Verify RSA-SHA256 signature on PIA server list response

The server list endpoint returns JSON on line 1 with a base64-encoded
RSA-SHA256 signature on lines 3+. This was previously ignored. Add
verifyServerList() that checks the signature against PIA's public
signing key before trusting the data. On failure the service aborts
and systemd restarts it.

Also bump RestartSec to 5m to avoid hammering PIA servers on repeated
failures, and add openssl to container dependencies.

common/network/pia-vpn/scripts.nix

@@ -1,5 +1,6 @@
 let
   caPath = ./ca.rsa.4096.crt;
+  pubKeyPath = ./pubkey.pem;
 in
 
 # Bash function library for PIA VPN WireGuard operations.
@@ -21,6 +22,23 @@ in
       fi
     }
 
+    # Verify the server list RSA-SHA256 signature (line 1 = JSON, lines 3+ = base64 signature).
+    verifyServerList() {
+      local raw=$1
+      local sig_file
+      sig_file=$(mktemp)
+      echo "$raw" | tail -n +3 | base64 -d > "$sig_file"
+      if ! echo -n "$(echo "$raw" | head -n 1 | tr -d '\n')" | \
+           openssl dgst -sha256 -verify "${pubKeyPath}" \
+           -signature "$sig_file"; then
+        echo "ERROR: Server list signature verification failed" >&2
+        rm -f "$sig_file"
+        return 1
+      fi
+      echo "Server list signature verified"
+      rm -f "$sig_file"
+    }
+
     fetchPIAToken() {
       local PIA_USER PIA_PASS resp
       echo "Reading PIA credentials..."
@@ -39,20 +57,20 @@ in
 
     choosePIAServer() {
       local serverLocation=$1
-      local servers servers_json totalservers serverindex
-      servers=$(mktemp)
+      local raw servers_json totalservers serverindex
       servers_json=$(mktemp)
       echo "Fetching PIA server list..."
-      curl -s $(proxy_args) \
-        "https://serverlist.piaservers.net/vpninfo/servers/v6" > "$servers"
-      head -n 1 "$servers" | tr -d '\n' > "$servers_json"
+      raw=$(curl -s $(proxy_args) \
+        "https://serverlist.piaservers.net/vpninfo/servers/v6")
+      verifyServerList "$raw"
+      echo "$raw" | head -n 1 | tr -d '\n' > "$servers_json"
 
       totalservers=$(jq -r \
         '.regions | .[] | select(.id=="'"$serverLocation"'") | .servers.wg | length' \
         "$servers_json")
       if ! [[ "$totalservers" =~ ^[0-9]+$ ]] || [ "$totalservers" -eq 0 ] 2>/dev/null; then
         echo "ERROR: Location \"$serverLocation\" not found." >&2
-        rm -f "$servers_json" "$servers"
+        rm -f "$servers_json"
         return 1
       fi
       echo "Found $totalservers WireGuard servers in region '$serverLocation'"
@@ -66,7 +84,7 @@ in
         "$servers_json")
       WG_SERVER_PORT=$(jq -r '.groups.wg | .[0] | .ports | .[0]' "$servers_json")
 
-      rm -f "$servers_json" "$servers"
+      rm -f "$servers_json"
       echo "Selected server $serverindex/$totalservers: $WG_HOSTNAME ($WG_SERVER_IP:$WG_SERVER_PORT)"
     }