diff --git a/aes/rtl/vhdl/aes_pkg.vhd b/aes/rtl/vhdl/aes_pkg.vhd
index 60dca15..a65b365 100644
--- a/aes/rtl/vhdl/aes_pkg.vhd
+++ b/aes/rtl/vhdl/aes_pkg.vhd
@@ -146,6 +146,8 @@ package aes_pkg is
   function bytesub    (input : std_logic_vector(7 downto 0)) return std_logic_vector;
   function invbytesub (input : std_logic_vector(7 downto 0)) return std_logic_vector;
 
+  function subword (input : t_datatable1d) return t_datatable1d;
+
   function subbytes (input : in t_datatable2d) return t_datatable2d;
   function invsubbytes (input : in t_datatable2d) return t_datatable2d;
 
@@ -203,6 +205,16 @@ package body aes_pkg is
   end function invbytesub;
 
 
+  function subword (input : t_datatable1d) return t_datatable1d is
+    v_data : t_datatable1d;
+  begin
+    for i in 0 to 3 loop
+      v_data(i) := c_sbox(to_integer(unsigned(input(i)(7 downto 4))))(to_integer(unsigned(input(i)(3 downto 0))));
+    end loop;
+    return v_data;
+  end function subword;
+
+
   function subbytes (input : in t_datatable2d) return t_datatable2d is
     variable v_data : t_datatable2d;
   begin