cryptmount.8.in

Cryptmount是对Linux系统下的文件系统以及用户设备、文档等进行加密的系统.

.\" cryptmount manual page
.\" $Revision: 117 $, $Date: 2006-07-30 10:20:45 +0100 (Sun, 30 Jul 2006) $
.\" Copyright (c) 2005,2006 RW Penney
.\"
.TH CRYPTMOUNT 8 "2006-07-17" "@PACKAGE_VERSION@" "User commands"
.SH NAME
cryptmount \- mount/unmount/configure an encrypted filing system

.\" --------------------------------
.SH SYNOPSIS

.BI "cryptmount [\-aclmu] [target ...]"
.LP
.BI "cryptmount [\-\-change\-password [target ...]]"
.LP
.BI "cryptmount [\-\-generate\-key \fIsize\fP [target ...]]"
.LP
.BI "cryptmount [\-\-prepare [target ...]]"
.LP
.BI "cryptmount [\-\-release [target ...]]"
___DELETE_CSWAP_0
.LP
.BI "cryptmount [\-\-swapon [target ...]]"
.LP
.BI "cryptmount [\-\-swapoff [target ...]]"
___END_CSWAP_0


.\" --------------------------------
.SH DESCRIPTION

.B
cryptmount
allows a user to mount an encrypted filing system
without requiring superuser privileges,
and assists the superuser in creating new encrypted filesystems.
After initial configuration of the filing system by the system administrator,
the user needs only to provide the decryption password for that filing sytem in order for
.B cryptmount
to automatically configure device-mapper and loopback targets before mounting the filing system.

.B 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 filing systems directly through
.B mount
(8).


.\" --------------------------------
.SH OPTIONS

.TP
.B \-a \-\-all
act on all available targets, e.g. for mounting all targets.
.TP
.B \-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 filing system.
.TP
.B \-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 filing system is in use, or if a non-root user tries
to unmount a filing system mounted by a different user.
.TP
.B \-l \-\-list
lists all available targets, including basic information about the filing system and mount point of each.
.TP
.B \-c \-\-change\-password
change the password protecting the decryption key for a given filing system.
.TP
.B \-\-generate\-key "\fIsize\fP"
setup a decryption key for a new filing system.
.IR size\fP
gives the length of the key in bytes.
.TP
.B \-\-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 filing system on an encrypted device.
.TP
.B \-\-release
releases all device-mapper and loopback devices associated with a particular target. This option is only available to the superuser.
___DELETE_CSWAP_0
.TP
.B \-\-swapon
enable the specified target for paging and swapping.
This option is only available to the superuser.
.TP
.B \-\-swapoff
disable the specified target for paging and swapping.
This option is only available to the superuser.
___END_CSWAP_0

.\" --------------------------------
.SH RETURN CODES
.B cryptmount
returns zero on success.
A non-zero value indicates a failure of some form, with specific values traceable, in principle, to different error conditions.


.\" --------------------------------
.SH EXAMPLE USAGE

Suppose that we wish to setup a new encrypted filing system, that will have a target-name of "opaque".
If we have a free disk partition available, say /dev/hdb63, then we can use this directly to store the encrypted filing system.
Alternatively, if we want to store the encrypted filing system within an ordinary file, we need to create space using a recipe such as:

.nf
    dd if=/dev/zero of=/home/opaque.fs bs=1M count=512
.fi

and then replace all occurences of '/dev/hdb63' 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 @etcdir@/cmtab,
which describes the encryption that will be used
to protect the filesystem itself and the access key, as follows:

.nf
    opaque {
        dev=/dev/hdb63 dir=/home/crypt
        fstype=ext2 fsoptions=defaults cipher=twofish
        keyfile=@etcdir@/opaque.key keyhash=md5 keycipher=bf\-cbc
    }
.fi

Here, we will be using the "twofish" algorithm to encrypt
the filing system itself,
with the "bf\-cbc" and "md5" algorithms being used
to protect the decryption key (to be stored in @etcdir@/opaque.key).

In order to generate a secret decryption key (in @etcdir@/opaque.key)
that will be used to encrypt the filing system itself,
we can execute, as root:

.nf
    cryptmount \-\-generate\-key 32 opaque
.fi

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:

.nf
    cryptmount \-\-prepare opaque
.fi

we will then be asked for the password that we used when setting up @etcdir@/opaque.key, which will enable
.B cryptmount
to setup a device-mapper target (/dev/mapper/opaque).

(Alternatively, if the encrypted filing system resides on a raw disk partition, one can use
.B cryptsetup
(8)
as follows:

.nf
    openssl enc \-d \-md md5 \-bf\-cbc @etcdir@/opaque.key | \\
    cryptsetup \-c twofish \-d /dev/stdin create opaque /dev/hdb63
.fi

to perform a similar process.)

We can now use standard tools to create the actual filing system on /dev/mapper/opaque:

.nf
    mke2fs /dev/mapper/opaque
.fi

(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

.nf
    cryptmount \-\-release opaque
    mkdir /home/crypt
.fi

(or using the equivalent recipe for
.B
cryptsetup
), the encrypted filing system is ready for use.
Ordinary users can mount it by typing

.nf
    cryptmount \-m opaque
.fi

or

.nf
    cryptmount opaque
.fi

and unmount it using

.nf
    cryptmount \-u opaque
.fi

.B 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.


.\" --------------------------------
.SH PASSWORD CHANGING

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:

.nf
    cryptmount \-\-change\-password opaque
.fi

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.)


___DELETE_FSCK_1
.\" --------------------------------
.SH FILESYTEM MAINTENANCE

For filesystems that are mounted on system-startup,
it is normal for checks on their integrity
to be performed automatically at regular intervals,
typically every few dozen mounts.
If
.B cryptmount
has not been compiled with the '\-\-enable\-fsck' option,
such checks need to be performed manually
for filesystems managed by
.B cryptmount
, which will require the involvement of both the system administrator
and the users who know the relevant access passwords
for the filing systems involved.
Suppose that we wish to check the filesystem associated with
.B cryptmount
target 'opaque', we first need to prepare the decryption devices:

.nf
    cryptmount \-\-prepare opaque
.fi

this will create a device-mapper target accessible via '/dev/mapper/opaque', on which we can then run standard checking utilities:

.nf
    fsck \-t auto /dev/mapper/opaque
.fi

After these tests have been completed, we can then release the devices:

.nf
    cryptmount \-\-release opaque
.fi

and continue using the filesystem as before.
___END_FSCK_1


.\" --------------------------------
.SH FILES
.I @etcdir@/cmtab
- main configuration file
.LP
.I @etcdir@/cmstatus
- record of mounted filesystems


.SH "SEE ALSO"
.BR cmtab (5),
.BR cryptsetup (8),
.BR dmsetup (8),
.BR fsck (8),
.BR mount (8),
.BR openssl (1)


.\" --------------------------------
.SH BUGS
The author would be grateful for any
.B constructive
suggestions and bug-reports, via <[e-mail]>


.\" --------------------------------
.SH COPYRIGHT NOTICE
.B cryptmount
is Copyright 2005-2006 RW Penney
.br
and is supplied with NO WARRANTY.
Licencing terms are as described in the file "COPYING"
within the cryptmount source distribution.