Fix winlegacy implementation of AES CBC with no padding

oscrypto/_win/symmetric.py

@@ -866,9 +866,7 @@ def _advapi32_encrypt(cipher, key, data, iv, padding):
 
         # Remove padding when not required. CryptoAPI doesn't support this, so
         # we just manually remove it.
-        if cipher == 'aes' and not padding:
-            if output[-16:] != (b'\x10' * 16):
-                raise ValueError('Invalid padding generated by OS crypto library')
+        if cipher == 'aes' and not padding and len(output) == len(data) + 16:
             output = output[:-16]
 
         return output
@@ -1059,17 +1057,17 @@ def _advapi32_decrypt(cipher, key, data, iv, padding):
     try:
         context_handle, key_handle = _advapi32_create_handles(cipher, key, iv)
 
-        # Add removed padding when not required. CryptoAPI doesn't support no
-        # padding, so we just add it back in
-        if cipher == 'aes' and not padding:
-            data += (b'\x10' * 16)
+        if cipher == 'aes' and not padding and len(data) % 16 != 0:
+            raise ValueError('Invalid data - ciphertext length must be a multiple of 16')
 
         buffer = buffer_from_bytes(data)
         out_len = new(advapi32, 'DWORD *', len(data))
         res = advapi32.CryptDecrypt(
             key_handle,
             null(),
-            True,
+            # To skip padding, we have to tell the API that this is not
+            # the final block
+            False if cipher == 'aes' and not padding else True,
             0,
             buffer,
             out_len