feat: automate SSH config ('known_hosts', 'authorized_keys' ...)

modules/ssh/extract-keys.nix

@@ -0,0 +1,24 @@
+{ lib, config, ... }:
+let
+  username = config.ssh.username;
+in
+{
+  # auto extract SSH keys
+  system.activationScripts.extractSshKeys = lib.stringAfter [ "etc" ] ''
+    HOST_KEY="/etc/ssh/ssh_host_ed25519_key.pub"
+    HOST_DIR="/home/${username}/nix/hosts/${config.networking.hostName}"
+
+    if [ -f "$HOST_KEY" ] && [ -d "$HOST_DIR" ]; then
+      cp "$HOST_KEY" "$HOST_DIR/ssh_host.pub"
+      chown ${username}:users "$HOST_DIR/ssh_host.pub"
+      chmod 644 "$HOST_DIR/ssh_host.pub"
+    fi
+
+    USER_KEY="/home/${username}/.ssh/id_ed25519.pub"
+    if [ -f "$USER_KEY" ] && [ -d "$HOST_DIR" ]; then
+      cp "$USER_KEY" "$HOST_DIR/ssh_user.pub"
+      chown ${username}:users "$HOST_DIR/ssh_user.pub"
+      chmod 644 "$HOST_DIR/ssh_user.pub"
+    fi
+  '';
+}