fscrypt

module
Version: v0.3.3 Latest Latest
Warning

This package is not in the latest version of its module.

Go to latest
Published: Feb 23, 2022 License: Apache-2.0

README

fscrypt GitHub version

Build Status GoDoc Go Report Card License

fscrypt is a high-level tool for the management of Linux native filesystem encryption. fscrypt manages metadata, key generation, key wrapping, PAM integration, and provides a uniform interface for creating and modifying encrypted directories. For a small low-level tool that directly sets policies, see fscryptctl.

Note that the kernel part of fscrypt (which is integrated into filesystems such as ext4) is also sometimes referred to as "fscrypt". To avoid confusion, this documentation calls the kernel part "Linux native filesystem encryption".

To use fscrypt, you must have a filesystem with encryption enabled and a kernel that supports reading/writing from that filesystem. Currently, ext4, F2FS, and UBIFS support native filesystem encryption. Ext4 has supported native filesystem encryption since v4.1, F2FS added support in v4.2, and UBIFS added support in v4.10. Other filesystems may add support for native encryption in the future. Filesystems may additionally require certain kernel configuration options to be set to use native encryption. See Runtime dependencies.

Table of contents

Alternatives to consider

Operating-system level storage encryption solutions work at either the filesystem or block device level. Linux native filesystem encryption (the solution configured by fscrypt) is filesystem-level; it encrypts individual directories. Only file contents and filenames are encrypted; non-filename metadata, such as timestamps, the sizes and number of files, and extended attributes, is not encrypted. Users choose which directories will be encrypted, and with what keys.

Before using fscrypt, you should consider other solutions:

  • dm-crypt/LUKS is block device level encryption: it encrypts an entire block device (and hence an entire filesystem) with one key. Unlocking this key will unlock the entire block device. dm-crypt/LUKS is usually configured using cryptsetup.

  • eCryptfs is an alternative filesystem-level encryption solution. It is a stacked filesystem, which means it sits on top of a real filesystem, rather than being directly integrated into the real filesystem. Stacked filesystems have a couple advantages (such as working on almost any real filesystem), but also some significant disadvantages. eCryptfs is usually configured using ecryptfs-utils.

  • The ZFS filesystem supports encryption in its own way (not compatible with fscrypt). ZFS encryption has some advantages; however, ZFS isn't part of the upstream Linux kernel and is less common than other filesystems, so this solution usually isn't an option.

Which solution to use? Here are our recommendations:

  • eCryptfs shouldn't be used, if at all possible. eCryptfs's use of filesystem stacking causes a number of issues, and eCryptfs is no longer actively maintained. The original author of eCryptfs recommends using Linux native filesystem encryption instead. The largest users of eCryptfs (Ubuntu and Chrome OS) have switched to dm-crypt or Linux native filesystem encryption.

  • If you need fine-grained control of encryption within a filesystem, then use fscrypt, or fscrypt together with dm-crypt/LUKS. If you don't need this, then use dm-crypt/LUKS.

    To understand this recommendation: consider that the main advantage of fscrypt is to allow different files on the same filesystem to be encrypted by different keys, and thus be unlockable, lockable, and securely deletable independently from each other. Therefore, fscrypt is useful in cases such as:

    • Multi-user systems, since each user's files can be encrypted with their own key that is unlocked by their own passphrase.

    • Single-user systems where it's not possible for all files to have the strongest level of protection. For example, it might be necessary for the system to boot up without user interaction. Any files that are needed to do so can only be encrypted by a hardware-protected (e.g. TPM-bound) key at best. If the user's personal files are located on the same filesystem, then with dm-crypt/LUKS the user's personal files would be limited to this weak level of protection. With fscrypt, the user's personal files could be fully protected using the user's passphrase.

    fscrypt isn't very useful in the following cases:

    • Single-user systems where the user is willing to enter a strong passphrase at boot time to unlock the entire filesystem. In this case, the main advantage of fscrypt would go unused, so dm-crypt/LUKS would be better as it would provide better security (due to ensuring that all files and all filesystem metadata are encrypted).

    • Any case where it is feasible to create a separate filesystem for every encryption key you want to use.

    Note: dm-crypt/LUKS and fscrypt aren't mutually exclusive; they can be used together when the performance hit of double encryption is tolerable. It only makes sense to do this when the keys for each encryption layer are protected in different ways, such that each layer serves a different purpose. A reasonable set-up would be to encrypt the whole filesystem with dm-crypt/LUKS using a TPM-bound key that is automatically unlocked at boot time, and also encrypt users' home directories with fscrypt using their login passphrases.

Threat model

Like other storage encryption solutions (including dm-crypt/LUKS and eCryptfs), Linux native filesystem encryption is primarily intended to protect the confidentiality of data from a single point-in-time permanent offline compromise of the disk. For a detailed description of the threat model, see the kernel documentation.

It's worth emphasizing that none of these encryption solutions protect unlocked encrypted files from other users on the same system (that's the job of OS-level access control, such as UNIX file permissions), or from the cloud provider you may be running a virtual machine on. By themselves, they also do not protect from "evil maid" attacks, i.e. non-permanent offline compromises of the disk.

Features

fscrypt is intended to improve upon the work in e4crypt by providing a more managed environment and handling more functionality in the background. fscrypt has a design document specifying its full architecture. See also the kernel documentation for Linux native filesystem encryption.

Briefly, fscrypt deals with protectors and policies. Protectors represent some secret or information used to protect the confidentiality of your data. The three currently supported protector types are:

  1. Your login passphrase, through PAM. The included PAM module (pam_fscrypt.so) can automatically unlock directories protected by your login passphrase when you log in, and lock them when you log out. IMPORTANT: before using a login protector, follow Setting up for login protectors.

  2. A custom passphrase. This passphrase is hashed with Argon2id, by default calibrated to use all CPUs and take about 1 second.

  3. A raw key file. See Using a raw key protector.

These protectors are mutable, so the information can change without needing to update any of your encrypted directories.

Policies represent the actual key passed to the kernel. This "policy key" is immutable and policies are (usually) applied to a single directory. Protectors then protect policies, so that having one of the protectors for a policy is enough to get the policy key and access the data. Which protectors protect a policy can also be changed. This allows a user to change how a directory is protected without needing to reencrypt the directory's contents.

Concretely, fscrypt contains the following functionality:

  • fscrypt setup - Creates /etc/fscrypt.conf and the /.fscrypt directory
    • This is the only functionality which always requires root privileges
  • fscrypt setup MOUNTPOINT - Gets a filesystem ready for use with fscrypt
  • fscrypt encrypt DIRECTORY - Encrypts an empty directory
  • fscrypt unlock DIRECTORY - Unlocks an encrypted directory
  • fscrypt lock DIRECTORY - Locks an encrypted directory
  • fscrypt purge MOUNTPOINT - Locks all encrypted directories on a filesystem
  • fscrypt status [PATH] - Gets detailed info about filesystems or paths
  • fscrypt metadata - Manages policies or protectors directly

See the example usage section below or run fscrypt COMMAND --help for more information about each of the commands.

Building and installing

fscrypt has a minimal set of build dependencies:

  • Go 1.11 or higher. Older versions may work but they are not tested or supported.
  • A C compiler (gcc or clang)
  • make
  • Headers for libpam. Install them with the appropriate package manager:
    • Debian/Ubuntu: sudo apt install libpam0g-dev
    • Red Hat: sudo yum install pam-devel
    • Arch: pam package (usually installed by default)

Once all the dependencies are installed, you can get the repository by running:

go get -d github.com/google/fscrypt/...

Running make in $GOPATH/src/github.com/google/fscrypt builds the binary (fscrypt) and PAM module (pam_fscrypt.so) in the bin/ directory.

Running sudo make install installs fscrypt into /usr/local/bin, pam_fscrypt.so into /usr/local/lib/security, and pam_fscrypt/config into /usr/local/share/pam-configs.

On Debian (and Debian derivatives such as Ubuntu), use sudo make install PREFIX=/usr to install into /usr instead of the default of /usr/local. Ordinarily you shouldn't manually install software into /usr, since /usr is reserved for Debian's own packages. However, Debian's PAM configuration framework only recognizes configuration files in /usr, not in /usr/local. Therefore, the PAM module will only work if you install into /usr. Note: if you later decide to switch to using the Debian package libpam-fscrypt, you'll have to first manually run sudo make uninstall PREFIX=/usr.

It is also possible to use make install-bin to only install the fscrypt binary, or make install-pam to only install the PAM files.

Alternatively, if you only want to install the fscrypt binary to $GOPATH/bin, simply run:

go get github.com/google/fscrypt/cmd/fscrypt

See the Makefile for instructions on how to further customize the build.

Runtime dependencies

To run, fscrypt needs the following libraries:

  • libpam.so (almost certainly already on your system)

In addition, fscrypt requires kernel support for encryption for your filesystem, and for some filesystems that a feature flag has been enabled in the on-disk filesystem superblock:

  • For ext4, the kernel must be v4.1 or later, and the kernel configuration must have either CONFIG_FS_ENCRYPTION=y (for kernels v5.1+) or CONFIG_EXT4_ENCRYPTION=y or =m (for older kernels). Also, the filesystem must have the encrypt feature flag enabled; see here for how to enable it.

  • For f2fs, the kernel must be v4.2 or later, and the kernel configuration must have either CONFIG_FS_ENCRYPTION=y (for kernels v5.1+) or CONFIG_F2FS_FS_ENCRYPTION=y (for older kernels). Also, the filesystem must have the encrypt feature flag enabled. It can be enabled at format time by mkfs.f2fs -O encrypt, or later by fsck.f2fs -O encrypt.

  • For UBIFS, the kernel must be v4.10 or later, and the kernel configuration must have either CONFIG_FS_ENCRYPTION=y (for kernels v5.1+) or CONFIG_UBIFS_FS_ENCRYPTION=y (for older kernels).

To check whether the needed option is enabled in your kernel, run:

zgrep -h ENCRYPTION /proc/config.gz /boot/config-$(uname -r) | sort | uniq

It is also recommended to use Linux kernel v5.4 or later, since this allows the use of v2 encryption policies. v2 policies have several security and usability improvements over v1 policies.

Be careful when using encryption on removable media, since filesystems with the encrypt feature cannot be mounted on systems with kernel versions older than the minimums listed above -- even to access unencrypted files!

If you configure fscrypt to use non-default features, other kernel prerequisites may be needed too. See Configuration file.

Configuration file

Running sudo fscrypt setup will create the configuration file /etc/fscrypt.conf if it doesn't already exist. It's a JSON file that looks like the following:

{
	"source": "custom_passphrase",
	"hash_costs": {
		"time": "52",
		"memory": "131072",
		"parallelism": "32"
	},
	"options": {
		"padding": "32",
		"contents": "AES_256_XTS",
		"filenames": "AES_256_CTS",
		"policy_version": "2"
	},
	"use_fs_keyring_for_v1_policies": false,
	"allow_cross_user_metadata": false
}

The fields are:

  • "source" is the default source for new protectors. The choices are "pam_passphrase", "custom_passphrase", and "raw_key".

  • "hash_costs" describes how difficult the passphrase hashing is. By default, fscrypt setup calibrates the hashing to use all CPUs and take about 1 second. The --time option to fscrypt setup can be used to customize this time when creating the configuration file.

  • "options" are the encryption options to use for new encrypted directories:

    • "padding" is the number of bytes by which filenames are padded before being encrypted. The choices are "32", "16", "8", and "4". "32" is recommended.

    • "contents" is the algorithm used to encrypt file contents. The choices are "AES_256_XTS", "AES_128_CBC", and "Adiantum". Normally, "AES_256_XTS" is recommended.

    • "filenames" is the algorithm used to encrypt file names. The choices are "AES_256_CTS", "AES_128_CTS", and "Adiantum". Normally, "AES_256_CTS" is recommended.

      To use algorithms other than "AES_256_XTS" for contents and "AES_256_CTS" for filenames, the needed algorithm(s) may need to be enabled in the Linux kernel's cryptography API. For example, to use Adiantum, CONFIG_CRYPTO_ADIANTUM must be set. Also, not all combinations of algorithms are allowed; for example, "Adiantum" for contents can only be paired with "Adiantum" for filenames. See the kernel documentation for more details about the supported algorithms.

    • "policy_version" is the version of encryption policy to use. The choices are "1" and "2". If unset, "1" is assumed. Directories created with policy version "2" are only usable on kernel v5.4 or later, but are preferable to version "1" if you don't mind this restriction.

  • "use_fs_keyring_for_v1_policies" specifies whether to add keys for v1 encryption policies to the filesystem keyrings, rather than to user keyrings. This can solve issues with processes being unable to access unlocked encrypted files. However, it requires kernel v5.4 or later, and it makes unlocking and locking encrypted directories require root. (The PAM module will still work.)

    The purpose of this setting is to allow people to take advantage of some of the improvements in Linux v5.4 on encrypted directories that are also compatible with older kernels. If you don't need compatibility with older kernels, it's better to not use this setting and instead (re-)create your encrypted directories with "policy_version": "2".

  • "allow_cross_user_metadata" specifies whether fscrypt will allow protectors and policies from other non-root users to be read, e.g. to be offered as options by fscrypt encrypt. The default value is false, since other users might be untrusted and could create malicious files. This can be set to true to restore the old behavior on systems where fscrypt metadata needs to be shared between multiple users. Note that this option is independent from the permissions on the metadata files themselves, which are set to 0600 by default; users who wish to share their metadata files with other users would also need to explicitly change their mode to 0644.

Setting up fscrypt on a filesystem

fscrypt needs some directories to exist on the filesystem on which encryption will be used:

  • MOUNTPOINT/.fscrypt/policies
  • MOUNTPOINT/.fscrypt/protectors

(If login protectors are used, these must also exist on the root filesystem.)

To create these directories, run fscrypt setup MOUNTPOINT. If MOUNTPOINT is owned by root, as is usually the case, then this command will require root.

There will be one decision you'll need to make: whether non-root users will be allowed to create fscrypt metadata (policies and protectors).

If you say y, then these directories will be made world-writable, with the sticky bit set so that users can't delete each other's files -- just like /tmp. If you say N, then these directories will be writable only by root.

Saying y maximizes the usability of fscrypt, and on most systems it's fine to say y. However, on some systems this may be inappropriate, as it will allow malicious users to fill the entire filesystem unless filesystem quotas have been configured -- similar to problems that have historically existed with other world-writable directories, e.g. /tmp. If you are concerned about this, say N. If you say N, then you'll only be able to run fscrypt as root to set up encryption on users' behalf, unless you manually set custom permissions on the metadata directories to grant write access to specific users or groups.

If you chose the wrong mode at fscrypt setup time, you can change the directory permissions at any time. To enable single-user writable mode, run:

sudo chmod 0755 MOUNTPOINT/.fscrypt/*

To enable world-writable mode, run:

sudo chmod 1777 MOUNTPOINT/.fscrypt/*

Setting up for login protectors

If you want any encrypted directories to be protected by your login passphrase, you'll need to:

  1. Secure your login passphrase (optional, but strongly recommended)
  2. Enable the PAM module (pam_fscrypt.so)

If you installed fscrypt from source rather than from your distro's package manager, you may also need to allow fscrypt to check your login passphrase.

Securing your login passphrase

Although fscrypt uses a strong passphrase hash algorithm, the security of login protectors is also limited by the strength of your system's passphrase hashing in /etc/shadow. On most Linux distributions, /etc/shadow by default uses SHA-512 with 5000 rounds, which is much weaker than what fscrypt uses.

To mitigate this, you should use a strong login passphrase.

If using a strong login passphrase is annoying because it needs to be entered frequently to run sudo, consider increasing the sudo timeout. That can be done by adding the following to /etc/sudoers:

Defaults timestamp_timeout=60

You should also increase the number of rounds that your system's passphrase hashing uses (though this doesn't increase security as much as choosing a strong passphrase). To do this, find the line in /etc/pam.d/passwd that looks like:

password	required	pam_unix.so sha512 shadow nullok

Append rounds=1000000 (or another number of your choice; the goal is to make the passphrase hashing take about 1 second, similar to fscrypt's default):

password	required	pam_unix.so sha512 shadow nullok rounds=1000000

Then, change your login passphrase to a new, strong passphrase:

passwd

If you'd like to keep the same login passphrase (not recommended, as the old passphrase hash may still be recoverable from disk), then instead run sudo passwd $USER and enter your existing passphrase. This re-hashes your existing passphrase with the new rounds.

Enabling the PAM module

To enable the PAM module pam_fscrypt.so, follow the directions for your Linux distro below. Enabling the PAM module is needed for login passphrase-protected directories to be automatically unlocked when you log in (and be automatically locked when you log out), and for login passphrase-protected directories to remain accessible when you change your login passphrase.

Enabling the PAM module on Debian or Ubuntu

The official libpam-fscrypt package for Debian (and Debian derivatives such as Ubuntu) will install a configuration file for Debian's PAM configuration framework to /usr/share/pam-configs/fscrypt. This file contains reasonable defaults for the PAM module. To automatically apply these defaults, run sudo pam-auth-update and follow the on-screen instructions.

This file also gets installed if you build and install fscrypt from source, but it is only installed to the correct location if you use make install PREFIX=/usr to install into /usr instead of the default of /usr/local.

Enabling the PAM module on Arch Linux

On Arch Linux, follow the recommendations at the Arch Linux Wiki.

We recommend using the Arch Linux package, either fscrypt (official) or fscrypt-git (AUR). If you instead install fscrypt manually using sudo make install, then in addition to the steps on the Wiki you'll also need to create /etc/pam.d/fscrypt.

Enabling the PAM module on other Linux distros

On all other Linux distros, follow the general guidance below to edit your PAM configuration files.

The fscrypt PAM module implements the Auth, Session, and Password types.

The Password functionality of pam_fscrypt.so is used to automatically rewrap a user's login protector when their unix passphrase changes. An easy way to get the working is to add the line:

password    optional    pam_fscrypt.so

after pam_unix.so in /etc/pam.d/common-password or similar.

The Auth and Session functionality of pam_fscrypt.so are used to automatically unlock directories when logging in as a user, and lock them when logging out. An easy way to get this working is to add the line:

auth        optional    pam_fscrypt.so

after pam_unix.so in /etc/pam.d/common-auth or similar, and to add the line:

session     optional    pam_fscrypt.so

after pam_unix.so in /etc/pam.d/common-session or similar, but before pam_systemd.so or any other module that accesses the user's home directory or which starts processes that access the user's home directory during their session.

To make pam_fscrypt.so print debugging messages to the system log, add the debug option. All hook types accept this option.

Allowing fscrypt to check your login passphrase

This step is only needed if you installed fscrypt from source code.

Some Linux distros use restrictive settings in /etc/pam.d/other that prevent programs from checking your login passphrase unless a per-program PAM configuration file grants access. This prevents fscrypt from creating any login passphrase-protected directories, even without auto-unlocking. To ensure that fscrypt will work properly (if you didn't install an official fscrypt package from your distro, which should have already handled this), also create a file /etc/pam.d/fscrypt containing:

auth        required    pam_unix.so

Backup, restore, and recovery

Encrypted files and directories can't be backed up while they are "locked", i.e. while they appear in encrypted form. They can only be backed up while they are unlocked, in which case they can be backed up like any other files. Note that since the encryption is transparent, the files won't be encrypted in the backup (unless the backup applies its own encryption).

For the same reason (and several others), an encrypted directory can't be directly "moved" to another filesystem. However, it is possible to create a new encrypted directory on the destination filesystem using fscrypt encrypt, then copy the contents of the source directory into it.

For directories protected by a custom_passphrase or raw_key protector, all metadata needed to unlock the directory (excluding the actual passphrase or raw key, of course) is located in the .fscrypt directory at the root of the filesystem that contains the encrypted directory. For example, if you have an encrypted directory /home/$USER/private that is protected by a custom passphrase, all fscrypt metadata needed to unlock the directory with that custom passphrase will be located in /home/.fscrypt if you are using a dedicated /home filesystem or in /.fscrypt if you aren't. If desired, you can back up the fscrypt metadata by making a copy of this directory, although this isn't too important since this metadata is located on the same filesystem as the encrypted directory(s).

pam_passphrase (login passphrase) protectors are a bit different as they are always stored on the root filesystem, in /.fscrypt. This ties them to the specific system and ensures that each user has only a single login protector. Therefore, encrypted directories on a non-root filesystem can't be unlocked via a login protector if the operating system is reinstalled or if the disk is connected to another system -- even if the new system uses the same login passphrase for the user.

Because of this, fscrypt encrypt will automatically generate a recovery passphrase when creating a login passphrase-protected directory on a non-root filesystem. The recovery passphrase is simply a custom_passphrase protector with a randomly generated high-entropy passphrase. Initially, the recovery passphrase is stored in a file in the encrypted directory itself; therefore, to use it you must record it in another secure location. It is strongly recommended to do this. Then, if ever needed, you can use fscrypt unlock to unlock the directory with the recovery passphrase (by choosing the recovery protector instead of the login protector).

If you really want to disable the generation of a recovery passphrase, use the --no-recovery option. Only do this if you really know what you are doing and are prepared for potential data loss.

Alternative approaches to supporting recovery of login passphrase-protected directories include the following:

  • Manually adding your own recovery protector, using fscrypt metadata add-protector-to-policy.

  • Backing up and restoring the /.fscrypt directory on the root filesystem. Note that after restoring the /.fscrypt directory, unlocking the login protectors will require the passphrases they had at the time the backup was made even if they were changed later, so make sure to remember these passphrase(s) or record them in a secure location. Also note that if the UUID of the root filesystem changed, you will need to manually fix the UUID in any .fscrypt/protectors/*.link files on other filesystems.

The auto-generated recovery passphrases should be enough for most users, though.

Encrypting existing files

fscrypt isn't designed to encrypt existing files, as this presents significant technical challenges and usually is impossible to do securely. Therefore, fscrypt encrypt only works on empty directories.

Of course, it is still possible to create an encrypted directory, copy files into it, and delete the original files. The mv command will even work, as it will fall back to a copy and delete (except on older kernels). However, beware that due to the characteristics of filesystems and storage devices, this may not properly protect the files, as their original contents may still be forensically recoverable from disk even after being deleted. It's much better to encrypt files from the very beginning.

There are only a few cases where copying files into an encrypted directory can really make sense, such as:

  • The source files are located on an in-memory filesystem such as tmpfs.

  • The confidentiality of the source files isn't important, e.g. they are system default files and the user hasn't added any personal files yet.

  • The source files are protected by a different fscrypt policy, the old and new policies are protected by only the same protector(s), and the old policy uses similar strength encryption.

If one of the above doesn't apply, then it's probably too late to securely encrypt your existing files.

As a best-effort attempt, you can use the shred program to try to erase the original files. Here are the recommended commands for "best-effort" encryption of an existing directory named "dir":

mkdir dir.new
fscrypt encrypt dir.new
cp -a -T dir dir.new
find dir -type f -print0 | xargs -0 shred -n1 --remove=unlink
rm -rf dir
mv dir.new dir

However, beware that shred isn't guaranteed to be effective on all storage devices and filesystems. For example, if you're using an SSD, "overwrites" of data typically go to new flash blocks, so they aren't really overwrites.

Note: for reasons similar to the above, changed or removed fscrypt protectors aren't guaranteed to be forensically unrecoverable from disk either. Thus, the use of weak or default passphrases should be avoided, even if changed later.

Example usage

All these examples assume there is an ext4 filesystem which supports encryption mounted at /mnt/disk. See here for how to enable encryption support on an ext4 filesystem.

Setting up fscrypt on a directory
# Check which directories on our system support encryption
>>>>> fscrypt status
filesystems supporting encryption: 1
filesystems with fscrypt metadata: 0

MOUNTPOINT  DEVICE     FILESYSTEM  ENCRYPTION   FSCRYPT
/           /dev/sda1  ext4        not enabled  No
/mnt/disk   /dev/sdb   ext4        supported    No

# Create the global configuration file. Nothing else necessarily needs root.
>>>>> sudo fscrypt setup
Defaulting to policy_version 2 because kernel supports it.
Customizing passphrase hashing difficulty for this system...
Created global config file at "/etc/fscrypt.conf".
Allow users other than root to create fscrypt metadata on the root filesystem?
(See https://github.com/google/fscrypt#setting-up-fscrypt-on-a-filesystem) [y/N] y
Metadata directories created at "/.fscrypt", writable by everyone.

# Start using fscrypt with our filesystem
>>>>> sudo fscrypt setup /mnt/disk
Allow users other than root to create fscrypt metadata on this filesystem? (See
https://github.com/google/fscrypt#setting-up-fscrypt-on-a-filesystem) [y/N] y
Metadata directories created at "/mnt/disk/.fscrypt", writable by everyone.

# Initialize encryption on a new empty directory
>>>>> mkdir /mnt/disk/dir1
>>>>> fscrypt encrypt /mnt/disk/dir1
The following protector sources are available:
1 - Your login passphrase (pam_passphrase)
2 - A custom passphrase (custom_passphrase)
3 - A raw 256-bit key (raw_key)
Enter the source number for the new protector [2 - custom_passphrase]: 2
Enter a name for the new protector: Super Secret
Enter custom passphrase for protector "Super Secret":
Confirm passphrase:
"/mnt/disk/dir1" is now encrypted, unlocked, and ready for use.

# We can see this created one policy and one protector for this directory
>>>>> fscrypt status /mnt/disk
ext4 filesystem "/mnt/disk" has 1 protector and 1 policy

PROTECTOR         LINKED  DESCRIPTION
7626382168311a9d  No      custom protector "Super Secret"

POLICY                            UNLOCKED  PROTECTORS
16382f282d7b29ee27e6460151d03382  Yes       7626382168311a9d
Quiet version
>>>>> sudo fscrypt setup --quiet --force --all-users
>>>>> sudo fscrypt setup /mnt/disk --quiet --all-users
>>>>> echo "hunter2" | fscrypt encrypt /mnt/disk/dir1 --quiet --source=custom_passphrase  --name="Super Secret"
Locking and unlocking a directory
# Write a file to our encrypted directory.
>>>>> echo "Hello World" > /mnt/disk/dir1/secret.txt
>>>>> fscrypt status /mnt/disk/dir1
"/mnt/disk/dir1" is encrypted with fscrypt.

Policy:   16382f282d7b29ee27e6460151d03382
Options:  padding:32 contents:AES_256_XTS filenames:AES_256_CTS policy_version:2
Unlocked: Yes

Protected with 1 protector:
PROTECTOR         LINKED  DESCRIPTION
7626382168311a9d  No      custom protector "Super Secret"

# Lock the directory.  Note: if using a v1 encryption policy instead
# of v2, you'll need 'sudo fscrypt lock /mnt/disk/dir1 --user=$USER'.
>>>>> fscrypt lock /mnt/disk/dir1
"/mnt/disk/dir1" is now locked.
>>>>> fscrypt status /mnt/disk/dir1
"/mnt/disk/dir1" is encrypted with fscrypt.

Policy:   16382f282d7b29ee27e6460151d03382
Options:  padding:32 contents:AES_256_XTS filenames:AES_256_CTS policy_version:2
Unlocked: No

Protected with 1 protector:
PROTECTOR         LINKED  DESCRIPTION
7626382168311a9d  No      custom protector "Super Secret"

# Now the filenames and file contents are inaccessible
>>>>> ls /mnt/disk/dir1
u,k20l9HrtrizDjh0zGkw2dTfBkX4T0ZDUlsOhBLl4P
>>>>> cat /mnt/disk/dir1/u,k20l9HrtrizDjh0zGkw2dTfBkX4T0ZDUlsOhBLl4P
cat: /mnt/disk/dir1/u,k20l9HrtrizDjh0zGkw2dTfBkX4T0ZDUlsOhBLl4P: Required key not available

# Unlocking the directory makes the contents available
>>>>> fscrypt unlock /mnt/disk/dir1
Enter custom passphrase for protector "Super Secret":
"/mnt/disk/dir1" is now unlocked and ready for use.
>>>>> fscrypt status /mnt/disk/dir1
"/mnt/disk/dir1" is encrypted with fscrypt.

Policy:   16382f282d7b29ee27e6460151d03382
Options:  padding:32 contents:AES_256_XTS filenames:AES_256_CTS policy_version:2
Unlocked: Yes

Protected with 1 protector:
PROTECTOR         LINKED  DESCRIPTION
7626382168311a9d  No      custom protector "Super Secret"
>>>>> cat /mnt/disk/dir1/secret.txt
Hello World
Quiet version
>>>>> fscrypt lock /mnt/disk/dir1 --quiet
>>>>> echo "hunter2" | fscrypt unlock /mnt/disk/dir1 --quiet
Protecting a directory with your login passphrase

First, ensure that you have properly set up your system for login protectors.

Then, you can protect directories with your login passphrase as follows:

# Select your login passphrase as the desired source.
>>>>> mkdir /mnt/disk/dir2
>>>>> fscrypt encrypt /mnt/disk/dir2
Should we create a new protector? [y/N] y
The following protector sources are available:
1 - Your login passphrase (pam_passphrase)
2 - A custom passphrase (custom_passphrase)
3 - A raw 256-bit key (raw_key)
Enter the source number for the new protector [2 - custom_passphrase]: 1
Enter login passphrase for joerichey:
"/mnt/disk/dir2" is now encrypted, unlocked, and ready for use.

# Note that the login protector actually sits on the root filesystem
>>>>> fscrypt status /mnt/disk/dir2
"/mnt/disk/dir2" is encrypted with fscrypt.

Policy:   fe1c92009abc1cff4f3257c77e8134e3
Options:  padding:32 contents:AES_256_XTS filenames:AES_256_CTS policy_version:2
Unlocked: Yes

Protected with 1 protector:
PROTECTOR         LINKED   DESCRIPTION
6891f0a901f0065e  Yes (/)  login protector for joerichey
>>>>> fscrypt status /mnt/disk
ext4 filesystem "/mnt/disk" has 2 protectors and 2 policies

PROTECTOR         LINKED   DESCRIPTION
7626382168311a9d  No       custom protector "Super Secret"
6891f0a901f0065e  Yes (/)  login protector for joerichey

POLICY                            UNLOCKED  PROTECTORS
16382f282d7b29ee27e6460151d03382  Yes       7626382168311a9d
fe1c92009abc1cff4f3257c77e8134e3  Yes       6891f0a901f0065e
>>>>> fscrypt status /
ext4 filesystem "/" has 1 protector(s) and 0 policy(ies)

PROTECTOR         LINKED  DESCRIPTION
6891f0a901f0065e  No      login protector for joerichey
Quiet version
>>>>> mkdir /mnt/disk/dir2
>>>>> echo "password" | fscrypt encrypt /mnt/disk/dir2 --source=pam_passphrase --quiet
Changing a custom passphrase
# First we have to figure out which protector we wish to change.
>>>>> fscrypt status /mnt/disk/dir1
"/mnt/disk/dir1" is encrypted with fscrypt.

Policy:   16382f282d7b29ee27e6460151d03382
Options:  padding:32 contents:AES_256_XTS filenames:AES_256_CTS policy_version:2
Unlocked: Yes

Protected with 1 protector:
PROTECTOR         LINKED  DESCRIPTION
7626382168311a9d  No      custom protector "Super Secret"

# Now specify the protector directly to the metadata command
>>>>> fscrypt metadata change-passphrase --protector=/mnt/disk:7626382168311a9d
Enter old custom passphrase for protector "Super Secret":
Enter new custom passphrase for protector "Super Secret":
Confirm passphrase:
Passphrase for protector 7626382168311a9d successfully changed.
Quiet version
>>>>> printf "hunter2\nhunter3" | fscrypt metadata change-passphrase --protector=/mnt/disk:7626382168311a9d --quiet
Using a raw key protector

fscrypt also supports protectors which use raw key files as the user-provided secret. These key files must be exactly 32 bytes long and contain the raw binary data of the key. Obviously, make sure to store the key file securely (and not in the directory you are encrypting with it). If generating the keys on Linux make sure you are aware of how randomness works and some common myths.

# Generate a 256-bit key file
>>>>> head --bytes=32 /dev/urandom > secret.key

# Now create a key file protector without using it on a directory. Note that we
# could also use `fscrypt encrypt --key=secret.key` to achieve the same thing.
>>>>> fscrypt metadata create protector /mnt/disk
Create new protector on "/mnt/disk" [Y/n] y
The following protector sources are available:
1 - Your login passphrase (pam_passphrase)
2 - A custom passphrase (custom_passphrase)
3 - A raw 256-bit key (raw_key)
Enter the source number for the new protector [2 - custom_passphrase]: 3
Enter a name for the new protector: Skeleton
Enter key file for protector "Skeleton": secret.key
Protector 2c75f519b9c9959d created on filesystem "/mnt/disk".
>>>>> fscrypt status /mnt/disk
ext4 filesystem "/mnt/disk" has 3 protectors and 2 policies

PROTECTOR         LINKED   DESCRIPTION
7626382168311a9d  No       custom protector "Super Secret"
2c75f519b9c9959d  No       raw key protector "Skeleton"
6891f0a901f0065e  Yes (/)  login protector for joerichey

POLICY                            UNLOCKED  PROTECTORS
16382f282d7b29ee27e6460151d03382  Yes       7626382168311a9d
fe1c92009abc1cff4f3257c77e8134e3  Yes       6891f0a901f0065e

# Finally, we could apply this key to a directory
>>>>> mkdir /mnt/disk/dir3
>>>>> fscrypt encrypt /mnt/disk/dir3 --protector=/mnt/disk:2c75f519b9c9959d
Enter key file for protector "Skeleton": secret.key
"/mnt/disk/dir3" is now encrypted, unlocked, and ready for use.
Quiet version
>>>>> head --bytes=32 /dev/urandom > secret.key
>>>>> fscrypt encrypt /mnt/disk/dir3 --key=secret.key --source=raw_key --name=Skeleton
Using multiple protectors for a policy

fscrypt supports the idea of protecting a single directory with multiple protectors. This means having access to any of the protectors is sufficient to decrypt the directory. This is useful for sharing data or setting up access control systems.

# Add an existing protector to the policy for some directory
>>>>> fscrypt status /mnt/disk
ext4 filesystem "/mnt/disk" has 3 protectors and 3 policies

PROTECTOR         LINKED   DESCRIPTION
7626382168311a9d  No       custom protector "Super Secret"
2c75f519b9c9959d  No       raw key protector "Skeleton"
6891f0a901f0065e  Yes (/)  login protector for joerichey

POLICY                            UNLOCKED  PROTECTORS
d03fb894584a4318d1780e9a7b0b47eb  No        2c75f519b9c9959d
16382f282d7b29ee27e6460151d03382  No        7626382168311a9d
fe1c92009abc1cff4f3257c77e8134e3  No        6891f0a901f0065e
>>>>> fscrypt status /mnt/disk/dir1
"/mnt/disk/dir1" is encrypted with fscrypt.

Policy:   16382f282d7b29ee27e6460151d03382
Options:  padding:32 contents:AES_256_XTS filenames:AES_256_CTS policy_version:2
Unlocked: No

Protected with 1 protector:
PROTECTOR         LINKED  DESCRIPTION
7626382168311a9d  No      custom protector "Super Secret"
>>>>> fscrypt metadata add-protector-to-policy --protector=/mnt/disk:2c75f519b9c9959d --policy=/mnt/disk:16382f282d7b29ee27e6460151d03382
WARNING: All files using this policy will be accessible with this protector!!
Protect policy 16382f282d7b29ee27e6460151d03382 with protector 2c75f519b9c9959d? [Y/n]
Enter key file for protector "Skeleton": secret.key
Enter custom passphrase for protector "Super Secret":
Protector 2c75f519b9c9959d now protecting policy 16382f282d7b29ee27e6460151d03382.
>>>>> fscrypt status /mnt/disk/dir1
"/mnt/disk/dir1" is encrypted with fscrypt.

Policy:   16382f282d7b29ee27e6460151d03382
Options:  padding:32 contents:AES_256_XTS filenames:AES_256_CTS policy_version:2
Unlocked: No

Protected with 2 protectors:
PROTECTOR         LINKED  DESCRIPTION
7626382168311a9d  No      custom protector "Super Secret"
2c75f519b9c9959d  No      raw key protector "Skeleton"

# Now the unlock command will prompt for which protector we want to use
>>>>> fscrypt unlock /mnt/disk/dir1
The available protectors are:
0 - custom protector "Super Secret"
1 - raw key protector "Skeleton"
Enter the number of protector to use: 1
Enter key file for protector "Skeleton": secret.key
"/mnt/disk/dir1" is now unlocked and ready for use.

# The protector can also be removed from the policy (if it is not the only one)
>>>>> fscrypt metadata remove-protector-from-policy --protector=/mnt/disk:2c75f519b9c9959d --policy=/mnt/disk:16382f282d7b29ee27e6460151d03382
WARNING: All files using this policy will NO LONGER be accessible with this protector!!
Stop protecting policy 16382f282d7b29ee27e6460151d03382 with protector 2c75f519b9c9959d? [y/N] y
Protector 2c75f519b9c9959d no longer protecting policy 16382f282d7b29ee27e6460151d03382.
Quiet version
>>>>> echo "hunter2" | fscrypt metadata add-protector-to-policy --protector=/mnt/disk:2c75f519b9c9959d --policy=/mnt/disk:16382f282d7b29ee27e6460151d03382 --key=secret.key --quiet
>>>>> fscrypt metadata remove-protector-from-policy --protector=/mnt/disk:2c75f519b9c9959d --policy=/mnt/disk:16382f282d7b29ee27e6460151d03382 --quiet --force

Contributing

We would love to accept your contributions to fscrypt. See the CONTRIBUTING.md file for more information about signing the CLA and submitting a pull request.

Troubleshooting

In general, if you are encountering issues with fscrypt, open an issue, following the guidelines in CONTRIBUTING.md. We will try our best to help.

I changed my login passphrase, now all my directories are inaccessible

Usually, the PAM module pam_fscrypt.so will automatically detect changes to a user's login passphrase and update the user's fscrypt login protector so that they retain access their login-passphrase protected directories. However, sometimes a user's login passphrase can become desynchronized from their fscrypt login protector. This can happen if root assigns the user a new passphrase without providing the old one, if the user's login passphrase is managed by an external system such as LDAP, if the PAM module is not installed, or if the PAM module is not properly configured. See Enabling the PAM module for how to configure the PAM module.

To fix a user's login protector, find the corresponding protector ID by running fscrypt status "/". Then, change this protector's passphrase by running:

fscrypt metadata change-passphrase --protector=/:ID
Directories using my login passphrase are not automatically unlocking

First, directories won't unlock if your session starts without password authentication. The most common case of this is public-key ssh login. To trigger a password authentication event, run su $USER -c exit.

If your session did start with password authentication, then the following may be causing the issue:

  • The PAM module might not be configured correctly. Ensure you have correctly configured the PAM module.

  • If your login passphrase recently changed, then it might have gotten out of sync with your login protector. To fix this, manually change your login protector's passphrase to get it back in sync with your actual login passphrase.

  • Due to a bug in sshd, encrypted directories won't auto-unlock when logging in with ssh using the ChallengeResponseAuthentication ssh authentication method, which is also called KbdInteractiveAuthentication. This ssh authentication method implements password authentication by default, so it might appear similar to PasswordAuthentication. However, only PasswordAuthentication works with fscrypt. To avoid this issue, make sure that your /etc/ssh/sshd_config file contains PasswordAuthentication yes, UsePAM yes, and either ChallengeResponseAuthentication no or KbdInteractiveAuthentication no.

Getting "encryption not enabled" on an ext4 filesystem

This is usually caused by your ext4 filesystem not having the encrypt feature flag enabled. The encrypt feature flag allows the filesystem to contain encrypted files. (It doesn't actually encrypt anything by itself.)

Before enabling encrypt on your ext4 filesystem, first ensure that all of the following are true for you:

  • You only need to use your filesystem on kernels v4.1 and later.

    (Kernels v4.0 and earlier can't mount ext4 filesystems that have the encrypt feature flag.)

  • Either you only need to use your filesystem on kernels v5.5 and later, or your kernel page size (run getconf PAGE_SIZE) and filesystem block size (run tune2fs -l /dev/device | grep 'Block size') are the same.

    (Both values will almost always be 4096, but they may differ if your filesystem is very small, if your system uses the PowerPC CPU architecture, or if you overrode the default block size when you created the filesystem. Only kernels v5.5 and later support ext4 encryption in such cases.)

  • Either you aren't using GRUB to boot directly off the filesystem in question, or you are using GRUB 2.04 or later.

    (Old versions of GRUB can't boot from ext4 filesystems that have encrypt enabled. If, like most people, you have a separate /boot partition, you are fine. You are also fine if you are using the GRUB Debian package 2.02-2 or later [not 2.02_beta*], including the version in Ubuntu 18.04 and later, since the patch to support encrypt was backported.)

After verifying all of the above, enable encrypt by running:

tune2fs -O encrypt /dev/device

If you need to undo this, first delete all encrypted files and directories on the filesystem. Then, run:

fsck -fn /dev/device
debugfs -w -R "feature -encrypt" /dev/device
fsck -fn /dev/device

If you've enabled encrypt but you still get the "encryption not enabled" error, then the problem is that ext4 encryption isn't enabled in your kernel config. See Runtime dependencies for how to enable it.

Getting "user keyring not linked into session keyring"

Some older versions of Ubuntu didn't link the user keyring into the session keyring, which caused problems with fscrypt.

To avoid this issue, upgrade to Ubuntu 20.04 or later.

Getting "Operation not permitted" when moving files into an encrypted directory

Originally, filesystems didn't return the correct error code when attempting to rename unencrypted files (or files with a different encryption policy) into an encrypted directory. Specifically, they returned EPERM instead of EXDEV, which caused mv to fail rather than fall back to a copy as expected.

This bug was fixed in version 5.1 of the mainline Linux kernel, as well as in versions 4.4 and later of the LTS (Long Term Support) branches of the Linux kernel; specifically v4.19.155, 4.14.204, v4.9.242, and v4.4.242.

If the kernel can't be upgraded, this bug can be worked around by explicitly copying the files instead, e.g. with cp.

IMPORTANT: Encrypting existing files can be insecure. Before doing so, read Encrypting existing files.

Getting "Package not installed" when trying to use an encrypted directory

Trying to create or open an encrypted file will fail with ENOPKG ("Package not installed") when the kernel doesn't support one or more of the cryptographic algorithms used by the file or its directory. Note that fscrypt encrypt and fscrypt unlock will still succeed; it's only using the directory afterwards that will fail.

The kernel will always support the algorithms that fscrypt uses by default. However, if you changed the contents and/or filenames encryption algorithms in /etc/fscrypt.conf, then you may run into this issue. To fix it, enable the needed CONFIG_CRYPTO_* options in your Linux kernel configuration. See the kernel documentation for details about which option(s) are required for each encryption mode.

Some processes can't access unlocked encrypted files

This issue is caused by a limitation in the original design of Linux native filesystem encryption which made it difficult to ensure that all processes can access unlocked encrypted files. This issue can manifest in many ways, such as:

  • SSH to a user with an encrypted home directory not working, even when that directory is already unlocked

  • Docker containers being unable to access encrypted files that were unlocked from outside the container

  • NetworkManager being unable to access certificates stored in the user's already-unlocked encrypted home directory

  • Other system services being unable to access already-unlocked encrypted files

  • sudo sessions being unable to access already-unlocked encrypted files

  • A user being unable to access encrypted files that were unlocked by root

If an OS-level error is shown, it is ENOKEY ("Required key not available").

To fix this issue, first run fscrypt status $dir, where $dir is your encrypted directory. If the output contains policy_version:2, then your issue is something else, so stop reading now. If the output contains policy_version:1 or doesn't contain any mention of policy_version, then you'll need to upgrade your directory(s) to policy version 2. To do this:

  1. Upgrade to Linux kernel v5.4 or later.

  2. Upgrade to fscrypt v0.2.7 or later.

  3. Run sudo fscrypt setup --force.

  4. Re-encrypt your encrypted directory(s). Since files cannot be (re-)encrypted in-place, this requires replacing them with new directories. For example:

      fscrypt unlock dir  # if not already unlocked
      mkdir dir.new
      fscrypt encrypt dir.new
      cp -a -T dir dir.new
      find dir -type f -print0 | xargs -0 shred -n1 --remove=unlink
      rm -rf dir
      mv dir.new dir
    

    You don't need to create a new protector. I.e., when fscrypt encrypt asks for a protector, just choose the one you were using before.

  5. fscrypt status on your directory(s) should now show policy_version:2, and the issue should be gone.

Note that once your directories are using policy version 2, they will only be usable with Linux kernel v5.4 and later and fscrypt v0.2.6 and later. So be careful not to downgrade your software past those versions.

This issue can also be fixed by setting "use_fs_keyring_for_v1_policies": true in /etc/fscrypt.conf, as described in Configuration file. This avoids needing to upgrade directories to policy version 2. However, this has some limitations, and the same kernel and fscrypt prerequisites still apply for this option to take effect. It is recommended to upgrade your directories to policy version 2 instead.

Users can access other users' unlocked encrypted files

This is working as intended. When an encrypted directory is unlocked (or locked), it is unlocked (or locked) for all users. Encryption is not access control; the Linux kernel already has many access control mechanisms, such as the standard UNIX file permissions, that can be used to control access to files.

Setting the mode of your encrypted directory to 0700 will prevent users other than the directory's owner and root from accessing it while it is unlocked. In fscrypt v0.2.5 and later, fscrypt encrypt sets this mode automatically.

Having the locked/unlocked status of directories be global instead of per-user may seem unintuitive, but it is actually the only logical way. The encryption is done by the filesystem, so in reality the filesystem either has the key or it doesn't. And once it has the key, any additional checks of whether particular users "have" the key would be OS-level access control checks (not cryptography) that are redundant with existing OS-level access control mechanisms.

Similarly, any attempt of the filesystem encryption feature to prevent root from accessing unlocked encrypted files would be pointless. On Linux systems, root is usually all-powerful and can always get access to files in ways that cannot be prevented, e.g. setuid() and ptrace(). The only reliable way to limit what root can do is via a mandatory access control system, e.g. SELinux.

The original design of Linux native filesystem encryption actually did put the keys into per-user keyrings. However, this caused a massive number of problems, as it's actually very common that encrypted files need to be accessed by processes running under different user IDs -- even if it may not be immediately apparent.

Getting "Required key not available" when backing up locked encrypted files

Encrypted files can't be backed up while locked; you need to unlock them first. For details, see Backup, restore, and recovery.

Originally, filesystems didn't conform to POSIX when reporting the size of encrypted symlinks, as they gave the size of the ciphertext symlink target rather than the size of the plaintext target. This would make the reported size of symlinks appear to be slightly too large when queried using lstat() or similar system calls. Most programs don't care about this, but in rare cases programs can depend on the filesystem reporting symlink sizes correctly.

This bug was fixed in version 5.15 of the mainline Linux kernel, as well as in versions 4.19 and later of the LTS (Long Term Support) branches of the Linux kernel; specifically v5.10.63, v5.4.145, and v4.19.207.

If the kernel can't be upgraded, the only workaround for this bug is to update any affected programs to not depend on symlink sizes being reported correctly.

Copyright 2017 Google Inc. under the Apache 2.0 License; see the LICENSE file for more information.

Author: Joe Richey joerichey@google.com

This is not an official Google product.

Directories

Path Synopsis
Package actions is the high-level interface to the fscrypt packages.
Package actions is the high-level interface to the fscrypt packages.
cmd
Package crypto manages all the cryptography for fscrypt.
Package crypto manages all the cryptography for fscrypt.
Package filesystem deals with the structure of the files on disk used to store the metadata for fscrypt.
Package filesystem deals with the structure of the files on disk used to store the metadata for fscrypt.
Package keyring manages adding, removing, and getting the status of encryption policy keys to/from the kernel.
Package keyring manages adding, removing, and getting the status of encryption policy keys to/from the kernel.
Package metadata contains all of the on disk structures.
Package metadata contains all of the on disk structures.
Package pam contains all the functionality for interfacing with Linux Pluggable Authentication Modules (PAM).
Package pam contains all the functionality for interfacing with Linux Pluggable Authentication Modules (PAM).
Package security manages: - Cache clearing (cache.go) - Privilege manipulation (privileges.go)
Package security manages: - Cache clearing (cache.go) - Privilege manipulation (privileges.go)
Package util contains useful components for simplifying Go code.
Package util contains useful components for simplifying Go code.

Jump to

Keyboard shortcuts

? : This menu
/ : Search site
f or F : Jump to
y or Y : Canonical URL