Add Separated-Key Architecture

[dvicory]

Overview

Closes #132.

This PR introduces a separated-key architecture that protects SOPS-encrypted secrets (including ZFS passphrase, passwords, API keys) from physical access attacks. By separating the boot SSH key from the runtime SSH key used for SOPS decryption, an attacker with physical access to the unencrypted /boot partition can no longer decrypt your secrets.

New hosts created with gen-new-host now use separated-key mode by default. Migration tools are provided for upgrading existing single-key hosts.

Security Model

The Problem: Single-Key Vulnerability

In traditional single-key mode, one SSH key (/boot/host_key) serves two purposes:

1. Initrd SSH: SSH into initrd to decrypt ZFS root pool
2. SOPS decryption: Decrypt secrets for system operation

Since /boot must be unencrypted, this creates a vulnerability:

• 🔓 Physical access to drive → Can read /boot/host_key
• 🔓 With boot key → Can decrypt all SOPS secrets (ZFS passphrase, passwords, API keys)
• 🔓 With secrets → Full system compromise, even while powered off

The Solution: Separated-Key Architecture

Boot key (/boot/host_key):

• Purpose: SSH host key for initrd environment only
• Location: Unencrypted /boot partition (required for remote decrypt)
• Security: Physical access = SSH MITM possible, but data remains safe at rest

Runtime key (/persist/etc/ssh/ssh_host_ed25519_key):

• Purpose: SSH host key for normal system + SOPS secrets decryption
• Location: Encrypted on ZFS pool in /persist
• Security: Physical access = cannot decrypt secrets without boot unlock

Configuration Modes

Single-key (legacy, discouraged):

sops.age.sshKeyPaths = ["/boot/host_key"];

Separated-key (default, recommended):

sops.age.sshKeyPaths = ["/persist/etc/ssh/ssh_host_ed25519_key"];

Core Implementation

Automatic Detection

Skarabox automatically detects separated-key mode when the runtime key path (/persist/etc/ssh/ssh_host_ed25519_key) is configured in sops.age.sshKeyPaths.

New Configuration Options

• skarabox.runtimeHostKeyPath - Path to runtime private key (default: "${hostDir}/runtime_host_key")
• skarabox.runtimeHostKeyPub - Runtime public key content

Key Installation

During system installation (install-on-beacon), the runtime key is:

1. Passed via --disk-encryption-keys to disko
2. Installed to /persist/etc/ssh/ssh_host_ed25519_key in disko's postMountHook
3. Configured for OpenSSH via services.openssh.hostKeys

Boot Dependency

/persist is marked as neededForBoot to ensure SOPS can access the runtime key early in boot process.

Breaking Changes

Default to Separated-Key Mode

gen-new-host now creates separated-key hosts by default. Use --single-key flag for legacy single-key mode.

Backward compatibility: Existing single-key hosts remain fully functional.

Migration Tools for Existing Hosts

Three new tools help migrate existing single-key hosts to separated-key architecture:

• enable-key-separation - Generates runtime keys and updates .sops.yaml configuration
• install-runtime-key - Deploys runtime key to /persist/etc/ssh/ on target host
• rotate-boot-key - Securely replaces boot key via partition wipe

Complete migration workflow is documented in docs/normal-operations.md.

Testing

• ✅ Fresh separated-key hosts created with gen-new-host
• ✅ Single-key hosts migrating to separated-key mode
• ✅ Boot key rotation with rotate-boot-key