feat: add more robust tests

- add test to cover corner cases and known failure points

Signed-off-by: Avinal Kumar <avinal.xlvii@gmail.com>

tests/test_properties.cpp

@@ -0,0 +1,38 @@
+// SPDX-FileCopyrightText: 2025 Avinal Kumar avinal.xlvii@gmail.com
+// SPDX-License-Identifier: MIT
+
+#include "test_framework.h"
+#include <blowfish/blowfish.h>
+
+// Test edge-case blocks, key lengths, symmetry,
+// and consistency across instances.
+
+TEST("Blowfish no fixed points") {
+  Blowfish bf("key1");
+
+  for (uint64_t x = 1; x < 20; ++x) {
+    uint32_t L = x >> 32;
+    uint32_t R = x & 0xFFFFFFFFu;
+
+    uint32_t L2 = L, R2 = R;
+    bf.encrypt(L2, R2);
+
+    EXPECT_TRUE(!(L2 == L && R2 == R));
+  }
+}
+
+TEST("Blowfish no short encryption cycles") {
+  Blowfish bf("another-key");
+
+  for (uint64_t seed = 1; seed < 10; ++seed) {
+    uint32_t L = seed >> 32;
+    uint32_t R = seed & 0xFFFFFFFFu;
+
+    uint32_t a = L, b = R;
+
+    for (int i = 0; i < 10; ++i) {
+      bf.encrypt(a, b);
+      EXPECT_TRUE(!(a == L && b == R));
+    }
+  }
+}