byte[] finalDigest = "final_digest".getBytes();

            when(mockMd5.digest()).thenReturn(innerDigest);
            when(mockMd5.digest(any(byte[].class))).thenReturn(finalDigest);

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

            assertArrayEquals(finalDigest, result);