I have a setup based on this, but I modified it to encrypt the SSH host key using the TPM. That way, I can detect a MiTM from an attacker who has stolen the drive or modified the boot policy because host key verification will fail.
/usr/lib/dracut/modules.d/46cryptssh:
#!/bin/bash
check() {
require_binaries sshd || return 1
return 0
}
depends() {
return 0
}
install() {
if [ ! -e /etc/ssh/dracut ]; then
mkdir /etc/ssh/dracut &&
tmp=$(mktemp -d) &&
head -c128 /dev/random > $tmp/passphrase &&
ssh-keygen -t ed25519 -f /etc/ssh/dracut/ssh_host_ed25519_key -N"$(base64
cryptsshd.service:
[Unit]
Description=OpenSSH Daemon for Disk Encryption Passphrase
DefaultDependencies=no
Before=cryptsetup.target
After=network-online.target
[Service]
Type=notify-reload
ExecStartPre=/bin/sh -c '/usr/bin/ssh-keygen -p -f /etc/ssh/ssh_host_ed25519_key \
-N "" -P "$(/usr/bin/tpm2_unseal -c /etc/ssh/seal -p pcr:sha256:0,7 | base64)"'
ExecStart=/usr/bin/sshd -D
KillMode=process
Restart=always
[Install]
WantedBy=sysinit.target
That encrypts the SSH host key using a password sealed with PCR7, which is invalidated if an attacker disables Secure Boot or tampers with the enrolled keys. Thus, an attacker can't extract the key from the drive or by modifying the kernel command line to boot to a shell (since that's not allowed without disabling secure boot).
It's still probably vulnerable to a cold boot attack, since the key is decrypted CPU-side. It would be interesting to perform the actual key operations on the TPM itself to prevent this.