From ef152cc53f98922466627ad728a89c066cd4de80 Mon Sep 17 00:00:00 2001
From: Ahmet Inan <xdsopl@gmail.com>
Date: Sun, 18 Jul 2021 04:48:32 +0200
Subject: [PATCH] scramble user data only using Xorshift32

---
 decode.cc | 13 ++++++-------
 encode.cc | 15 +++++++--------
 2 files changed, 13 insertions(+), 15 deletions(-)

diff --git a/decode.cc b/decode.cc
index 296de66..eac1533 100644
--- a/decode.cc
+++ b/decode.cc
@@ -11,6 +11,7 @@ Copyright 2021 Ahmet Inan <inan@aicodix.de>
 namespace DSP { using std::abs; using std::min; using std::cos; using std::sin; }
 #include "bip_buffer.hh"
 #include "resampler.hh"
+#include "xorshift.hh"
 #include "trigger.hh"
 #include "complex.hh"
 #include "decibel.hh"
@@ -173,8 +174,6 @@ struct Decoder
 	static const int mls1_len = 255;
 	static const int mls1_off = - mls1_len / 2;
 	static const int mls1_poly = 0b100101011;
-	static const int mls3_poly = 0b10001000000001011;
-	static const int mls4_poly = 0b10111010010000001;
 	static const int buffer_len = (rows_max + 8) * (symbol_len + guard_len);
 	static const int search_pos = buffer_len - 4 * (symbol_len + guard_len);
 	DSP::ReadPCM<value> *pcm;
@@ -422,17 +421,14 @@ struct Decoder
 		for (int i = 0; i < buffer_len; ++i)
 			tdom[i] = resam[i] * osc();
 
-		CODE::MLS seq3(mls3_poly), seq4(mls4_poly);
 		cmplx *cur = tdom + symbol_pos - (code_rows + 1) * (symbol_len + guard_len);
 		fwd(fdom, cur);
 		for (int j = 0; j < code_rows; ++j) {
 			for (int i = 0; i < code_cols; ++i)
 				head[bin(i+code_off)] = fdom[bin(i+code_off)];
 			fwd(fdom, cur += symbol_len+guard_len);
-			for (int i = 0; i < code_cols; ++i) {
-				cmplx con = fdom[bin(i+code_off)] / head[bin(i+code_off)];
-				cons[code_cols*j+i] = cmplx(con.real() * nrz(seq3()), con.imag() * nrz(seq4()));
-			}
+			for (int i = 0; i < code_cols; ++i)
+				cons[code_cols*j+i] = fdom[bin(i+code_off)] / head[bin(i+code_off)];
 		}
 		value precision = 16;
 		if (1) {
@@ -553,6 +549,9 @@ int main(int argc, char **argv)
 		return 1;
 	}
 	const int data_len = code_len - (12 * 16 + 21600) / 8;
+	CODE::Xorshift32 scrambler;
+	for (int i = 0; i < data_len; ++i)
+		output_data[i] ^= scrambler();
 	for (int i = 0; i < data_len; ++i)
 		output_file.put(output_data[i]);
 	delete []output_data;
diff --git a/encode.cc b/encode.cc
index 30fc31a..f4efcfb 100644
--- a/encode.cc
+++ b/encode.cc
@@ -7,6 +7,7 @@ Copyright 2021 Ahmet Inan <inan@aicodix.de>
 #include <iostream>
 #include <cassert>
 #include <cmath>
+#include "xorshift.hh"
 #include "complex.hh"
 #include "utils.hh"
 #include "bitman.hh"
@@ -35,8 +36,6 @@ struct Encoder
 	static const int mls1_len = 255;
 	static const int mls1_poly = 0b100101011;
 	static const int mls2_poly = 0b100101010001;
-	static const int mls3_poly = 0b10001000000001011;
-	static const int mls4_poly = 0b10111010010000001;
 	DSP::WritePCM<value> *pcm;
 	DSP::FastFourierTransform<symbol_len, cmplx, 1> bwd;
 	DSP::FastFourierTransform<4*symbol_len, cmplx, -1> fwd4;
@@ -239,13 +238,10 @@ struct Encoder
 			code[i] = nrz(CODE::get_le_bit(inp, i));
 		ldpcenc(code, code + bch_bits);
 		interleave();
-		CODE::MLS seq3(mls3_poly), seq4(mls4_poly);
 		for (int j = 0; j < code_rows; ++j) {
-			for (int i = 0; i < code_cols; ++i) {
-				cmplx con = mod_map(bint+mod_bits*(code_cols*j+i));
-				con = cmplx(con.real() * nrz(seq3()), con.imag() * nrz(seq4()));
-				fdom[bin(i+code_off)] *= con;
-			}
+			for (int i = 0; i < code_cols; ++i)
+				fdom[bin(i+code_off)] *=
+					mod_map(bint+mod_bits*(code_cols*j+i));
 			symbol();
 		}
 		schmidl_cox();
@@ -345,6 +341,9 @@ int main(int argc, char **argv)
 	uint8_t *input_data = new uint8_t[code_len];
 	for (int i = 0; i < data_len; ++i)
 		input_data[i] = input_file.get();
+	CODE::Xorshift32 scrambler;
+	for (int i = 0; i < data_len; ++i)
+		input_data[i] ^= scrambler();
 
 	DSP::WriteWAV<value> output_file(output_name, output_rate, output_bits, output_chan);
 	output_file.silence(output_rate);