eCryptfs: a Stacked Cryptographic Filesystem

As with any filesystem, you should make regular backups of your data when using eCryptfs. This is done easily and securely by unmounting eCryptfs and reading the lower encrypted files.

eCryptfs protects only the confidentiality of data at rest that is outside the control of the trusted host environment. You should use access control mechanisms properly, such as SELinux on the trusted host in order to regulate access to the decrypted files.

eCryptfs will, by default, preserve all of the information necessary to access the decrypted contents of the files in the contents of the lower files themselves. All that is required is the key used to create the files in the first place. You should take measures to protect this key. If applications, such as incremental backup utilities, are configured to read only the lower encrypted files, these utilities do not need to apply any further encryption to the files in order to ensure data confidentiality.

If you are using a passphrase, follow common best practices in selecting and protecting your passphrase (for instance, see www.iusmentis.com/security/passphrasefaq). I recommend using the public key mode of operation instead of passphrase mode whenever possible. When using a public key module, make a backup copy of your key file and store it in a physically secure location. Should you lose your key, nobody will be able to retrieve your data. Do not store unprotected copies of your passphrase or your public key file on the same media as your encrypted data.

You are free to choose among the symmetric encryption ciphers that are available through the Linux kernel cryptographic API. eCryptfs recommends AES-128 as the default cipher. If you have hardware acceleration available on your machine, and if it is supported by the selected cipher in the kernel cryptographic API, eCryptfs encryption and decryption operations will be hardware-accelerated automatically.

You should take measures to ensure that sensitive data is not written to secondary storage in unencrypted form. Applications that write out sensitive temporary data should be configured so that they write only under an eCryptfs mountpoint. You also should use dm-crypt to encrypt the swap space with a random key. The details are beyond the scope of this article, but commands to set it up take the following form:

# cryptsetup -c aes-cbc-plain -d /dev/random create \
  swap /dev/SWAPDEV
# mkswap /dev/mapper/swap
# swapon -p 1 /dev/mapper/swap

SWAPDEV is the swap block device on your machine (refer to your /etc/fstab file if you are not sure which device currently is used for swap). You can create simple boot scripts to set up the encrypted swap space automatically, run ecryptfsd and perform eCryptfs mounts. Consult your distribution's documentation for more details on writing boot scripts and using dm-crypt with a random key to encrypt your swap space.

Note that current releases of eCryptfs encrypt only the file contents. Metadata about the file—for instance, the size, the name, permissions and extended attributes—are all readable by anyone with access to the lower encrypted file. Future work on eCryptfs will include encryption or obfuscation of some of this metadata.

Using block device encryption together with eCryptfs can combine the security provided by both mechanisms while offering the flexibility of having seamless access to individual encrypted lower files, although this roughly doubles the processing overhead of encrypting and decrypting the data. If only the contents of the files on secondary storage require confidentiality, eCryptfs by itself is, in most cases, sufficient.

eCryptfs was designed to support a host of advanced key management and policy features. The development road map for eCryptfs includes multiple keys per file, different keys and ciphers for different files depending on the application creating the file and the location where the file is being written, integrity enforcement and more extensive interoperability with existing key infrastructures and key management devices. These features will become available as they are implemented in future versions of the Linux kernel.

eCryptfs is a flexible kernel-native solution that cryptographically enforces data confidentiality on secondary storage devices. eCryptfs can be deployed on existing filesystems with minimal effort. The individual encrypted files can be transferred to other hosts running eCryptfs and accessed transparently using the proper key. The eCryptfs key management mechanism is highly extensible. eCryptfs is suitable to use as a strong and convenient data-confidentiality enforcement component to help secure data managed in Linux environments.