diff --git a/oletools/msodde.py b/oletools/msodde.py
index 69eac6c..0df0a8c 100644
--- a/oletools/msodde.py
+++ b/oletools/msodde.py
@@ -61,7 +61,9 @@ import olefile
 from oletools import ooxml
 from oletools import xls_parser
 from oletools import rtfobj
+from oletools import oleid
 from oletools.common.log_helper import log_helper
+from oletools.common.errors import FileIsEncryptedError
 
 # -----------------------------------------------------------------------------
 # CHANGELOG:
@@ -438,17 +440,18 @@ def process_doc_stream(stream):
     return result_parts
 
 
-def process_doc(filepath):
+def process_doc(ole):
     """
     find dde links in word ole (.doc/.dot) file
 
+    Checks whether files is ppt and returns empty immediately in that case
+    (ppt files cannot contain DDE-links to my knowledge)
+
     like process_xml, returns a concatenated unicode string of dde links or
     empty if none were found. dde-links will still begin with the dde[auto] key
     word (possibly after some whitespace)
     """
     logger.debug('process_doc')
-    ole = olefile.OleFileIO(filepath, path_encoding=None)
-
     links = []
     for sid, direntry in enumerate(ole.direntries):
         is_orphan = direntry is None
@@ -886,9 +889,20 @@ def process_file(filepath, field_filter_mode=None):
         if xls_parser.is_xls(filepath):
             logger.debug('Process file as excel 2003 (xls)')
             return process_xls(filepath)
+
+        # encrypted files also look like ole, even if office 2007+ (xml-based)
+        # so check for encryption, first
+        ole = olefile.OleFileIO(filepath, path_encoding=None)
+        oid = oleid.OleID(ole)
+        if oid.check_encrypted().value:
+            log.debug('is encrypted - raise error')
+            raise FileIsEncryptedError(filepath)
+        elif oid.check_powerpoint().value:
+            log.debug('is ppt - cannot have DDE')
+            return u''
         else:
             logger.debug('Process file as word 2003 (doc)')
-            return process_doc(filepath)
+            return process_doc(ole)
 
     with open(filepath, 'rb') as file_handle:
         if file_handle.read(4) == RTF_START: