We don’t know — we may never know — what former U.S. House speaker Dennis Hastert did that was so bad, if anything. What we do know: Prosecutors say he was willing to pay $3.5 million to cover it up. The former lawmaker was indicted this week on charges that he lied to the FBI and tried to disguise cash withdrawals paid to an unnamed party identified only as “Individual A” to “compensate for and conceal his prior misconduct against Individual A.” Hastert’s “prior misconduct” was not explained.

While “the indictment did not spell out the exact nature of the ‘prior misconduct’ by Hastert,” The Washington Post reported, “… it noted that before entering state and federal politics in 1981, Has­tert served for more than a decade as a teacher and wrestling coach at Yorkville High School in Illinois.”

[The Fix: Why Dennis Hastert got indicted by the feds]

The indictment’s allusion to Hastert’s tenure as a wrestling coach and teacher was provocative. So was its reference to Individual A. Individual A claimed to have known Hastert “most of” his or her life. Individual A met with Hastert repeatedly “in or about 2010" and “discussed past misconduct” by Hastert against him or her that “had occurred years earlier.” And, allegedly, Hastert paid Individual A off.

Indeed, there was far more media interest in what the indictment didn’t say than in what it did. Individual A got as much attention as the relatively mundane details about money allegedly moving in and out of Hastert’s bank accounts. What kind of “misconduct” could the Republican from Illinois have been a part of as a coach and teacher?

The U.S. attorney for the Northern District of Illinois, who brought the charges, provided no comment. And Hastert, who has yet to comment, provided no answers.

It was “Denny Hastert’s dark secret,” as the Chicago Tribune said in an editorial. “The formal charges against the affable Illinois Republican are as baffling as they are astonishing …. This isn’t just yet another Illinois politician winding up in the crosshairs of federal prosecutors. The underlying narrative leaves much to the imagination.”

The reference sent reporters scurrying to talk with someone familiar with the legislator’s coaching years. The Associated Press turned up David Corwin, whose son wrestled for Hastert at Yorkville. Corwin: “You won’t get anyone to say anything bad about him out here.”

The only accusation against Hastert that got real traction during his years in the House of Representatives concerned his alleged use of earmarks to benefit his property in Illinois, which occurred long after he left coaching. And it’s hard to blackmail someone for activities already publicized.

Blackmail is, after all, a bizarre crime of relatively recent vintage. When blackmailers are successful — unlike, say, bank robbers — no one but their victims know; when blackmailers fail, their intended victims, whose dirty laundry is aired, are humiliated anyway.

“Blackmail, a wonderfully curious offense, is the favorite of clever criminal law theorists,” according to a 2009 academic paper, “Competing Theories of Blackmail: An Empirical Research Critique of Criminal Law Theory.” “It criminalizes the threat to do something that would not be criminal if one did it.” That is, it would be perfectly legal for Individual A to stand up and accuse Hastert of misconduct, but it’s illegal to demand hush money as a condition of keeping quiet.

Blackmail couldn’t really exist until the rise of capitalism and modern social mores. A king was always a king, whether or not he behaved badly, but a businessman or politician who lost his reputation could face ruin — as Josh Duggar is finding out.

“It was a crime that only emerged in the 19th century,” Angus McLaren, a professor emeritus of history at the University of Victoria and the author of “Sexual Blackmail: A Modern History,” told the New York Times in 2009 after David Letterman was blackmailed for his extramarital affairs. “If one was an aristocrat, say, you couldn’t lose your position because of trifling with the housemaids.”

How to respond to a blackmailer is always a conundrum. Pay and cover up — and maybe pay again later — or admit misdeeds and face the music? Letterman’s response was, arguably, masterful: He called the authorities, foiled the $2 million plot, confessed on national television and played the incident for laughs.

Robert “Joe” Halderman, center, at his arraignment in New York City in 2009 for an alleged blackmail plot against David Letterman. (Marc A. Hermann-Pool/Getty Images)

Had Letterman had sex with women who worked for him? “My response to that is: ‘Yes, I have,'” the host said to laughter and applause. “… Would it be embarrassing if it were made public? Perhaps it would, perhaps it would — especially for the women.” Cue more laughter and applause.

Letterman added: “But that’s a decision for them to make, if they want to go public…. What you don’t want is a guy saying, ‘Well, I know you had sex with women, so I would like $2 million or I’m going to make trouble for you.'”

Depending on what Hastert’s alleged misconduct was, he may not have had the option of ‘fessing up, Letterman-style. An affair between consenting adults? No big deal — society’s changing attitudes toward sex permitted Letterman to bare all. America would not have chuckled had, say, Johnny Carson made a similiar mea culpa in the 1960s. And gay public figures, long considered targets for blackmail, couldn’t simply walk out of the closet in the decades before Stonewall. Many feel they still can’t.

When it comes to sex, blackmail is sometimes a crime with an expiration date. “After a spike during the moralistic years between the world wars, sexual blackmail has lost some of its zing and sting,” the New York Times wrote.

If Hastert committed a crime, however, the zing and sting of publicly revealing what he was blackmailed for may be as painful as federal charges. Whatever his next move is, it will demand legal creativity to keep alleged misconduct from becoming public. If Hastert chooses to go to trial, the government will presumably have to explain who Individual A is. If Hastert negotiates a deal and a plea, the information could still come out in the criminal information document that usually accompanies a plea, or in a sentencing memorandum later.

As Daniel Ellsberg — he of the Pentagon Papers — explained in a 1959 paper on the crime while an economist at Harvard University: “The answer to successful blackmail is not within the scope of logic: It is an art.”

Read the entire indictment