VHDL: las señales obtienen el valor U a pesar de que hay otro valor asignado

Estoy implementando el algoritmo IDEA usando VHDL, tengo un problema en mi módulo generador de claves, cuando ejecuto el simulador obtengo valores U en todas las señales, aunque les asigno otros valores.

    library IEEE;
use IEEE.STD_LOGIC_1164.ALL;

-- Uncomment the following library declaration if using
-- arithmetic functions with Signed or Unsigned values
--use IEEE.NUMERIC_STD.ALL;

-- Uncomment the following library declaration if instantiating
-- any Xilinx primitives in this code.
--library UNISIM;
--use UNISIM.VComponents.all;

entity keygenerator is
    Port ( round : in  STD_LOGIC_VECTOR (3 downto 0);
           key : in  STD_LOGIC_VECTOR (127 downto 0);
           keyout1 : out  STD_LOGIC_VECTOR (15 downto 0);
           keyout2 : out  STD_LOGIC_VECTOR (15 downto 0);
           keyout3 : out  STD_LOGIC_VECTOR (15 downto 0);
           keyout4 : out  STD_LOGIC_VECTOR (15 downto 0);
           keyout5 : out  STD_LOGIC_VECTOR (15 downto 0);
           keyout6 : out  STD_LOGIC_VECTOR (15 downto 0));
end keygenerator;

architecture Behavioral of keygenerator is

  SIGNAL key0 : std_logic_vector (127 downto 0);
  SIGNAL key1 : std_logic_vector (127 downto 0);
  SIGNAL key2 : std_logic_vector (127 downto 0);
  SIGNAL key3 : std_logic_vector (127 downto 0);
  SIGNAL key4 : std_logic_vector (127 downto 0);
  SIGNAL key5 : std_logic_vector (127 downto 0);
  SIGNAL key6 : std_logic_vector (95 downto 0);

  signal output : std_logic_vector (95 downto 0);


begin

  process (round, key)

  begin
    key0 <= key;
    key1 <= key0(102 downto 0) & key0(127 downto 103);
    key2 <= key1(102 downto 0) & key1(127 downto 103);
    key3 <= key2(102 downto 0) & key2(127 downto 103);
    key4 <= key3(102 downto 0) & key3(127 downto 103);
    key5 <= key4(102 downto 0) & key4(127 downto 103);
    key6 <= key5(102 downto 7); 

    case round is
      when "0000" => output <= key0(127 downto 32);
      when "0001" => output <= key0(31 downto 0) & key1(127 downto 64);
      when "0010" => output <= key1(63 downto 0) & key2(127 downto 96);
      when "0011" => output <= key2(95 downto 0);
      when "0100" => output <= key3(127 downto 32);
      when "0101" => output <= key3(31 downto 0) & key4(127 downto 64);
      when "0110" => output <= key4(63 downto 0) & key5(127 downto 96);
      when "0111" => output <= key5(95 downto 0);
      when "1000" => output <= key6;
      when others => output <= (others => 'X');
    end case;

  end process;

  keyout6 <= output(15 downto 0);
  keyout5 <= output(31 downto 16);
  keyout4 <= output(47 downto 32);
  keyout3 <= output(63 downto 48);
  keyout2 <= output(79 downto 64);
  keyout1 <= output(95 downto 80);

end Behavioral;

Ese es mi banco de pruebas:

    LIBRARY ieee;
USE ieee.std_logic_1164.ALL;

-- Uncomment the following library declaration if using
-- arithmetic functions with Signed or Unsigned values
--USE ieee.numeric_std.ALL;

ENTITY tb_keygenerator IS
END tb_keygenerator;

ARCHITECTURE behavior OF tb_keygenerator IS 

    -- Component Declaration for the Unit Under Test (UUT)

    COMPONENT keygenerator
    PORT(
         round : IN  std_logic_vector(3 downto 0);
         key : IN  std_logic_vector(127 downto 0);
         keyout1 : OUT  std_logic_vector(15 downto 0);
         keyout2 : OUT  std_logic_vector(15 downto 0);
         keyout3 : OUT  std_logic_vector(15 downto 0);
         keyout4 : OUT  std_logic_vector(15 downto 0);
         keyout5 : OUT  std_logic_vector(15 downto 0);
         keyout6 : OUT  std_logic_vector(15 downto 0)
        );
    END COMPONENT;


   --Inputs
   signal round : std_logic_vector(3 downto 0) := (others => '0');
   signal key : std_logic_vector(127 downto 0) := (others => '0');

   --Outputs
   signal out1 : std_logic_vector(15 downto 0);
   signal out2 : std_logic_vector(15 downto 0);
   signal out3 : std_logic_vector(15 downto 0);
   signal out4 : std_logic_vector(15 downto 0);
   signal out5 : std_logic_vector(15 downto 0);
   signal out6 : std_logic_vector(15 downto 0);
   -- No clocks detected in port list. Replace <clock> below with 
   -- appropriate port name 

   constant I_period : time := 10 ns;

BEGIN

  -- Instantiate the Unit Under Test (UUT)
   uut: keygenerator PORT MAP (
          round => round,
          key => key,
          keyout1 => out1,
          keyout2 => out2,
          keyout3 => out3,
          keyout4 => out4,
          keyout5 => out5,
          keyout6 => out6
        );

   -- Clock process definitions
   I_process :process
   begin
    key <= X"12345678912345678912345678912345";
    round <="1100";
    wait for I_period/2;
    key <= X"12345678912345678912345678912345";
    round <="1001";
    wait for I_period/2;
   end process;


END;