ENCFS(1) Encrypted Filesystem ENCFS(1)NAMEencfs - mounts or creates an encrypted virtual filesystem
SYNOPSISencfs [--version] [-s] [-f] [-v⎪--verbose] [-i MINUTES⎪--idle=MINUTES]
[--extpass=program] [-S⎪--stdinpass] [--anykey] [--forcedecode]
[-d⎪--fuse-debug] [--public] [--no-default-flags] [--ondemand]
[--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 pre‐
sented 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 inac‐
tivity. The period is specified in minutes, so the shortest time‐
out period that can be requested is one minute. EncFS will not
automatically unmount if there are files open within the filesys‐
tem, even if they are open in read-only mode. However simply hav‐
ing 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.
-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 speci‐
fying --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 consider‐
ing 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 it's reference to it. If
someone attempts to access the filesystem again, the extpass pro‐
gram is used to prompt the user for the password. If this suc‐
ceeds, then the filesystem becomes available again.
--reverse
Normally EncFS provides a plaintext view of data on demand. Nor‐
mally 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/.encfs5 which contains the filesystem information.
Together, the two can be used to reproduce the unencrypted data:
ENCFS5_CONFIG=/home/me/.encfs5 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.
--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 pass‐
word, 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 cre‐
ation 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.
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 cre‐
ate 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. Therefor any limitations of the
host filesystem will likely be inherited by EncFS (or possibly be fur‐
ther 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 then 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 sup‐
port 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 dif‐
ferent versions of encfs, as long as they are compatible with the cur‐
rent 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
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
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 tar‐
get 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, it rougly 64k iterations of the key deriva‐
tion 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 ver‐
sion 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 automati‐
cally based on what supported algorithms EncFS found in the encryp‐
tion 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 lenths 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 simi‐
lar 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 opera‐
tion, 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 ini‐
tialization vector, but in a deterministic way so that block N in
one file is encoded in the same was as block N in another file.
That made it possible for someone to tell if two files were identi‐
cal (or parts of the file were identical) by comparing the encoded
data.
With per-file initialization vectors, each file gets its own 64bit
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 Initial‐
ization Vectors and Filename Initialization Vector Chaining. Basi‐
cally 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 filesys‐
tem 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.
Attacks
The primary goal of EncFS is to protect data off-line. That is, pro‐
vide 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 therefor modify EncFS, or FUSE, or the ker‐
nel 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 modifi‐
cations 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 MER‐
CHANTABILITY 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>.
SEE ALSOencfsctl(1)1.7.3 2009-11-29 ENCFS(1)
