isabella232 / fscryptctl

Small C tool for Linux filesystem encryption

fscryptctl

fscryptctl is a low-level tool written in C that handles raw keys and manages policies for Linux filesystem encryption, specifically the "fscrypt" kernel interface which is supported by the ext4, f2fs, and UBIFS filesystems.

fscryptctl is mainly intended for embedded systems which can't use the full-featured fscrypt tool, or for testing or experimenting with the kernel interface to Linux filesystem encryption. fscryptctl does not handle key generation, key stretching, key wrapping, or PAM integration. Most users should use the fscrypt tool instead, which supports these features and generally is much easier to use.

As fscryptctl is intended for advanced users, you should read the kernel documentation for filesystem encryption before using fscryptctl.

Table of Contents

• Building and Installing
• Runtime Dependencies
• Features
• Example Usage
• Contributing
• Legal

Building and Installing

To build fscryptctl, run make. The only build dependencies are GNU Make and a C compiler (only C99 is needed).

To install fscryptctl, run sudo make install.

See the Makefile for compilation and installation options.

Runtime Dependencies

fscryptctl doesn't link to any libraries (other than libc), so its only runtime dependencies are the kernel and filesystem support for encryption. In most cases that means the kernel must have been built CONFIG_FS_ENCRYPTION=y, and a command like tune2fs -O encrypt must have been run on the filesystem.

Since v1.0, fscryptctl only supports v2 filesystem encryption policies. This means that it must be used with Linux kernel 5.4 or later. If you need support for v1 encryption policies, use an earlier version of fscryptctl. However, be aware that v1 had some significant usability and security limitations.

For more information about the kernel and filesystem prerequisites, see the fscrypt documentation, including the troubleshooting tips.

Features

fscryptctl has the following commands:

• fscryptctl add_key - add an encryption key to a filesystem
• fscryptctl remove_key - remove an encryption key from a filesystem
• fscryptctl key_status - get the status of an encryption key on a filesystem
• fscryptctl get_policy - get the encryption policy of a file or directory
• fscryptctl set_policy - set the encryption policy of an empty directory

Run fscryptctl --help for full usage details.

The add_key command accepts the encryption key in binary on standard input. It is critical that this be a real cryptographic key (and not a passphrase, for example), since fscryptctl doesn't do key stretching itself. Obviously, don't store the raw encryption key alongside the encrypted files. (If you need support for passphrases, use fscrypt instead of fscryptctl.)

After running the add_key command to add an encryption key to a filesystem, you can use the set_policy command to create an encrypted directory on that filesystem. The encryption key is specified by the 32-character hex "key identifier" that was printed by add_key. The directory must be empty.

Example Usage

# Create and mount an ext4 filesystem that supports encryption.
# (Alternatively, use `tune2fs -O encrypt` on an existing ext4 filesystem.)
# (For f2fs, use `mkfs.f2fs -O encrypt` or `fsck.f2fs -O encrypt`.)
> mkfs.ext4 -O encrypt /dev/vdb
> mount /dev/vdb /mnt

# Generate a random 512-bit key and store it in a file.
> head -c 64 /dev/urandom > /tmp/key

# Add the key to the filesystem.
> fscryptctl add_key /mnt < /tmp/key
f12fccad977328d20a16c79627787a1c

# Get the status of the key on the filesystem.
> fscryptctl key_status f12fccad977328d20a16c79627787a1c /mnt
Present (user_count=1, added_by_self)

# Create an encrypted directory that uses the key.
> fscryptctl set_policy f12fccad977328d20a16c79627787a1c /mnt/dir

# Show the directory's encryption policy that was just set.
> fscryptctl get_policy /mnt/dir
Encryption policy for /mnt/dir:
        Policy version: 2
        Master key identifier: f12fccad977328d20a16c79627787a1c
        Contents encryption mode: AES-256-XTS
        Filenames encryption mode: AES-256-CTS
        Flags: PAD_32

# Create some files in the encrypted directory.
> echo foo > /mnt/dir/foo
> mkdir /mnt/dir/bar

# Remove the encryption key from the filesystem.
# (Alternatively, unmounting the filesystem will remove the key too.)
> fscryptctl remove_key f12fccad977328d20a16c79627787a1c /mnt

# Get the status of the key on the filesystem.
> fscryptctl key_status f12fccad977328d20a16c79627787a1c /mnt
Absent

# The directory is now locked.  So the filenames are shown in encrypted form,
# and files can't be opened or created.
> ls /mnt/dir
AcbnATV97HZzxlmWNoErWS8QkdgTzMzbPU5hjs7XwvyralC5fQCtQA
qXT50ks2,3RzC8kqJ5FvnHgxS6oL2UDa8nsVkCFmoUQQygA3nWzxfA
> cat /mnt/dir/qXT50ks2,3RzC8kqJ5FvnHgxS6oL2UDa8nsVkCFmoUQQygA3nWzxfA
cat: /mnt/dir/qXT50ks2,3RzC8kqJ5FvnHgxS6oL2UDa8nsVkCFmoUQQygA3nWzxfA: Required key not available
> mkdir /mnt/dir/foobar
mkdir: cannot create directory ‘/mnt/dir/foobar’: Required key not available

# Re-adding the key restores access to the files.
> fscryptctl add_key /mnt < /tmp/key
f12fccad977328d20a16c79627787a1c
> ls /mnt/dir
bar foo
> cat /mnt/dir/foo
foo

Contributing

We would love to accept your contributions to fscryptctl. See the CONTRIBUTING.md file for more information.

Legal

Copyright 2017, 2020 Google LLC. Licensed under the Apache 2.0 License; see the LICENSE file for more information.

Authors: Joe Richey ([email protected]), Eric Biggers ([email protected])

This is not an official Google product.