/usr/share/doc/ecryptfs-utils/ecryptfs-faq.html

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eCryptfs
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<h1>eCryptfs</h1>
<h3>FAQ</h3>

<hr>

<a href="http://ecryptfs.org">Main Page</a>

<hr>

<table width="640">
<tr>
<td>

<p>

<h3>Index</h3>

<ul>

<li><a href="#novelty">What is novel about eCryptfs?</a></li>

<li><a href="#versions">What versions of the kernel have eCryptfs
support?</a></li>

<li><a href="#deployment">Will eCryptfs by itself protect all my
data?</a></li>

<li><a href="#access_lower">Can I access the lower files while
eCryptfs is mounted?</a></li>

<li><a href="#options">What kernel options do I need to enable to
build eCryptfs?</a></li>

<li><a href="#compatibility">On what filesystems can I expect eCryptfs
to function?</a></li>

<li><a href="#stack">Why is the kernel stack such an issue with
eCryptfs?</a></li>

<li><a href="#ecryptfsd">What is <code>ecryptfsd</code>?</a></li>

<li><a href="#ecryptfs-manager">What is
<code>ecryptfs-manager</code>?</a></li>

<li><a href="#nonroot">Do I have to be root to mount
eCryptfs?</a></li>

<li><a href="#xattr">How do I store the metadata in the extended
attribute region of the lower file?</a></li>

<li><a href="#encryptedview">I am using the <code>-o xattr</code>
option, but my backup tools do not preserve extended attributes. How
can I back up the lower files?</a></li>

<li><a href="#sparse">What about sparse files?</a></li>

<li><a href="#passphrase">How should I select my passphrase?</a></li>

<li><a href="#protectkey">How can I protect my key?</a></li>

<li><a href="#lostkey">I forgot my password/lost my key! What can I do
to recover my data?</a></li>

<li><a href="#compare">How does eCryptfs compare with other Linux disk
encryption solutions?</a></li>

<li><a href="#no-ecryptfsac">Once one user can access an eCryptfs
file, any users with permission can also access the file. Should not
eCryptfs require all users to have the key in order to access the
files?</a></li>

<li><a href="#initcipher">&quot;<code>Unable to allocate crypto cipher
with name [---]; rc = [-2]</code>&quot;</a></li>

<li><a href="#baddir">&quot;<code>Error mounting eCryptfs; rc = [-2];
strerr = [No such file or directory]</code>&quot;</a></li>

<li><a href="#einval">&quot;<code>Error mounting eCryptfs; rc = [-22];
strerr = [Invalid argument]</code>&quot;</a></li>

<li><a href="#keyproblem">&quot;<code>ecryptfs_parse_options: Could
not find key with description: [deadbeaf...]&quot;</code></a></li>

<li><a href="#sigsize">&quot;<code>ecryptfs_parse_packet_set: Expected
signature of size [8]; read size [7]&quot;</code></a></li>

<li><a href="#nothere">My question isn't answered here.</a></li>

</ul>
</p>

</p>

<a name="novelty">

<p><h3>Q. What is novel about eCryptfs?</h3></p>

<p>
Well, nothing, to be honest. All of the techniques used in eCryptfs
are directly based on cryptographic technology that was widely known
and in practical use in the 1970's.
</p>

<p>
Security problems often arise when software tries to ``invent its own
crypto'' by deviating from what has been in common practical use for a
lengthy period of time. eCryptfs sticks to tried-and-true encryption
technology.
</p>

<p>
In terms of per-file key management, eCryptfs simply uses the methods
of PGP (created by Philip Zimmermann in 1991 and formally specified as
a public standard in RFC2440 in 1998) and takes the obvious and
conceptually trivial step of applying those methods within a
filesystem service in the kernel. eCryptfs employs the well-weathered
encryption techniques that have been in common use in the community
for over two decades. Other cryptographic filesystems published and
widely used in the 1990's use the same basic approach to encrypting
files. eCryptfs just happens to be the first such filesystem to make
it upstream in the Linux kernel.
</p>

<a name="versions">

<p><h3>Q. What versions of the kernel have eCryptfs support?</h3></p>

<p>
Linux kernel versions 2.6.19 and later have eCryptfs support. The
official mainline kernel is supported and is in active development.
</p>

<a name="deployment">

<p><h3>Q. Will eCryptfs by itself protect all my data?</h3></p>

<p>
eCryptfs is just one component in a comprehensive set of mechanisms to
protect the confidentiality of your data. Simply mounting eCryptfs
over a directory in your home directory will probably not provide
sufficient coverage for everything your applications will write to
disk. For instance, applications that produce and store thumbnails of
your images may write the thumbnails to an unprotected location.
</p>

<p>
Sensitive application data will typically wind up in the following
locations, although some applications will write data to other
locations not listed here:
</p>

<ul>
<li>Anywhere in your home directory</li>
<li>The /tmp directory</li>
<li>The /var directory</li>
<li>The swap device</li>
</ul>

<p>
The /tmp directory and the swap device can be easily protected with
dm-crypt using a key randomly generated when the system is booted,
since the information in those locations does not need to persist
between reboots. eCryptfs must mount the /var directory prior to any
daemons or other system applications reading from or writing to that
location (including the syslog utility). eCryptfs must also mount over
the user's home directory prior to the user logging into the system.
</p>

<p>
You will need to consider other applications that diverge from
traditional paths for storing data on a case-by-case basis. Analyzing
application behavior with the kernel auditing system is one way to
profile the behavior of an application, and explicit SE Linux rules
that only allow applications to write to encrypted mountpoints helps
prevent inadvertent information leakage. We recommend always using
eCryptfs together with appropriate Mandatory Access Control (MAC)
mechanisms to ensure that your sensitive data is always encrypted.
</p>

<p>
Proper deployment of a comprehensive per-file encryption mechanism is
a task best tackled by the entire Linux distribution. The eCryptfs
team is working closely with various major Linux distributions to help
ensure that eCryptfs is properly used as one component of a
comprehensive data protection strategy.
</p>

<a name="access_lower">

<p><h3>Q. Can I access the lower files while eCryptfs is mounted?</h3></p>

<p>
Accessing the lower files during an active eCryptfs mount is somewhat
like accessing a block device on which ext3 is mounted. The kernel
allows it, and it may work (depending on what you do with the data),
but it is not a good idea.
</p>

<a name="options">

<p><h3>Q. What kernel options do I need to enable to build
eCryptfs?</h3></p>

<p>
<code>
Code maturity level options  ---><br>
&nbsp;&nbsp;[*] Prompt for development and/or incomplete code/drivers<br>
<br>
Security options  ---><br>
&nbsp;&nbsp;&lt;M&gt; Enable access key retention support<br>
<br>
Cryptographic options  ---><br>
&nbsp;&nbsp;&lt;M&gt;   MD5 digest algorithm<br>
&nbsp;&nbsp;&lt;M&gt;   AES cipher algorithms<br>
<br>
File systems  ---><br>
&nbsp;&nbsp;Miscellaneous filesystems  ---><br>
&nbsp;&nbsp;&nbsp;&nbsp;&lt;M&gt; eCrypt filesystem layer support (EXPERIMENTAL)<br>
<br>
Recommended .config options (some options not available in older kernels):<br>
CONFIG_EXPERIMENTAL=y<br>
CONFIG_KEYS=y<br>
CONFIG_CRYPTO=y<br>
CONFIG_CRYPTO_ALGAPI=y<br>
CONFIG_CRYPTO_BLKCIPHER=y<br>
CONFIG_CRYPTO_HASH=y<br>
CONFIG_CRYPTO_MANAGER=y<br>
CONFIG_CRYPTO_MD5=y<br>
CONFIG_CRYPTO_ECB=y<br>
CONFIG_CRYPTO_CBC=y<br>
CONFIG_CRYPTO_AES=y<br>
CONFIG_ECRYPT_FS=m
</code>
</p>

<p>
Newer versions of the Linux kernel now have a ``Layered filesystems''
submenu under the ``File systems'' menu, where eCryptfs and Unionfs
reside.
</p>

<p>
Make certain that you have loaded all of the crypto modules that you
need to run eCryptfs. This includes <code>ecb</code>,
<code>cbc</code>, <code>md5</code>, and at least one popular symmetric
cipher, like <code>aes</code>.
</p>

<a name="compatibility">

<p><h3>Q. On what filesystems can I expect eCryptfs to function?</h3></p>

<p>
eCryptfs has been well tested on EXT3, EXT4, XFS and it should work well on
other popular local filesystems such as JFS,  ReiserFS, and so
forth. Changes in the 2.6.24 kernel make eCryptfs more functional on
NFS and CIFS, although there is still a little more work to do in
order to make eCryptfs function as well on networked filesystems as it
currently works on local filesystems. This <a
href="https://bugs.launchpad.net/ecryptfs/+bug/277578">bug</a>
tracks the issues around making eCryptfs work on top of NFS, CIFS, Samba
and WebDAV.
</p>

<a name="stack">

<p><h3>Q. Why is the kernel stack such an issue with eCryptfs?</h3></p>

<p>
eCryptfs is a stacked filesystem. This implies that eCryptfs adds on
top of whatever call stack exists with current filesystems. Each
process in the Linux kernel has a fixed maximum stack size (4k+4k or
8k). Some filesystems (such as XFS) push the limit of the stack by
themselves; adding eCryptfs on top may cause a stack overflow on these
filesystems. If you wish to use eCryptfs on XFS, I recommend that you
first perform stress tests to help determine whether your specific
configuration will lead to a kernel process stack overflow.
</p>

<a name="nonroot">

<p><h3>Q. Do I have to be root to mount eCryptfs?</h3></p>

<p>
eCryptfs mounts can be set up to be done by non-root users, using
the <code>ecryptfs-setup-private</code> utility.  The root user
can also setup mount points in <code>/etc/fstab</code>, but the
non-root users will need to manually load their keys into the
kernel keyring.
</p>

<a name="xattr">

<p><h3>Q. How do I store the metadata in the extended attribute region
of the lower file?</h3></p>

<p>
If your kernel has support for it, mount with the <code>-o
xattr</code> option. Be sure to preserve the extended attributes in
the lower files, or you will lose your data. Bear in mind that many
userspace utilities such as <code>tar</code> lack extended attribute
support, and so you need to use utilities like <code>star</code> with
the proper options instead.
</p>

<a name="encryptedview">

<p><h3>Q. I am using the <code>-o xattr</code> option, but my backup
tools do not preserve extended attributes. How can I back up the lower
files?</h3></p>

<p>
Mount with the <code>-o encrypted_view</code> flag and read the files
from under the eCryptfs mount point. The files read will be encrypted,
and the cryptographic metadata will be in the headers of the encrypted
files that are passed through, even if this metadata is actually
stored in the extended attribute regions of the lower files.
</p>

<a name="sparse">

<p><h3>Q. What about sparse files?</h3></p>

<p>
eCryptfs does not currently support sparse files. Sequences of
encrypted extents with all 0's could be interpreted as sparse regions
in eCryptfs without too much implementation complexity. However, this
would open up a possible attack vector, since the fact that certain
segments of data are all 0's could betray strategic information that
the user does not necessarily want to reveal to an attacker. For
instance, if the attacker knows that a certain database file with
patient medical data keeps information about viral infections in one
region of the file and information about diabetes in another section
of the file, then the very fact that the segment for viral infection
data is populated with data at all would reveal that the patient has a
viral infection.
</p>

<a name="passphrase">

<p><h3>Q. How should I select my passphrase?</h3></p>

<p>
There are plenty of good guides out there to help you choose a strong
passphrase. Here is one, for instance: <a
href="http://www.iusmentis.com/security/passphrasefaq/">http://www.iusmentis.com/security/passphrasefaq/</a>.
</p>

<a name="protectkey">

<p><h3>Q. How can I protect my key?</h3></p>

<p>
Make a copy and store it in a physically secure location. For
instance, copy your public/private keypair to a USB flash drive or
write your passphrase onto a sheet of paper. Then, lock the drive and
paper in your desk drawer or put them in a safe deposit box (depending
on the sensitivity of the data that the keys protect). Future versions
of eCryptfs userspace utilities may implement key splitting functions
to provide even more paranoid levels of key protection.
</p>

<p>
Do not store your keys under the same physical security context in
which you are storing your media. It should be much harder for an
attacker to get to your keys than it is for him to get to your media.
</p>

<p>
When you use public key mode and generate a new key using
<code>ecryptfs-manager</code>, the generated key file is the one that
you must back up in order to access your files.
</p>

<p>
When mounting with a new key, I recommend performing a full mount,
creating a new file, unmounting, clearing the user session keyring
(<code>keyctl clear @u</code>), mounting again, and then trying to
access the newly created file. This minimizes the likelihood that you
will mistype a passphrase and create files that you will not be able
to later recover. When mounting in passphrase mode, make sure that the
ecryptfs_sig value matches between mounts. To help avoid the pitfall
of mistyping a passphrase on mount, eCryptfs stores a cache of
previous ecryptfs_sig values and warns the user if a mount passphrase
does not match any passphrases used for previous mounts.
</p>

<a name="lostkey">

<p><h3>Q. I forgot my password/lost my key! What can I do to recover
my data?</h3></p>

<p>
Nothing; you're screwed. (<a
href="http://www.cskk.ezoshosting.com/cs/goodstuff/bs-spc.html">Apologies</a>
to Bruce Schneier).
</p>

<p>
If you have forgotten your passphrase, your only hope is that you
chose a weak passphrase in the first place. There is an outside chance
that you might be able to perform a successful dictionary attack to
recover your passphrase. If you manage to recover your passphrase that
way, then you may as well have not been bothering to encrypt your data
in the first place, since a malicious attacker could have done the
exact same thing to recover your passphrase.
</p>

<p>
If you selected a strong passphrase or lost your key file, you are
completely out of luck. Nobody can help you recover your data.
</p>

<a name="compare">

<p><h3>Q. How does eCryptfs compare with other Linux disk encryption
solutions?</h3></p>

<p>
eCryptfs is an actual filesystem. Some other popular disk encryption
technologies are not filesystems; they are block device encryption
layers (they provide what appears to be a physical block device to
some actual filesystem). There is no filesystem logic in these
layers. A few of the more well-known block device encryption layers
include dm-crypt, Truecrypt, and Loop-AES. Perhaps the best thing
about block device-layer encryption is that it is an order of
magnitude simpler to implement than filesystem-layer
encryption. Another advantage of block device-layer encryption is that
it will encrypt the entire filesystem, including all of the filesystem
metadata. However, for many use cases, this can turn out to be more of
a disadvantage than an advantage.
</p>

<p>
While eCryptfs uses a powerful and flexible approach to protecting
filesystem content, block device-layer encryption technology is still
required to protect swap space and certain databases that use their
own block device partition. The table below provides a
compare-and-constrast of the two technologies. I anticipate that block
device encryption will be the best solution for some people, while
stacked filesystem encryption will be the best solution for
others. Sometimes it even makes sense to use them both together, to
combine the comprehensive full-disk encryption of a block device layer
encryption technology with the transparent per-file encryption
provided by eCryptfs (this will result in double-encryption of the
file contents).
</p>

<br>

<table border=1 cellspacing=5 cellpadding=4>

<tr>
 <td width="50%"><center><b>Block Device Encryption</b></center></td>
 <td width="50%"><center><b>Stacked Filesystem Encryption</b></center></td>
</tr>

<tr>
 <td>Simple in concept and implementation; just transform blocks as
 they pass through.</td>
 <td>High level of design complexity; meticulous handling of internal
 filesystem primitives required.</td>
</tr>

<tr>
 <td>Must allocate a block device to dedicate for the entire
 filesystem.</td>
 <td>Stacks on top of existing mounted filesystems; requires no special
 on-disk storage allocation effort.</td>
</tr>

<tr>
 <td>Everything in the filesystem incurs the cost of encryption and
 decryption, regardless of the confidentiality requirements for the
 data.</td>
 <td>Selective encryption of the contents of only the sensitive
 files.</td>
</tr>

<tr>
 <td>Fully protects the confidentiality of the directory structures,
 superblocks, file sizes, file permissions, and so forth.</td>
 <td>Cannot keep all filesystem metadata confidential. Since stacked
 filesystems encrypt on a per-file basis, attackers will know the
 approximate file sizes, for instance.</td>
</tr>

<tr>
 <td>Coarse granularity; only fixed per-mountpoint encryption policies
 are possible.</td>
 <td>Fine granularity; flexible per-file encryption policies are
 possible.</td>
</tr>

<tr>
 <td>No notion of ``encrypted files.'' Individual files must be
 re-encrypted via a userspace application before written to backups,
 sent via email, etc.</td>
 <td>Individual encrypted files can be accessed transparently by
 applications; no additional work needed on the part of applications
 before moving the files to another location.</td>
</tr>

<tr>
 <td>Clients cannot use directly on networked filesystems; encryption
 must be set up and managed on the server, or the client must encase
 all of his files in a loopback mount, losing the per-file granularity
 from the perspective of other clients.</td>
 <td>Clients can stack on locally mounted networked filesystems;
 individual files are sent to the server and stored in encrypted
 form.</td> </tr>
<tr>
 <td>Can protect databases that use their own dedicated block device.</td>
 <td>Can only protect databases that write their tables to regular
 files in an existing filesystem.</td>
</tr>

<tr>
 <td>Used to protect swap space.</td>
 <td>Not designed to protect swap space; we recommend using block
 device encryption to protect swap space while using eCryptfs on the
 filesystem.</td>
</tr>

<tr>
 <td>Possible to hide the fact that the partition is encrypted.</td>
 <td>The fact that encrypted data exists on the device is obvious to an
 observer.</td>
</tr>

<tr>
 <td>Filesystem-agnostic; any filesystem will work on an encrypted
 block device.</td>
 <td>Can only be expected to work with existing filesystems that are
 upstream in the official Linux kernel.</td>
</tr>

</table>

<p>
EncFS is another popular cryptographic filesystem that behaves much
like a stacked filesystem. EncFS is a userspace filesystem, and so
individual page reads and writes require additional context switches
between kernel and userspace. One advantage a userspace cryptographic
filesystem is that it is possible to use symmetric ciphers implemented
in userspace libraries, but the frequent context switching impacts
performance.
</p>

<a name="no-ecryptfsac">

<p><h3>Once one user can access an eCryptfs file, any users with
permission can also access the file. Should not eCryptfs require all
users to have the key in order to access the files?</h3></p>

<p>
eCryptfs deliberately makes no attempt to re-implement the
discretionary and mandatory access control mechanisms already present
in the Linux kernel. eCryptfs will simply require that a File
Encryption Key (FEK) be associated with any given inode in order to
decrypt the contents of the file on disk. This prevents an attacker
from accessing the file contents outside the context of the trusted
host environment; for instance, by removing the storage device or by
booting a live CD. This is the only type of unauthorized access that
eCryptfs is intended to prevent.
</p>

<p>
Once eCryptfs has associated that FEK with the inode, it does not
impose any additional restrictions on who or what can access the
files, deferring to the standard user/group/other permissions,
capabilities, SE Linux type enforcement, and so forth to regulate
access to the files. eCryptfs maintains no pedigree regarding how the
FEK found its way to the inode, so it has no way of knowing that any
particular UID should or should not be able to open the file, nor
should eCryptfs do such a thing.
</p>

<p>
Having eCryptfs impose additional access control onto the decrypted
file contents in a trusted host environment would provide no
additional security while introducing unintended usability issues. For
instance, a user may wish to share his decrypted files with certain
other users on the system without having to share his key with them or
add their keys to a set of keys wrapping the inode's FEK. Users expect
to be able to accomplish such a task via users, groups, capabilities,
and types, and eCryptfs defers access control decisions on trusted
host environments to these existing access control mechanisms.
</p>

<a name="initcipher">

<p><h3>Q. &quot;<code>Unable to allocate crypto cipher with name
[---]; rc = [-2]</code>&quot;</h3></p>

<p>
Make sure that you have enabled the kernel crypto API and that you
have built the ciphers, hashes, and chaining modes that you wish to
use. This will usually be md5, aes, cbc, and ecb. Also, make sure that
the requested key size is valid for your cipher.
</p>

<a name="baddir">

<p><h3>Q. &quot;<code>Error mounting eCryptfs; rc = [-2]; strerr = [No
such file or directory]</code>&quot;</h3></p>

<p>
Make sure that both the source and destination directories that you
provide to the mount command exist.
</p>

<a name="einval">

<p><h3>Q. &quot;<code>Error mounting eCryptfs; rc = [-22]; strerr =
[Invalid argument]</code>&quot;</a></h3></p>

<p>
Check your system log for the real problem.
</p>

<a name="keyproblem">

<p><h3>Q. &quot;<code>ecryptfs_parse_options: Could not find key with
description: [deadbeaf...]&quot;</code></h3></p>

<p>
If the mount fails and the message &quot;<code>ecryptfs_parse_options:
Could not find key with description: [deadbeaf...]&quot;</code> is in
your system logs, then there was a problem inserting your mount key
into your kernel <i>user session</i> keyring.
</p>

<p>
After a mount attempt, run <code>keyctl_show</code>. You should see
something like this:
</p>

<pre>
# keyctl show
Session Keyring
       -3 lswrv----------     0     0  keyring: _uid_ses.0
        2 lswrv----------     0     0   \_ keyring: _uid.0
892244932 lswrv----------     0     0       \_ user: deadbeef...
</pre>

<p>
Where <code>deadbeef</code> is the signature that corresponds with
your key. If you don't see this, then there is a problem with your
keyring setup.
</p>

<p>
If you su to root, be sure to initiate the session by using the -
flag.
</p>

<p>
Finally, try linking your user keyring into your active session
keyring:
</p>

<pre>
# keyctl link @u @s
</pre>

<a name="sigsize">

<p><h3>Q. &quot;<code>ecryptfs_parse_packet_set: Expected
signature of size [8]; read size [7]</code>&quot;</h3></p>

<p>
Older versions of eCryptfs shipping in older kernels had a minor bug
where eCryptfs would only write out and read in 7 of the 8 key
signature characters to the metadata of the lower file. This violates
the eCryptfs spec, so newer versions of eCryptfs correct this bug and
refuse to read files that do not conform to the spec.
</p>

<p>
The current correctly implemented version of eCryptfs cannot read
files created with the early nonconformant and buggy release. If you
have any files created with the earlier version, you will need to boot
with the earlier version of eCryptfs and copy the decrypted files to
secure location (e.g., a loopback mount image protected with
dm-crypt). You will then need to copy the data from the secure
location into an eCryptfs mount using the most recent kernel release.
</p>

<p>
Note that the Versions of eCryptfs from 2.6.24 and on will be able to
read files created by earlier versions, back through to 2.6.24, as
indicated in the ecryptfs-utils package README file:
</p>

<p>
<i>
eCryptfs is still in a developmental stage. When you upgrade the
eCryptfs kernel module, it is possible that the eCryptfs file format
has been updated. For this reason you should copy your files to an
unencrypted location and then copy the files back into the new
eCryptfs mount point to migrate the files.  File format version 3 and
beyond (in kernel version 2.6.24) is expected to remain readable,
however.
</i>
</p>

<a name="nothere">

<p><h3>Q. My question isn't answered here.</h3></p>

<p>
Ask a question on <a
href="http://stackexchange.com/search?q=ecryptfs">StackExchange</a>.
File a bug on <a
href="https://bugs.launchpad.net/ecryptfs">Launchpad</a>.
Discuss an issue on the <a
href="http://vger.kernel.org/vger-lists.html#ecryptfs">mailing list</a>.
</p>

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