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>
<link linkend="opt-boot.initrd.luks.devices._name__.device">boot.initrd.luks.devices.crypted.device</link> = "/dev/disk/by-uuid/3f6b0024-3a44-4fde-a43a-767b872abe5d";
<xref linkend="opt-fileSystems"/>."/".device = "/dev/mapper/crypted";
</programlisting>
  Should grub be used as bootloader, and <filename>/boot</filename> is located
  on an encrypted partition, it is necessary to add the following grub option:
<programlisting><xref linkend="opt-boot.loader.grub.enableCryptodisk"/> = true;</programlisting>
 </para>
</section>