Alice's message is indeed fairly secure against brute-force attacks. But maybe not quite as safe as we have supposed here. When we start thinking about years of brute-force attack, we really need to consider Moore's Law. Moore's Law claims (generally) that computing power doubles every 18 months. For each of the last 40 years, people have declared an imminent termination of Moore's Law, but let's suppose it continues on course. That means that 30 years from now, the TLA (and the elderly Mallory) will have a million times the computing power they now have. So using the supercomputers of 2030, Alice's message can be brute-forced in just 125 years. Still probably not too much cause for Alice to worry, but what about the supercomputers of 2045 that will be able to break Alice's message in only a month? Nonetheless, Alice will not likely worry all that much about this brute-force attack, but it is noteworthy that 45 years is quite a bit shorter than 125 million years.