diff --git a/chunks.py b/chunks.py
index 2d1aa3d..934397e 100644
--- a/chunks.py
+++ b/chunks.py
@@ -1,3 +1,4 @@
+import string
 import threading
 import time
 from utils import CHUNK_WIDTH, CHUNK_HEIGHT, Position
@@ -65,6 +66,20 @@ class ChunkDiff():
 	
 	def empty(self):
 		return not bool(self._chars)
+	
+	def legitimate(self):
+		for i, char in self._chars.items():
+			if not (isinstance(char, str) and len(char) == 1 and ord(char) > 31 and (char not in string.whitespace or char == " ")):
+				return False
+		else:
+			return True
+	
+	def diff(self, chunk):
+		diffs = {}
+		for pos, char in self._chars.items():
+			diffs[pos] = chunk._chars.get(pos, " ")
+		
+		return ChunkDiff.from_dict(diffs)
 
 def jsonify_changes(changes):
 	dchanges = []
diff --git a/client.py b/client.py
index 587bcfc..4216acb 100644
--- a/client.py
+++ b/client.py
@@ -116,7 +116,7 @@ class Client():
 			elif i == "\n":   self.map_.newline()
 			#elif i in string.digits + string.ascii_letters + string.punctuation + " ":
 				#self.map_.write(i)
-			elif isinstance(i, str) and len(i) == 1 and (i not in string.whitespace or i == " "):
+			elif isinstance(i, str) and len(i) == 1 and ord(i) > 31 and (i not in string.whitespace or i == " "):
 				self.map_.write(i)
 	
 	def connection_thread(self):
diff --git a/server.py b/server.py
index b1c3a5c..574196a 100644
--- a/server.py
+++ b/server.py
@@ -35,12 +35,41 @@ class WotServer(WebSocket):
 	def handle_save_changes(self, dchanges):
 		changes = dejsonify_changes(dchanges)
 		
-		with pool:
-			pool.apply_changes(changes)
+		# check whether changes are correct (exclude certain characters)
+		# if not correct, send corrections them back to sender
+		legitimate_changes = []
+		illegitimate_changes = []
+		for chunk in changes:
+			if chunk[1].legitimate():
+				legitimate_changes.append(chunk)
+			else:
+				illegitimate_changes.append(chunk)
 		
-		for client in clients:
-			if client:
-				client.send_changes(changes)
+		if legitimate_changes:
+			with pool:
+				pool.apply_changes(legitimate_changes)
+			
+			for client in clients:
+				if client:
+					client.send_changes(legitimate_changes)
+		
+		if illegitimate_changes:
+			reverse_changes = self.reverse_changes(changes)
+			reverse_dchanges = jsonify_changes(reverse_changes)
+			message = {"type": "apply-changes", "data": reverse_dchanges}
+			self.sendMessage(json.dumps(message))
+	
+	def reverse_changes(self, changes):
+		with pool:
+			reverse_changes = []
+			for chunk in changes:
+				pos = chunk[0]
+				change = chunk[1]
+				real_chunk = pool.get(pos) or pool.create(pos)
+				reverse_change = change.diff(real_chunk.as_diff())
+				reverse_changes.append((pos, reverse_change))
+		
+		return reverse_changes
 	
 	def send_changes(self, changes):
 		changes = [chunk for chunk in changes if chunk[0] in self.loaded_chunks]