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// Copyright 2019 ETH Zurich and University of Bologna.
// Copyright and related rights are licensed under the Solderpad Hardware
// License, Version 0.51 (the "License"); you may not use this file except in
// compliance with the License. You may obtain a copy of the License at
// http://solderpad.org/licenses/SHL-0.51. Unless required by applicable law
// or agreed to in writing, software, hardware and materials distributed under
// this License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR
// CONDITIONS OF ANY KIND, either express or implied. See the License for the
// specific language governing permissions and limitations under the License.
//
// Author: Michael Schaffner <schaffner@iis.ee.ethz.ch>, ETH Zurich
// Date: 26.04.2019
//
// Description: This is a parametric LFSR with precomputed coefficients for
// LFSR lengths from 4 to 64bit.
// Additional block cipher layers can be instantiated to non-linearly transform
// the pseudo-random LFSR sequence at the output, and hence break the shifting
// patterns. The additional cipher layers can only be used for an LFSR width
// of 64bit, since the block cipher has been designed for that block length.
module lfsr #(
parameter int unsigned LfsrWidth = 64, // [4,64]
parameter int unsigned OutWidth = 8, // [1,LfsrWidth]
parameter logic [LfsrWidth-1:0] RstVal = '1, // [1,2^LfsrWidth-1]
// 0: disabled, the present cipher uses 31, but just a few layers (1-3) are enough
// to break linear shifting patterns
parameter int unsigned CipherLayers = 0,
parameter bit CipherReg = 1'b1 // additional output reg after cipher
) (
input logic clk_i,
input logic rst_ni,
input logic en_i,
output logic [OutWidth-1:0] out_o
);
// Galois LFSR feedback masks
// Automatically generated with get_lfsr_masks.py
// Masks are from https://users.ece.cmu.edu/~koopman/lfsr/
localparam logic [63:0] Masks [4:64] = '{64'hC,
64'h1E,
64'h39,
64'h7E,
64'hFA,
64'h1FD,
64'h3FC,
64'h64B,
64'hD8F,
64'h1296,
64'h2496,
64'h4357,
64'h8679,
64'h1030E,
64'h206CD,
64'h403FE,
64'h807B8,
64'h1004B2,
64'h2006A8,
64'h4004B2,
64'h800B87,
64'h10004F3,
64'h200072D,
64'h40006AE,
64'h80009E3,
64'h10000583,
64'h20000C92,
64'h400005B6,
64'h80000EA6,
64'h1000007A3,
64'h200000ABF,
64'h400000842,
64'h80000123E,
64'h100000074E,
64'h2000000AE9,
64'h400000086A,
64'h8000001213,
64'h1000000077E,
64'h2000000123B,
64'h40000000877,
64'h8000000108D,
64'h100000000AE9,
64'h200000000E9F,
64'h4000000008A6,
64'h80000000191E,
64'h100000000090E,
64'h2000000000FB3,
64'h4000000000D7D,
64'h80000000016A5,
64'h10000000000B4B,
64'h200000000010AF,
64'h40000000000DDE,
64'h8000000000181A,
64'h100000000000B65,
64'h20000000000102D,
64'h400000000000CD5,
64'h8000000000024C1,
64'h1000000000000EF6,
64'h2000000000001363,
64'h4000000000000FCD,
64'h80000000000019E2};
// this S-box and permutation P has been taken from the Present Cipher,
// a super lightweight block cipher. use the cipher layers to add additional
// non-linearity to the LFSR output. note one layer does not fully correspond
// to the present cipher round, since the key and rekeying function is not applied here.
//
// See also:
// "PRESENT: An Ultra-Lightweight Block Cipher", A. Bogdanov et al., Ches 2007
// http://www.lightweightcrypto.org/present/present_ches2007.pdf
// this is the sbox from the present cipher
localparam logic[15:0][3:0] Sbox4 = {4'h2, 4'h1, 4'h7, 4'h4,
4'h8, 4'hF, 4'hE, 4'h3,
4'hD, 4'hA, 4'h0, 4'h9,
4'hB, 4'h6, 4'h5, 4'hC };
// these are the permutation indices of the present cipher
localparam logic[63:0][5:0] Perm = {6'd63, 6'd47, 6'd31, 6'd15, 6'd62, 6'd46, 6'd30, 6'd14,
6'd61, 6'd45, 6'd29, 6'd13, 6'd60, 6'd44, 6'd28, 6'd12,
6'd59, 6'd43, 6'd27, 6'd11, 6'd58, 6'd42, 6'd26, 6'd10,
6'd57, 6'd41, 6'd25, 6'd09, 6'd56, 6'd40, 6'd24, 6'd08,
6'd55, 6'd39, 6'd23, 6'd07, 6'd54, 6'd38, 6'd22, 6'd06,
6'd53, 6'd37, 6'd21, 6'd05, 6'd52, 6'd36, 6'd20, 6'd04,
6'd51, 6'd35, 6'd19, 6'd03, 6'd50, 6'd34, 6'd18, 6'd02,
6'd49, 6'd33, 6'd17, 6'd01, 6'd48, 6'd32, 6'd16, 6'd00};
function automatic logic [63:0] sbox4_layer(logic [63:0] in);
logic [63:0] out;
//for (logic [4:0] j = '0; j<16; j++) out[j*4 +: 4] = sbox4[in[j*4 +: 4]];
// this simulates much faster than the loop
out[0*4 +: 4] = Sbox4[in[0*4 +: 4]];
out[1*4 +: 4] = Sbox4[in[1*4 +: 4]];
out[2*4 +: 4] = Sbox4[in[2*4 +: 4]];
out[3*4 +: 4] = Sbox4[in[3*4 +: 4]];
out[4*4 +: 4] = Sbox4[in[4*4 +: 4]];
out[5*4 +: 4] = Sbox4[in[5*4 +: 4]];
out[6*4 +: 4] = Sbox4[in[6*4 +: 4]];
out[7*4 +: 4] = Sbox4[in[7*4 +: 4]];
out[8*4 +: 4] = Sbox4[in[8*4 +: 4]];
out[9*4 +: 4] = Sbox4[in[9*4 +: 4]];
out[10*4 +: 4] = Sbox4[in[10*4 +: 4]];
out[11*4 +: 4] = Sbox4[in[11*4 +: 4]];
out[12*4 +: 4] = Sbox4[in[12*4 +: 4]];
out[13*4 +: 4] = Sbox4[in[13*4 +: 4]];
out[14*4 +: 4] = Sbox4[in[14*4 +: 4]];
out[15*4 +: 4] = Sbox4[in[15*4 +: 4]];
return out;
endfunction : sbox4_layer
function automatic logic [63:0] perm_layer(logic [63:0] in);
logic [63:0] out;
// for (logic [7:0] j = '0; j<64; j++) out[perm[j]] = in[j];
// this simulates much faster than the loop
out[Perm[0]] = in[0];
out[Perm[1]] = in[1];
out[Perm[2]] = in[2];
out[Perm[3]] = in[3];
out[Perm[4]] = in[4];
out[Perm[5]] = in[5];
out[Perm[6]] = in[6];
out[Perm[7]] = in[7];
out[Perm[8]] = in[8];
out[Perm[9]] = in[9];
out[Perm[10]] = in[10];
out[Perm[11]] = in[11];
out[Perm[12]] = in[12];
out[Perm[13]] = in[13];
out[Perm[14]] = in[14];
out[Perm[15]] = in[15];
out[Perm[16]] = in[16];
out[Perm[17]] = in[17];
out[Perm[18]] = in[18];
out[Perm[19]] = in[19];
out[Perm[20]] = in[20];
out[Perm[21]] = in[21];
out[Perm[22]] = in[22];
out[Perm[23]] = in[23];
out[Perm[24]] = in[24];
out[Perm[25]] = in[25];
out[Perm[26]] = in[26];
out[Perm[27]] = in[27];
out[Perm[28]] = in[28];
out[Perm[29]] = in[29];
out[Perm[30]] = in[30];
out[Perm[31]] = in[31];
out[Perm[32]] = in[32];
out[Perm[33]] = in[33];
out[Perm[34]] = in[34];
out[Perm[35]] = in[35];
out[Perm[36]] = in[36];
out[Perm[37]] = in[37];
out[Perm[38]] = in[38];
out[Perm[39]] = in[39];
out[Perm[40]] = in[40];
out[Perm[41]] = in[41];
out[Perm[42]] = in[42];
out[Perm[43]] = in[43];
out[Perm[44]] = in[44];
out[Perm[45]] = in[45];
out[Perm[46]] = in[46];
out[Perm[47]] = in[47];
out[Perm[48]] = in[48];
out[Perm[49]] = in[49];
out[Perm[50]] = in[50];
out[Perm[51]] = in[51];
out[Perm[52]] = in[52];
out[Perm[53]] = in[53];
out[Perm[54]] = in[54];
out[Perm[55]] = in[55];
out[Perm[56]] = in[56];
out[Perm[57]] = in[57];
out[Perm[58]] = in[58];
out[Perm[59]] = in[59];
out[Perm[60]] = in[60];
out[Perm[61]] = in[61];
out[Perm[62]] = in[62];
out[Perm[63]] = in[63];
return out;
endfunction : perm_layer
////////////////////////////////////////////////////////////////////////
// lfsr
////////////////////////////////////////////////////////////////////////
logic [LfsrWidth-1:0] lfsr_d, lfsr_q;
assign lfsr_d =
(en_i) ? (lfsr_q>>1) ^ ({LfsrWidth{lfsr_q[0]}} & Masks[LfsrWidth][LfsrWidth-1:0]) : lfsr_q;
always_ff @(posedge clk_i or negedge rst_ni) begin : p_regs
//$display("%b %h", en_i, lfsr_d);
if (!rst_ni) begin
lfsr_q <= LfsrWidth'(RstVal);
end else begin
lfsr_q <= lfsr_d;
end
end
////////////////////////////////////////////////////////////////////////
// block cipher layers
////////////////////////////////////////////////////////////////////////
if (CipherLayers > unsigned'(0)) begin : g_cipher_layers
logic [63:0] ciph_layer;
localparam int unsigned NumRepl = ((64+LfsrWidth)/LfsrWidth);
always_comb begin : p_ciph_layer
automatic logic [63:0] tmp;
tmp = 64'({NumRepl{lfsr_q}});
for(int unsigned k = 0; k < CipherLayers; k++) begin
tmp = perm_layer(sbox4_layer(tmp));
end
ciph_layer = tmp;
end
// additiona output reg after cipher
if (CipherReg) begin : g_cipher_reg
logic [OutWidth-1:0] out_d, out_q;
assign out_d = (en_i) ? ciph_layer[OutWidth-1:0] : out_q;
assign out_o = out_q[OutWidth-1:0];
always_ff @(posedge clk_i or negedge rst_ni) begin : p_regs
if (!rst_ni) begin
out_q <= '0;
end else begin
out_q <= out_d;
end
end
// no outreg
end else begin : g_no_out_reg
assign out_o = ciph_layer[OutWidth-1:0];
end
// no block cipher
end else begin : g_no_cipher_layers
assign out_o = lfsr_q[OutWidth-1:0];
end
////////////////////////////////////////////////////////////////////////
// assertions
////////////////////////////////////////////////////////////////////////
`ifndef VERILATOR
// pragma translate_off
initial begin
// these are the LUT limits
assert(OutWidth <= LfsrWidth) else
$fatal(1,"OutWidth must be smaller equal the LfsrWidth.");
assert(RstVal > unsigned'(0)) else
$fatal(1,"RstVal must be nonzero.");
assert((LfsrWidth >= $low(Masks)) && (LfsrWidth <= $high(Masks))) else
$fatal(1,"Unsupported LfsrWidth.");
assert(Masks[LfsrWidth][LfsrWidth-1]) else
$fatal(1, "LFSR mask is not correct. The MSB must be 1." );
assert((CipherLayers > 0) && (LfsrWidth == 64) || (CipherLayers == 0)) else
$fatal(1, "Use additional cipher layers only in conjunction with an LFSR width of 64 bit." );
end
all_zero: assert property (
@(posedge clk_i) disable iff (!rst_ni) en_i |-> lfsr_d)
else $fatal(1,"Lfsr must not be all-zero.");
// pragma translate_on
`endif
endmodule // lfsr
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