No code is unbreakable. Mathematicians may be able design codes that can't be cracked by all the computational power available on earth, but that won't guarantee the security of the encoded data. The cipher can still be stolen, clearances forged, or the data intercepted when authorized users access it. Designing a perfect system for protecting secrets is impossible in the same way that it's impossible to design a perfectly efficient engine. It’s practically a law of thermodynamics: if someone can decode something, someone else can steal it.

Codes and security protocols can’t ensure than any secret is safe. But by making it difficult to get at information—by raising the cost of stealing it—they still deter people from trying to steal secrets. Secrets, in other words, are effectively safe when obtaining them is more trouble than its worth, when trying to steal them costs more than the value of the information they contain.

That calculation is at the root of the privacy issues raised by new information technology and social media. In the past, ordinary people could be confident that the neither the government, nor private corporations and individuals were spying on them. It simply wasn’t worth anyone’s effort to spy on the average citizen. During the Cold War, the East German government did spy on nearly all its citizens as a way of keeping control, but that took a huge, unprecedented network of spies and informants. Now, however, as more of our personal data is uploaded to central, easily searchable databases, the cost of collecting it and using it—whether to monitor us or impersonate us—is dropping quickly. As the price of spying drops, it's becoming feasible to spy on everyone, all the time.

One reason to monitor what people do, of course, is to catch criminals and prevent terrorism. It's on these grounds that the Obama administration has asked for the authority to access our e-mail and browser logs without having to obtain a court order. National security concerns are also behind the decision to invest—alongside Google—in Recorded Future, a company which searches for patterns in publicly available data to keep track of events of all kinds and “assemble actual real-time dossiers on people.” But the price of that power—the flip side of the homeland security coin—is that it also gives the government the ability to suppress dissent  and stay in power. It's not that our leaders are any less trustworthy or honest than they used to be, but that the temptation to spy on us is growing as it becomes easier to do. Sooner or later, governments—perhaps even with the best of intentions—end up use whatever powers they have to increase their control their control over the population.

That's what makes Dana Priest and William Arkin’s investigative series on our vast and growing intelligence apparatus—which I would have included in my recent list of essential readings if it had been published earlier—so disturbing. Priest and Arkin report that since September 11 we have started work on 33 new building complexes around Washington for high-level intelligence work. They estimate that there are more than 850,000 people—including some 250,000 private contractors—who hold “Top Secret” security clearance. That doesn't surprise me, since I had Top Secret Security clearance at one time myself—at the age of 20, as a student intern at the State Department.

But the fact that it is increasingly hard to keep secrets cuts both ways. As my fellow blogger Lea Carpenter says, we may be facing “the end of secrecy” in another way too. There’s simply no way that 850,000 people can keep vast quantities of classified information to themselves. Not when a 22 year-old junior intelligence analyst can leak 150,000 diplomatic cables and classified videos by copying them onto what looked like Lady Gaga CDs. Big conspiracies are simply too difficult to maintain now. That means that we have the same power to monitor the government it has to monitor us. We should be able to see any threat to our freedom coming—if we pay attention.

We will have to pay attention. The end of secrecy—and privacy—means we'll have take pains to make sure the government doesn’t abuse the power information technology gives it. After all, we have, as Ben Franklin once said, not a monarchy, but a republic—if we can keep it.