HTTPS Keyscript

A keyscript and initramfs-tools helpers to allow a machine with a LUKS-encrypted boot drive to passwordlessly boot by fetching a remote key over HTTPS.

It is intended to protect against information disclosure in the event of accidental loss or untargeted theft - the key can be removed from the webserver as soon as the loss is noticed. It's not at all secure if you're trying to protect yourself against someone deliberately trying to obtain your information.

Features

• Falls back to asking for a passphrase if the remote key can't be retrieved
• Compatible with the Plymouth splash screen
• Caches keys across devices using the kernel keyring

Example

1. Create keyfile.

dd if=/dev/random bs=1c count=256 | base64 > unencrypted_keyfile
cat unencrypted_keyfile | openssl enc -base64 -aes-256-cbc -md sha256 -e -salt -out encrypted_keyfile -k somepassphrase
rm unencrypted_keyfile

2. Make encrypted_keyfile available somewhere on a webserver (e.g. https://example.org/encrypted_keyfile).
3. Install the keyscript and initramfs hooks.

wget https://github.com/stupidpupil/https-keyscript/releases/download/v1.0.1/https-keyscript_1.0.1_all.deb
sudo dpkg -i https-keyscript_1.0.1_all.deb

4. Add keyfile to LUKS slots.

busybox sh /lib/cryptsetup/scripts/wget_or_ask "somepassphrase:https://example.org/encrypted_keyfile" > unencrypted_keyfile
sudo cryptsetup luksAddKey /dev/someDevice unencrypted_keyfile
rm unencrypted_keyfile

5. Update the /dev/someDevice entry in /etc/crypttab with the option keyscript=wget_or_ask and with a 'key file' field like somepassphrase:https://example.org/encrypted_keyfile.
6. sudo update-initramfs -u

(You might also need to add the initramfs option to the crypttab in step 5 due to systemd's lack of support for keyscripts; see the crypttab man page.)

Caching

The keyscript uses the kernel keyring to cache decrypted keys for 60 seconds, avoiding multiple HTTPS requests where multiple devices have the same key file specified in /etc/crypttab. This feature depends on the keyutils package.

Alternatives

Mandos provides a system with better client authentication and automated detection of a client going offline for an unexpected period of time.

netkeyscript uses link-local IPv6 UDP packets with an unencrypted passphrase.

clevis provides a dracut-based unlocker for use with the tang keyserver.

There are a number of variations on using dropbear or similar to allow remote, but not unattended, restarts with an encrypted boot.

License

This collection of scripts is licensed under the GNU GPLv3.