FileVault is a method of using encryption with volumes on Mac
computers. Encryption and decryption are performed on the fly.
Versions and key features FileVault was introduced with Mac OS X
Panther, and could only be applied to a user's home directory, not the startup
volume. The operating system uses an encrypted sparse disk image to present a
volume for the home directory. Mac OS X Leopard and Mac OS X Snow Leopard use
more modern sparse bundle disk images which spread the data over 8 MB files
within a bundle. OS X Lion and newer offer FileVault 2,
which is a significant redesign. This encrypts the entire OS X startup volume
and typically includes the home directory, abandoning the disk image
approach. For this approach to disk encryption, authorised users'
information is loaded from a separate non-encrypted boot volume.
Apple refers to the original iteration of FileVault as legacy FileVault.
FileVault The original version of FileVault was
added in OS X Panther to encrypt a user's home directory.
= Master passwords and recovery keys = When FileVault is enabled the system
invites the user to create a master password for the computer. If a user
password is forgotten, the master password or recovery key may be used to
decrypt the files instead. = Migration =
Migration of FileVault home directories is subject to two limitations:
there must be no prior migration to the target computer
the target must have no existing user accounts.
If Migration Assistant has already been used or if there are user accounts on
the target: before migration, FileVault must be
disabled at the source. If transferring FileVault data from a
previous Mac that uses 10.4 using the built-in utility to move data to a new
machine, the data continues to be stored in the old sparse image format, and the
user must turn FileVault off and then on again to re-encrypt in the new sparse
bundle format. = Manual encryption =
Instead of using FileVault to encrypt a user's home directory, using Disk
Utility a user can create an encrypted disk image themselves and store any
subset of their home directory in there. This encrypted image behaves similar to
a Filevault encrypted home directory, but is under the user's maintenance.
Encrypting only a part of a user's home directory might be problematic when
applications need access to the encrypted files, which will not be
available until the user mounts the encrypted image. This can be mitigated
to a certain extent by making symbolic links for these specific files.
= Limitations and issues = Backups
These limitations apply to versions of Mac OS X prior to v10.7 only.
Without Mac OS X Server, Time Machine will back up a FileVault home directory
only while the user is logged out. In such cases, Time Machine is limited to
backing up the home directory in its entirety. Using Mac OS X Server as a
Time Machine destination, backups of FileVault home directories occur while
users are logged in. Because FileVault restricts the ways in
which other users' processes can access the user's content, some third party
backup solutions can back up the contents of a user's FileVault home
directory only if other parts of the computer are excluded.
Issues Several shortcomings were identified in
Legacy FileVault. Its security can be broken by cracking either 1024-bit RSA
or 3DES-EDE. Legacy FileVault used the CBC mode of
operation; FileVault 2 uses stronger XTS-AESW mode. Another issue is storage
of keys in the Mac OS X "safe sleep" mode. A study published in 2008 found
data remanence in dynamic random access memory, with data retention of seconds
to minutes at room temperature and much longer times when memory chips were
cooled to low temperature. The study authors were able to use a cold boot
attack to recover cryptographic keys for several popular disk encryption systems,
including FileVault, by taking advantage of redundancy in the way keys are stored
after they have been expanded for efficient use, such as in key
scheduling. The authors recommend that computers be powered down, rather than
be left in a "sleep" state, when not in physical control by the owner.
Early versions of FileVault automatically stored the user's
passphrase in the system keychain, requiring the user to notice and
manually disable this security hole. In 2006, following a talk at the 23rd
Chaos Communication Congress titled Unlocking FileVault: An Analysis of
Apple's Encrypted Disk Storage System, Jacob Appelbaum & Ralf-Philipp Weinmann
released VileFault which decrypts encrypted Mac OS X disk image files.
FileVault 2 = Security =
FileVault uses the user's login password as the encryption pass phrase. It uses
the AES-XTS mode of AES with 128 bit blocks and a 256 bit key to encrypt the
disk, as recommended by NIST. Only unlock-enabled users can start or unlock
the drive. Once unlocked, other users may also use the computer until it is
shut down. = Performance =
The I/O performance penalty for using FileVault 2 was found to be in the order
of 20 – 30% when using CPUs with the AES instruction set, such as the Intel Core
i. Performance deterioration will be larger for CPUs without this instruction
set, such as older Core CPUs. = Master passwords and recovery keys =
When FileVault 2 is enabled while the system is running, the system creates
and displays a recovery key for the computer, and optionally offers the user
to store the key with Apple. The 120 bit recovery key is encoded with all letters
and numbers 1 through 9, and read from random, and therefore relies on the
security of the PRNG used in OS X. During a cryptanalysis in 2012, this
mechanism was found safe. Changing the recovery key is not
possible without re-encrypting the Filevault volume.
= Validation = Users who use FileVault 2 in OS X 10.9
and above can validate their key correctly works after encryption by
running sudo fdesetup validaterecovery in Terminal after encryption has
finished. The key must be in form xxxx-xxxx-xxxx-xxxx-xxxx-xxxx and will
return true if correct. Starting the OS with FileVault 2 without
a user account If a volume to be used for startup is
erased and encrypted before clean installation of OS X 10.7.4 or 10.8:
there is a password for the volume the clean system will immediately behave
as if FileVault was enabled after installation
there is no recovery key, no option to store the key with Apple
when the computer is started, Disk Password will appear at the EfiLoginUI –
this may be used to unlock the volume and start the system
the running system will present the traditional login window.
Apple describes this type of approach as Disk Password—based DEK.
See also Apple Keychain
BitLocker Truecrypt
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^ "Archived - Mac OS X 10.3, 10.4: Transferring data with Setup Assistant /
Migration Assistant FAQ". Apple support. Apple. Retrieved January 21, 2013.
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CrashPlan support. CrashPlan. Retrieved January 21, 2013.
^ Jacob Appelbaum, Ralf-Philipp Weinmann. "Unlocking FileVault: An
Analysis of Apple's disk encryption". Retrieved March 31, 2007.
^ J. Alex Halderman; et al.. "Lest We Remember: Cold Boot Attacks on
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Apple's disk encryption system". ^ a b Apple, Inc. "Best Practices for
Deploying FileVault 2". Apple, Inc. p. 28. Retrieved September 5, 2012.
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