add map-to-point to G1

include/mcl/bn.hpp

@@ -108,6 +108,46 @@ bool getGoodRepl(Vec& v, const mpz_class& x)
 	}
 }
 
+template<class Fp>
+struct HashMapToG1 {
+	Fp c1; // sqrt(-3)
+	Fp c2; // (-1 + sqrt(-3)) / 2
+	HashMapToG1()
+	{
+		if (!Fp::squareRoot(c1, -3)) throw cybozu::Exception("HashMapToG1:c1");
+		c2 = (c1 - 1) / 2;
+	}
+	int legendre(const Fp& x) const
+	{
+		return gmp::legendre(x.getMpz(), Fp::getOp().mp);
+	}
+	/*
+		P.-A. Fouque and M. Tibouchi,
+		"Indifferentiable hashing to Barreto Naehrig curves, " in Proc. Int. Conf. Cryptol. Inform. Security Latin Amer., 2012, vol. 7533, pp.1-17.
+		algorithm 1 in "Software Implementation of an Attribute-Based Encryption Scheme", IEEE. trans. on computesr, vol. 64, No.5, May 2015
+	*/
+	void calc(mcl::EcT<Fp>& P, const Fp& t, int b) const
+	{
+		Fp w = t * t + b;
+		w = (t / (w + 1)) * c1;
+		bool negative = legendre(t) < 0;
+		Fp x, y;
+		for (int i = 0; i < 3; i++) {
+			switch (i) {
+			case 0: x = c2 - t * w; break;
+			case 1: x = -1 - x; break;
+			case 2: x = 1 + 1 / (w * w); break;
+			}
+			y = x * x * x + b;
+			if (Fp::squareRoot(y, y)) {
+				if (negative) Fp::neg(y, y);
+				P.set(x, y);
+				return;
+			}
+		}
+	}
+};
+
 template<class Fp>
 struct ParamT {
 	typedef Fp2T<Fp> Fp2;