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_vectors.cpp

@@ -0,0 +1,25 @@
+// SPDX-FileCopyrightText: 2025 Avinal Kumar avinal.xlvii@gmail.com
+// SPDX-License-Identifier: MIT
+
+#include "test_framework.h"
+#include <blowfish/blowfish2.h>
+
+// Known Answer Tests (KAT): Validate Blowfish output against fixed reference
+// vectors to ensure algorithmic correctness.
+TEST("Blowfish2 Known Answer Vector")
+{
+    Blowfish2 bf("abcdefghijklmnopqrstuvwxyz");
+
+    uint64_t L = 0x424C4F5742463231ULL; // "BLOWBF21" (just a stable block)
+    uint64_t R = 0x4649534832463231ULL; // "FISH2F21"
+
+    uint64_t Lc = L, Rc = R;
+    bf.encrypt(Lc, Rc);
+
+    EXPECT_EQ(Lc, 0xF69E30BC3A4E8B0AULL);
+    EXPECT_EQ(Rc, 0x31E5F507F5412293ULL);
+
+    bf.decrypt(Lc, Rc);
+    EXPECT_EQ(Lc, L);
+    EXPECT_EQ(Rc, R);
+}