In the case of the Playpen child pornography site, a judge ordered the FBI to disclose a hacking technique. Tor

The judge who is reviewing the case of Playpen website visitors accused of pedophilia demanded that the FBI disclose the source codes of the software that helped them disclose visitors to the resource and get information about them.

At the end of 2015, as a result of a special operation, the FBI seized and controlled within a few weeks one of the most popular (as they say) sites with child pornography Playpen. During this time, the feds were catching live bait by hapless visitors to the resource.
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Since the site worked as a hidden service on the Tor network, the FBI had to develop and apply a system that allows not only revealing the real IP address of visitors, but also infecting their computers with malware.

Lawyers for one of the people convicted in this case announced that using a website with child pornography to catch criminals is in itself a "business offense" and compared this technique to how the anti-drug department was selling drugs to catch drug addicts.



At the request of lawyers who want to better understand the actions of the feds and check how they comply with the laws, the judge ordered to disclose the source code and the scheme of the programs with which the FBI revealed the identity of visitors to the site. True, the FBI has previously happened to ignore the orders of the court , so time will tell how events will develop.