3 ms·

This has been a "need" for a long time for home users with FDE. The go-to software solution for Linux has been: https://github.com/gsauthof/dracut-sshd https:/

• hnarn 8mo ago
  Sure, if you're using dracut, which is not true for "Linux" in general.
  • benttoothpaste 8mo ago
    Most Linux distros are not Arch either. It would be nice to have more support for this use case in general - like something one can configure easily during the initial OS setup. I use OpenSuse so I had to use the guide for Fedora, but there were some differences as far as I remember.
  • mmh0000 8mo ago
    Dracut is used by default on: Fedora, RHEL, CentOS, Rocky, Alma, Arch, and Gentoo Dracut is available on: Debian and Ubuntu That covers most common Linux distros. - Personally, I'm using this on Fedora.
    • hnarn 8mo ago
      ”Fedora, RHEL, CentOS, Rocky, Alma” are all the same distro group: Enterprise Linux (EL). That it's ”available” on other distros is irrelevant, almost anything is ”available” as a non standard choice for any distro.
• NobodyNada 8mo ago
  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 < $tmp/passphrase)" && tpm2_createprimary -C o -c $tmp/primary.ctx && tpm2_pcrread -o $tmp/pcr.bin sha256:0,7 tpm2_createpolicy --policy-pcr -l sha256:0,7 -f $tmp/pcr.bin -L $tmp/pcr.policy tpm2_create -C $tmp/primary.ctx -L $tmp/pcr.policy -i $tmp/passphrase -c $tmp/seal.ctx && tpm2_evictcontrol -C o -c $tmp/seal.ctx -o /etc/ssh/dracut/seal || { rm -r $tmp /etc/ssh/dracut exit 1 } rm -r $tmp fi for file in /etc/ssh/dracut/*; do inst_simple "$file" "/etc/ssh/${file#/etc/ssh/dracut/}" done mkdir -p -m 0700 "$initdir"/root/.ssh /usr/bin/install -m 600 /etc/ssh/dracut_authorized_keys "$initdir"/root/.ssh/authorized_keys inst_binary /usr/sbin/sshd inst_binary /usr/sbin/ssh-keygen inst_binary /usr/bin/tpm2_unseal inst_binary /usr/bin/base64 inst_simple /usr/lib/libtss2-tcti-device.so inst_simple "$moddir"/cryptsshd.service "$systemdsystemunitdir"/cryptsshd.service inst_simple "$moddir"/sshd_config /etc/ssh/sshd_config inst_binary /usr/lib/ssh/sshd-session inst_binary /usr/lib/ssh/sshd-auth mkdir -p -m 0755 "$initdir"/var/empty/sshd mkdir -p -m 0755 "$initdir"/usr/share/empty.sshd mkdir -p -m 0755 "$initdir"/var/log touch "$initdir"/var/log/lastlog systemctl -q --root "$initdir" enable cryptsshd } cryptsshd.service: [Unit] Description=OpenSSH Daemon for Disk Encryption Passphrase DefaultDependencies=no Before=cryptsetup.target After=network-online.target [Service] Type=notify-reload ExecStartPre=/bin/sh -c '/usr/bin/ssh-keygen -p -f /etc/ssh/ssh_host_ed25519_key \ -N "" -P "$(/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.