On-line Encrypted Backups for Your Laptop

 in
Protect your laptop files with FUSE and an on-line storage service.

Or:


tar xzvf rlog-1.3.7.tgz
cd rlog-1.3.7
./configure && make
make install
cd ..
tar xzvf encfs-1.3.2-1.tgz
cd encfs-1.3.2
./configure && make
make install

The first time you attempt to mount a raw filesystem to an encrypted filesystem, EncFS will ask you what level of cryptography you desire and what passphrase to use. The same command is used to create an encrypted filesystem and to mount one. Subsequent mounts of the raw filesystem with EncFS will prompt you only for the passphrase. Initial mounting and remounting of EncFS on a rawfs (backed at the time by sshfs) is shown here:

$ encfs ~/rawfs ~/backupfs
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... will select standard mode.
?> 

Standard configuration selected.

Configuration finished.  
The filesystem ... has the following properties:
Filesystem cipher: "ssl/blowfish", version 2:1:1
Filename encoding: "nameio/block", version 3:0:1
Key Size: 160 bits
Block Size: 512 bytes
Each file contains 8 byte header with unique IV data
Filenames encoded using IV chaining mode.

Now you will need to enter a password ...
You will need to remember this password, ...
no recovery mechanism.  
However, the password can be changed
later using encfsctl.

New Encfs Password: 
Verify Encfs Password: 
$ date > backupfs/datetest.txt
$ cat backupfs/datetest.txt
Fri Aug 24 20:44:33 EDT 2007
$ ls -l rawfs
total 4
-rw-rw---- 1 ben 505 37 2007-08-24 06:27 K9dmA...
$ fusermount -u backupfs
$ encfs ~/rawfs ~/backupfs
EncFS Password: 
$ ls -l ~/backupfs
-rw-rw---- 1 ben 505 29 2007-08-24 06:27 datetest.txt

Performing the Backup: Synchronization

We now have a ~/backupfs filesystem that encrypts anything written to it and stores it on an on-line storage system somewhere. A great tool for keeping your on-line backup up to date is rsync(1).

The rsync manual page states: “The rsync remote-update protocol allows rsync to transfer just the differences between two sets of files across the network connection.”

In our case, both the data to be backed up and the place to which we are backing up appear through the Linux kernel. Because ~/backupfs needs to read and write to the Internet, we very much would like to limit the amount of data that is written to it.

Some differences between a normal Linux kernel filesystem like ext3 and our layered setup might have to be worked around with command-line options to rsync. Listing 3 shows an rsync on an EncFS, which is using sshfs to provide the on-line storage. The first time rsync is run, the whole file is uploaded to the on-line storage. The second time, only some metadata is sent and received.

The -a option to rsync is similar to the -a option to the cp command; it attempts to preserve everything in the source filesystem at the destination. The --no-g command-line option to rsync tells it not to try to sync the destination file's group to the source file's group. In this case, the sshfs does not allow me to change the group of the destination file, so rsync would generate a warning when it failed to set the remote file's group. The --delete-after cleans up any files that exist only in the on-line storage filesystem. In this case, I also use --include to sync only the plain-text files. This can be quite handy for keeping backups of only OpenOffice.org documents in a larger filesystem.

Another rsync option that can be invaluable is --modify-window=n, where the parameter n is the number of seconds that the two timestamps can differ between the local and remote files and still be considered the same. When using a filesystem showing on-line storage, the modification time might range from not being perfectly accurate to being a few days off. Setting the --modify-window correctly can hide these slight timestamp drifts or large fixed timestamp offsets and allow rsync to continue to work efficiently.

Conclusion

Running EncFS on top of OmniFS requires some special parameters when first mounting the EncFS. The main issue I found with using the default settings for EncFS was that file contents, when read back, would sometimes have trailing garbage. When using OmniFS and first creating the EncFS, choose expert mode, cipher=AES, keysize=256, blocksize=4096, filename encoding=Stream, filename IV chaining=Yes, per-file IV=no and block authentication code headers=no. The main issues seem to stem from the per file IV settings and something going missing with the round-trip latency of OmniFS. Listing 4 shows some combinations of expert mode settings to EncFS when using OmniFS as the base filesystem and the resulting filesystem interaction.

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