Initial release

backend/app/core/security.py

@@ -0,0 +1,59 @@
+import hmac
+import hashlib
+import time
+from urllib.parse import parse_qsl
+
+from app.core.config import settings
+
+# Reject initData older than this many seconds (24 hours)
+_MAX_AGE_SECONDS = 86_400
+
+
+def validate_init_data(init_data: str) -> dict | None:
+    """Validate Telegram WebApp initData using HMAC-SHA256.
+
+    Also rejects payloads whose auth_date is older than _MAX_AGE_SECONDS to
+    prevent indefinite replay of leaked initData strings.
+    """
+    try:
+        # parse_qsl already URL-decodes each value. Decoding the whole query
+        # string first can turn escaped separators inside JSON values into real
+        # separators and make Telegram's HMAC check fail.
+        parsed = dict(parse_qsl(init_data, keep_blank_values=True))
+        hash_value = parsed.pop("hash", "")
+        if not hash_value:
+            return None
+
+        # Validate auth_date freshness before doing any crypto work
+        auth_date_str = parsed.get("auth_date")
+        if not auth_date_str:
+            return None
+        try:
+            age = time.time() - int(auth_date_str)
+            if age > _MAX_AGE_SECONDS:
+                return None
+        except ValueError:
+            return None
+
+        data_check_string = "\n".join(
+            f"{k}={v}" for k, v in sorted(parsed.items())
+        )
+
+        secret_key = hmac.new(
+            b"WebAppData",
+            settings.bot_token.encode(),
+            hashlib.sha256,
+        ).digest()
+
+        expected = hmac.new(
+            secret_key,
+            data_check_string.encode(),
+            hashlib.sha256,
+        ).hexdigest()
+
+        if not hmac.compare_digest(expected, hash_value):
+            return None
+
+        return parsed
+    except Exception:
+        return None