eCryptfs: A stacked cryptographic filesystem for Linux

eCryptfs is free software. Please see the file COPYING for details.
For documentation, please see the files in the doc/ subdirectory.  For
building and installation instructions please see the INSTALL file.

Lead developers:
	Tyler Hicks (kernel)
	Dustin Kirkland (userspace)
Contributors:
	Michael Halcrow
	Michael C. Thompson
	Kent Yoder
	Trevor Highland
	Theresa Nelson
Former maintainers:
	Phillip Hellewell
	Michael Halcrow
Web Site: http://ecryptfs.org/

As you should do with any filesystem, make sure to maintain a backup
copy of any data you write into eCryptfs.  In addition, you should
reliably store your secret keys in a secure location.  eCryptfs uses
industry-standard cryptographic ciphers, key generation, and
passphrase protection mechanisms; without your salt/passphrase or
private key, nobody will be able to retrieve your data.

eCryptfs requires the userspace tools available at:

<http://ecryptfs.org/downloads.html>

Requirements include:
 - Kernel version 2.6.19 or above
  - Build crypto API support with at least one symmetric key cipher
  - Build key retention support
  - If you have an older kernel, try the latest eCryptfs full package
    release to build a stand-alone kernel module
 - Userspace tools obtainable from the site listed above; requirements
   for the userspace tools are:
  - keyutils
   - David Howells' userspace keyring headers and libraries (version
     1.0 or higher)
   - Finding its way into some distros
   - Obtainable from <http://people.redhat.com/~dhowells/keyutils> 


KERNEL BUILD OPTIONS

Code maturity level options  --->
  [*] Prompt for development and/or incomplete code/drivers

Security options  --->
  <M> Enable access key retention support

Cryptographic options  --->
  <M>   CBC support
  <M>   ECB support
  <M>   MD5 digest algorithm
  <M>   AES cipher algorithms

File systems  --->
  Miscellaneous filesystems  --->
    <M> eCrypt filesystem layer support (EXPERIMENTAL)


NOTES

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.


BUILD AND INSTALL INSTRUCTIONS

1) Consult the INSTALL file. In the majority of cases, this step
   should involve the standard procedure for Linux packages:
     % ./configure --prefix=/usr && make && make install

2) Make sure that you have sysfs mounted and that a valid entry for
   the sysfs mount point is in your /etc/mtab file.


INTERACTIVE MOUNTING

Create a new directory into which eCryptfs will write its encrypted
files (i.e., /root/crypt).  Then, create the mount point directory
(i.e., /mnt/crypt).  Here is an example of how you might mount
eCryptfs:

mount -t ecryptfs /root/crypt /mnt/crypt

Note that you can also perform a layover mount (essentially,
converting an existing path into an eCryptfs mount point) to reduce
the likelihood of contention between eCryptfs and other applications
for access to the lower files; this is generally the recommended way
to mount eCryptfs:

mount -t ecryptfs /secret /secret

Navigate the menus to select your preferred mount options.


NON-INTERACTIVE MOUNTING

eCryptfs supports mounting through command-line options. You can do so
by passing groups of name=value attribute pairs as a -o option to the
mount helper.  Each key type has a set of name=value pairs associated
with it.  For instance, the ``passphrase'' key type can accept a
``passwd=XXX'' name=value pair and a ``salt=XXX'' name=value pair.
Attributes that apply to a particular key type immediately follow the
key type specifier ``key=XXX'' and are separated by colons.
Attributes that do not apply to any specific key type (general
attributes) are separated by comma's.  The option string will take the
form:

 name1=value1,key=alias:name2=value2:name3=value3,name4=value4,name5=value5

name1=value1, name4=value4, and name5=value5 are all generic global
attributes that can apply to any key type.  name2=value2 and
name3=value3 are specific to the key type.

A BNF grammar for the option set follows:

<NULL> ::=
<letter> ::= (a | b | c | ... | z)
<letter-string> ::= <letter> (<letter-string> | <NULL>)
<character> ::= <any printable ASCII character that is not , or :>
<character-elem> ::= (<character> | <NULL>)
<character-string> ::= <character-elem> (<character-string> | <NULL>)
<name> ::= <letter-string>
<value> ::= <character-string>
<attr> ::= <name> "=" <value>
<comma-attr-list> ::= <attr> ("," <comma-attr-list> | <NULL>)
<colon-attr-list> ::= <attr> (":" <colon-attr-list> | <NULL>)
<key-attr-list> ::= "key=" <letter-string> (<colon-attr-list> | <NULL>)
<attr-list> ::= (<comma-attr-list> | <key-attr-list>)
<options-set> ::= <attr-list> (<attr-list> | <NULL>)

Values read from a file should be specified with name value pairs. For
example, a passphrase may be specified in a file by adding the following
line to the file:

passwd=<passphrase>

Currently supported aliases and attributes parsed by the mount helper include:

 key=passphrase:
     passphrase_passwd=<passphrase>
     passphrase_passwd_file=/path/to/file
     passphrase_passwd_fd=<file descriptor>
     passphrase_salt=<salt value>

 key=openssl:
     openssl_keyfile=/path/to/key
     openssl_passwd=<passphrase>
     openssl_passwd_file=/path/to/file
     openssl_passwd_fd=<file descriptor>

The following general attributes are valid kernel mount options,
and may also be acted upon by the mount helper, mount.ecryptfs:

 ecryptfs_cipher=<cipher>
        Currently supported ciphers include:
         aes
         blowfish
         des3_ede
         cast5
         cast6
         twofish

 ecryptfs_key_bytes=<key bytes>
	With the exception of AES-192, eCryptfs requires that the
	keysize be a multiple of the block size.

 ecryptfs_sig=<hex signature>
	The signature for the FEKEK to use to encrypt the FEK for newly
	created files.  A key with description <sig> should be in the
	user's session keyring.  The mount helper, with its key modules,
	should make sure that the key gets placed there.

 ecryptfs_passthrough
	Allows for non-eCryptfs files to be read and written from within an
	eCryptfs mount. This option is disabled by default.

 ecryptfs_xattr_metadata
	When set, newly created files will have their cryptographic
	metadata stored in the extended attribute region of the file rather
	than the header (requires kernel support).

 ecryptfs_encrypted_view
	When set, this option causes eCryptfs to present applications a
	view of encrypted files as if the cryptographic metadata were
	stored in the file header, whether the metadata is actually stored
	in the header or in the extended attributes.

 ecryptfs_hmac
        When set, eCryptfs will include HMAC integrity enforcement to
        its files (requires kernel support).

 ecryptfs_debug=<value>
	A value greater than 0 enables kernel debug messages, see NOTES
	below.

The following general attributes are parsed only by the mount helper,
mount.ecryptfs:

 no_sig_cache
        Do not perform any key signature cache checks. By default,
        when mounting, the eCryptfs mount helper will look for the key
        signature in <$HOME/.ecryptfs/sig-cache.txt>. If the key
        signature is not found, then the user will get a warning and
        will be prompted on whether to continue and whether to add the
        new key signature to the sig-cache.txt file. This behavior can
        be suppressed with the no_sig_cache option.

If you wish to have the same passphrase used in previous passphrase
mounts and store it in a file (*not* recommended unless you can
provide sufficient protection of the file itself), you can take the
following steps:

(Previous mount; specify passphrase on command line)
mount -t ecryptfs /mnt/dev /mnt/dir -o key=passphrase:passphrase_passwd=my_passphrase

(Next mount; have passphrase read from a file)
echo "passphrase_passwd=my_passphrase" > /mnt/secureusb/my_passphrase
mount -t ecryptfs /mnt/dev /mnt/dir -o key=passphrase:passphrase_passwd_file=/mnt/secureusb/my_passphrase

Saving your unencrypted passphrase to a file on the same disk that
contains your encrypted files defeats the purpose of using a
cryptographic filesystem in the first place.  You should instead store
the file that contains your passphrase on a physically secure medium,
such as a USB flash drive that you keep locked in a drawer, if you
choose to store it to a file at all.

In general, it is probably best to just type in your passphrase via
stdin every time you need to perform a mount.  Future versions of
eCryptfs will allow hardware token devices, such as a TPM chip, to
protect your secret keys.


TESTING A NEW MOUNT POINT

Try writing a new file:

echo "Hello, World" > /secret/hello.txt

The operation will complete.  When you unmount eCryptfs, you will
Notice that there is a new file in /secret that is at least 12288
bytes in size (depending on your host page size).  This is the
encrypted underlying file for what you just wrote.  To test reading,
from start to finish, you need to clear the user session keyring:

keyctl clear @u

Then umount /secret and mount again per the instructions given above.

cat /secret/hello.txt


PAM MODULE

You can use the PAM module to automatically use a key based on your
login passphrase, which can then be used to perform an eCryptfs mount
non-interactively.

Perform an eCryptfs mount as root, using your user login:

root# mount -t ecryptfs /secret /secret

Then, grab your raw mount parameters from 

root# grep "ecryptfs" /etc/mtab

You should get something like this:

---
/secret /secret ecryptfs rw,ecryptfs_sig=deadbeefdeadbeef,ecryptfs_key_bytes=16,ecryptfs_cipher=aes 0 0
---

Add ``user'' and ``noauto'' to the mount options:

---
/secret /secret ecryptfs user,noauto,rw,ecryptfs_sig=deadbeefdeadbeef,ecryptfs_key_bytes=16,ecryptfs_cipher=aes 0 0
---

Append your edited line to your /etc/fstab file. Now it's time to test
the mount.

root# umount /secret

Verify that eCryptfs is not mounted. Then, log in as the regular
user. Manually add your passphrase to the user session keyring via the
ecryptfs-manager utility. Then, perform the mount, while telling the
mount application to not call the mount helper:

user# mount -i /secret

Verify that eCryptfs mounted correctly. Then, unmount:

user# umount /secret

Clear your user session keyring:

user# keyctl clear @u

Add the mount command into your login script (e.g., ~/.bash_profile):

---
mount -i /secret
---

Finally, add this to the appropriate /etc/pam* file (e.g.,
/etc/pam.d/login) after the call to pam_unix.so:

---
auth required pam_ecryptfs.so
---

You will need to insert this intelligently into your PAM stack so that
PAM actually calls pam_ecryptfs.so. Depending on your distribution,
this may involve, for instance, changing the context of pam_unix.so
from ``sufficient'' to ``required'' and moving pam_deny.so to another
location in the stack. Be mindful of the security implications of any
changes you make to your PAM stack.

From another window, try logging in as the user. If all went well, the
eCryptfs PAM module will insert a key derived from your login
passphrase into the user session keyring. Then, the login script will
perform the mount, using the parameters in your /etc/fstab.

The script src/utils/ecryptfs-setup-pam.sh attempts to automate
setting up PAM mounts on Red Hat-based distros. Use that for
inspiration in setting up your own eCryptfs PAM mount.


NOTES

eCryptfs shipping in kernel version 2.6.19 does not support public
key.  To determine what your current kernel supports, load the
ecryptfs module and view the contents of fs/ecryptfs/version_str under
your sysfs mount point.

Do not run eCryptfs in higher verbosity levels unless you are doing so
for the sole purpose of development, since secret values will be written
out to the system log in that case.


TROUBLESHOOTING

For the most up-to-date support resources, see
<http://ecryptfs.org/support.html>.


BUGS

Please send bug reports to the Launchpad bug tracker:
 * https://bugs.launchpad.net/ecryptfs/+filebug

For kernel bugs, please follow the procedure
detailed in the kernel documentation (Documentation/oops-tracing.txt)
to help us figure out what is happening.


Mike Halcrow
[email protected]