diff --git a/lib/include/chiaki/ecdh.h b/lib/include/chiaki/ecdh.h
index b96907e..440fcb8 100644
--- a/lib/include/chiaki/ecdh.h
+++ b/lib/include/chiaki/ecdh.h
@@ -27,6 +27,8 @@
 extern "C" {
 #endif
 
+#define CHIAKI_ECDH_SECRET_SIZE 32
+
 typedef struct chiaki_ecdh_t
 {
 	struct ec_group_st *group;
@@ -36,7 +38,7 @@ typedef struct chiaki_ecdh_t
 CHIAKI_EXPORT ChiakiErrorCode chiaki_ecdh_init(ChiakiECDH *ecdh);
 CHIAKI_EXPORT void chiaki_ecdh_fini(ChiakiECDH *ecdh);
 CHIAKI_EXPORT ChiakiErrorCode chiaki_ecdh_get_local_pub_key(ChiakiECDH *ecdh, uint8_t *key_out, size_t *key_out_size, const uint8_t *handshake_key, uint8_t *sig_out, size_t *sig_out_size);
-CHIAKI_EXPORT ChiakiErrorCode chiaki_ecdh_derive_secret(ChiakiECDH *ecdh, uint8_t *secret_out, size_t *secret_out_size, const uint8_t *remote_key, size_t remote_key_size, const uint8_t *handshake_key, const uint8_t *remote_sig, size_t remote_sig_size);
+CHIAKI_EXPORT ChiakiErrorCode chiaki_ecdh_derive_secret(ChiakiECDH *ecdh, uint8_t *secret_out, const uint8_t *remote_key, size_t remote_key_size, const uint8_t *handshake_key, const uint8_t *remote_sig, size_t remote_sig_size);
 CHIAKI_EXPORT ChiakiErrorCode chiaki_ecdh_set_local_key(ChiakiECDH *ecdh, const uint8_t *private_key, size_t private_key_size, const uint8_t *public_key, size_t public_key_size);
 
 #ifdef __cplusplus
diff --git a/lib/src/ecdh.c b/lib/src/ecdh.c
index ae84c23..786e3c7 100644
--- a/lib/src/ecdh.c
+++ b/lib/src/ecdh.c
@@ -108,7 +108,7 @@ CHIAKI_EXPORT ChiakiErrorCode chiaki_ecdh_get_local_pub_key(ChiakiECDH *ecdh, ui
 	return CHIAKI_ERR_SUCCESS;
 }
 
-CHIAKI_EXPORT ChiakiErrorCode chiaki_ecdh_derive_secret(ChiakiECDH *ecdh, uint8_t *secret_out, size_t *secret_out_size, const uint8_t *remote_key, size_t remote_key_size, const uint8_t *handshake_key, const uint8_t *remote_sig, size_t remote_sig_size)
+CHIAKI_EXPORT ChiakiErrorCode chiaki_ecdh_derive_secret(ChiakiECDH *ecdh, uint8_t *secret_out, const uint8_t *remote_key, size_t remote_key_size, const uint8_t *handshake_key, const uint8_t *remote_sig, size_t remote_sig_size)
 {
 	EC_POINT *remote_public_key = EC_POINT_new(ecdh->group);
 	if(!remote_public_key)
@@ -120,14 +120,12 @@ CHIAKI_EXPORT ChiakiErrorCode chiaki_ecdh_derive_secret(ChiakiECDH *ecdh, uint8_
 		return CHIAKI_ERR_UNKNOWN;
 	}
 
-	int r = ECDH_compute_key(secret_out, *secret_out_size, remote_public_key, ecdh->key_local, NULL);
+	int r = ECDH_compute_key(secret_out, CHIAKI_ECDH_SECRET_SIZE, remote_public_key, ecdh->key_local, NULL);
 
 	EC_POINT_free(remote_public_key);
 
-	if(r <= 0)
+	if(r != CHIAKI_ECDH_SECRET_SIZE)
 		return CHIAKI_ERR_UNKNOWN;
 
-	*secret_out_size = (size_t)r;
-
 	return CHIAKI_ERR_SUCCESS;
 }
\ No newline at end of file
diff --git a/lib/src/nagare.c b/lib/src/nagare.c
index c2067db..94ac8cf 100644
--- a/lib/src/nagare.c
+++ b/lib/src/nagare.c
@@ -200,10 +200,12 @@ static void nagare_takion_data_expect_bang(ChiakiNagare *nagare, uint8_t *buf, s
 
 	CHIAKI_LOGI(nagare->log, "Nagare bang looks good so far\n");
 
-//	chiaki_ecdh_derive_secret(&nagare->session->ecdh,
-//			ecdh_pub_key_buf.buf, ecdh_pub_key_buf.size,
-//			nagare->session->handshake_key,
-//			ecdh_sig_buf.buf, ecdh_sig_buf.size);
+	uint8_t secret[CHIAKI_ECDH_SECRET_SIZE];
+	chiaki_ecdh_derive_secret(&nagare->session->ecdh,
+			secret,
+			ecdh_pub_key_buf.buf, ecdh_pub_key_buf.size,
+			nagare->session->handshake_key,
+			ecdh_sig_buf.buf, ecdh_sig_buf.size);
 
 error:
 	chiaki_mirai_signal(&nagare->bang_mirai, true);
diff --git a/test/gkcrypt.c b/test/gkcrypt.c
index 30a9d6f..57751ce 100644
--- a/test/gkcrypt.c
+++ b/test/gkcrypt.c
@@ -53,13 +53,12 @@ static MunitResult test_ecdh(const MunitParameter params[], void *user)
 	munit_assert_memory_equal(sizeof(local_public_key_sig), local_public_key_sig_result, local_public_key_sig);
 
 	uint8_t secret_result[128];
-	size_t secret_result_size = sizeof(secret_result);
-	chiaki_ecdh_derive_secret(&ecdh, secret_result, &secret_result_size,
+	chiaki_ecdh_derive_secret(&ecdh, secret_result,
 			remote_public_key, sizeof(remote_public_key),
 			handshake_key,
 			remote_public_key_sig, sizeof(remote_public_key_sig));
 
-	munit_assert_size(secret_result_size, ==, sizeof(secret));
+	munit_assert_size(CHIAKI_ECDH_SECRET_SIZE, ==, sizeof(secret));
 	munit_assert_memory_equal(sizeof(secret), secret_result, secret);
 
 	chiaki_ecdh_fini(&ecdh);