Verify RSA-SHA256 signatures on all PIA API responses

Every PIA API response includes a trailing RSA-SHA256 signature
(line 1 = JSON, lines 3+ = base64-encoded signature) which was
previously ignored entirely. Add verifyPIAResponse() that checks
each response against PIA's public signing key before trusting
the data. On verification failure the service aborts and systemd
restarts it.

Also bump RestartSec to 5m to avoid hammering PIA servers on
repeated failures.

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.
@@ -21,14 +22,33 @@ in
       fi
     }
 
+    # Verify a raw PIA API response (line 1 = JSON, lines 3+ = base64 RSA-SHA256 signature).
+    verifyPIAResponse() {
+      local raw=$1 label=$2
+      local sig_file
+      sig_file=$(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: $label signature verification failed" >&2
+        rm -f "$sig_file"
+        return 1
+      fi
+      echo "$label signature verified"
+      rm -f "$sig_file"
+    }
+
     fetchPIAToken() {
-      local PIA_USER PIA_PASS resp
+      local PIA_USER PIA_PASS raw resp
       echo "Reading PIA credentials..."
       PIA_USER=$(sed '1q;d' /run/agenix/pia-login.conf)
       PIA_PASS=$(sed '2q;d' /run/agenix/pia-login.conf)
       echo "Requesting PIA authentication token..."
-      resp=$(curl -s $(proxy_args) -u "$PIA_USER:$PIA_PASS" \
+      raw=$(curl -s $(proxy_args) -u "$PIA_USER:$PIA_PASS" \
         "https://www.privateinternetaccess.com/gtoken/generateToken")
+      verifyPIAResponse "$raw" "generateToken"
+      resp=$(echo "$raw" | head -n 1)
       PIA_TOKEN=$(echo "$resp" | jq -r '.token')
       if [[ -z "$PIA_TOKEN" || "$PIA_TOKEN" == "null" ]]; then
         echo "ERROR: Failed to fetch PIA token: $resp" >&2
@@ -39,20 +59,20 @@ 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 $(proxy_args) \
+        "https://serverlist.piaservers.net/vpninfo/servers/v6")
+      verifyPIAResponse "$raw" "serverList"
+      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 +86,7 @@ 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)"
     }
 
@@ -77,14 +97,16 @@ in
     }
 
     authorizeKeyWithPIAServer() {
-      local addKeyResponse
+      local raw addKeyResponse
       echo "Sending addKey request to $WG_HOSTNAME ($WG_SERVER_IP:$WG_SERVER_PORT)..."
-      addKeyResponse=$(curl -s -G $(proxy_args) \
+      raw=$(curl -s -G $(proxy_args) \
         --connect-to "$WG_HOSTNAME::$WG_SERVER_IP:" \
         --cacert "${caPath}" \
         --data-urlencode "pt=$PIA_TOKEN" \
         --data-urlencode "pubkey=$PUBLIC_KEY" \
         "https://$WG_HOSTNAME:$WG_SERVER_PORT/addKey")
+      verifyPIAResponse "$raw" "addKey"
+      addKeyResponse=$(echo "$raw" | head -n 1)
       local status
       status=$(echo "$addKeyResponse" | jq -r '.status')
       if [[ "$status" != "OK" ]]; then
@@ -158,13 +180,15 @@ in
     }
 
     reservePortForward() {
-      local payload_and_signature
+      local raw payload_and_signature
       echo "Requesting port forward signature from $WG_HOSTNAME..."
-      payload_and_signature=$(curl -s -m 5 \
+      raw=$(curl -s -m 5 \
         --connect-to "$WG_HOSTNAME::$WG_SERVER_IP:" \
         --cacert "${caPath}" \
         -G --data-urlencode "token=$PIA_TOKEN" \
         "https://$WG_HOSTNAME:19999/getSignature")
+      verifyPIAResponse "$raw" "getSignature"
+      payload_and_signature=$(echo "$raw" | head -n 1)
       local status
       status=$(echo "$payload_and_signature" | jq -r '.status')
       if [[ "$status" != "OK" ]]; then
@@ -195,14 +219,16 @@ in
     }
 
     refreshPIAPort() {
-      local bindPortResponse
+      local raw bindPortResponse
       echo "Refreshing port forward binding with $WG_HOSTNAME..."
-      bindPortResponse=$(curl -Gs -m 5 \
+      raw=$(curl -Gs -m 5 \
         --connect-to "$WG_HOSTNAME::$WG_SERVER_IP:" \
         --cacert "${caPath}" \
         --data-urlencode "payload=$PORT_PAYLOAD" \
         --data-urlencode "signature=$PORT_SIGNATURE" \
         "https://$WG_HOSTNAME:19999/bindPort")
+      verifyPIAResponse "$raw" "bindPort"
+      bindPortResponse=$(echo "$raw" | head -n 1)
       echo "bindPort response: $bindPortResponse"
     }
   '';