HMACT64 hmac = new HMACT64(TEST_KEY);
            byte[] result = hmac.engineDigest();

            assertArrayEquals(finalDigest, result);
            verify(mockMd5, times(1)).digest(); // First call for inner digest
            verify(mockMd5, times(2)).update(any(byte[].class)); // Once in constructor with ipad, once in engineDigest with opad
            verify(mockMd5, times(1)).digest(innerDigest); // Second call for final digest

    void testEngineDigest() {
        // Test engineDigest()
        HMACT64 hmac = new HMACT64(TEST_KEY);
        hmac.engineUpdate(TEST_DATA, 0, TEST_DATA.length);
        byte[] result = hmac.engineDigest();

        assertNotNull(result);
        assertEquals(16, result.length); // MD5 produces 16 bytes
    }

    @Test
    void testEngineDigestWithBuffer() {
        // Test engineDigest(byte[] buf, int offset, int len)