add mclBn{Fr,Fp}_getLittleEndian

include/mcl/bn.h

@@ -245,6 +245,14 @@ MCLBN_DLL_API void mclBnFr_setInt32(mclBnFr *y, int x);
 MCLBN_DLL_API int mclBnFr_setLittleEndian(mclBnFr *x, const void *buf, mclSize bufSize);
 MCLBN_DLL_API int mclBnFp_setLittleEndian(mclBnFp *x, const void *buf, mclSize bufSize);
 
+/*
+	write a value as little endian
+	return written size if success else 0
+	@note buf[0] = 0 and return 1 if the value is zero
+*/
+MCLBN_DLL_API mclSize mclBnFr_getLittleEndian(void *buf, mclSize maxBufSize, const mclBnFr *x);
+MCLBN_DLL_API mclSize mclBnFp_getLittleEndian(void *buf, mclSize maxBufSize, const mclBnFp *x);
+
 // set (buf mod r) to x
 // return 0 if bufSize <= (byte size of Fr * 2) else -1
 MCLBN_DLL_API int mclBnFr_setLittleEndianMod(mclBnFr *x, const void *buf, mclSize bufSize);

include/mcl/impl/bn_c_impl.hpp

@@ -144,6 +144,10 @@ int mclBnFr_setLittleEndian(mclBnFr *x, const void *buf, mclSize bufSize)
 	cast(x)->setArrayMask((const char *)buf, bufSize);
 	return 0;
 }
+mclSize mclBnFr_getLittleEndian(void *buf, mclSize maxBufSize, const mclBnFr *x)
+{
+	return cast(x)->getLittleEndian(buf, maxBufSize);
+}
 int mclBnFr_setLittleEndianMod(mclBnFr *x, const void *buf, mclSize bufSize)
 {
 	bool b;
@@ -595,6 +599,11 @@ int mclBnFp_setLittleEndianMod(mclBnFp *x, const void *buf, mclSize bufSize)
 	cast(x)->setArray(&b, (const char *)buf, bufSize, mcl::fp::Mod);
 	return b ? 0 : -1;
 }
+
+mclSize mclBnFp_getLittleEndian(void *buf, mclSize maxBufSize, const mclBnFp *x)
+{
+	return cast(x)->getLittleEndian(buf, maxBufSize);
+}
 int mclBnFp_isEqual(const mclBnFp *x, const mclBnFp *y)
 {
 	return *cast(x) == *cast(y);

test/bn_c_test.hpp

@@ -664,6 +664,37 @@ CYBOZU_TEST_AUTO(mapToG2)
 	CYBOZU_TEST_ASSERT(mclBnG2_isEqual(&P1, &P2));
 }
 
+CYBOZU_TEST_AUTO(getLittleEndian)
+{
+	const struct {
+		const char *in;
+		uint8_t out[16];
+		size_t size;
+	} tbl[] = {
+		{ "0", { 0 }, 1 },
+		{ "1", { 1 }, 1 },
+		{ "0x1200", { 0x00, 0x12 }, 2 },
+		{ "0x123400", { 0x00, 0x34, 0x12 }, 3 },
+		{ "0x1234567890123456ab", { 0xab, 0x56, 0x34, 0x12, 0x90, 0x78, 0x56, 0x34, 0x12 }, 9 },
+	};
+	for (size_t i = 0; i < CYBOZU_NUM_OF_ARRAY(tbl); i++) {
+		size_t n;
+		mclBnFr x;
+		CYBOZU_TEST_ASSERT(!mclBnFr_setStr(&x, tbl[i].in, strlen(tbl[i].in), 0));
+		uint8_t buf[128];
+		n = mclBnFr_getLittleEndian(buf, tbl[i].size, &x);
+		CYBOZU_TEST_EQUAL(n, tbl[i].size);
+		CYBOZU_TEST_EQUAL_ARRAY(buf, tbl[i].out, n);
+
+		n = mclBnFr_getLittleEndian(buf, tbl[i].size + 1, &x);
+		CYBOZU_TEST_EQUAL(n, tbl[i].size);
+		CYBOZU_TEST_EQUAL_ARRAY(buf, tbl[i].out, n);
+
+		n = mclBnFr_getLittleEndian(buf, tbl[i].size - 1, &x);
+		CYBOZU_TEST_EQUAL(n, 0);
+	}
+}
+
 void G1onlyTest(int curve)
 {
 	printf("curve=%d\n", curve);