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cmtab: static information about filesystems managed by cryptmount

cmtab - static information about filesystems managed by cryptmount


Information about the encrypted filing systems managed by cryptmount is contained in the file /etc/cryptmount/cmtab. Each filesystem is labelled by a target name which can be used as an argument to cryptmount and which appears in /etc/cryptmount/cmtab in front of a list of parameters describing where that filesystem is stored, and how it is encrypted. The format of the cmtab is flexible, with the description of each target being delimited by braces, parameters being specified by KEY=VALUE pairs, and white-space being freely usable. Comments are prefixed by a '#' character, and can start at any point in a line, lasting to the end of the line. The backslash character '\' can be used to ignore any special significance of the following character, for example to include a space in a filename. /etc/cryptmount/cmtab contains entries of the following form: TARGET_NAME { dev=DEVICE flags=FLAG,FLAG,... startsector=STARTSECTOR numsectors=NUMSECTORS loop=LOOPDEV dir=MOUNT_POINT fstype=TYPE mountoptions=MOPT,MOPT,... fsckoptions=FOPT;FOPT;... cipher=CIPHER ivoffset=IVOFFSET keyformat=KEYFORMAT keyfile=KEYFILE keyhash=KEYHASH keycipher=KEYCIPHER keymaxlen=KEYMAXLEN passwdretries=NUMATTEMPTS } wherein the fields 'flags', 'startsector', 'numsectors', 'loop', 'ivoffset', 'keyformat', 'keymaxlen' and 'passwdretries' are optional. The fields in this structure have the following meaning: TARGET_NAME is the name that cryptmount uses to refer to a particular filing system. The special name "_DEFAULTS_" may be used to set default values in subsequent targets for various parameters such as 'flags', 'fstype', 'mountoptions', 'cipher', 'keyformat', 'keyhash', 'keycipher', 'keymaxlen', 'passwdretries'. DEVICE is the name of the raw device (e.g. /dev/hdb63) or ordinary file (e.g. /home/secretiveuser/private.fs) that contains the encrypted filing system. FLAG is a configuration switch, such as * "user" (any user can mount), * "nouser" (only root can mount), * "fsck" (automatically check filesystem before mounting), * "nofsck" (don't check filesystem before mounting), * "mkswap" (format swap partition before use), * "nomkswap" (don't format swap partition). This parameter is optional and defaults to "user,fsck,nomkswap". STARTSECTOR is the number of sectors (512-byte blocks) into DEVICE at which the filing system is to start. This parameter is optional, and defaults to zero. NUMSECTORS gives the total length of the filing system in sectors (512-byte blocks). This parameter is optional, and defaults to -1 which is shorthand for the total available length of DEVICE. LOOPDEV can be used to specify a particular loopback device (e.g. /dev/loop0) used when DEVICE is an ordinary file. This parameter is optional and defaults to "auto". MOUNT_POINT is the directory onto which the encrypted filing system will be mounted. TYPE is the filing system type (as used by mount (8)). This must be set to "swap" if the device is to be used as an encrypted swap partition. MOPT is a filesystem mounting option, as used by mount (8). MOPT can typically be "default", "noatime", "noexec", "nosuid", "ro", "sync" etc. FOPT is a filesystem-checking option understood by fsck (8). FOPT can typically be "-C", "-V" etc. CIPHER is the encryption algorithm used on the DEVICE. The available algorithms are determined by the system kernel. This parameter is optional and defaults to "aes-cbc-plain". KEYFORMAT specifies which encryption engine is used to manage the KEYFILE. The available engines are determined when cryptmount is built, but may include "libgcrypt", "luks", and "openssl-compat", in addition to "builtin" and "raw". This parameter is optional: if absent, "builtin" will be used on first generating the key, with an automatic choice being made when reading a pre-existing key. KEYFILE is an ordinary file that contains the key used by the CIPHER algorithm to decrypt the filing system. This key is itself encrypted in a way specified by the KEYHASH and KEYCIPHER IVOFFSET is the offset added to the sector-number used in constructing the cipher algorithm's initialization vector. This parameter is optional, and defaults to 0. KEYHASH is the hashing algorithm used to turn the user's password into the decryption key used by the KEYCIPHER algorithm. The available hashing algorithms are determined by the chosen key- encryption engine specified by KEYFORMAT . This parameter is optional and the default depends on the value of KEYFORMAT . KEYCIPHER is the encryption algorithm used to secure the decryption key of the filing system itself. The available key-encryption algorithms are determined by the chosen key-encryption engine specified by KEYFORMAT. This parameter is optional and the default depends on the value of KEYFORMAT. KEYMAXLEN is the maximum number of bytes of the decryption key that will be read from KEYFILE . This parameter is optional, and defaults to 0, indicating that the full length of KEYFILE should be read. NUMATTEMPTS is the number of password-entry attempts that can be made before cryptmount will exit with an error-code when trying to mount or configure the target.


cryptmount supports a variety of different ways of protecting the access key associated with each encrypted filesystem. For most users, the default "builtin" keymanager will provide a good level of security and flexibility. Alternative keymanagers offer a wider choice of different password-hashing schemes and compatibility with other encryption tools. The strengths and weaknesses of the different keymanagers are discussed below. builtin This keymanager is supported by cryptmount-2.0 or later, and uses a separate key-file. A password-based key derivation function (PBKDF) using the SHA1 hashing algorithm, together with blowfish-cbc encryption is used to protect the filesystem key. That key-derivation function was changed in cryptmount-4.0 to improve the security of new keyfiles, while preserving compatibility with existing keyfiles. If you need to write keyfiles in the previous format, you can specify "keyformat=builtin:0". The KEYHASH and KEYCIPHER parameters are ignored. libgcrypt This keymanager is supported by cryptmount-1.1 or later, and uses a separate key-file. A password-based key derivation function (PBKDF) is used to protect the filesystem key, with any hashing or cipher algorithm supported by the installed version of the libgcrypt library being available. luks This keymanager is supported by cryptmount-3.1 or later, and provided compatibility with the Linux Unified Key Setup (LUKS) disk-format. Instead of a separate keyfile, LUKS uses a header within the encrypted filesystem itself. It is advisable to choose the same value for both the 'dev' and 'keyfile' parameters, or leave 'keyfile' unspecified. As with all cryptmount filesystems, the 'dev' parameter may point to either a raw disk partition or an ordinary file. However, because of the filesystem structure assumed by LUKS, it is strongly recommended that you do not use either the 'startsector' or 'numsector' parameters. openssl/openssl-compat This keymanager has been supported since the earliest release of cryptmount, and uses a separate keyfile which is compatible with the format used by the 'openssl' command-line encryption tool. Since cryptmount-3.0 this file-format has been provided via the libgcrypt library, and is preferably specified by "keyformat=openssl-compat". A password-based key derivation function (PBKDF) is used to protect the filesystem key, with a choice of hashing or cipher algorithms being available. Most algorithms supported by the 'openssl' command-line tool should be available, provided the underlying algorithms are available within libgcrypt. password This keymanager is supported by cryptmount-4.0 or later, and does not require any separate keyfile, but instead derives the filesystem key directly from the user's password. This means that it is not possible to change the access password without re-encrypting the entire filesystem. The 'keyhash' and 'keycipher' parameters are ignored. raw This keymanager is supported by cryptmount-1.1 or later, and uses a separate keyfile where the access key is stored directly and without any encryption. This keymanager is most useful for managing encrypted swap partitions, where the keyfile can be chosen as /dev/random, and hence where the access key will be different every time it is read. If the keyfile is an ordinary file, it offers minimal security, and should preferably be stored separately from the disk containing the encrypted filesystem, e.g. on a USB flash disk.


Because cryptmount needs to operate with setuid privileges, it is very important that its configuration file is kept secure. Ideally /etc/cryptmount/cmtab should be managed only by the system administrator, and all key-files should be readable only by their owner. cryptmount makes basic checks on the security of /etc/cryptmount/cmtab each time it runs, and will refuse to operate unless the following conditions are met: * cmtab must be owned by root * cmtab must be a regular file * cmtab must not be globally writable * the directory containing cmtab must be owned by root * the directory containing cmtab must not be globally writable In addition, for each target within /etc/cryptmount/cmtab, all paths must be absolute (i.e. starting with '/'). When using unencrypted keyfiles (i.e. when KEYFORMAT is "raw"), it is recommended that the KEYFILE is stored with access permissions no less restrictive than 0600, or on a removable device such as a USB flash- disk. (With recent versions of cryptmount the "builtin" key-format should be portable between different installations and vastly more secure than "raw" keyfiles.) It is very important that you do not lose or damage the KEYFILE as this file is essential to providing access to your encrypted filesystem. You are strongly advised to consider keeping a backup of your KEYFILE in some form.


When the 'mkswap' option is selected for a particular target within /etc/cryptmount/cmtab, cryptmount will attempt to automatically format an encrypted swap partition whenever you run "cryptmount --swapon <target>". This is often useful when there is no need to preserve swap data between reboots, such as when not using the kernel's hibernation features. Because reformatting will destroy any existing data on the chosen swap partition, cryptmount will do some basic checking on the first megabyte of the partition, based on the degree of randomness (entropy) in the current contents. If the partition looks like it contains pure noise, or has been zeroed, then the partition will be formatted automatically. If cryptmount determines that the partition may contain non-random data, then it will ask you to run 'mkswap' manually. As there is no fool-proof way of determining whether a partition (especially after encryption) contains valuable data, you should be very careful about the raw device chosen for any target on which you select the 'mkswap' option.
The following example of /etc/cryptmount/cmtab consists of five targets, using a variety of encryption algorithms and storing their filesystems in different ways, including a target representing an encrypted swap partition: # /etc/cryptmount/cmtab # example file - please modify before use _DEFAULTS_ { passwdretries=3 # allow 3 password attempts by default } basic { dev=/home/secretiveuser/crypt.fs dir=/home/secretiveuser/crypt # where to mount loop=auto # find free loop-device fstype=ext3 mountoptions=default cipher=aes-cbc-plain # filesystem encryption keyfile=/home/secretiveuser/crypt.key # use default sha1/blowfish key-encryption: keyformat=builtin } partition { dev=/dev/hdb62 # use whole disk partition dir=/mnt/crypt62 fstype=ext3 mountoptions=nosuid,noexec cipher=serpent-cbc-plain # information about file used to store decryption key: keyfile=/etc/cryptmount/crypt_hdb62.key keyformat=openssl # use OpenSSL key-encryption keyhash=md5 keycipher=bf-cbc # encryption of key file } subset { dev=/dev/hdb63 startsector=512 numsectors=16384 # use subset of partition dir=/mnt/encrypted\ subset\ of\ hdb fstype=reiserfs mountoptions=defaults cipher=twofish-cbc-plain # filesystem encryption # information about file used to store decryption key: keyfile=/etc/cryptmount/crypt_hdb63.key keyformat=libgcrypt keyhash=md5 keycipher=blowfish-cbc # encryption of key file } encswap { # encrypted swap partition dev=/dev/hdb63 startsector=16896 numsectors=1024 # use subset of partition fstype=swap flags=mkswap cipher=twofish-cbc-plain # read fresh 16-byte key from /dev/random whenever used: keyfile=/dev/random keymaxlen=16 keyformat=raw } luks { # partition created by cryptsetup-luks dev=/dev/hdb63 dir=/mnt/luks-partition keyformat=luks keyfile=/dev/hdb63 fstype=ext3 } # end of cmtab The 'basic' target uses an ordinary file "/home/secretiveuser/crypt.fs" to store the encrypted filesystem, perhaps within a normal user's home directory. A loopback device will be automatically allocated (because of the "loop=auto") by cryptmount to turn this into a block-special device, before mounting. The decryption key for the filesystem is also stored in this user's home directory, making it easier for them to change the password protecting the key. The 'partition' target uses a whole disk partition to store the encrypted filing system, with the decryption key stored in the main cryptmount configuration directory. The 'subset' target is similar to the 'partition' target except that it does not use a whole disk partition. This would allow other groups of blocks within that partition to be used for other filesystems managed via cryptmount or dmsetup. The 'encswap' target uses a subset of blocks within a disk partition to form an encrypted swap device. A new encryption key is read from the system random-number generator /dev/random every time the target is used. The 'luks' target provides access to an encrypted partition created by the 'cryptsetup-luks' utility. cryptmount will be able to mount and unmount the partition, but password-changing and advanced LUKS features must be accessed through cryptsetup
/etc/cryptmount/cmtab - main configuration file


cryptmount(8), cryptmount-setup(8), cryptsetup(8), dmsetup(8), openssl(1)


cryptmount is Copyright 2005-2009 RW Penney and is supplied with NO WARRANTY. Licencing terms are as described in the file "COPYING" within the cryptmount source distribution. CMTAB(5)

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