In the bad-old-days of American intelligence, J. Edgar Hoover maintained a notorious "Sex Deviate" file filled with salacious bits of information on the sexual proclivities of prominent Americans: actors, columnists, activists, members of Congress, and even presidents. Sometimes this information could be immediately useful—as when Hoover's right hand Cartha DeLoach proudly reported that the Bureau had learned of a truculent senator caught driving drunk with a "good looking broad."  The senator, DeLoach explained, was promptly made "aware that we had the information, and we never had trouble with him on appropriations since." But Hoover could be patient as well: In the 1940s, the FBI investigated and wiretapped columnist and suspected German spy Inga Arvad, who happened to be conducting an affair with a young naval ensign named John Kennedy.  When Kennedy won the Democratic presidential nomination 17 years later, the Arvad dossier was immediately moved to Hoover's personal office file. Sometimes the information was used to discredit Hoover's political enemies through targeted leaks; on other occasions, the threat of exposure was enough.

The National Security Agency has clearly learned to apply Hoover's tactics in the war on terrorism: In a new Huffington Post story based on documents leaked by Edward Snowden, Glenn Greenwald reports that the NSA discussed how Islamist "radicalizers" could be discredited by exploiting information—presumably obtained through electronic surveillance—about their online sexual activities. 

Critically, the six "radicalizers" mentioned in the NSA document do not appear to have been directly involved in terrorism: They are described only as preachers of extremist ideas. Indeed, the document notes that the three English-speaking "radicalizers," one of whom is identified as a "U.S. person," seem to have had minimal contact with members of violent groups, and one is characterized as explicitly condeming violence against civillians.  Nevertheless, the Agency urged that such "vulnerabilities" as "online promiscuity" or "viewing sexually explicit material online" could be used to discredit these "radicalizers" by exposing them as hypocrites.

While it's not clear whether the NSA—or one of its client agencies—ever carried through on this plan, even compiling and disseminating such derogatory information about a U.S. person guilty of no more than vile (but nevertheless First Amendment protected) speech would be, prima facie, illegal. But given the breadth of NSA's collection, records of the online sexual habits of millions of others who might one day be deemed "radicalizers" of one stripe or another are almost certainly sitting in a database waiting to be mined and deployed.

The report underscores one of the primary reasons intelligence surveillance in particular is so susceptible to abuse. In criminal investigations, the paradigmatic government use of information gleaned from wiretaps or other forms of spying is as evidence in a criminal trial, where the government's actions are subject to eventual scrutiny and legal accountability. But in the realm of intelligence, only rarely are the fruits of surveillance used in criminal prosecution—eliminating the primary "back end" institutional check on government spying. Often, the point is precisely to make use of information from intercepts in ways that can never be directly or publicly traced to government. A target whose reputation or career is torpedoed by embarassing disclosures may never know whether they were the victim of an intelligence operation or simple bad luck.

The U.S. legal system is ill-designed to guard against such tactics: Our primary safeguard—indeed, in most cases the only safeguard—against violations of the Fourth Amendment is the "exclusionary rule," which prohibits evidence derived from illicit surveillance from being used against a defendant at trial.  When, instead, intelligence agencies use surveillance to attack their targets through means other than prosecution, no court or judge is ever likely to review their work. You may not be engaged in any criminal conduct, but does that really mean you have "nothing to hide" from government?  Check your browser history before answering too confidently.