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/bf2_test_properties.cpp

@@ -0,0 +1,35 @@
+// SPDX-FileCopyrightText: 2025 Avinal Kumar avinal.xlvii@gmail.com
+// SPDX-License-Identifier: MIT
+
+#include "test_framework.h"
+#include <blowfish/blowfish2.h>
+
+TEST("Blowfish2 no fixed points") {
+  Blowfish2 bf("fixed-point-check");
+
+  for (uint64_t i = 1; i < 50; ++i) {
+    uint64_t L = i * 0x123456789ABCDEFULL;
+    uint64_t R = i * 0xFEDCBA987654321ULL;
+
+    uint64_t L2 = L, R2 = R;
+    bf.encrypt(L2, R2);
+
+    EXPECT_TRUE(!(L == L2 && R == R2));
+  }
+}
+
+TEST("Blowfish2 no short cycles") {
+  Blowfish2 bf("cycle-check");
+
+  for (uint64_t seed = 1; seed <= 5; ++seed) {
+    uint64_t L0 = seed;
+    uint64_t R0 = seed * 12345;
+
+    uint64_t L = L0, R = R0;
+
+    for (int iter = 0; iter < 12; ++iter) {
+      bf.encrypt(L, R);
+      EXPECT_TRUE(!(L == L0 && R == R0));
+    }
+  }
+}