Ubuntu Manpage: encfs - mounts or creates an encrypted virtual filesystem

NAME

       encfs - mounts or creates an encrypted virtual filesystem

SYNOPSIS

       encfs [--version] [-s] [-f] [-v|--verbose] [-i MINUTES|--idle=MINUTES] [--extpass=program]
       [-S|--stdinpass] [--anykey] [--forcedecode] [-d|--fuse-debug] [--public] [--no-default-flags]
       [--ondemand] [--delaymount] [--reverse] [--standard] [-o FUSE_OPTION] rootdir mountPoint [-- [Fuse Mount
       Options]]

DESCRIPTION

       EncFS creates a virtual encrypted filesystem which stores encrypted data in the rootdir directory and
       makes the unencrypted data visible at the mountPoint directory.  The user must supply a password which is
       used to (indirectly) encrypt both filenames and file contents.

       If EncFS is unable to find a supported filesystem at the specified rootdir, then the user will be asked
       if they wish to create a new encrypted filesystem at the specified location.  Options will be presented
       to the user allowing some control over the algorithms to use.  As EncFS matures, there may be an
       increasing number of choices.

OPTIONS

-i, --idle=MINUTES
Enable automatic unmount of the filesystem after a period of inactivity. The period is specified in
minutes, so the shortest timeout period that can be requested is one minute. EncFS will not
automatically unmount if there are files open within the filesystem, even if they are open in read-
only mode. However simply having files open does not count as activity.

-f The -f (foreground) option causes EncFS to run in the foreground. Normally EncFS spawns off as a
daemon and runs in the background, returning control to the spawning shell. With the -f option, it
will run in the foreground and any warning/debug log messages will be displayed on standard error.
In the default (background) mode, all log messages are logged via syslog.

-v, --verbose
Causes EncFS to enable logging of various debug channels within EncFS. Normally these logging
messages are disabled and have no effect. It is recommended that you run in foreground (-f) mode
when running with verbose enabled.

-t, --syslogtag
This option allows to set the syslog tag which will be used when messages are logged via syslog. By
default the syslog tag is set to encfs.

-s The -s (single threaded) option causes EncFS to run in single threaded mode. By default, EncFS runs
in multi-threaded mode. This option is used during EncFS development in order to simplify debugging
and allow it to run under memory checking tools.

-d, --fuse-debug
Enables debugging within the FUSE library. This should only be used if you suspect a problem within
FUSE itself (not EncFS), as it generates a lot of low-level data and is not likely to be very helpful
in general problem tracking. Try verbose mode (-v) first, which gives a higher level view of what is
happening within EncFS.

--forcedecode
This option only has an effect on filesystems which use MAC block headers. By default, if a block is
decoded and the stored MAC doesn't match what is calculated, then an IO error is returned to the
application and the block is not returned. However, by specifying --forcedecode, only an error will
be logged and the data will still be returned to the application. This may be useful for attempting
to read corrupted files.

--public
Attempt to make encfs behave as a typical multi-user filesystem. By default, all FUSE based
filesystems are visible only to the user who mounted them. No other users (including root) can view
the filesystem contents. The --public option does two things. It adds the FUSE flags "allow_other"
and "default_permission" when mounting the filesystem, which tells FUSE to allow other users to
access the filesystem, and to use the ownership permissions provided by the filesystem. Secondly,
the --public flag changes how encfs's node creation functions work - as they will try and set
ownership of new nodes based on the caller identification.

Warning: In order for this to work, encfs must be run as root -- otherwise it will not have the
ability to change ownership of files. I recommend that you instead investigate if the fuse
allow_other option can be used to do what you want before considering the use of --public.

--ondemand
Mount the filesystem on-demand. This currently only makes sense in combination with --idle and
--extpass options. When the filesystem becomes idle, instead of exiting, EncFS stops allowing access
to the filesystem by internally dropping its reference to it. If someone attempts to access the
filesystem again, the extpass program is used to prompt the user for the password. If this succeeds,
then the filesystem becomes available again.

--delaymount
Do not mount the filesystem when encfs starts; instead, delay mounting until first use. This option
only makes sense with --ondemand.

--require-macs
If creating a new filesystem, this forces block authentication code headers to be enabled. When
mounting an existing filesystem, this causes encfs to exit if block authentication code headers are
not enabled.

This can be used to improve security in case the ciphertext is vulnerable to tampering, by preventing
an attacker from disabling MACs in the config file.

--reverse
Normally EncFS provides a plaintext view of data on demand: it stores enciphered data and displays
plaintext data. With --reverse it takes as source plaintext data and produces enciphered data on-
demand. This can be useful for creating remote encrypted backups, where you do not wish to keep the
local files unencrypted.

For example, the following would create an encrypted view in /tmp/crypt-view.

encfs --reverse /home/me /tmp/crypt-view

You could then copy the /tmp/crypt-view directory in order to have a copy of the encrypted data. You
must also keep a copy of the file /home/me/.encfs6.xml which contains the filesystem information.
Together, the two can be used to reproduce the unencrypted data:

ENCFS6_CONFIG=/home/me/.encfs6.xml encfs /tmp/crypt-view /tmp/plain-view

Now /tmp/plain-view contains the same data as /home/me

Note that --reverse mode only works with limited configuration options, so many settings may be
disabled when used.

--nocache
Disable the kernel's cache of file attributes. Setting this option makes EncFS pass "attr_timeout=0"
and "entry_timeout=0" to FUSE. This makes sure that modifications to the backing files that occour
outside EncFS show up immediately in the EncFS mount. The main use case for "--nocache" is reverse
mode.

--standard
If creating a new filesystem, this automatically selects standard configuration options, to help with
automatic filesystem creation. This is the set of options that should be used unless you know what
you're doing and have read the documentation.

When not creating a filesystem, this flag does nothing.

-o FUSE_ARG
Pass through FUSE args to the underlying library. This makes it easy to pass FUSE options when
mounting EncFS via mount (and /etc/fstab). Eg:

mount encfs#/home/me-crypt /home/me -t fuse -o kernel_cache

Note that encfs arguments cannot be set this way. If you need to set encfs arguments, create a
wrapper, such as encfs-reverse;

#!/bin/sh
encfs --reverse "$@"

Then mount using the script path

mount encfs-reverse#/home/me /home/me-crypt -t fuse

-- The -- option tells EncFS to send any remaining arguments directly to FUSE. In turn, FUSE passes the
arguments to fusermount. See the fusermount help page for information on available commands.

--no-default-flags
Encfs adds the FUSE flags "use_ino" and "default_permissions" by default, as of version 1.2.2,
because that improves compatibility with some programs. If for some reason you need to disable one
or both of these flags, use the option --no-default-flags.

The following command lines produce the same result:

encfs raw crypt
encfs --no-default-flags raw crypt -- -o use_ino,default_permissions

--extpass=program
Specify an external program to use for getting the user password. When the external program is
spawned, the environment variable "RootDir" will be set to contain the path to the root directory.
The program should print the password to standard output.

EncFS takes everything returned from the program to be the password, except for a trailing newline
(\n) which will be removed.

For example, specifying --extpass=/usr/lib/ssh/ssh-askpass will cause EncFS to use ssh's password
prompt program.

Note: EncFS reads at most 2k of data from the password program, and it removes any trailing newline.
Versions before 1.4.x accepted only 64 bytes of text.

-S, --stdinpass
Read password from standard input, without prompting. This may be useful for scripting encfs mounts.

Note that you should make sure the filesystem and mount points exist first. Otherwise encfs will
prompt for the filesystem creation options, which may interfere with your script.

--anykey
Turn off key validation checking. This allows EncFS to be used with secondary passwords. This could
be used to store a separate set of files in an encrypted filesystem. EncFS ignores files which do
not decode properly, so files created with separate passwords will only be visible when the
filesystem is mounted with their associated password.

Note that if the primary password is changed (using encfsctl), the other passwords will not be usable
unless the primary password is set back to what it was, as the other passwords rely on an invalid
decoding of the volume key, which will not remain the same if the primary password is changed.

Warning: Use this option at your own risk.

ENVIRONMENT VARIABLES

       ENCFS6_CONFIG
           Which config file (typically named .encfs6.xml) to use.  By default, the config file is read from the
           encrypted directory.  Using this option allows to store the config file separated from the  encrypted
           files.

           Warning: If you lose the config file, the encrypted file contents are irrecoverably lost. It contains
           the  master key encrypted with your password. Without the master key, recovery is impossible, even if
           you know the password.

EXAMPLES

       Create a new encrypted filesystem.  Store  the  raw  (encrypted)  data  in  "~/.crypt"  ,  and  make  the
       unencrypted data visible in "~/crypt".  Both directories are in the home directory in this example.  This
       example shows the full output of encfs as it asks the user if they wish to create the filesystem:

           % encfs ~/.crypt ~/crypt
           Directory "/home/me/.crypt" does not exist, create (y,n)?y
           Directory "/home/me/crypt" does not exist, create (y,n)?y
           Creating new encrypted volume.
           Please choose from one of the following options:
            enter "x" for expert configuration mode,
            enter "p" for pre-configured paranoia mode,
            anything else, or an empty line will select standard mode.
           ?>

           Standard configuration selected.
           Using cipher Blowfish, key size 160, block size 512
           New Password: <password entered here>
           Verify: <password entered here>

       The  filesystem  is  now mounted and visible in ~/crypt.  If files are created there, they can be seen in
       encrypted form in ~/.crypt.  To unmount the filesystem, use fusermount with the -u (unmount) option:

           % fusermount -u ~/crypt

       Another example.  To mount the same filesystem, but have fusermount name the mount point  '/dev/foo'  (as
       shown  in  df  and  other tools which read /etc/mtab), and also request kernel-level caching of file data
       (which are both special arguments to fusermount):

           % encfs ~/.crypt ~/crypt -- -n /dev/foo -c

       Or, if you find strange behavior under some particular program when working in an  encrypted  filesystem,
       it may be helpful to run in verbose mode while reproducing the problem and send along the output with the
       problem report:

           % encfs -v -f ~/.crypt ~/crypt 2> encfs-report.txt

       In  order  to  avoid leaking sensitive information through the debugging channels, all warnings and debug
       messages (as output in verbose mode)  contain  only  encrypted  filenames.   You  can  use  the  encfsctl
       program's decode function to decode filenames if desired.

CAVEATS

       EncFS is not a true filesystem.  It does not deal with any of the actual storage or maintenance of files.
       It  simply translates requests (encrypting or decrypting as necessary) and passes the requests through to
       the underlying host filesystem.  Therefore any limitations of the host filesystem will  be  inherited  by
       EncFS (or possibly be further limited).

       One  such  limitation  is filename length.  If your underlying filesystem limits you to N characters in a
       filename, then EncFS will limit you to approximately 3*(N-2)/4.   For  example  if  the  host  filesystem
       limits  to  256  characters,  then  EncFS  will  be  limited to 190 character filenames.  This is because
       encrypted filenames are always longer than plaintext filenames.

FILESYSTEM OPTIONS

       When EncFS is given a root directory which does not contain an existing EncFS filesystem,  it  will  give
       the  option  to  create one.  Note that options can only be set at filesystem creation time.  There is no
       support for modifying a filesystem's options in-place.

       If you want to upgrade a filesystem to use newer features, then you need to create a new  filesystem  and
       mount both the old filesystem and new filesystem at the same time and copy the old to the new.

       Multiple  instances  of encfs can be run at the same time, including different versions of encfs, as long
       as they are compatible with the current FUSE module on your system.

       A choice is provided for two pre-configured settings ('standard' and 'paranoia'), along  with  an  expert
       configuration mode.

       Standard mode uses the following settings:
           Cipher: AES
           Key Size: 192 bits
           PBKDF2 with 1/2 second runtime, 160 bit salt
           Filesystem Block Size: 1024 bytes
           Filename Encoding: Block encoding with IV chaining
           Unique initialization vector file headers
           File holes passed through

       Paranoia mode uses the following settings:
           Cipher: AES
           Key Size: 256 bits
           PBKDF2 with 3 second runtime, 160 bit salt
           Filesystem Block Size: 1024 bytes
           Filename Encoding: Block encoding with IV chaining
           Unique initialization vector file headers
           Message Authentication Code block headers
           External IV Chaining
           File holes passed through

       In  the expert / manual configuration mode, each of the above options is configurable.  Here is a list of
       current options with some notes about what they mean:

Key Derivation Function

As of version 1.5, EncFS now uses PBKDF2 as the default key derivation function. The number of
iterations in the keying function is selected based on wall clock time to generate the key. In standard
mode, a target time of 0.5 seconds is used, and in paranoia mode a target of 3.0 seconds is used.

On a 1.6Ghz AMD 64 system, roughly 64k iterations of the key derivation function can be handled in half a
second. The exact number of iterations to use is stored in the configuration file, as it is needed to
remount the filesystem.

If an EncFS filesystem configuration from 1.4.x is modified with version 1.5 (such as when using encfsctl
to change the password), then the new PBKDF2 function will be used and the filesystem will no longer be
readable by older versions.

Cipher
Which encryption algorithm to use. The list is generated automatically based on what supported
algorithms EncFS found in the encryption libraries. When using a recent version of OpenSSL, Blowfish
and AES are the typical options.

Blowfish is an 8 byte cipher - encoding 8 bytes at a time. AES is a 16 byte cipher.

Cipher Key Size
Many, if not all, of the supported ciphers support multiple key lengths. There is not really much
need to have enormous key lengths. Even 160 bits (the default) is probably overkill.

Filesystem Block Size
This is the size (in bytes) that EncFS deals with at one time. Each block gets its own
initialization vector and is encoded in the cipher's cipher-block-chaining mode. A partial block at
the end of a file is encoded using a stream mode to avoid having to store the filesize somewhere.

Having larger block sizes reduces the overhead of EncFS a little, but it can also add overhead if
your programs read small parts of files. In order to read a single byte from a file, the entire
block that contains that byte must be read and decoded, so a large block size adds overhead to small
requests. With write calls it is even worse, as a block must be read and decoded, the change applied
and the block encoded and written back out.

The default is 512 bytes as of version 1.0. It was hard coded to 64 bytes in version 0.x, which was
not as efficient as the current setting for general usage.

Filename Encoding
New in 1.1. A choice is given between stream encoding of filename and block encoding. The advantage
of stream encoding is that the encoded filenames will be as short as possible. If you have a
filename with a single letter, it will be very short in the encoded form, where as block encoded
filenames are always rounded up to the block size of the encryption cipher (8 bytes for Blowfish and
16 bytes for AES).

The advantage of block encoding mode is that filename lengths all come out as a multiple of the
cipher block size. This means that someone looking at your encrypted data can't tell as much about
the length of your filenames. It is on by default, as it takes a similar amount of time to using the
stream cipher. However stream cipher mode may be useful if you want shorter encrypted filenames for
some reason.

Prior to version 1.1, only stream encoding was supported.

Filename Initialization Vector Chaining
New in 1.1. In previous versions of EncFS, each filename element in a path was encoded separately.
So if "foo" encoded to "XXX", then it would always encode that way (given the same encryption key),
no matter if the path was "a/b/foo", or "aa/foo/cc", etc. That meant it was possible for someone
looking at the encrypted data to see if two files in different directories had the same name, even
though they wouldn't know what that name decoded to.

With initialization vector chaining, each directory gets its own initialization vector. So "a/foo"
and "b/foo" will have completely different encoded names for "foo". This features has almost no
performance impact (for most operations), and so is the default in all modes.

Note: One significant performance exception is directory renames. Since the initialization vector
for filename encoding depends on the directory path, any rename requires re-encoding every filename
in the tree of the directory being changed. If there are thousands of files, then EncFS will have to
do thousands of renames. It may also be possible that EncFS will come across a file that it can't
decode or doesn't have permission to move during the rename operation, in which case it will attempt
to undo any changes it made up to that point and the rename will fail.

Per-File Initialization Vectors
New in 1.1. In previous versions of EncFS, each file was encoded in the same way. Each block in a
file has always had its own initialization vector, but in a deterministic way, so that block N in one
file was encoded in the same way as block N in another file. That made it possible for someone to
tell if two files were identical (or parts of the file were identical) by comparing the encoded data.

With per-file initialization vectors, each file gets its own 64-bit random initialization vector, so
that each file is encrypted in a different way.

This option is enabled by default.

External IV Chaining
New in 1.1.3. This option is closely related to Per-File Initialization Vectors and Filename
Initialization Vector Chaining. Basically it extends the initialization vector chaining from
filenames to the per-file initialization vector.

When this option is enabled, the per-file initialization vector is encoded using the initialization
vector derived from the filename initialization vector chaining code. This means that the data in a
file becomes tied to the filename. If an encrypted file is renamed outside of encfs, it will no
longer be decodable within encfs. Note that unless Block MAC headers are enabled, the decoding error
will not be detected and will result in reading random looking data.

There is a cost associated with this. When External IV Chaining is enabled, hard links will not be
allowed within the filesystem, as there would be no way to properly decode two different filenames
pointing to the same data.

Also, renaming a file requires modifying the file header. So renames will only be allowed when the
user has write access to the file.

Because of these limits, this option is disabled by default for standard mode (and enabled by default
for paranoia mode).

Block MAC headers
New to 1.1. If this is enabled, every block in every file is stored along with a cryptographic
checksum (Message Authentication Code). This makes it virtually impossible to modify a file without
the change being detected by EncFS. EncFS will refuse to read data which does not pass the checksum,
and will log the error and return an IO error to the application.

This adds substantial overhead (default being 8 bytes per filesystem block), plus computational
overhead, and is not enabled by default except in paranoia mode.

When this is not enabled and if EncFS is asked to read modified or corrupted data, it will have no
way to verify that the decoded data is what was originally encoded.

File-hole pass-through
Make encfs leave holes in files. If a block is read as all zeros, it will be assumed to be a hole
and will be left as 0's when read (not deciphered). This is required if accessing encfs using the
SMB protocol.

Enabled by default. Can be disabled in expert mode.

Attacks

The primary goal of EncFS is to protect data off-line. That is, provide a convenient way of storing
files in a way that will frustrate any attempt to read them if the files are later intercepted.

Some algorithms in EncFS are also meant to frustrate on-line attacks where an attacker is assumed to be
able to modify the files.

The most intrusive attacks, where an attacker has complete control of the user's machine (and can
therefore modify EncFS, or FUSE, or the kernel itself) are not guarded against. Do not assume that
encrypted files will protect your sensitive data if you enter your password into a compromised computer.
How you determine that the computer is safe to use is beyond the scope of this documentation.

That said, here are some example attacks and data gathering techniques on the filesystem contents along
with the algorithms EncFS supports to thwart them:

Attack: modifying a few bytes of an encrypted file (without knowing what they will decode to).
EncFS does not use any form of XOR encryption which would allow single bytes to be modified without
affecting others. Most modifications would affect dozens or more bytes. Additionally, MAC Block
headers can be used to identify any changes to files.

Attack: copying a random block of one file to a random block of another file.
Each block has its own [deterministic] initialization vector.

Attack: copying block N to block N of another file.
When the Per-File Initialization Vector support is enabled (default in 1.1.x filesystems), a copied
block will not decode properly when copied to another file.

Attack: copying an entire file to another file.
Can be prevented by enabling External IV Chaining mode.

Attack: determine if two filenames are the same by looking at encrypted names.
Filename Initialization Vector chaining prevents this by giving each file a 64-bit initialization
vector derived from its full path name.

Attack: compare if two files contain the same data.
Per-File Initialization Vector support prevents this.

DISCLAIMER

       This library is distributed in the hope that it will be useful, but WITHOUT ANY  WARRANTY;  without  even
       the  implied  warranty  of  MERCHANTABILITY  or  FITNESS  FOR  A PARTICULAR PURPOSE.  Please refer to the
       "COPYING" file distributed with EncFS for complete details.

AUTHORS

       EncFS was written by Valient Gough <vgough@pobox.com>.