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)
@@ -704,7 +705,10 @@ 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)
+            # 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)}"
@@ -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
@@ -1045,24 +1050,43 @@ 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
 
+    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()
+            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:
     return {

tests/channels/test_weixin_channel.py

@@ -7,12 +7,15 @@ from unittest.mock import AsyncMock
 
 import pytest
 
+import nanobot.channels.weixin as weixin_mod
 from nanobot.bus.queue import MessageBus
 from nanobot.channels.weixin import (
     ITEM_IMAGE,
     ITEM_TEXT,
     MESSAGE_TYPE_BOT,
     WEIXIN_CHANNEL_VERSION,
+    _decrypt_aes_ecb,
+    _encrypt_aes_ecb,
     WeixinChannel,
     WeixinConfig,
 )
@@ -340,3 +343,63 @@ async def test_send_media_falls_back_to_upload_param_url(tmp_path) -> None:
     cdn_url = cdn_post.await_args_list[0].args[0]
     assert cdn_url.startswith(f"{channel.config.cdn_base_url}/upload?encrypted_query_param=enc-need-fallback")
     assert "&filekey=" in cdn_url
+
+
+def test_decrypt_aes_ecb_strips_valid_pkcs7_padding() -> None:
+    key_b64 = "MDEyMzQ1Njc4OWFiY2RlZg=="  # base64("0123456789abcdef")
+    plaintext = b"hello-weixin-padding"
+
+    ciphertext = _encrypt_aes_ecb(plaintext, key_b64)
+    decrypted = _decrypt_aes_ecb(ciphertext, key_b64)
+
+    assert decrypted == plaintext
+
+
+class _DummyDownloadResponse:
+    def __init__(self, content: bytes, status_code: int = 200) -> None:
+        self.content = content
+        self.status_code = status_code
+
+    def raise_for_status(self) -> None:
+        return None
+
+
+@pytest.mark.asyncio
+async def test_download_media_item_uses_full_url_when_present(tmp_path) -> None:
+    channel, _bus = _make_channel()
+    weixin_mod.get_media_dir = lambda _name: tmp_path
+
+    full_url = "https://cdn.example.test/download/full"
+    channel._client = SimpleNamespace(
+        get=AsyncMock(return_value=_DummyDownloadResponse(content=b"raw-image-bytes"))
+    )
+
+    item = {
+        "media": {
+            "full_url": full_url,
+            "encrypt_query_param": "enc-fallback-should-not-be-used",
+        },
+    }
+    saved_path = await channel._download_media_item(item, "image")
+
+    assert saved_path is not None
+    assert Path(saved_path).read_bytes() == b"raw-image-bytes"
+    channel._client.get.assert_awaited_once_with(full_url)
+
+
+@pytest.mark.asyncio
+async def test_download_media_item_falls_back_to_encrypt_query_param(tmp_path) -> None:
+    channel, _bus = _make_channel()
+    weixin_mod.get_media_dir = lambda _name: tmp_path
+
+    channel._client = SimpleNamespace(
+        get=AsyncMock(return_value=_DummyDownloadResponse(content=b"fallback-bytes"))
+    )
+
+    item = {"media": {"encrypt_query_param": "enc-fallback"}}
+    saved_path = await channel._download_media_item(item, "image")
+
+    assert saved_path is not None
+    assert Path(saved_path).read_bytes() == b"fallback-bytes"
+    called_url = channel._client.get.await_args_list[0].args[0]
+    assert called_url.startswith(f"{channel.config.cdn_base_url}/download?encrypted_query_param=enc-fallback")