From 241518f369efce64046be36a15fcb722b00e9477 Mon Sep 17 00:00:00 2001
From: tomsmeding <tom.smeding@gmail.com>
Date: Fri, 7 Oct 2016 20:33:18 +0200
Subject: Working AES encrypt and decrypt!

---
 gf28.cpp | 119 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 119 insertions(+)
 create mode 100644 gf28.cpp

(limited to 'gf28.cpp')

diff --git a/gf28.cpp b/gf28.cpp
new file mode 100644
index 0000000..e65c20a
--- /dev/null
+++ b/gf28.cpp
@@ -0,0 +1,119 @@
+#include <cassert>
+#include "gf28.h"
+
+using namespace std;
+
+int GF28::reduce(int v,int m){
+	assert(m);
+	while(true){
+		int sh=__builtin_clz(m)-__builtin_clz(v);
+		if(sh<0)break;
+		v^=m<<sh;
+		if(v==0)break;
+	}
+	return v;
+}
+
+uint8_t GF28::multiply(uint8_t x,uint8_t y){
+	if(x==0||y==0)return 0;
+	int res=0;
+	int addend=x;
+	while(y){
+		if(y&1)res^=addend;
+		addend<<=1;
+		if(addend&0x100)addend^=modulus;
+		y>>=1;
+	}
+	return res;
+}
+
+GF28::GF28()
+	:value(0){}
+
+GF28::GF28(int v)
+	:value(reduce(v,modulus)){}
+
+GF28::operator uint8_t() const {
+	return value;
+}
+
+GF28& GF28::operator+=(GF28 o){
+	value^=o.value;
+	return *this;
+}
+
+GF28& GF28::operator-=(GF28 o){
+	value^=o.value;
+	return *this;
+}
+
+GF28& GF28::operator<<=(int n){ //multiplication by x^n
+	assert(n>=0);
+	value<<=n;
+	if(value&0x100)value^=modulus;
+	return *this;
+}
+
+GF28 GF28::operator+(GF28 o) const {
+	return GF28(value^o.value);
+}
+
+GF28 GF28::operator-(GF28 o) const {
+	return GF28(value^o.value);
+}
+
+GF28 GF28::operator*(GF28 o) const {
+	if(value==0||o.value==0)return GF28(0);
+	GF28 res;
+	GF28 addend(*this);
+	while(o.value){
+		if(o.value&1)res+=addend;
+		addend<<=1;
+		o.value>>=1;
+	}
+	return res;
+}
+GF28 GF28::operator<<(int n) const {
+	return GF28(*this)<<=n;
+}
+
+bool GF28::operator==(GF28 o) const {
+	return value==o.value;
+}
+
+GF28 GF28::inverse() const {
+	if(value==0)return *this;
+	int x=1,y=0,x2=0,y2=1,r=modulus,r2=value;
+	while(r2!=0){
+		assert(r!=0);
+		int ex=__builtin_clz(r2)-__builtin_clz(r);
+		if(ex<0){
+			swap(x,x2);
+			swap(y,y2);
+			swap(r,r2);
+		} else {
+			int xn=x^(x2<<ex); x=x2; x2=xn;
+			int yn=y^(y2<<ex); y=y2; y2=yn;
+			int rn=r^(r2<<ex); r=r2; r2=rn;
+		}
+	}
+	assert(r==1);
+	return GF28(y);
+}
+
+ostream& operator<<(ostream &os,GF28 p){
+	if(os.flags()&ios_base::hex){
+		return os<<p.value;
+	}
+	if(p.value==0)return os<<'0';
+	bool first=true;
+	for(int m=1<<16,i=16;m!=0;m>>=1,i--){
+		if(p.value&m){
+			if(!first)os<<'+';
+			first=false;
+			if(i==0)os<<'1';
+			else os<<"x^"<<i;
+		}
+	}
+	return os;
+}
-- 
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