Array Indexing possible in VHDL?

You can create an indexed type

hi dave, thank you so much for the informative reply! actually i'm trying to implement XTEA:http://en.wikipedia.org/wiki/xtea

Library IEEE;
USE IEEE.std_logic_1164.all;
use ieee.numeric_std.all;
entity XTEA is    
Port     (
    clk:in std_logic;
    input: in unsigned(63 downto 0);
    key : in unsigned(127 downto 0);
    Encoder_output,Decoder_output: out unsigned(63 downto 0)
        );
end entity;
architecture rtl of XTEA is
signal k0,k1,k2,k3: unsigned(31 downto 0);
signal     n:integer:=0;
signal key_array: unsigned (3 downto 0);
type key_t is array (0 to 3) of unsigned(31 downto 0);    
signal k : key_t;
begin
process(clk,input)
variable z,z_up,z_mid,z_down,z_total,y,y_up,y_mid,y_down,y_total: unsigned (31 downto 0);
variable sum: unsigned (31 downto 0) := x"00000000";
variable delta: unsigned (31 downto 0) := x"9e3779b9";
variable test,test1,test2,test3 : unsigned (31 downto 0);
variable natvar,natvar1,natvar2,natvar3 : integer;
begin
        --divide the key into 4 parts    
        k(3)<=key(127 downto 96);
        k(2)<=key(95 downto 64);
        k(1)<=key(63 downto 32);
        k(0)<=key(31 downto 0);
    
                    
if(rising_edge(clk)) then
    if (n=0) then
        --divide input into y and z
        y:= input(63 downto 32);
        z:= input (31 downto 0);
        --sum:=x"00000000";         -- try take this out later
        elsif (n=32) then
        --for decode:delta is delta*2^5 for 32=(2^5) round 
        sum:=delta(26 downto 0)&"00000";
        end if;
--encode        
if (n <32) then
test:=sum and x"00000003";
test1:="00000000000" & sum(31 downto 11);    --shift by 11 bits to the right
natvar:=to_integer(test);
natvar1:=to_integer(test1);
z_total:=(((z(27 downto 0)&"0000") XOR ("00000"&z(31 downto 5)))+z) XOR(   sum + k( natvar  downto natvar )   );
y:=y+z_total;
            sum:=sum+delta;
y_total:=(((y(27 downto 0)&"0000") XOR ("00000"&y(31 downto 5)))+y) XOR (sum+ k(natvar1 downto natvar1  ) );
z:=z+y_total;
            
            Encoder_output<=y&z;    
            
--decode            
elsif(n<64) then
        if(n=32) then
        sum:=delta*32;
        end if; 
test2:=sum and x"00000003";
test3:="00000000000" & sum(31 downto 11);    --shift by 11 bits to the right
natvar2:=to_integer(test2);
natvar3:=to_integer(test3);    
    
test1:=sum and x"00000003";
natvar1:=to_integer(test1);
y_total:=(((y(27 downto 0)&"0000") XOR ("00000"&y(31 downto 5)))+y) XOR (sum+ k(natvar3 downto natvar3));
z:=z-y_total;
            sum:=sum-delta;
z_total:=(((z(27 downto 0)&"0000") XOR ("00000"&z(31 downto 5)))+z) XOR(sum + k(natvar2 downto natvar2));
y:=y+z_total;
            Decoder_output<=y&z;
        end if;
            n<=n+1; --counter
    end if;
    
end process;
end rtl;

The cause of the error is different from what you assume. This expression can't work, because it results in a subarray rather than a single array element k( natvar downto natvar ). Do you mean k( natvar)?

On another note - you're indexing into K (that has 4 elements) with a 32 bit integer. What happens when natvar2 or 3 are greater than 3?

Thank you FvM for pointing out my mistakes. yes i meant k(natvar)...also the

delta*32

 delta(26 downto 5) & "00000"

Apparently the integer counter is optimized to 0 to 63 range and overflows. If you want to stop operation after one pass, you should do explicitely.