cryptmount - mount/unmount/configure an encrypted filesystem
cryptmount TARGET [TARGET ...]
cryptmount --unmount TARGET [TARGET ...]
cryptmount --change-password TARGET
cryptmount --generate-key SIZE TARGET
cryptmount --swapon TARGET
cryptmount --swapoff TARGET
cryptmount allows an encrypted filesystem to be mounted or unmounted, without requiring superuser privileges, and assists the superuser in creating new encrypted filesystems. After initial configuration of the filesystem by the system administrator, the user needs only to provide the decryption password for that filing system in order for cryptmount to automatically configure device-mapper and loopback targets before mounting the filesystem.
cryptmount was written in response to differences between the newer device-mapper infrastructure of the linux-2.6 kernel series, and the older cryptoloop infrastructure which allowed ordinary users access to encrypted filesystems directly through mount (8).
-a --all
act on all available targets, e.g. for mounting all targets.
-m --mount
mount the specified target, configuring any required device-mapper or loopback devices. The user will be asked to supply a password to unlock the decryption key for the filesystem.
-u --unmount
unmount the specified target, and deconfigure any underlying device-mapper or loopback devices. No password is required, although the operation will fail if the filesystem is in use, or if a non-root user tries to unmount a filesystem mounted by a different user.
-S --status
provide information on whether the specified target is currently mounted or not
-l --list
lists all available targets, including basic information about the filesystem and mount point of each.
-c --change-password
change the password protecting the decryption key for a given filesystem.
-g --generate-key size
setup a decryption key for a new filesystem. size gives the length of the key in bytes.
-e --reuse-key existing-target
setup a decryption key for a new filesystem, using an existing key from another filesystem, for example to translate between different file-formats for storing a single key. This option is only available to the superuser.
-f --config-fd num
read configuration information about targets from file-descriptor num instead of the default configuration file. This option is only available to the superuser.
-w --passwd-fd num
read passwords from file-descriptor num instead of from the terminal, e.g. for using cryptmount within scripts or GUI wrappers. Each password is read once only, in contrast to terminal-based operation where new passwords would be requested twice for verification.
-p --prepare
prepare all the device-mapper and loopback devices needed to access a target, but do not mount. This is intended to allow the superuser to install a filesystem on an encrypted device.
-r --release
releases all device-mapper and loopback devices associated with a particular target. This option is only available to the superuser.
-s --swapon
enable the specified target for paging and swapping. This option is only available to the superuser.
-x --swapoff
disable the specified target for paging and swapping. This option is only available to the superuser.
-k --key-managers
list all the available formats for protecting the filesystem access keys.
-B --system-boot
setup all targets which have declared a "bootaction" parameter. This will typically be used to automatically mount encrypted filesystems, or setup encrypted swap partitions, on system startup. This option is only available to the superuser.
-Q --system-shutdown
close-down all targets which have declared a "bootaction" parameter. This is essentially the opposite of the "--system-boot" option.
-n --safetynet
attempts to close-down any mounted targets that should normally have been shutdown with --unmount or --swapoff. This option is only available to the superuser, and intended exclusively for use during shutdown/reboot of the operating system.
-v --version
show the version-number of the installed program.
cryptmount returns zero on success. A non-zero value indicates a failure of some form, as follows:
|
1 |
unrecognized command-line option; |
|||
|
2 |
unrecognized filesystem target name; |
|||
|
3 |
failed to execute helper program; |
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|
100 |
insufficient privilege; |
|||
|
101 |
security failure in installation. |
In order to create a new encrypted filesystem managed by cryptmount, you can use the supplied ’cryptmount-setup’ program, which can be used by the superuser to interactively configure a basic setup.
Alternatively,
a manual setup allows more control of configuration
settings. Before doing so, one should ensure that kernel
support for /dev/loop and /dev/mapper is available, e.g. via
modprobe -a loop dm-crypt
Now suppose that we wish to setup a new encrypted
filesystem, that will have a target-name of
"opaque". If we have a free disk partition
available, say /dev/sdb63, then we can use this directly to
store the encrypted filesystem. Alternatively, if we want to
store the encrypted filesystem within an ordinary file, we
need to create space using a recipe such as:
dd if=/dev/zero of=/home/opaque.fs bs=1M count=512
and then replace all occurrences of ’/dev/sdb63’ in the following with ’/home/opaque.fs’. (/dev/urandom can be used in place of /dev/zero, debatably for extra security, but is rather slower.)
First, we need to add an entry in /etc/cryptmount/cmtab, which describes the encryption that will be used to protect the filesystem itself and the access key, as follows:
opaque {
dev=/dev/sdb63 dir=/home/crypt
fstype=ext2 mountoptions=defaults cipher=twofish
keyfile=/etc/cryptmount/opaque.key
keyformat=builtin
}
Here, we will be using the "twofish" algorithm to encrypt the filesystem itself, with the built-in key-manager being used to protect the decryption key (to be stored in /etc/cryptmount/opaque.key).
In order to generate a secret decryption key (in /etc/cryptmount/opaque.key) that will be used to encrypt the filesystem itself, we can execute, as root:
cryptmount --generate-key 32 opaque
This will generate a 32-byte (256-bit) key, which is known to be supported by the Twofish cipher algorithm, and store it in encrypted form after asking the system administrator for a password.
If we now execute, as root:
cryptmount --prepare opaque
we will then be asked for the password that we used when setting up /etc/cryptmount/opaque.key, which will enable cryptmount to setup a device-mapper target (/dev/mapper/opaque). (If you receive an error message of the form device-mapper ioctl cmd 9 failed: Invalid argument , this may mean that you have chosen a key-size that isn’t supported by your chosen cipher algorithm. You can get some information about suitable key-sizes by checking the output from "more /proc/crypto", and looking at the "min keysize" and "max keysize" fields.)
We can now use standard tools to create the actual filesystem on /dev/mapper/opaque:
mke2fs /dev/mapper/opaque
(It may be advisable, after the filesystem is first mounted, to check that the permissions of the top-level directory created by mke2fs are appropriate for your needs.)
After executing
cryptmount
--release opaque
mkdir /home/crypt
the encrypted filesystem is ready for use. Ordinary users can mount it by typing
cryptmount -m opaque
or
cryptmount opaque
and unmount it using
cryptmount -u opaque
cryptmount keeps a record of which user mounted each filesystem in order to provide a locking mechanism to ensure that only the same user (or root) can unmount it.
After a filesystem has been in use for a while, one may want to change the access password. For an example target called "opaque", this can be performed by executing:
cryptmount --change-password opaque
After successfully supplying the old password, one can then choose a new password which will be used to re-encrypt the access key for the filesystem. (The filesystem itself is not altered or re-encrypted.)
cryptmount can be used to provide easy access to encrypted filesystems compatible with the Linux Unified Key Setup (LUKS) capabilities of the cryptsetup application.
In order to access an existing LUKS partition, an entry needs to be created within /etc/cryptmount/cmtab. For example, if the hard-disk partition /dev/sdb62 is used to contain a LUKS encrypted ext3 filesystem, an entry of the form:
LUKS {
keyformat=luks
dev=/dev/sdb62 keyfile=/dev/sdb62
dir=/home/luks-dir fstype=ext3
}
would allow this to be mounted via cryptmount beneath /home/luks-dir by executing
cryptmount LUKS
cryptmount will also allow any user that knows one of the access-passwords to change their password via
cryptmount --change-password LUKS
cryptmount also provides basic support for creating new LUKS encrypted filesystems, which can be placed within ordinary files as well as disk partitions, via the ’--generate-key’ recipe shown above. However, to exploit the full range of functionality within LUKS, such as for adding multiple passwords, one needs to use cryptsetup
It is strongly recommended that you do not attempt to use LUKS support in combination with cryptmount’s features for storing multiple encrypted filesystems within a single disk partition or an ordinary file. This is because of assumptions within the cryptsetup-luks design that the LUKS key-material is always stored at the beginning of the disk partition.
/etc/cryptmount/cmtab - main configuration file
/run/cryptmount.status - record of mounted filesystems
cmtab(5), cryptmount-setup(8), cryptsetup(8), mount(8)
cryptmount
is Copyright 2005-2023, RW Penney
and is supplied with NO WARRANTY. Licencing terms are as
described in the file "COPYING" within the
cryptmount source distribution.