nixpkgs/nixos/doc/manual/configuration/luks-file-systems.xml

<section xmlns="http://docbook.org/ns/docbook"
         xmlns:xlink="http://www.w3.org/1999/xlink"
         xmlns:xi="http://www.w3.org/2001/XInclude"
         version="5.0"
         xml:id="sec-luks-file-systems">

<title>LUKS-Encrypted File Systems</title>

<para>NixOS supports file systems that are encrypted using
<emphasis>LUKS</emphasis> (Linux Unified Key Setup).  For example,
here is how you create an encrypted Ext4 file system on the device
<filename>/dev/disk/by-uuid/3f6b0024-3a44-4fde-a43a-767b872abe5d</filename>:

<screen>
# cryptsetup luksFormat /dev/disk/by-uuid/3f6b0024-3a44-4fde-a43a-767b872abe5d

WARNING!
========
This will overwrite data on /dev/disk/by-uuid/3f6b0024-3a44-4fde-a43a-767b872abe5d irrevocably.

Are you sure? (Type uppercase yes): YES
Enter LUKS passphrase: ***
Verify passphrase: ***

# cryptsetup luksOpen /dev/disk/by-uuid/3f6b0024-3a44-4fde-a43a-767b872abe5d crypted
Enter passphrase for /dev/disk/by-uuid/3f6b0024-3a44-4fde-a43a-767b872abe5d: ***

# mkfs.ext4 /dev/mapper/crypted
</screen>

To ensure that this file system is automatically mounted at boot time
as <filename>/</filename>, add the following to
<filename>configuration.nix</filename>:

<programlisting>
boot.initrd.luks.devices.crypted.device = "/dev/disk/by-uuid/3f6b0024-3a44-4fde-a43a-767b872abe5d";
fileSystems."/".device = "/dev/mapper/crypted";
</programlisting>

</para>

</section>