The Roman Emperor Caligula wrote the law in small writing high up in a corner of a public building so that no one could read what the law was. If no one knew the law, the emperor had a freer hand in saying who had violated it. It was a clever and naked power grab.

The U.S. Department of Justice—with an assist from Congress—is taking a page from Caligula.

Sure, you can find our nation’s criminal laws. But if you’re charged with a crime, for too many cases, good luck to you in figuring out what our laws mean. And federal prosecutors will force you into an untenable risk of prison if you aren’t willing to cut a deal with them and plead guilty.

A recent example is the case of David Nosal, a recruiter with the firm Korn/Ferry. He was convicted in federal court of violating a law that prohibits hacking—the Computer Fraud and Abuse Act. Yet, as media stories have pointed out, Nosal didn’t do anything close to hacking. The law prohibits anyone using a computer that “exceeds authorized access.”

Nosal left a recruiting firm to start a new one. When he left, he asked his friends at his old company to give him some information from the company’s computers. That’s not a great thing to do. But, as the U.S. Court of Appeals for the Ninth Circuit said last year, in an earlier appeal in Nosal’s case, it isn’t clear that that’s “exceed[ing] authorized access” as the law is written.

Nosal will have a hearing later this year to determine if the charges should be dismissed. If they are, he’ll likely walk free. If they aren’t, he’ll go to prison. That’s a high-stakes gamble to see if what he did was a crime.

Most people in Nosal’s situation would take a plea. Right now, the vast majority of cases resolve by having the defendant plead guilty. It’s more than 90 percent in most parts of the country.

One part of the problem is with Congress. Because members of Congress have to show that they’re doing something — in addition to not passing either gun control legislation or a budget — each year new federal crimes roll out of Washington. Between 2000 and 2007, for example, Congress made up 452 new crimes — about 57 each year. Too many of these new crimes are unclear. And, in our legal system, the way you find out what an unclear law means is to have someone challenge it, often all the way to a court of appeals.

A person who pleads guilty gets time off his or her sentence because of the guilty plea. This comes in a few ways: The sentencing guidelines are lower when someone accepts responsibility by taking a plea, and federal prosecutors give concessions in many cases to induce a plea.

If someone wants to challenge what a new law means, he or she loses those benefits. The incentive to plea is great, so the law rarely gets clarified. This is more than a theoretical problem; federal judges are taking notice. For example, federal law prohibits “harboring” an illegal alien. Last year, the Seventh Circuit shot down a prosecution in U.S. v. Costello for harboring. There, a woman was accused of harboring an illegal alien —her boyfriend — when she gave him a ride to her house from the bus station. Exactly what “harboring” meant was unclear, because federal law hadn’t been developed enough to answer the question.

The government’s response was typical: “Trust us. We’re the Department of Justice; we aren’t going to prosecute anyone who ought not be prosecuted.” Judge Richard Posner — probably our most interesting federal appellate court judge — skewered the government on this point:

“The government tells us not to worry: we judges can rely on prosecutors to avoid bringing cases at the outer margin of the government’s sweeping definition of “harboring.” But this case is at the outer margin. No doubt it was brought because the Justice Department suspects that the defendant was involved in her boyfriend’s drug dealings, but cannot prove it, so the Department reaches into its deep arsenal (the 4000-plus federal crimes) and finds a crime that she doubtless never heard of that it can pin on her.”

Or consider Judge Alex Kozinski’s comments in Nosal’s case: “The government assures us that, whatever the scope of the CFAA, it won’t prosecute minor violations. But we shouldn’t have to live at the mercy of our local prosecutor.…And it’s not clear we can trust the government when a tempting target comes along. Take the case of the mom who posed as a 17-year-old boy and cyber-bullied her daughter’s classmate. The Justice Department prosecuted her…for violating MySpace’s terms of service, which prohibited lying about identifying information, including age.…Lying on social media websites is common: People shave years off their age, add inches to their height and drop pounds from their weight. The difference between puffery and prosecution may depend on whether you happen to be someone an AUSA has reason to go after.”

If even some federal judges don’t trust the government, it’s hard to say why the rest of us should.

Matthew G. Kaiser is an adjunct professor of law at Georgetown University Law Center, a former assistant federal public defender, author of the Federal Criminal Appeals blog and a partner at the Washington-based Kaiser Law Firm.