Let's say that Joe hates John. Joe knows that John is security-aware and has encrypted all of his hard disks and storage devices. John keeps the decryption phrase in his head.

Joe makes an anonymous telephone call or otherwise reports John to the police, claiming that he knows for sure that John has child porn.

Will the cops now go home to John and demand that he gives the cops all of his hardware and tells them the decryption phrase for them to go through it in the headquarters? My skin starts crawling at the thought of my data being looked through by anyone, including cops, who might be making copies and then leak them due to malice or incompetence.

Even if they find nothing illegal, John will feel extremely violated after that experience, and must assume that all of his data has been copied/stolen/compromised. He initially refused to tell them the decryption phrase, but they threaten to lock him up indefinitely if he did not. They may also have turned off their bodycams and smacked him around a bit.

And even if he consistently refuses to tell them the decryption phrase, and they are unable to brute-force it (who knows what kind of hardware they have access to these days?), they still may have taken copies of the encrypted data for "future brute-force attempts", so John will always wonder if his data is compromised or not, even though he didn't even actually have any child porn or anything else that's illegal.

Is this really how things are done? I have a feeling that it's exactly how it happens, as long as the reporter is convincing/serious-sounding.

  • 3
    I don't practice actively in this area, but recall from law school and bar exam prep that there is an elaborate case law (that took a full page flow chart to summarize) governing when anonymous tips do and do not constitute probable cause under the 4th Amendment and applicable state law. Corroborating facts and the informant's prior track record are among the factors.
    – ohwilleke
    Jan 14, 2022 at 0:42
  • Re "cops, who might be making copies..." AFAIK computer forensic people remove the hard drive(s) and make a copy to examine. That's on the same basis as any forensic evidence: not to contaminate it. And there may be files, or relics, that are not normally accessible to the casual user. So in that sense, yes, they do make copies. Note that in UK two police officers were recently jailed for distributing photos of crime victims. Jan 14, 2022 at 9:46

3 Answers 3


One of the leading U.S. Supreme Court cases on probable cause in circumstances in which there is an anonymous informant is Illinois v. Gates, 462 U.S. 213 (1983). In that case it allowed the use anonymous informants’ tips as part of the factual basis substantiating a probable-cause determination, which is the standard necessary to issue a search warrant. One summary of this case explains:

In Gates, an anonymous informant wrote a letter to the police claiming that Lance and Sue Gates were selling drugs, hiding $100,000 of drugs in their basement, bragging about not having to work, and planning a trip to Florida to buy over $100,000 worth of drugs. The letter specified the approximate timing for the trip, as well as some other details of the couple’s regular travel arrangements. The police corroborated some of these details—including the timing of the upcoming trip as well as its initiation in real time, through their own investigation, and ultimately acquired a warrant to search the couple’s car and home, where they found drugs, weapons, and other contraband.

The Supreme Court held that the combination in Gates of the anonymous informant’s tip and police corroboration of some of the information that the tip provided were sufficient to make out probable cause to justify the issuance of the search warrant. This was true, moreover, despite the fact that what the police corroborated were primarily innocent sorts of details—in other words, details that did not (in the absence of the criminal accusations) necessarily point to criminal activity.

More recently, the U.S. Supreme Court held in the case of Navarette v. California, 572 U.S. 393 (2014), that an anonymous 911 call could, under the circumstances of the case, provide a basis for establishing probable cause. As on summary of the case explains:

someone called 911 saying she had been run off the road by a pickup truck. The anonymous caller gave the 911 operator the make, model and license plate number of the pickup truck as well as the location of where she was allegedly ran off the road. Police stopped the driver of the truck, smelled marijuana and discovered 30 pounds of marijuana. Navarette was arrested but argued the police had no probable cause for the initial stop as they had no way of establishing the identity or reliably of the anonymous caller.

The majority of the Supreme Court did not agree, holding that police can stop someone if the information obtained from an anonymous informant provides enough detail to create reasonable suspicion of a crime. In Justice Thomas’ majority opinion, he concluded that the caller was an eyewitness who was forced off the road leading police to believe the pickup truck driver was drunk. Thomas also felt it was reasonable to rely on the information given by anonymous 911 callers because the calls are recorded and the service can track and identify callers.

A similar analysis would apply in the case in the question.


In order to seize or search John's computer hardware lawfully, the police would need a warrant. To obtain this they would neeed to convince a judge that there was probable cause. An unknown anonymous informant is usually not sufficient to establish probable cause. If police officers testify, under oath, that the informant is credible because the same informant has provided accurate information in the past, that might be enough.

Given the facts stated in the question, the police could not honestly testify to that. The police might lie, police sometimes do. But they are not likely to tell that kind of lie unless either they have their own reasons for strongly disliking John, or they have what they think is good reason for believing that John is guilty, even if those reasons are not admissible.

Police know that reports, particularly anonymous reports, are often lies. Most police do not particularly want to convict innocent citizens, and very few want a public and possibly embarrassing investigation where the charges are proved false.

In the situation describes, most police would not try to obtain a warrant unless there was some oyhrt reason for suspecting John, beyond a singe anonymous accusation.


No one answer fits all jurisdictions. The US has fairly strong protections against arbitrary searches, owing to the Fourth Amendment prohibition against unreasonable searches. See Alabama v. White, 496 U.S. 325 for the requirements on search warrants based no anonymous tips.

The first thing is that the police will not just go to John's house and conduct a search, they will go to court to get a search warrant. The circumstances surrounding White differ slightly (a vehicle search of a reported drug transportation in progress) so that the warrant requirement was not an immediate problem for the police. To go to your house and search your computer, there must be a warrant issued by the court. The warrant can only be issued if there is "probable cause", and pursuant to White, there must be sufficient supporting evidence. In other words, an anonymous type declaring that there is evidence of a crime at some place would not suffice for a legal search.

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