diff --git a/include/mcl/gmp_util.hpp b/include/mcl/gmp_util.hpp
index 2b581a3..78cafc1 100644
--- a/include/mcl/gmp_util.hpp
+++ b/include/mcl/gmp_util.hpp
@@ -416,14 +416,15 @@ inline mpz_class abs(const mpz_class& x)
 	return ::abs(x);
 #endif
 }
-template<class RG>
-void getRand(mpz_class& z, size_t bitSize, RG& rg)
+
+inline void getRand(mpz_class& z, size_t bitSize, fp::RandGen rg = fp::RandGen())
 {
+	if (rg.isZero()) rg = fp::RandGen::get();
 	assert(bitSize > 1);
 	const size_t rem = bitSize & 31;
 	const size_t n = (bitSize + 31) / 32;
 	std::vector<uint32_t> buf(n);
-	rg.read(buf.data(), n);
+	rg.read(buf.data(), n * sizeof(buf[0]));
 	uint32_t v = buf[n - 1];
 	if (rem == 0) {
 		v |= 1U << 31;
@@ -434,9 +435,10 @@ void getRand(mpz_class& z, size_t bitSize, RG& rg)
 	buf[n - 1] = v;
 	setArray(z, &buf[0], n);
 }
-template<class RG>
-void getRandPrime(mpz_class& z, size_t bitSize, RG& rg, bool setSecondBit = false, bool mustBe3mod4 = false)
+
+inline void getRandPrime(mpz_class& z, size_t bitSize, fp::RandGen rg = fp::RandGen(), bool setSecondBit = false, bool mustBe3mod4 = false)
 {
+	if (rg.isZero()) rg = fp::RandGen::get();
 	assert(bitSize > 2);
 	do {
 		getRand(z, bitSize, rg);
diff --git a/include/mcl/paillier.hpp b/include/mcl/paillier.hpp
index faab203..03e44cb 100644
--- a/include/mcl/paillier.hpp
+++ b/include/mcl/paillier.hpp
@@ -24,9 +24,9 @@ public:
 		g = 1 + _n;
 		n2 = _n * _n;
 	}
-	template<class RG>
-	void enc(mpz_class& c, const mpz_class& m, RG& rg) const
+	void enc(mpz_class& c, const mpz_class& m, mcl::fp::RandGen rg = mcl::fp::RandGen()) const
 	{
+		if (rg.isZero()) rg = mcl::fp::RandGen::get();
 		if (primeBitSize == 0) throw cybozu::Exception("paillier:PublicKey:not init");
 		mpz_class r;
 		mcl::gmp::getRand(r, primeBitSize, rg);
@@ -56,9 +56,9 @@ public:
 	/*
 		the size of prime is half of bitSize
 	*/
-	template<class RG>
-	void init(size_t bitSize, RG& rg)
+	void init(size_t bitSize, mcl::fp::RandGen rg = mcl::fp::RandGen())
 	{
+		if (rg.isZero()) rg = mcl::fp::RandGen::get();
 		primeBitSize = bitSize / 2;
 		mpz_class p, q;
 		mcl::gmp::getRandPrime(p, primeBitSize, rg);
diff --git a/test/gmp_test.cpp b/test/gmp_test.cpp
index 2d3e563..22c80dd 100644
--- a/test/gmp_test.cpp
+++ b/test/gmp_test.cpp
@@ -21,3 +21,11 @@ CYBOZU_TEST_AUTO(testBit)
 	}
 }
 
+CYBOZU_TEST_AUTO(getRandPrime)
+{
+	for (int i = 0; i < 10; i++) {
+		mpz_class z;
+		mcl::gmp::getRandPrime(z, i * 10 + 3);
+		CYBOZU_TEST_ASSERT(mcl::gmp::isPrime(z));
+	}
+}
diff --git a/test/paillier_test.cpp b/test/paillier_test.cpp
index 3791452..31d2b26 100644
--- a/test/paillier_test.cpp
+++ b/test/paillier_test.cpp
@@ -1,19 +1,17 @@
 #include <cybozu/test.hpp>
-#include <cybozu/random_generator.hpp>
 #include <mcl/paillier.hpp>
 
 CYBOZU_TEST_AUTO(paillier)
 {
-	cybozu::RandomGenerator rg;
 	using namespace mcl::paillier;
 	SecretKey sec;
-	sec.init(2048, rg);
+	sec.init(2048);
 	PublicKey pub;
 	sec.getPublicKey(pub);
 	mpz_class m1("12342340928409"), m2("23049820498204");
 	mpz_class c1, c2, c3;
-	pub.enc(c1, m1, rg);
-	pub.enc(c2, m2, rg);
+	pub.enc(c1, m1);
+	pub.enc(c2, m2);
 	std::cout << std::hex << "c1=" << c1 << "\nc2=" << c2 << std::endl;
 	pub.add(c3, c1, c2);
 	mpz_class d1, d2, d3;