null +++ b/debian/README.Debian @@ -0,0 +1,326 @@ +Cryptsetup for Debian +===================== + +Table of Contents +----------------- + +* 1. Introduction into Cryptsetup for Debian +* 2. Encrypted swap partition(s) +* 3. Insecure mode/owner for keys +* 4. Cryptsetup and udev +* 5. Useful keyscripts: askpass and passdev +* 6. The `check` option +* 7. Cryptsetup and Splashy +* 8. Remotely unlock encrypted rootfs +* 9. Backup the LUKS header +* 10. Changing the boot order of cryptdisks init scripts +* 11. Unlocking LUKS devices from GRUB +* 12. Credits + + +1. Introduction into Cryptsetup for Debian +------------------------------------------ + + Cryptsetup is a command-line interface for configuring encrypted block +devices via dm-crypt, a kernel device-mapper target. For documentation about +the cryptsetup tool, see manpage of cryptsetup(8) and the frequently asked +questions at `/usr/share/doc/cryptsetup/FAQ.gz`. + + The Debian cryptsetup package provides the initscript `/etc/init.d/cryptdisks` +and a configuration file `/etc/crypttab` for automatically configuring encrypted +devices at boot time. The applications cryptdisks_start and cryptdisks_stop +are provided to process crypttab configured devices manually. See the manpages +of crypttab(5), cryptdisks_start(8) and cryptdisks_stop(8) for more information. + + The luksformat script provides a simple interface for creating an encrypted +device that follows the LUKS standard and for putting a file system onto the +encrypted device. See man luksformat(8) for more information. + + If you wish to perform a Debian installation to an encrypted root, you might +be interested in using a version of Debian Installer with partman-crypto, +which will install the system and setup cryptsetup and initramfs-tools. + + For instructions about how to encrypt your root filesystem and integrate +cryptsetup into initramfs on a running system, see +`/usr/share/doc/cryptsetup-initramfs/README.initramfs.gz`. + + +2. Encrypted swap partition(s) +------------------------------ + + An encrypted swap partition prevents spying on plaintext secrets (passwords) +that may be written to disk when memory is swapped to disk. + + To encrypt your swap partitions, you'll first have to deactivate your swap: + + swapoff -a + + You'll have to add an entry for every swap partition in `/etc/crypttab`. Be +sure to place the source device (here `/dev/sde9`) with your swap devices: + + # <target name> <source device> <key file> <options> + cswap1 /dev/sde9 /dev/urandom swap,cipher=aes-xts-plain64,size=256,hash=sha1 + + Now you need to change the swap devices in `/etc/fstab` to the encrypted swap +device names (`/dev/mapper/cswap1` in this example). + + # <file system> <mount point> <type> <options> <dump> <pass> + /dev/sde9 none swap sw 0 0 + +becomes + + # <file system> <mount point> <type> <options> <dump> <pass> + /dev/mapper/cswap1 none swap sw 0 0 + + Then, you need to start the cryptsetup swap devices and reactivate swap: + + cryptdisks_start cswap1 + swapon -a + + And finally, if `/dev/sde9` was previously used as resume device, you should +disable it (the new swap partition is mapped with a non-persistent key hence +can't be used for resuming after suspend to disk). With initramfs-tools 0.130 +and later, this can be done with + + echo "RESUME=none" >/etc/initramfs-tools/conf.d/resume + update-initramfs -u + + That's it! You have a crypted swap device. Note that `/dev/urandom` provides +only pseudo-random entropy. So if you're paranoid rather use `/dev/random` as +source for random data. Be aware though that `/dev/random` might not provide +enough random bytes for your key, causing your system to hang at boot, waiting +for more entropy. Moving mouse and keyboard typing might help in this case. + + Read the crypttab(5) manpage for more information, for example options to use +a different encryption algorithm than the default. + + +3. Insecure mode/owner for keys +------------------------------- + + Any key that is stored somewhere to be used with cryptsetup should have the +mode 400 (`-r--------`) and root as owner/group. `chown root.root keyfile` and +`chmod 400 keyfile` will do the trick for you. + + If a key is stored on a vfat filesystem (very common for removable media), +chmod and chown will not work. The vfat filesystem (and several others too) +does not support file permissions and ownership. Instead, you should use the +uid, gid and umask options in `/etc/fstab` to ensure secure permissions for +the key. + + As an example, assume that `/dev/sdg8` is the removable media containing +keyfiles on a vfat filesystem and that it is going to be mounted on +`/media/flash0`. The configuration in `/etc/fstab` should then be something +like this: + + # <file system> <mount point> <type> <options> <dump> <pass> + /dev/sdg8 /media/flash0 vfat uid=0,gid=0,umask=277 0 0 + + If you are using udev, it might be a good idea to use the `/dev/disk/by-label` +links instead of `/dev/sdg8` as the link will work no matter in which order the +media is inserted and detected. + + +4. Cryptsetup and udev +---------------------- + + As a workaround for some yet-to-be-fixed race condition in kernel, +device-mapper or udev, cryptsetup currently runs udevsettle. + + This leads to problems if you invoke cryptsetup as part of a udev rule. +udevsettle waits until queued kernel/udev events are processed and the +"run programs" have finished. Due to cryptsetup itself being a "run +program" in this case, this ends in a deadlock. + + Therefore cryptsetup should be detached directly after invocation in this +case, so that it runs asynchronously. + + +5. Useful keyscripts: askpass and passdev +----------------------------------------- + + The cryptsetup package ships with several keyscripts. Keyscripts may be +configured in `/etc/crypttab` in order to provide the key required to unlock +the device. The shipped keyscripts are located at `/lib/cryptsetup/scripts`. + + Some keyscripts have an own README file at `/usr/share/doc/cryptsetup/`. + + Two special keyscripts, worth being mentioned here, are askpass and passdev. + + Askpass is located at `/lib/cryptsetup/askpass`. It's a simple helper program +that supports different methods (console, fifo, splashy, ...) to prompt for a +passphrase, and prints the result to stdout. The syntax is: + + /lib/cryptsetup/askpass PROMPT + + Passdev will wait for a given device to appear, mount it read-only, read the +key, and unmount the device. See `/usr/share/doc/cryptsetup-initramfs/README.initramfs.gz` +for more information about passdev. + + +6. The `check` option +--------------------- + + The `check` option in crypttab allows one to configure checks to be run +against the target device after cryptsetup has been invoked. +The default check `blkid` can check for any known filesystem type, as it uses +blkid from util-linux. you can check for a particular filesystem by giving for +example `checkargs=ext4` or `checkargs=swap` as an option in `/etc/crypttab`. + + Please send us your checks, if you write new ones. If they are generally +useful, we will include them in the package. + + See man crypttab(5) for more information about the checksystem. + + +7. Cryptsetup and Splashy +------------------------- + + Splashy support in cryptsetup is currently somehow limited. Splashy is known +to freeze at the password dialog for encrypted non-root filesystems. Only the +password dialog for the encrypted root filesystem works. + + It seems like splashy freezes for any input dialog in initscripts while +input dialogs at initramfs stage seem to work. This leads to the assumption +that the bug is somewhere in splashy and neither in cryptsetups initscripts +nor in askpass. + + +8. Remotely unlock encrypted rootfs +----------------------------------- + + Thanks to Chris <debian@x.ray.net> it's possible to install a dropbear SSH +server into the initramfs, connect to this SSH server during execution of +initramfs early in the boot process, and unlock encrypted devices - even +the root device - before the boot process continues. (Note that in order +to force an arbitrary device to be processed at initramfs stage you +might need to set the `initramfs` option in its crypttab entry; see +crypttab(5) for details.) + + This way it is possible to use an encrypted root filesystem on headless +systems where no physical access is available during boot process. + + Dropbear 0.52-1 or later is required for this to work. (Since 2015.68-1 the +functionality has its own binary package `dropbear-initramfs`.) Consult +`/usr/share/doc/dropbear-initramfs/README.initramfs` from the dropbear-initramfs +package for information how to install and configure the dropbear SSH server +into the initramfs. + + You can then unlock the disk remotely via SSH with + + ssh -tF ~/.luks/ssh.conf root@remote.system.com cryptroot-unlock + + Or, using a local gpg-encrypted key file: + + gpg --decrypt ~/.luks/remote.key.gpg | ssh -TF ~/.luks/ssh.conf root@remote.system.com cryptroot-unlock + + When its standard input is a TTY, `cryptroot-unlock` keeps prompting for +passphrases until there are no more devices to unlock; otherwise you'll +need to invoke it as many times as there are devices to unlock. + + That's it. Now that all required encrypted devices are unlocked, the +remote system should continue with the boot process. + + You can also use the following authorized_keys(5) options in +`/etc/dropbear-initramfs/authorized_keys` to restrict access and avoid +users poking around: + + no-port-forwarding,no-agent-forwarding,no-X11-forwarding,command="/bin/cryptroot-unlock" ssh-rsa ... + +(Be sure to rebuild the initrd afterwards: `update-initramfs -u -k all`) + + +9. Backup the LUKS header +------------------------- + + WARNING: This information might be outdated. Please read the cryptsetup FAQ +at `/usr/share/doc/cryptsetup/FAQ.gz` for up-to-date information on how to +backup the LUKS header. + + The LUKS header is located at the beginning of every LUKS encrypted device. +It stores information such as used cipher, hash, etc. But most importantly, +the header contains eight keyslots, which do keep an encrypted version of the +LUKS masterkey. the data on an encrypted LUKS partition is encrypted with this +masterkey. thus, there's no way to restore the data once the masterkey is +lost. For that reason, one might want to backup the LUKS header in order to +prevent accidental data loss. + + On the other hand keeping a backup of the LUKS header isn't recommended for +security reasons. The reason is, that LUKS was designed with key revocation in +mind. Once the LUKS header is copied to a backup, revoking a (possibly +compromised) passphrase or keyfile from the keyslot isn't enough anymore. the +revoked passphrase/keyfile can easily be reactived by writing back the header +backup to the device. + + Beginning with version 1.1.0, cryptsetup has support for the commands +luksHeaderBackup and luksHeaderRestore. If you want to store a backup of your +LUKS header with the mentioned drawbacks in mind, do the following: + + Prepare a ramdisk to store the backup temporarely. You should do that in order +to prevent any hardware caching functions or filesystem jounals to copy the +backup around to places you cannot control. If you want to store the backup +permanently, write it to a read-only medium like CD immediately from ramdisk, +without your burning program writing an intermediate image to some temp dir. + + To actually backup the header, use the following command: + + cryptsetup luksHeaderBackup <luks-device> --header-backup-file <destination-on-ramdisk> + + That's it. But once again, keep in mind all the security implications when +doing LUKS header backups. In general it's better to backup the data from +encrypted LUKS devices to another encrypted LUKS device. That way you can +manage the keyslots for both original and backup device independently. + + +10. Changing the boot order of cryptdisks init scripts +----------------------------------------------------- + + In order to support non-standard setups, it might be necessary to change the +order of init scripts in the boot process. Cryptsetup already installs two +init scripts, cryptdisks-early and cryptdisks, in order to support some complex +setups. For example, both "lvm on luks" and "luks on lvm" are supported that +way. + + If your system isn't supported by the default order of init scripts in the +boot process, you need to change the boot process on your own. In some cases +it might be enough to change the LSB dependency headers at initscripts, see +`/etc/init.d/README` for more information about that. For more complex setups, +more intrusive changes are required. For example, adding a third cryptdisks +init script might help. See the log of bugreport [#576646] and [discussion on +debian-devel] for further information. + +[#576646]: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=576646 +[discussion on debian-devel]: https://lists.debian.org/debian-devel/2010/06/msg00021.html + + +11. Unlocking LUKS devices from GRUB +------------------------------------ + +GRUB has been able to unlock LUKS1 devices since early in Jessie's +release cycle. This feature removes the need for a separate cleartext +`/boot` partition, hence enables "real" full disk encryption. However +cryptsetup >=2.1 uses LUKS version 2 by default, which GRUB 2.02 doesn't +support. In other words, as of Buster it is not possible to unlock from +GRUB new LUKS devices formatted with the default parameters. + +Neither Jessie nor Stretch's installers natively support unlocking from +GRUB, hence users already had to implement various workarounds to enable +it. **Former workarounds won't work anymore with LUKS2**. Integration +between LUKS and GRUB is documented at +<https://cryptsetup-team.pages.debian.net/cryptsetup/encrypted-boot.html>, +including recipes to enable the feature starting from the usual +"encrypted LVM" partitioning method of the Debian Installer -- both with +LUKS1 (pre-Buster) and LUKS2 (Buster and later) devices. + +12. Credits +----------- + + People who contributed to the Debian cryptsetup package: + +* Guilhem Moulin <guilhem@debian.org> +* Jonas Meurer <jonas@freesources.org> +* David Härdeman <david@hardeman.nu> +* Bastian Kleineidam <calvin@debian.org> +* Michael Gebetsroither <michael.geb@gmx.at> + + -- Jonas Meurer <jonas@freesources.org>, Sun, 09 Jun 2019 15:01:09 +0200 |