feat(image): remote LUKS unlock via a Tor onion service in the initramfs

Encrypted image setups can now bake a Tor onion service into the initramfs
so the dropbear unlock shell stays reachable behind NAT or a dynamic IP.
When the user opts in, configure_encryption installs tor + busybox, drops
the mkinitcpio hooks (ordered `netconf tor dropbear encryptssh`), generates
the v3 onion keys offline in the image chroot, and prints the stable
.onion address. Unlock with `torsocks ssh root@<onion-address>`.

The runtime hook syncs the clock via NTP first (RTC-less boards boot at
1970, which Tor's consensus checks reject) and starts the onion service
pointing at dropbear on 127.0.0.1:22.

Hardening baked in from an adversarial review of the shipped path:
- cmdline.txt boot path (RPi4-class firmware boot) now sets the same
  ip=::::<host>:eth0:dhcp net.ifnames=0 params as the boot.txt path, so
  the initramfs actually gets a network and the onion can publish.
- the initramfs bakes in libnss_dns.so.2 so the NTP hostname resolves.
- the hook extracts DHCP DNS with sed instead of sourcing the lease files,
  which would run attacker-controlled DHCP option strings as root pre-boot.
- NTP is attempted unconditionally (bounded), not gated on DHCP-provided DNS.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

lim/image/tor.py

@@ -0,0 +1,79 @@
+"""Tor onion service in the initramfs for remote LUKS unlock.
+
+The onion keys are generated offline inside the image chroot
+(``tor --DisableNetwork 1`` writes them without touching the network)
+and baked into the initramfs by the "tor" mkinitcpio hook, so the
+dropbear unlock shell stays reachable under a stable .onion address
+even behind NAT or a dynamic IP.
+"""
+
+from pathlib import Path
+
+from lim import config, packages, runner, ui
+from lim.errors import LimError
+from lim.image.plan import ImagePlan
+from lim.image.session import chroot_bash, install_packages
+
+# Paths inside the image (relative to the mounted root).
+ONION_DIR = "etc/tor/initramfs-onion"
+TORRC_PATH = "etc/tor/initramfs-torrc"
+
+# An empty -f torrc keeps the image's /etc/tor/torrc (User tor, ...) out of
+# the keygen run; with DisableNetwork the keys appear within a second, the
+# loop only cushions slow qemu-emulated chroots.
+_KEYGEN_SCRIPT = f"""
+mkdir -p /{ONION_DIR}
+chmod 0700 /{ONION_DIR}
+: > /tmp/tor-keygen-torrc
+tor -f /tmp/tor-keygen-torrc --DisableNetwork 1 \\
+    --DataDirectory /tmp/tor-keygen-data \\
+    --HiddenServiceDir /{ONION_DIR} \\
+    --HiddenServicePort "22 127.0.0.1:22" \\
+    --SocksPort 0 --RunAsDaemon 0 --Log "notice stderr" &
+tor_pid=$!
+for _ in $(seq 1 30); do
+    [ -s /{ONION_DIR}/hostname ] && break
+    sleep 1
+done
+kill "$tor_pid" 2>/dev/null || true
+rm -rf /tmp/tor-keygen-data /tmp/tor-keygen-torrc
+[ -s /{ONION_DIR}/hostname ]
+"""
+
+
+def _install_initcpio_files(root: Path) -> None:
+    source_dir = config.CONFIGURATION_PATH / "initcpio"
+    for source, target in (
+        (source_dir / "tor_install", root / "etc/initcpio/install/tor"),
+        (source_dir / "tor_hook", root / "etc/initcpio/hooks/tor"),
+        (source_dir / "torrc", root / TORRC_PATH),
+    ):
+        ui.info(f"Installing {target}...")
+        runner.run(
+            ["install", "-D", "-m", "0644", str(source), str(target)], sudo=True
+        )
+
+
+def _generate_onion_keys(root: Path) -> str:
+    hostname_path = root / ONION_DIR / "hostname"
+    if hostname_path.is_file():
+        ui.info("Onion keys already exist, keeping the existing address.")
+    else:
+        ui.info("Generating onion service keys (offline, inside the chroot)...")
+        chroot_bash(root, _KEYGEN_SCRIPT, error_msg="Onion key generation failed.")
+        if not hostname_path.is_file():
+            raise LimError(f"Onion key generation produced no {hostname_path}.")
+    return hostname_path.read_text().strip()
+
+
+def configure_tor_unlock(plan: ImagePlan, root: Path) -> str:
+    """Install everything the "tor" mkinitcpio hook bakes in; return the onion address."""
+    ui.info("Setting up remote unlock via Tor onion service...")
+    install_packages(
+        plan.distribution, root, " ".join(packages.get_packages("server/tor"))
+    )
+    _install_initcpio_files(root)
+    onion_address = _generate_onion_keys(root)
+    ui.success(f"Onion unlock address: {onion_address}")
+    ui.info(f"Unlock later with: torsocks ssh root@{onion_address}")
+    return onion_address