Chapter 12. Encrypted File System

12.1. Mounting a File System as Encrypted
12.2. Additional Information

Red Hat Enterprise Linux 6 now supports eCryptfs, a "pseudo-file system" which provides data and filename encryption on a per-file basis. The term "pseudo-file system" refers to the fact that eCryptfs does not have an on-disk format; rather, it is a file system layer that resides on top of an actual file system. The eCryptfs layer provides encryption capabilities.

eCryptfs works like a bind mount, as it intercepts file operations that write to the underlying (i.e. encrypted) file system. The eCryptfs layer adds a header to the metadata of files in the underlying file system. This metadata describes the encryption for that file, and eCryptfs encrypts file data before it is passed to the encrypted file system. Optionally, eCryptfs can also encrypt filenames.

eCryptfs is not an on-disk file system; as such, there is no need to create it via tools such as mkfs. Instead, eCryptfs is initiated by issuing a special mount command. To manage file systems protected by eCryptfs, the ecryptfs-utils package must be installed first.

12.1. Mounting a File System as Encrypted

The easiest way to encrypt a file system with eCryptfs and mount it is interactively. To start this process, execute the following command:

mount -t ecryptfs /source /destination

Encrypting a directory heirarchy (i.e. /source) with eCryptfs means mounting it to a mount point encrypted by eCryptfs (i.e. /destination). All file operations to /destination will be passed encrypted to the underlying /source file system. In some cases, however, it may be possible for a file operation to modify /source directly without passing through the eCryptfs layer; this could lead to inconsistencies.

This is why for most environments, Red Hat recommends that both /source and /destination be identical. For example:

mount -t ecryptfs /home /home

This effectively means encrypting a file system and mounting it on itself. Doing so helps ensure that all file operations to /home pass through the eCryptfs layer.

During the interactive encryption/mount process, mount will allow the following settings to be configured:

Encryption key type; openssl, tspi, or passphrase. When choosing passphrase, mount will ask for one.
Cipher; aes, blowfish, des3_ede, cast6, or cast5.
Key bytesize; 16, 32, 24
Whether or not plaintext passthrough is enabled
Whether or not filename encryption is enabled

After the last step of an interactive mount, mount will display all the selections made and perform the mount. This output consists of the command-line option equivalents of each chosen setting. For example, mounting /home with a key type of passphrase, aes cipher, key bytesize of 16 with both plaintext passthrough and filename encryption disabled, the output would be:

Attempting to mount with the following options:
  ecryptfs_unlink_sigs
  ecryptfs_key_bytes=16
  ecryptfs_cipher=aes
  ecryptfs_sig=c7fed37c0a341e19
Mounted eCryptfs

The options in this display can then be passed directly to the command line to encrypt and mount a file system using the same configuration. To do so, use each option as an argument to the -o option of mount. For example:

mount -t ecryptfs /home /home -o ecryptfs_unlink_sigs \ ecryptfs_key_bytes=16 ecryptfs_cipher=aes ecryptfs_sig=c7fed37c0a341e19^[3]

^[3] This is a single command split into multiple lines, to accommodate printed and PDF versions of this document. All concatenated lines — preceded by the backslash (\) — should be treated as one command, sans backslashes.