diff --git a/boards/models/post/sync.py b/boards/models/post/sync.py
--- a/boards/models/post/sync.py
+++ b/boards/models/post/sync.py
@@ -5,11 +5,12 @@ from boards.utils import get_file_mimety
 from django.db import transaction
 from boards.models import KeyPair, GlobalId, Signature, Post, Tag
 
-EXCEPTION_NODE = 'Sync node returned an error: {}'
-EXCEPTION_OP = 'Load the OP first'
-EXCEPTION_DOWNLOAD = 'File was not downloaded'
-EXCEPTION_HASH = 'File hash does not match attachment hash'
-EXCEPTION_SIGNATURE = 'Invalid model signature for {}'
+EXCEPTION_NODE = 'Sync node returned an error: {}.'
+EXCEPTION_OP = 'Load the OP first.'
+EXCEPTION_DOWNLOAD = 'File was not downloaded.'
+EXCEPTION_HASH = 'File hash does not match attachment hash.'
+EXCEPTION_SIGNATURE = 'Invalid model signature for {}.'
+EXCEPTION_AUTHOR_SIGNATURE = 'Model {} has no author signature.'
 ENCODING_UNICODE = 'unicode'
 
 TAG_MODEL = 'model'
@@ -166,10 +167,10 @@ class SyncManager:
                 tag_content = tag_model.find(TAG_CONTENT)
 
                 content_str = et.tostring(tag_content, ENCODING_UNICODE)
-                signatures = SyncManager._verify_model(content_str, tag_model)
 
                 tag_id = tag_content.find(TAG_ID)
                 global_id, exists = GlobalId.from_xml_element(tag_id)
+                signatures = SyncManager._verify_model(global_id, content_str, tag_model)
 
                 if exists:
                     print('Post with same ID already exists')
@@ -253,7 +254,7 @@ class SyncManager:
         return et.tostring(response, ENCODING_UNICODE)
 
     @staticmethod
-    def _verify_model(content_str, tag_model):
+    def _verify_model(global_id, content_str, tag_model):
         """
         Verifies all signatures for a single model.
         """
@@ -261,11 +262,16 @@ class SyncManager:
         signatures = []
 
         tag_signatures = tag_model.find(TAG_SIGNATURES)
+        has_author_signature = False
         for tag_signature in tag_signatures:
             signature_type = tag_signature.get(ATTR_TYPE)
             signature_value = tag_signature.get(ATTR_VALUE)
             signature_key = tag_signature.get(ATTR_KEY)
 
+            if global_id.key_type == signature_type and\
+                    global_id.key == signature_key:
+                has_author_signature = True
+
             signature = Signature(key_type=signature_type,
                                   key=signature_key,
                                   signature=signature_value)
@@ -274,6 +280,8 @@ class SyncManager:
                 raise SyncException(EXCEPTION_SIGNATURE.format(content_str))
 
             signatures.append(signature)
+        if not has_author_signature:
+            raise SyncException(EXCEPTION_AUTHOR_SIGNATURE.format(content_str))
 
         return signatures