unit-tests: Add tests to ensure our Curve25519/448 implementations don't return all-zero secrets

Note that wolfSSL before 5.9.2 required building with
WOLFSSL_ECDHX_SHARED_NOT_ZERO, which was added with 5.3.0, to get and
explicit check.  Since the plugin validates the public key, the test case
fails nonetheless.

src/libstrongswan/tests/Makefile.am

@@ -64,6 +64,8 @@ libstrongswan_tests_SOURCES = tests.h tests.c \
   suites/test_rng_tester.c \
   suites/test_mgf1.c \
   suites/test_prf_plus.c \
+  suites/test_curve25519.c \
+  suites/test_curve448.c \
   suites/test_ed25519.c \
   suites/test_ed448.c \
   suites/test_signature_params.c \

src/libstrongswan/tests/suites/test_curve25519.c

@@ -0,0 +1,51 @@
+/*
+ * Copyright (C) 2026 Tobias Brunner
+ *
+ * Copyright (C) secunet Security Networks AG
+ *
+ * This program is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License as published by the
+ * Free Software Foundation; either version 2 of the License, or (at your
+ * option) any later version.  See <http://www.fsf.org/copyleft/gpl.txt>.
+ *
+ * This program is distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
+ * or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * for more details.
+ */
+
+#include "test_suite.h"
+
+static chunk_t all_zero = chunk_from_chars(
+	0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,
+	0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00);
+
+START_TEST(test_reject_all_zero_secret)
+{
+	key_exchange_t *ke;
+	chunk_t secret;
+
+	ke = lib->crypto->create_ke(lib->crypto, CURVE_25519);
+	ck_assert(ke);
+
+	/* implementations might already reject setting the public key */
+	ignore_result(ke->set_public_key(ke, all_zero));
+	ck_assert(!ke->get_shared_secret(ke, &secret));
+
+	ke->destroy(ke);
+}
+END_TEST
+
+Suite *curve25519_suite_create()
+{
+	Suite *s;
+	TCase *tc;
+
+	s = suite_create("curve25519");
+
+	tc = tcase_create("curve25519_secret");
+	tcase_add_test(tc, test_reject_all_zero_secret);
+	suite_add_tcase(s, tc);
+
+	return s;
+}

src/libstrongswan/tests/suites/test_curve448.c

@@ -0,0 +1,53 @@
+/*
+ * Copyright (C) 2026 Tobias Brunner
+ *
+ * Copyright (C) secunet Security Networks AG
+ *
+ * This program is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License as published by the
+ * Free Software Foundation; either version 2 of the License, or (at your
+ * option) any later version.  See <http://www.fsf.org/copyleft/gpl.txt>.
+ *
+ * This program is distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
+ * or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * for more details.
+ */
+
+#include "test_suite.h"
+
+static chunk_t all_zero = chunk_from_chars(
+	0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,
+	0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,
+	0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,
+	0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00);
+
+START_TEST(test_reject_all_zero_secret)
+{
+	key_exchange_t *ke;
+	chunk_t secret;
+
+	ke = lib->crypto->create_ke(lib->crypto, CURVE_448);
+	ck_assert(ke);
+
+	/* implementations might already reject setting the public key */
+	ignore_result(ke->set_public_key(ke, all_zero));
+	ck_assert(!ke->get_shared_secret(ke, &secret));
+
+	ke->destroy(ke);
+}
+END_TEST
+
+Suite *curve448_suite_create()
+{
+	Suite *s;
+	TCase *tc;
+
+	s = suite_create("curve448");
+
+	tc = tcase_create("curve448_secret");
+	tcase_add_test(tc, test_reject_all_zero_secret);
+	suite_add_tcase(s, tc);
+
+	return s;
+}

src/libstrongswan/tests/tests.h

@@ -61,6 +61,8 @@ TEST_SUITE_DEPEND(mgf1_sha1_suite_create, XOF, XOF_MGF1_SHA1)
 TEST_SUITE_DEPEND(mgf1_sha256_suite_create, XOF, XOF_MGF1_SHA256)
 TEST_SUITE_DEPEND(prf_plus_suite_create, KDF, KDF_PRF_PLUS)
 TEST_SUITE_DEPEND(fetch_http_suite_create, FETCHER, "http://")
+TEST_SUITE_DEPEND(curve25519_suite_create, KE, CURVE_25519)
+TEST_SUITE_DEPEND(curve448_suite_create, KE, CURVE_448)
 TEST_SUITE_DEPEND(ed25519_suite_create, PRIVKEY_GEN, KEY_ED25519)
 TEST_SUITE_DEPEND(ed448_suite_create, PRIVKEY_GEN, KEY_ED448)
 TEST_SUITE(signature_params_suite_create)