diff --git a/app_secure_client.py b/app_secure_client.py
index be3ed2a..e848dfb 100644
--- a/app_secure_client.py
+++ b/app_secure_client.py
@@ -106,8 +106,8 @@ class Message:
         self.read_to_buffer(content_len)
         data = self._recv_buffer[:content_len]
         # decrypt data
-        iv = bytes(data[:16])
-        encrypted = bytes(data[16:])
+        iv = bytes(data[:AES.block_size])
+        encrypted = bytes(data[AES.block_size:])
         key = self.sessionkey + self.sessionkey[::-1]
         cipher = AES.new(key, AES.MODE_CFB, iv)
         data = cipher.decrypt(encrypted)
diff --git a/lib_secure_server.py b/lib_secure_server.py
index 146be6e..e70f92f 100644
--- a/lib_secure_server.py
+++ b/lib_secure_server.py
@@ -282,8 +282,8 @@ class Message:
             self._set_selector_events_mask("r")
         else:
             print(f"Receiving request (encrypted)")
-            iv = data[:16]
-            encrypted = data[16:]
+            iv = data[:AES.block_size]
+            encrypted = data[AES.block_size:]
             key = self.sessionkey + self.sessionkey[::-1]
             cipher = AES.new(key, AES.MODE_CFB, iv)
             data = cipher.decrypt(encrypted)