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_roundtrip.cpp

@@ -0,0 +1,27 @@
+// SPDX-FileCopyrightText: 2025 Avinal Kumar avinal.xlvii@gmail.com
+// SPDX-License-Identifier: MIT
+
+#include "test_framework.h"
+#include <blowfish/blowfish.h>
+#include <random>
+
+// Ensure encrypt -> decrypt returns original plaintext
+// for fixed and randomized test blocks.
+TEST("Blowfish encrypt/decrypt roundtrip") {
+  Blowfish bf("test-key");
+
+  std::mt19937_64 rng(12345);
+
+  for (int i = 0; i < 500; ++i) {
+    uint64_t block = rng();
+    uint32_t L = block >> 32;
+    uint32_t R = block & 0xFFFFFFFFu;
+
+    uint32_t L2 = L, R2 = R;
+    bf.encrypt(L2, R2);
+    bf.decrypt(L2, R2);
+
+    EXPECT_EQ(L, L2);
+    EXPECT_EQ(R, R2);
+  }
+}