Title: Linux $HOME encryption with ecryptfs

	Title: Linux $HOME encryption with ecryptfs
	Author: Solène
	Date: 12 March 2023
	Tags: linux encryption privacy
	Description: In this article, you will learn how to encrypt a user home
	directory, or a single directory using ecryptfs

	# Introduction

	In this article, I'd like to share with you about the Linux specific
	feature ecryptfs, which allows users to have encrypted directories.

	While disk encryption done with cryptsetup/LUKS is very performant and
	secure, there are some edge cases in which you may want to use
	ecryptfs, whether the disk is LUKS encrypted or not.

	I've been able to identify a few use cases making ecryptfs relevant:

	* a multi-user system, people want their files to be private (and full
	disk encryption wouldn't help here)
	* an encrypted disk on which you want to have an encrypted directory
	that is only available when needed (preventing a hacked live computer
	to leak important files)
	* a non-encrypted disk on which you want to have an encrypted
	directory/$HOME instead of reinstalling with full disk encryption

	ecryptfs official website

	# Full $HOME Encryption

	In this configuration, you want all the files in the $HOME directory of
	your user to be encrypted. This works well and especially as it
	integrates with PAM (the "login manager" in Linux) so it unlocks the
	files upon login.

	I tried the following setup on Gentoo Linux, the setup is quite
	standard for any Linux distribution packaging ecryptfs-utils.

	## Setup

	As I don't want to duplicate documentation effort, let me share two
	links explaining how to set up the home encryption for a user.

	Gentoo Wiki: Encrypt a home directory with ECryptfs
	ArchWiki: eCryptfs

	Both guides are good, they will explain thoroughly how to set up
	ecryptfs for a user.

	However, here is a TLDR version:

	1. install ecryptfs-utils and make sure ecryptfs module is loaded at
	boot
	2. modify `/etc/pam.d/system-auth` to add ecryptfs unlocking at login
	(3 lines are needed, at specific places)
	3. run `ecryptfs-migrate-home -u $YOUR_USER` as root to convert the
	user home directory into an encrypted version
	4. delete the old unencrypted home which should be named after
	`/home/YOUR_USER.xxxxx` where xxxxx are random characters (make sure
	you have backups)

	After those steps, you should be able to log in with your user, `mount`
	outputs should show a dedicated entry for the home directory.

	# Private directory encryption

	In this configuration, you will have ecryptfs encrypting a single
	directory named `Private` in the home directory.

	That can be useful if you already have an encrypted disk, but you have
	very secret files that must be encrypted when you don't need them, this
	will protect file leak on a compromised running system, except if you
	unlock the directory while the system is compromised.

	This can also be used on a thrashable system (like my netbook) that
	isn't encrypted, but I may want to save a few files there that are
	private.

	## Setup

	That part is really easy:

	1. install a package named `ecryptfs-utils` (may depend on your
	distribution)
	2. run `ecryptfs-setup-private --noautomount`
	3. Type your login password
	4. Press enter to use an auto generated mount passphrase (you don't use
	this one to unlock the directory)
	5. Done!

	The mount passphrase is used in addition to the login passphrase to
	encrypt the files, you may need it if you have to unlock backuped
	encrypted files, so better save it in your password manager if you make
	backup of the encrypted files.

	You can unlock the access to the directory `~/Private` by typing
	`ecryptfs-mount-private` and type your login password.
	Congratulations, now you have a local safe for your files!

	# Performance

	Ecryptfs was available in older Ubuntu installer releases as an option
	to encrypt a user home directory without the full disk, it seems it has
	been abandoned due to performance reasons.

	I didn't make extensive benchmarks here, but I compared the writing
	speed of random characters into a file on an unencrypted ext4
	partition, and the ecryptfs private directory on the same disk. On the
	unencrypted directory, it was writing at 535 MB/s while on the ecryptfs
	it was only writing at 358 MB/s, that's almost 33% slower. However,
	it's still fast enough for a daily workstation. I didn't measure the
	time to read or browse many files, but it must be slower. A LUKS
	encrypted disk should only have a performance penalty of a few percent,
	so ecryptfs is really not efficient in comparison, but it's still fast
	enough if you don't do database operation on it.

	# Security shortcoming

	There are extra security shortcomings coming with ecryptfs: when using
	your encrypted files unlocked, they may be copied in swap or in
	temporary directories, or in cache.

	If you use the Private encrypted directories, for instance, you should
	think that most image reader will create a thumbnail in your HOME
	directory, so pictures in Private may have a local copy that is
	available outside the encrypted directory. Some text editors may cache
	a backup file in another directory.

	If your system is running a bit out of memory, data may be written to
	the swap file, if it's not encrypted then one may be able to recover
	files that were opened during that time. There is a command
	`ecryptfs-setup-swap` from the ecryptfs package which check if the swap
	files are encrypted, and if not, propose to encrypt them using LUKS.

	One major source of leakage is the `/tmp/` directory, that may be used
	by programs to make a temporary copy of an opened file. It may be safe
	to just use a `tmpfs` filesystem for it.

	Finally, if you only have a Private directory encrypted, don't forget
	that if you use a file browser to delete a file, it may end up in a
	trash directory on the unencrypted filesystem.

	# Troubleshooting

	## setreuid: Operation not permitted

	If you get the error `setreuid: Operation not permitted` when running
	ecryptfs commands, this mean the ecryptfs binaries aren't using suid
	bit. On Gentoo, you have to compile `ecryptfs-utils` with the USE
	suid.

	# Conclusion

	Ecryptfs is can be useful in some real life scenarios, and doesn't have
	much alternative. It's especially user-friendly when used to encrypt
	the whole home because users don't even have to know about it.

	Of course, for a private encrypted directory, the most tech-savvy can
	just create a big raw file and format it in LUKS, and mount it on need,
	but this mean you will have to manage the disk file as a separate
	partition with its own size, and scripts to mount/umount the volume,
	while ecryptfs offers an easy secure alternative with a performance
	drawback.