The FBI is resisting calls to reveal how it identified people who used a child pornography site on the Tor anonymising network.
The agency was ordered to share details by a Judge presiding over a case involving one alleged user of the site.
Defence lawyers said they need the information to see if the FBI exceeded its authority when indentifying users.
But the Department of Justice (DoJ), acting for the FBI, said the details were irrelevant to the case.
“Knowing how someone unlocked the front door provides no information about what that person did after entering the house,” wrote FBI agent Daniel Alfin in court papers filed by the DoJ which were excerpted on the Vice news site.
The Judge ordered the FBI to hand over details during a court hearing in late February.
The court case revolves around a “sting” the FBI carried out in early 2015 when it seized a Tor-based site called Playpen that traded in images and videos of child sexual abuse. The agency kept the site going for 13 days and used it to grab information about visitors who took part in discussion threads about images of child abuse.
Tor, aka The Onion Router, aims to hide the identity of people who use it by bouncing traffic through many different routers and encrypting it at each step. As well as letting people browse the web anonymously it has also given rise to hidden sites that sit on the Tor network.
The defence team is keen to see if the FBI’s use of technical tricks to identify people on Tor exceeded the authorisations it got in a warrant to run the sting. It has argued that the investigative technique used by the FBI amounted to “gross misconduct”.
The lawyers also want more details to ensure that their client has been properly identified.
In its legal filing, Mr Alfin said defence lawyers were “wrong” to think more information would help them work out if the FBI went further than it was allowed. It said the FBI had already provided details for the defence experts to work this out.
He also said he was confident that the defendant was properly identified. The DoJ documents conclude by asking the Judge to reconsider the order to reveal more information.