diff --git a/bignum.c b/bignum.c
index d2ebd6350..a0364309b 100644
--- a/bignum.c
+++ b/bignum.c
@@ -156,6 +156,15 @@ void bn_write_le(const bignum256 *in_number, uint8_t *out_number)
 	}
 }
 
+// copies number a to b
+void bn_copy(bignum256 *a, bignum256 *b)
+{
+	int i;
+	for (i = 0; i < 9; i++) {
+		b->val[i] = a->val[i];
+	}
+}
+
 // sets a bignum to zero.
 void bn_zero(bignum256 *a)
 {
diff --git a/bignum.h b/bignum.h
index 65bea7ba7..fd040c8dc 100644
--- a/bignum.h
+++ b/bignum.h
@@ -53,6 +53,8 @@ void bn_read_le(const uint8_t *in_number, bignum256 *out_number);
 
 void bn_write_le(const bignum256 *in_number, uint8_t *out_number);
 
+void bn_copy(bignum256 *a, bignum256 *b);
+
 void bn_zero(bignum256 *a);
 
 int bn_is_zero(const bignum256 *a);
diff --git a/tests.c b/tests.c
index f1e10ba2b..82d47ab8c 100644
--- a/tests.c
+++ b/tests.c
@@ -153,6 +153,18 @@ START_TEST(test_bignum_write_le)
 }
 END_TEST
 
+START_TEST(test_bignum_copy)
+{
+	bignum256 a;
+	bignum256 b;
+
+	bn_read_be(fromhex("c55ece858b0ddd5263f96810fe14437cd3b5e1fbd7c6a2ec1e031f05e86d8bd5"), &a);
+	bn_copy(&a, &b);
+
+	ck_assert_int_eq(bn_is_equal(&a, &b), 1);
+}
+END_TEST
+
 // from https://github.com/bitcoin/bitcoin/blob/master/src/test/data/base58_keys_valid.json
 START_TEST(test_base58)
 {
@@ -2403,6 +2415,7 @@ Suite *test_suite(void)
 	tcase_add_test(tc, test_bignum_equal);
 	tcase_add_test(tc, test_bignum_read_le);
 	tcase_add_test(tc, test_bignum_write_le);
+	tcase_add_test(tc, test_bignum_copy);
 	suite_add_tcase(s, tc);
 
 	tc = tcase_create("base58");