Subversion Repositories aes_decrypt_fpga

////////////////////////////////////////////////////////////////// ////
////                                                                                                                            ////
//// AES Decryption Core for FPGA                                                                       ////
////                                                                                                                            ////
//// This file is part of the AES Decryption Core for FPGA project      ////
//// http://www.opencores.org/cores/xxx/                                                        ////
////                                                                                                                            ////
//// Description                                                                                                        ////
//// Implementation of  AES Decryption Core for FPGA according to       ////
//// core specification document.                                                                       ////
////                                                                                                                            ////
//// To Do:                                                                                                             ////
//// -                                                                                                                          ////
////                                                                                                                            ////
//// Author(s):                                                                                                         ////
//// - scheng, schengopencores@opencores.org                                            ////
////                                                                                                                            ////
//////////////////////////////////////////////////////////////////////
////                                                                                                                            ////
//// Copyright (C) 2009 Authors and OPENCORES.ORG                                       ////
////                                                                                                                            ////
//// This source file may be used and distributed without                       ////
//// restriction provided that this copyright statement is not          ////
//// removed from the file and that any derivative work contains        ////
//// the original copyright notice and the associated disclaimer.       ////
////                                                                                                                            ////
//// This source file is free software; you can redistribute it         ////
//// and/or modify it under the terms of the GNU Lesser General         ////
//// Public License as published by the Free Software Foundation;       ////
//// either version 2.1 of the License, or (at your option) any         ////
//// later version.                                                                                             ////
////                                                                                                                            ////
//// This source is distributed in the hope that it will be             ////
//// useful, but WITHOUT ANY WARRANTY; without even the implied         ////
//// warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR            ////
//// PURPOSE. See the GNU Lesser General Public License for more        ////
//// details.                                                                                                           ////
////                                                                                                                            ////
//// You should have received a copy of the GNU Lesser General          ////
//// Public License along with this source; if not, download it         ////
//// from http://www.opencores.org/lgpl.shtml                                           ////
////                                                                                                                            //// ///
///////////////////////////////////////////////////////////////////
////                                                                                                                            ////
//// This module implements the Galois field multipliers                        ////
//// for x2, x3, x9, xb, xd, xe. Used in InvMixCol_slice.                       ////
//// See section 4.2 of FIPS-197 specification for details.                     ////
////                                                                                                                            ////
////////////////////////////////////////////////////////////////////////
module gfmul_inv(
        input   [7:0]   d,
        output  [7:0]   x2,
        output  [7:0]   x3,
        output  [7:0]   x9,
        output  [7:0]   xb,
        output  [7:0]   xd,
        output  [7:0]   xe
        );
        // Multiplier over GF(256)
        // Generates -  x2, 3 for cipher
        //                       -      x9, xb, xd, xe for inverse cipher
        
        function byte unsigned xtime(byte unsigned x);
        // Multiplication by 2 over GF(256)
        // Refer to FIPS-197 spec section 4.2.1 on definition of GF(256) multiplication
                xtime = (x[7])? (x<<1) ^ 8'h1b : x<<1;
        endfunction

        function byte unsigned GFmul3(byte unsigned x);
        // Multiply by 3 over GF(256)
        // 3*x = 2*x +x
                GFmul3 = xtime(x) ^ x;
        endfunction
        
        function byte unsigned GFmul4(byte unsigned x);
        // Multiply by 4 over GF(256)
        // 4*x = 2*(2*x)
                GFmul4 = xtime(xtime(x));
        endfunction

        function byte unsigned GFmul8(byte unsigned x);
        // Multiply by 8 over GF(256)
        // 8*x = 2*(4*x)
                GFmul8 = xtime(GFmul4(x));
        endfunction

        function byte unsigned GFmul9(byte unsigned x);
        // Multiply by 9 over GF(256)
        // 9*x = 8*x + x
        // Addition over GF(256) is xor
                GFmul9 = GFmul8(x) ^ x;
        endfunction

        function byte unsigned GFmulb(byte unsigned x);
        // Multiply by 0xb over GF(256)
        // b*x = 8*x + 2*x +x
                GFmulb = GFmul8(x) ^ xtime(x) ^ x;
        endfunction

        function byte unsigned GFmuld(byte unsigned x);
        // Multiply by 0xd over GF(256)
        // d*x = 8*x + 4*x + x
                GFmuld = GFmul8(x) ^ GFmul4(x) ^ x;
        endfunction

        function byte unsigned GFmule(byte unsigned x);
        // Multiply by 0xe over GF(256)
        // e*x = 8*x + 4*x +2*x
                GFmule = GFmul8(x) ^ GFmul4(x) ^ xtime(x);
        endfunction
        
        assign x2 = xtime(d);
        assign x3 = GFmul3(d);
        assign x9 = GFmul9(d);
        assign xb = GFmulb(d);
        assign xd = GFmuld(d);
        assign xe = GFmule(d);
        
endmodule