fix(weixin): correct PKCS7 unpadding for AES-ECB; support full_url for media download

nanobot/channels/weixin.py

@@ -685,9 +685,10 @@ class WeixinChannel(BaseChannel):
         """Download + AES-decrypt a media item. Returns local path or None."""
         try:
             media = typed_item.get("media") or {}
-            encrypt_query_param = media.get("encrypt_query_param", "")
+            encrypt_query_param = str(media.get("encrypt_query_param", "") or "")
+            full_url = str(media.get("full_url", "") or "").strip()
 
-            if not encrypt_query_param:
+            if not encrypt_query_param and not full_url:
                 return None
 
             # Resolve AES key (media-download.ts:43-45, pic-decrypt.ts:40-52)
@@ -704,11 +705,14 @@ class WeixinChannel(BaseChannel):
             elif media_aes_key_b64:
                 aes_key_b64 = media_aes_key_b64
 
-            # Build CDN download URL with proper URL-encoding (cdn-url.ts:7)
-            cdn_url = (
-                f"{self.config.cdn_base_url}/download"
-                f"?encrypted_query_param={quote(encrypt_query_param)}"
-            )
+            # Prefer server-provided full_url, fallback to encrypted_query_param URL construction.
+            if full_url:
+                cdn_url = full_url
+            else:
+                cdn_url = (
+                    f"{self.config.cdn_base_url}/download"
+                    f"?encrypted_query_param={quote(encrypt_query_param)}"
+                )
 
             assert self._client is not None
             resp = await self._client.get(cdn_url)
@@ -727,7 +731,8 @@ class WeixinChannel(BaseChannel):
             ext = _ext_for_type(media_type)
             if not filename:
                 ts = int(time.time())
-                h = abs(hash(encrypt_query_param)) % 100000
+                hash_seed = encrypt_query_param or full_url
+                h = abs(hash(hash_seed)) % 100000
                 filename = f"{media_type}_{ts}_{h}{ext}"
             safe_name = os.path.basename(filename)
             file_path = media_dir / safe_name
@@ -1045,23 +1050,42 @@ def _decrypt_aes_ecb(data: bytes, aes_key_b64: str) -> bytes:
         logger.warning("Failed to parse AES key, returning raw data: {}", e)
         return data
 
+    decrypted: bytes | None = None
+
     try:
         from Crypto.Cipher import AES
 
         cipher = AES.new(key, AES.MODE_ECB)
-        return cipher.decrypt(data)  # pycryptodome auto-strips PKCS7 with unpad
+        decrypted = cipher.decrypt(data)
     except ImportError:
         pass
 
-    try:
-        from cryptography.hazmat.primitives.ciphers import Cipher, algorithms, modes
+    if decrypted is None:
+        try:
+            from cryptography.hazmat.primitives.ciphers import Cipher, algorithms, modes
 
-        cipher_obj = Cipher(algorithms.AES(key), modes.ECB())
-        decryptor = cipher_obj.decryptor()
-        return decryptor.update(data) + decryptor.finalize()
-    except ImportError:
-        logger.warning("Cannot decrypt media: install 'pycryptodome' or 'cryptography'")
+            cipher_obj = Cipher(algorithms.AES(key), modes.ECB())
+            decryptor = cipher_obj.decryptor()
+            decrypted = decryptor.update(data) + decryptor.finalize()
+        except ImportError:
+            logger.warning("Cannot decrypt media: install 'pycryptodome' or 'cryptography'")
+            return data
+
+    return _pkcs7_unpad_safe(decrypted)
+
+
+def _pkcs7_unpad_safe(data: bytes, block_size: int = 16) -> bytes:
+    """Safely remove PKCS7 padding when valid; otherwise return original bytes."""
+    if not data:
         return data
+    if len(data) % block_size != 0:
+        return data
+    pad_len = data[-1]
+    if pad_len < 1 or pad_len > block_size:
+        return data
+    if data[-pad_len:] != bytes([pad_len]) * pad_len:
+        return data
+    return data[:-pad_len]
 
 
 def _ext_for_type(media_type: str) -> str: