added fe_cswap for DH

src/fe.c

@@ -187,6 +187,77 @@ void fe_cmov(fe f, const fe g, unsigned int b) {
     f[9] = f9 ^ x9;
 }
 
+/*
+	Replace (f,g) with (g,f) if b == 1;
+	replace (f,g) with (f,g) if b == 0.
+
+	Preconditions: b in {0,1}.
+*/
+
+void fe_cswap(fe f,fe g,unsigned int b) {
+	int32_t f0 = f[0];
+	int32_t f1 = f[1];
+	int32_t f2 = f[2];
+	int32_t f3 = f[3];
+	int32_t f4 = f[4];
+	int32_t f5 = f[5];
+	int32_t f6 = f[6];
+	int32_t f7 = f[7];
+	int32_t f8 = f[8];
+	int32_t f9 = f[9];
+	int32_t g0 = g[0];
+	int32_t g1 = g[1];
+	int32_t g2 = g[2];
+	int32_t g3 = g[3];
+	int32_t g4 = g[4];
+	int32_t g5 = g[5];
+	int32_t g6 = g[6];
+	int32_t g7 = g[7];
+	int32_t g8 = g[8];
+	int32_t g9 = g[9];
+	int32_t x0 = f0 ^ g0;
+	int32_t x1 = f1 ^ g1;
+	int32_t x2 = f2 ^ g2;
+	int32_t x3 = f3 ^ g3;
+	int32_t x4 = f4 ^ g4;
+	int32_t x5 = f5 ^ g5;
+	int32_t x6 = f6 ^ g6;
+	int32_t x7 = f7 ^ g7;
+	int32_t x8 = f8 ^ g8;
+	int32_t x9 = f9 ^ g9;
+	b = -b;
+	x0 &= b;
+	x1 &= b;
+	x2 &= b;
+	x3 &= b;
+	x4 &= b;
+	x5 &= b;
+	x6 &= b;
+	x7 &= b;
+	x8 &= b;
+	x9 &= b;
+	f[0] = f0 ^ x0;
+	f[1] = f1 ^ x1;
+	f[2] = f2 ^ x2;
+	f[3] = f3 ^ x3;
+	f[4] = f4 ^ x4;
+	f[5] = f5 ^ x5;
+	f[6] = f6 ^ x6;
+	f[7] = f7 ^ x7;
+	f[8] = f8 ^ x8;
+	f[9] = f9 ^ x9;
+	g[0] = g0 ^ x0;
+	g[1] = g1 ^ x1;
+	g[2] = g2 ^ x2;
+	g[3] = g3 ^ x3;
+	g[4] = g4 ^ x4;
+	g[5] = g5 ^ x5;
+	g[6] = g6 ^ x6;
+	g[7] = g7 ^ x7;
+	g[8] = g8 ^ x8;
+	g[9] = g9 ^ x9;
+}
+
 
 
 /*

src/fe.h

@@ -26,6 +26,7 @@ void fe_copy(fe h, const fe f);
 int fe_isnegative(const fe f);
 int fe_isnonzero(const fe f);
 void fe_cmov(fe f, const fe g, unsigned int b);
+void fe_cswap(fe f, fe g, unsigned int b);
 
 void fe_neg(fe h, const fe f);
 void fe_add(fe h, const fe f, const fe g);