The SubBytes phase of AES involves splitting the input into bytes and passing each through the Substitution Box or S-Box we created earlier
in our last tutorial. Unlike DES, AES uses the same S-Box for all bytes. The AES S-Box implements inverse multiplication in Galois Field 28.
Here, each byte of our key is passed through the subbyte and a new output is being generated. This operation provides the non-linearity in the cipher. The S-box used is derived from the multiplicative inverse over GF(28), known to have good non-linearity properties.
watch the video below for demonstration in logism.
`timescale 1 ns/1 ps module SubBytes # ( parameter DATA_W = 128, //data width parameter NO_BYTES = DATA_W >> 3 //no of bytes = data width / 8 ) ( input clk, //system clock input reset, //asynch active low reset input valid_in, //input valid signal input [DATA_W-1:0] data_in, //input data output reg valid_out, //output valid signal output [DATA_W-1:0] data_out //output data ) ; genvar i; generate //generating sbox roms for (i=0; i< NO_BYTES ; i=i+1) begin : ROM SBox ROM(clk,reset,valid_in,data_in[(i*8)+7:(i*8)],data_out[(i*8)+7:(i*8)]); end endgenerate always@(posedge clk or negedge reset) //valid out register if(!reset)begin valid_out <= 1'b0; end else begin valid_out <= valid_in; end endmodule
