Wisconsin Adopts Virtual-Certainty Private-Search Rule for Hash-Flagged CSAM Files

Introduction

State v. Michael Joseph Gasper (2026 WI 3) presented the Wisconsin Supreme Court with a recurring Fourth Amendment problem in modern child-exploitation investigations: when an electronic service provider (“ESP”) uses automated hash-based tools (here, Microsoft’s PhotoDNA) to identify and report suspected child sexual abuse material (“CSAM”) to the National Center for Missing and Exploited Children (“NCMEC”), may government analysts open and view the specific file attached to a CyberTip without first obtaining a warrant?

The parties were the State of Wisconsin (Plaintiff-Appellant) and Michael Joseph Gasper (Defendant-Respondent-Petitioner), charged with ten counts of possessing child pornography and nine counts of child exploitation based on content found on his cell phone after a warrant-backed search. Gasper sought suppression on the theory that DOJ and local law enforcement conducted warrantless searches when they were the first humans to view the single flagged 16-second Snapchat video included in the CyberTip.

The circuit court granted suppression, emphasizing privacy interests in cell phones and expressing skepticism about hashing reliability (especially MD5). The court of appeals reversed, largely reasoning that Gasper lacked a reasonable expectation of privacy due to Snapchat’s terms of service. The Wisconsin Supreme Court affirmed the reversal—but on different grounds—vacating the court of appeals’ reasoning and announcing a clarified application of the private search doctrine to hash-flagged CSAM.

Summary of the Opinion

The court held that Snapchat performed a private search when it scanned and flagged a single 16-second video as CSAM using PhotoDNA. When DOJ and local law enforcement later opened and viewed only that same file without a warrant, they did not exceed the private search because (i) any expectation of privacy in the file had already been frustrated by the private search, and (ii) there was virtual certainty that viewing the file would reveal nothing of significance beyond what the private scan had already identified. Therefore, no Fourth Amendment search occurred, and suppression was unwarranted.

The court expressly stated that it affirmed the court of appeals’ ultimate result on other grounds and that the court of appeals’ published reasoning was vacated and “has no precedential value.”

Analysis

Precedents Cited

1) Foundational Fourth Amendment principles

  • Camara v. Mun. Ct. of City & Cnty. of San Francisco: Cited for the Fourth Amendment’s purpose of safeguarding privacy against arbitrary governmental invasions.
  • Katz v. United States (Harlan, J., concurring) and Oliver v. United States: Referenced for the “reasonable expectation of privacy” framework.
  • Florida v. Jimeno and State v. Tullberg: Reinforced that the Fourth Amendment’s “touchstone” is reasonableness and that warrantless searches are presumptively unreasonable.
  • Riley v. California and Carpenter v. United States: Discussed in the procedural history (and circuit court reasoning) as emphasizing heightened privacy concerns in digital contexts; the Supreme Court of Wisconsin ultimately did not need to decide the case under that rubric because it resolved the dispute under the private search doctrine.

2) The private search doctrine: the controlling line

  • United States v. Jacobsen: The central authority. The opinion draws from Jacobsen’s rule that the Fourth Amendment is generally inapplicable to private searches, and government reinspection is permissible unless it exceeds the private search; the key inquiry is whether government obtained information whose privacy expectation “has not already been frustrated,” and whether there was “virtual certainty” that nothing else of significance would be found.
  • Walter v. United States: Used as a contrast case. In Walter, projecting films revealed more than the private party had seen, constituting a significant expansion. Wisconsin distinguished Walter because Snapchat’s scan had already inspected and segregated the single file reported, making it virtually certain government viewing would not reveal anything beyond what the scan had already disclosed.
  • Coolidge v. New Hampshire and Burdeau v. McDowell: Cited for the proposition that government may view what a private party has made available without offending the Fourth Amendment.
  • Skinner v. Ry. Lab. Execs. Ass'n: Noted for the “instrument or agent of the Government” limitation (not argued here).
  • United States v. Ginglen: Cited for the basic separation between private searches and government searches.
  • State v. Payano-Roman: Placed the burden on Gasper, once the State asserted a private search, to prove a government search occurred.

3) Hash-scanning and the “virtual certainty” split in digital CSAM cases

  • United States v. Miller: Adopted as persuasive: hash-based identification can create “virtual certainty” that government viewing of the flagged file reveals nothing more than what the private scan already established.
  • United States v. Reddick: Also adopted as persuasive: opening a file following a hash match “merely confirmed” suspected contraband.
  • United States v. Wilson and United States v. Maher: Explicitly rejected in reasoning: the Wisconsin court criticized their focus on law enforcement learning “details” upon viewing, calling that approach inconsistent with Jacobsen’s scope/virtual-certainty inquiry and warning it would create an “unworkable” standard that paradoxically recognizes privacy in the “details” of CSAM after a private scan has already frustrated privacy in the file itself.
  • Supporting “virtual certainty” framework and definitions: United States v. Rivera-Morales, United States v. Lichtenberger, United States v. Ackerman, and United States v. Runyan.
  • Hash definition and reliability references: United States v. Reddick, United States v. Larman, United States v. Miller.
  • Wisconsin’s own CyberTip analogy: State v. Silverstein (identified citizen informant analogy).

4) Good-faith / exclusionary rule (not reached by the majority, but framed)

  • Herring v. United States: Quoted for the idea that exclusion requires sufficiently culpable police conduct to justify deterrence.
  • Hiibel v. Sixth Jud. Dist. Ct. Nev., Humboldt Cnty.: Cited for the proposition that Fourth Amendment rights are against the government, not private parties.
  • Miranda v. Arizona: Mentioned factually regarding Gasper’s custodial statements.

Legal Reasoning

  1. Identifying the operative doctrine. The court characterized the case as turning on the private search doctrine, not the third-party doctrine and not a categorical “cell phone search” theory under Riley v. California. It emphasized the Fourth Amendment’s function as a restraint on government power and its inapplicability to completed private searches.
  2. Finding a private search occurred despite no human viewing. The key move is doctrinal: the court declined to require “human eyes” to establish a private search. Snapchat’s automated scanning “scanned, opened, and flagged” the single file as CSAM, and Gasper did not argue Snapchat was a government actor.
  3. Determining whether government exceeded the private search. Under United States v. Jacobsen, the inquiry is whether government action exceeded the private search’s scope—i.e., whether government obtained information whose privacy expectation had not already been frustrated. The court held Gasper failed to meet his burden because law enforcement viewed only the same single file that Snapchat flagged and provided, and there was “virtual certainty” nothing of significance beyond CSAM would be revealed by watching that file.
  4. Rejecting Wilson/Maher’s “details” approach. The opinion reasons that it is inconsistent to say privacy is frustrated in a file identified as CSAM by a private scan yet insist police viewing exceeds the private search because it reveals the “particulars” of CSAM. The court treated that as a misapplication of Jacobsen’s scope analysis and warned it would effectively create a subjective, after-the-fact constitutional test.
  5. Correcting the trial court’s hashing/MD5 premise. The court held the circuit court’s MD5-based findings were clearly erroneous: the record reflected PhotoDNA use, not MD5, and no evidence supported collision concerns in this case. Importantly, the court also suggested that even if reliability could be contested, the private search doctrine analysis focuses on what the private party searched and what information was already revealed/frustrated.
  6. Limiting the holding. The court stressed the narrow factual frame: one segregated, flagged 16-second video was provided; no unfettered account access occurred; Gasper did not argue police viewed more than the provided video.

Impact

1) A clear Wisconsin rule for CyberTip practice. The decision supplies Wisconsin law enforcement and courts with a concrete standard: when an ESP’s hash-based tool flags and forwards a specific file as CSAM, officers may open and view that same file without a warrant if their review does not exceed what the private scan has already revealed and there is “virtual certainty” the file contains nothing else of significance.

2) Wisconsin aligns with Miller/Reddick and rejects Wilson/Maher. The court takes a side in an acknowledged federal split, potentially influencing how Wisconsin courts treat future digital-private-search questions (including other automated detection tools).

3) Reduced litigation focus on platform “terms of service.” Because the court vacated the court of appeals’ reasoning and stated the analysis “need not turn” on terms-of-service violations, future suppression litigation in Wisconsin should pivot away from contractual privacy-waiver theories and toward (a) scope of private scan, (b) segregation of reported files, and (c) whether police went beyond what was reported.

4) Practical investigative sequencing. The opinion validates a common investigative sequence: confirm the CyberTip file before seeking broader warrants. The concurrences emphasize this may be less intrusive than immediately seeking expansive warrants to search accounts, homes, and devices.

5) Pressure points for future cases. The holding is narrow on its facts; it invites future disputes where (i) multiple files are provided, (ii) providers share broader account access, (iii) scanning methods are novel or contested for error rates, or (iv) law enforcement uses the tip to rummage beyond the provided, flagged file.

Complex Concepts Simplified

Private search doctrine
If a private party searches something first (without acting as the government), the Fourth Amendment generally does not apply to that private search. Police may repeat what the private party already exposed without a warrant so long as police do not go further than what the private party’s search revealed.
“Virtual certainty”
A common-sense standard derived from United States v. Jacobsen: if it is virtually certain that police inspection will reveal nothing meaningful beyond what the private search already revealed, police inspection does not count as a new Fourth Amendment search.
Hash / PhotoDNA
A hash is often described as a file’s “digital fingerprint.” Traditional hashes (like MD5) change drastically if a file changes. PhotoDNA is designed to detect “visually similar” contraband even if the file has minor alterations.
NCMEC / CyberTip
NCMEC receives provider reports of suspected child exploitation. A “CyberTip” is the report forwarded to law enforcement containing account identifiers and, at times, the flagged file itself.
Scope of the search
The “scope” question asks whether police looked at more than what the private party searched/exposed. Here, police viewed only the single file Snapchat flagged and provided—nothing else in Gasper’s account or phone without a warrant.

Conclusion

State v. Gasper establishes a Wisconsin rule for hash-flagged CSAM CyberTips: when an ESP’s automated scan identifies and forwards a specific file as CSAM, law enforcement does not conduct a Fourth Amendment search by opening and viewing that same file without a warrant, so long as the viewing does not exceed the private search and there is “virtual certainty” the file contains nothing else of significance beyond what the private scan revealed. The decision vacates the court of appeals’ terms-of-service-based rationale, aligns Wisconsin with United States v. Miller and United States v. Reddick, and rejects the “details-based expansion” approach associated with United States v. Wilson and United States v. Maher.