fscrypt

fscrypt

fscrypt is a high-level tool written in Go for the management of Linux filesystem encryption. This tool 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.

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 Linux filesystem encryption. Ext4 has supported Linux filesystem encryption since v4.1, F2FS added support in v4.2, and UBIFS added support in v4.10. Note that only certain configurations of the Linux kernel enable encryption, and other filesystems may add support for encryption.

Most of the testing for fscrypt has been done with ext4 filesystems. However, the kernel uses a common userspace interface, so this tool should work with all existing and future filesystems which support encryption. If there is a problem using fscrypt with other filesystems, please open an issue.

Other encryption solutions

It is important to distinguish Linux filesystem encryption from two other encryption solutions: eCryptfs and dm-crypt.

Currently, dm-crypt encrypts an entire block device with a single master key. dm-crypt can be used with or without fscrypt. All filesystem data (including all filesystem metadata) is encrypted with this single key when using dm-crypt, while fscrypt only encrypts the filenames and file contents in a specified directory. Note that using both dm-crypt and fscrypt simultaneously will give the protections and benefits of both; however, this may cause a decrease in your performance, as file contents are encrypted twice.

Once example of a reasonable setup could involve using dm-crypt with a TPM or Secure boot key, while using fscrypt for the contents of a home directory. This would still encrypt the entire drive, but would also tie the encryption of a user's personal documents to their passphrase.

On the other hand, eCryptfs is another form of filesystem encryption on Linux; it encrypts a filesystem directory with some key or passphrase. eCryptfs sits on top of an existing filesystem. This makes eCryptfs an alternative choice if your filesystem or kernel does not support native filesystem encryption.

Also note that fscrypt does not support or setup either eCryptfs or dm-crypt. For these tools, use ecryptfs-utils for eCryptfs or cryptsetup for dm-crypt.

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 which should be read to understand the full architecture of fscrypt.

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 (see troubleshooting below)
2. A custom passphrase
3. A raw key file

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.

Specifically, fscrypt contains the following functionality:

• fscrypt setup - Initializes the fscrypt.conf file
  • This is the only functionality which 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 purge MOUNTPOINT - Removes keys for a filesystem before unmounting
• fscrypt status [PATH] - Gets detailed info about filesystems or paths
• fscrypt metadata - Manages policies or protectors directly

The following functionality is planned:

• fscrypt backup - Manages backups of the fscrypt metadata
• fscrypt recovery - Manages recovery keys for directories
• fscrypt cleanup - Scans filesystem for unused policies/protectors
• A PAM module to support login passphrase changes (see below)

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

Building

fscrypt is written in Go, so to build the program you will need to setup Go, setup your GOPATH, and clone the repository into the correct location by running:

go get github.com/google/fscrypt

Alternatively, just copy or checkout the source into $GOPATH/src/github.com/google/fscrypt and run:

go build github.com/google/fscrypt

You will also want to add $GOPATH/bin to your $PATH.

fscrypt has the following build dependencies:

• make
• A C compiler (gcc or clang)
• Go
• Argon2 Passphrase Hash, a C library which can be installed (both the header argon2.h and library libargon2) by running:

  >>>>> git clone https://github.com/P-H-C/phc-winner-argon2 argon2
  >>>>> cd argon2
  >>>>> make
  >>>>> sudo make install
  

• Headers for libblkid (specifically blkid/blkid.h) and libpam (specifically security/pam_appl.h). These can be installed with your appropriate package manager. - sudo apt-get install libblkid-dev libpam0g-dev - sudo yum install libblkid-devel pam-devel - pam and util-liux packages for Arch

Once this is setup, you can run make fscrypt to build the executable in the current directory. See the Makefile for instructions on building a static executable. The C libraries used by fscrypt will be dynamically linked by default.

Running and Installing

fscrypt has the following runtime dependencies:

• Kernel support for filesystem encryption (this will depend on your kernel configuration and specific filesystem)
• libargon2 (see the above installation instructions for Argon2), unless you built a static executable.
• libblkid and libpam (which are almost certainly already on your system), unless you built a static executable.

Installing it just requires placing it in your path or running make install. Change $GOBIN to change the install location of fscrypt. By default, fscrypt is installed to $GOPATH/bin.

Note about stability

fscrypt follows semantic versioning. As such, all versions below 1.0.0 should be considered development versions. This means no guarantees are make about the stability of APIs or formats of config files. As the on-disk metadata structures use Protocol Buffers, we don't expect to break backwards compatibility for metadata, but we give no guarantees.

Example Usage

All these examples assumes we have ext4 filesystems mounted at / and /mnt/disk which both support encryption and that /mnt/disk contains directories we want to encrypt.

Setting up fscrypt on a directory

# Check which directories on our system support encryption
>>>>> fscrypt status
2 filesystem(s) on this system support encryption

MOUNTPOINT            DEVICE     FILESYSTEM  STATUS
/                     /dev/sda1  ext4        encryption not enabled
/mnt/disk             /dev/sdb   ext4        not setup with fscrypt

# Create the global configuration file. Nothing else needs root.
>>>>> sudo fscrypt setup
Create "/etc/fscrypt.conf"? [Y/n] y
Customizing passphrase hashing difficulty for this system...
Created global config file at "/etc/fscrypt.conf".

# Start using fscrypt with our filesystem
>>>>> fscrypt setup /mnt/disk
Metadata directories created at "/mnt/disk/.fscrypt".
Filesystem "/mnt/disk" (/dev/sdb) ready for use with ext4 encryption.

# Initialize encryption on a new empty directory
>>>>> mkdir /mnt/disk/dir1
>>>>> fscrypt encrypt /mnt/disk/dir1
Should we create a new protector? [Y/n] y
Your data can be protected with one of the following sources:
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(s) and 1 policy(ies)

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

POLICY            UNLOCKED  PROTECTORS
7626382168311a9d  Yes       7626382168311a9d

Quiet Version

>>>>> sudo fscrypt setup --quiet --force
>>>>> fscrypt setup /mnt/disk --quiet
>>>>> echo "hunter2" | fscrypt encrypt /mnt/disk/dir1 --quiet --source=custom_passphrase  --name="Super Secret"

Locking and unlocking a directory

As noted in the troubleshooting below, we (as of now) have to unmount a filesystem after purging its keys to clear the necessary caches.

# 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:   16382f282d7b29ee
Unlocked: Yes

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

# Purging, unmounting, and remounting a filesystem locks all the files.
>>>>> fscrypt purge /mnt/disk
WARNING: This may make data encrypted with fscrypt inaccessible.
Purge all policy keys from "/mnt/disk" (this will lock all encrypted directories) [y/N] y
All keys purged for "/mnt/disk".
Filesystem "/mnt/disk" should now be unmounted.
>>>>> umount /mnt/disk
>>>>> mount /mnt/disk
>>>>> fscrypt status /mnt/disk/dir1
"/mnt/disk/dir1" is encrypted with fscrypt.

Policy:   16382f282d7b29ee
Unlocked: No

Protected with 1 protector(s):
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:   16382f282d7b29ee
Unlocked: Yes

Protected with 1 protector(s):
PROTECTOR         LINKED  DESCRIPTION
7626382168311a9d  No      custom protector "Super Secret"
>>>>> cat /mnt/disk/dir1/secret.txt
Hello World

Quiet Version

>>>>> fscrypt purge /mnt/disk --quiet --force
>>>>> umount /mnt/disk
>>>>> mount /mnt/disk
>>>>> printf "hunter2" | fscrypt unlock /mnt/disk/dir1 --quiet

Protecting a directory with your login passphrase

As noted above and in the troubleshooting below, fscrypt cannot (yet) detect when your login passphrase changes. So if you protect a directory with your login passphrase, you may have to do additional work when you change your system passphrase.

# Login passphrases also require that fscrypt is setup on the root directory
>>>>> sudo fscrypt setup /
Filesystem "/" (/dev/dm-1) ready for use with ext4 encryption.

# 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
Your data can be protected with one of the following sources:
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:   fe1c92009abc1cff
Unlocked: Yes

Protected with 1 protector(s):
PROTECTOR         LINKED   DESCRIPTION
6891f0a901f0065e  Yes (/)  login protector for joerichey
>>>>> fscrypt status /mnt/disk
ext4 filesystem "/mnt/disk" has 3 protector(s) and 3 policy(ies)

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

POLICY            UNLOCKED  PROTECTORS
16382f282d7b29ee  Yes       7626382168311a9d
fe1c92009abc1cff  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/dir1 --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:   16382f282d7b29ee
Unlocked: Yes

Protected with 1 protector(s):
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
Your data can be protected with one of the following sources:
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 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
16382f282d7b29ee  Yes       7626382168311a9d
fe1c92009abc1cff  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 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
d03fb894584a4318  No        2c75f519b9c9959d
16382f282d7b29ee  No        7626382168311a9d
fe1c92009abc1cff  No        6891f0a901f0065e
>>>>> fscrypt status /mnt/disk/dir1
"/mnt/disk/dir1" is encrypted with fscrypt.

Policy:   16382f282d7b29ee
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:16382f282d7b29ee
WARNING: All files using this policy will be accessible with this protector!!
Protect policy 16382f282d7b29ee 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 16382f282d7b29ee.
>>>>> fscrypt status /mnt/disk/dir1
"/mnt/disk/dir1" is encrypted with fscrypt.

Policy:   16382f282d7b29ee
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:16382f282d7b29ee
WARNING: All files using this policy will NO LONGER be accessible with this protector!!
Stop protecting policy 16382f282d7b29ee with protector 2c75f519b9c9959d? [y/N] y
Protector 2c75f519b9c9959d no longer protecting policy 16382f282d7b29ee.

Quiet Version

>>>>> echo "hunter2" | fscrypt metadata add-protector-to-policy --protector=/mnt/disk:2c75f519b9c9959d --policy=/mnt/disk:16382f282d7b29ee --key=secret.key --quiet
>>>>> fscrypt metadata remove-protector-from-policy --protector=/mnt/disk:2c75f519b9c9959d --policy=/mnt/disk:16382f282d7b29ee --quiet --force

Contributing

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

Troubleshooting

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

We do not currently support the changing of the login passphrase. This will change when the appropriate module is completed. Until then, you can fix it by first finding the necessary protector (with fscrypt status PATH) and then running:

fscrypt metadata change-passphrase --protector=MOUNTPOINT:ID

I can still see files or filenames after running fscrypt purge MOUNTPOINT

You need to unmount MOUNTPOINT to clear the necessary caches. See fscrypt purge --help for more information

Getting "encryption not enabled" on an ext4 filesystem.

Getting this error on an ext4 system usually means the filesystem has not been setup for encryption. To setup a filesystem to support encryption, first check that your block size is equal to your page size by comparing the outputs of getconf PAGE_SIZE and tune2fs -l /dev/device | grep 'Block size'. If these are not the same, DO NOT ENABLE ENCRYPTION.

To turn on the encryption feature flag for your filesystem, run

tune2fs -O encrypt /dev/device

This command may require root privileges. Once the flag is enabled, older kernels may not be able to mount the filesystem. Note that there was a bug in an older kernel version that allowed encryption policies to be set on ext4 filesystems without enabling this encryption feature flag.

Legal

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.