feat(sync): read-only PHP endpoint serving the encrypted snapshot

Alternative to WebDAV, and the base for an Android client. The server never decrypts: it stores and serves opaque bytes, so a compromise yields ciphertext plus size and mtime. GET only for now, with a strong ETag and If-None-Match; PUT answers 405 explicitly so a client fails loudly rather than believing it wrote. A bad token answers 404 like a missing snapshot, so an unauthenticated caller learns nothing. HTTPS is required - the bearer token rides a header on every call.
2026-08-14 17:41:34 +01:00
parent c74b708b39
commit ec4fa433a9
4 changed files with 168 additions and 0 deletions
@@ -0,0 +1,102 @@
+<?php
+/**
+ * Read-only sync endpoint for the vault snapshot.
+ *
+ * The snapshot is an OPAQUE encrypted container (pm-encrypted-export-v1):
+ * PBKDF2-SHA256 600k + AES-GCM, produced and read by the clients. This script
+ * never decrypts anything and never needs to — it stores and serves bytes.
+ * A compromised server yields ciphertext plus the metadata below, nothing more.
+ *
+ * Deliberately read-only for now: GET only. The write path (PUT with If-Match)
+ * comes with the two-way client and is refused explicitly rather than silently
+ * ignored, so a client that tries it fails loudly instead of believing it wrote.
+ */
+
+declare(strict_types=1);
+
+require __DIR__ . '/config.php';   // defines SYNC_TOKEN and SYNC_FILE
+
+// ---------------------------------------------------------------------------
+// Transport
+// ---------------------------------------------------------------------------
+
+// The token travels in a header on every request; without TLS it is readable by
+// anyone on the path, and so is the ciphertext. Refuse rather than leak.
+$https = (!empty($_SERVER['HTTPS']) && $_SERVER['HTTPS'] !== 'off')
+      || (($_SERVER['HTTP_X_FORWARDED_PROTO'] ?? '') === 'https')
+      || in_array($_SERVER['REMOTE_ADDR'] ?? '', ['127.0.0.1', '::1'], true);
+if (!$https) {
+    http_response_code(403);
+    header('Content-Type: application/json');
+    echo json_encode(['error' => 'HTTPS required']);
+    exit;
+}
+
+header('Content-Type: application/json');
+// This endpoint is for native clients, never a browser page: no CORS, no cache.
+header('Cache-Control: no-store');
+header('X-Content-Type-Options: nosniff');
+
+// ---------------------------------------------------------------------------
+// Authentication
+// ---------------------------------------------------------------------------
+
+/** Bearer token, compared in constant time so a wrong guess leaks no timing. */
+function client_token(): string {
+    $h = $_SERVER['HTTP_AUTHORIZATION'] ?? $_SERVER['REDIRECT_HTTP_AUTHORIZATION'] ?? '';
+    if (stripos($h, 'Bearer ') === 0) return substr($h, 7);
+    return '';
+}
+
+if (!hash_equals(SYNC_TOKEN, client_token())) {
+    // 404, not 401: an unauthenticated caller learns nothing about what lives
+    // here — same reasoning as the desktop server's loopback lockdown.
+    http_response_code(404);
+    echo json_encode(['error' => 'Not found']);
+    exit;
+}
+
+// ---------------------------------------------------------------------------
+// Routing
+// ---------------------------------------------------------------------------
+
+$method = $_SERVER['REQUEST_METHOD'] ?? 'GET';
+
+if ($method === 'HEAD' || $method === 'GET') {
+    if (!is_file(SYNC_FILE)) {
+        // 404 = "no snapshot yet". The desktop client already treats this as
+        // "first sync, remote is empty" rather than an error.
+        http_response_code(404);
+        echo json_encode(['error' => 'No snapshot yet']);
+        exit;
+    }
+
+    $blob = file_get_contents(SYNC_FILE);
+    if ($blob === false) {
+        http_response_code(500);
+        echo json_encode(['error' => 'Read failed']);
+        exit;
+    }
+
+    // Strong ETag over the bytes. The write path will require If-Match against
+    // it, which is what stops two devices from clobbering each other.
+    $etag = '"' . hash('sha256', $blob) . '"';
+    header('ETag: ' . $etag);
+    header('Last-Modified: ' . gmdate('D, d M Y H:i:s', (int)filemtime(SYNC_FILE)) . ' GMT');
+
+    // Save the round trip when the client already holds this version.
+    if (trim($_SERVER['HTTP_IF_NONE_MATCH'] ?? '') === $etag) {
+        http_response_code(304);
+        exit;
+    }
+
+    header('Content-Length: ' . strlen($blob));
+    if ($method === 'HEAD') exit;
+    echo $blob;
+    exit;
+}
+
+// Everything else — including PUT — is not implemented yet.
+http_response_code(405);
+header('Allow: GET, HEAD');
+echo json_encode(['error' => 'Read-only endpoint']);