luksrku

luksrku is a tool that allows you to remotely unlock LUKS disks during boot up from within your initrd. The intention is to have full-disk-encryption with LUKS-rootfs running headlessly. You should be able to remotely unlock their LUKS cryptographic file systems when you know they have been (legitimately) rebooted.

This works as follows: The luksrku client (which needs unlocking) and luksrku server (which holds all the LUKS keys) share a secret. The client either knows the address of the server or it can issue a broadcast in the network to find the correct one. With the help of the shared secret, a TLS connection is established between the client and a legitimate server (who also knows the same secret). The server then tells the client all the LUKS passphrases, which performs luksOpen on all volumes.

Security

luksrku uses TLSv1.3-PSK with forward-secrecy key shares (i.e., ECDHE). The curves that are used for key agreement are X448 and X25519. TLS_CHACHA20_POLY1305_SHA256 or TLS_AES_256_GCM_SHA384 are accepted as cipher suites.

The TLS PSKs are 256 bit long and randomly generated (/dev/urandom). Likewise, the LUKS passphrases are based on 256 bit long secrets, also generated from /dev/urandom, and are converted to Base64 for easier handling (when setting up everything initially).

The binary protocol that runs between server and client is intentionally extremely simple to allow for easy code review. It exclusively uses fixed message lengths. There are two portions to it, an UDP and a TCP portion:

Via UDP, a client broadcasts its client UUID (randomly generated when creating the client in the database) on the network (port 23170). A server then can check if it’s key database contains that client’s LUKS keys. If it does, the server will respond with a fixed unicast UDP datagram. The client receives this datagram and tries to establish a TCP connection to that server on the luksrku port 23170. This connection is secured using TLSv1.3-PSK, i.e., even when the UDP messages are spoofed/forged, a successful connection will only then happen if the server and client share the same, previously defined, PSK.

For persistent storage, the key database is encrypted, using AES256-GCM. A 128 bit randomized initialization vector is used and all data is authenticated with a 128 bit authentication tag. Key derivation is done using scrypt with N = 262144 = 2^18, r = 8, p = 1 (although this is flexible in code and can be easily adapted).

When the key database is not in use, the server encrypts all LUKS passphrases and PSKs in-memory (again, using AES256-GCM). A large, 1 MiB pre-key is also kept in memory. The AES key is derived from this pre-key using PBKDF2-HMAC-SHA256 and an iteration count that results in ~25ms key derivation. While it might seem nonsensical to encrypt memory and have the key right next to the encrypted data, the reason for this this is to thwart cold-boot attacks. A successful cold-boot attack would require a complete and perfect 1 MiB snapshot of the pre-key (or an acquisition in the short timeframe where the key vault is open) – something that is difficult to do because of naturally occurring bit errors during cold boot acquisition.

Dependencies

OpenSSL v1.1 is required for luksrku as well as pkg-config.

Usage

The help pages of luksrku are fairly well documented, i.e.:

$ ./luksrku
error: no command supplied

Available commands:
    ./luksrku edit     Interactively edit a key database
    ./luksrku server   Start a key server process
    ./luksrku client   Unlock LUKS volumes by querying a key server

For further help: ./luksrku (command) --help

luksrku version v0.02-45-gf01ec97d6b-dirty

Then, for each command, you have an own help page:

$ ./luksrku edit --help
usage: luksrku edit [-v] [filename]

Edits a luksrku key database.

positional arguments:
  filename       Database file to edit.

optional arguments:
  -v, --verbose  Increase verbosity. Can be specified multiple times.

$ ./luksrku server --help
usage: luksrku server [-p port] [-s] [-v] filename

Starts a luksrku key server.

positional arguments:
  filename              Database file to load keys from.

optional arguments:
  -p port, --port port  Port that is used for both UDP and TCP communication.
                        Defaults to 23170.
  -s, --silent          Do not answer UDP queries for clients trying to find a
                        key server, only serve key database using TCP.
  -v, --verbose         Increase verbosity. Can be specified multiple times.

$ ./luksrku client --help
usage: luksrku client [-t secs] [-p port] [--no-luks] [-v] filename [hostname]

Connects to a luksrku key server and unlocks local LUKS volumes.

positional arguments:
  filename              Exported database file to load TLS-PSKs and list of
                        disks from.
  hostname              When hostname is given, auto-searching for suitable
                        servers is disabled and only a connection to the given
                        hostname is attempted.

optional arguments:
  -t secs, --timeout secs
                        When searching for a keyserver and not all volumes can
                        be unlocked, abort after this period of time, given in
                        seconds. Defaults to 60 seconds.
  -p port, --port port  Port that is used for both UDP and TCP communication.
                        Defaults to 23170.
  --no-luks             Do not call LUKS/cryptsetup. Useful for testing
                        unlocking procedure.
  -v, --verbose         Increase verbosity. Can be specified multiple times.

Example

First, you need to create a server key database. For this you use the editor:

$ ./luksrku edit
> add_host my_host

Now there’s a host “my_host” in the key database. At any point you can inspect the database by using the “list” command:

Keydb version 2, server database, 1 hosts.
    Host 1: "my_host" UUID e7ff6e3d-1793-48f6-b43b-9c7bb0348622 -- 0 volumes:

You’ll see that the host has no volumes associated with it. Determine the UUID of the LUKS device that you want luksrku to decrypt, then add this volume with the name you want it to have after unlocking. In our case, the UUID is 18de9f14-2914-4a8b-9b46-b7deacbfbe8a and we want it to decrypt as “crypt-root”:

> add_volume my_host crypt-root 18de9f14-2914-4a8b-9b46-b7deacbfbe8a
LUKS passphrase of crypt-root / 18de9f14-2914-4a8b-9b46-b7deacbfbe8a: 5DySDFcpVtBRoIMNv7mrLqlozPYeq7X5kPmB3M1wsW8A

At this point, luksrku will tell you, in clear text, the LUKS passphrase that you need to add to the volume. Then, you save the server database:

> save server.bin
Database passphrase:

It asks you for a passphrase that is needed to decrypt the file. On disk it’s always stored encrypted. Using an encrypted server database is highly recommended.

For the client, you export the client portion of the database:

> export my_host my_host.bin
Client passphrase:

Note that client databases can also be encrypted, but they’re less critical than the server database. The client database does not contain the LUKS passphrases, it only contains the required TLS-PSK so that a successful connection to a luksrku server can be established.

With these two in place, you can now start a luksrku server:

$ ./luksrku server server.bin
Database passphrase:
[I]: Serving luksrku database for 1 hosts.

And on your client, when you want the LUKS disks to be unlocked:

$ ./luksrku client my_host.bin

Integration into initramfs

Using luksrku as part of your initramfs is quite easy. You’ll need a server somewhere in your network and an exported client database. On the client, you copy the client database file into /etc/luksrku-client.bin.

Then, install luksrku globally by performing make install as root and install the initramfs script by running install in the initramfs/ subdirectory. You’ll only need to install that once.

# make install
strip luksrku
cp luksrku /usr/local/sbin/
chown root:root /usr/local/sbin/luksrku
chmod 755 /usr/local/sbin/luksrku
# cd initramfs
# ./install

Finally, have initramfs recreate your initial ramdisk:

# update-initramfs -u

That’s it, it should now work.

Legacy version

luksrku has undergone an extensive rewrite of the internal code. The current version is fully incompatible to previous (<= v0.02) versions. Servers and clients will not recognize each other, database formats and the creation of database works entirely different.

License

GNU GPL-3.