How Hash Matching Works and How a Single File Becomes a Federal Case.

Digital forensics tools such as the Axiom and EnCase use the process of “hash matching” to identify files of interest. Law enforcement officers use these tools to compare the hash values of the files found on a suspect’s device to “reference hash values.”

A reference hash value is a hash value that has been pre-calculated for a known file. Law enforcement officers have access to a list of pre-calculated hash values for files that contain child sexual abuse material (CSAM), as well as other known prohibited content.

When the investigator or investigator’s software compares the hash values of the files on the suspect’s device to a reference hash, a match will either be found, or it won’t. If a match is found, it indicates a high probability that the file on the suspect’s device is a copy of the known file. In these cases, an investigator can immediately determine the identity of the image or video and the suspect can face the consequences of a match.

How Often Can a Single File Start a Federal Investigation?

A single hash match can start a federal investigation. If a machine matches a hash value, the platform may immediately report the file to the CyberTipline, and NCMEC then forwards that report to law enforcement, which can lead to a federal investigation. The federal investigation can lead to prosecution for child sexual abuse material (CSAM).

Can Law Enforcement Examine an Image Automatically or in Person Without a Warrant?

Within the U.S. court system, circuit splits exist with regard to whether law enforcement can examine an image after an automatic hash match without a warrant. Specifically, some circuits have ruled that law enforcement can view an image after a computer matches the hash value to a reference hash value, while other circuits have ruled that law enforcement needs a warrant before viewing the image.

How Does Hash Matching Work?

As previously mentioned, “hashing” is a form of data encoding that maps a file’s contents into a fixed-length numerical value. At the same time, the same file will always produce the same hash value under one specific algorithm.

Hashing can be viewed as a form of “fingerprinting,” but while a criminal fingerprint is unique to a particular human being, a file’s hash value is unique to the underlying file data (i.e., its bytes).

As a result, if two different files have the same hash value, it is overwhelmingly likely that they are byte-for-byte duplicates, though rare collisions are mathematically possible. This has enormous implications for criminal investigations. In the context of child sexual abuse material (CSAM), file hash matching allows law enforcement to identify illegal images or videos without visually reviewing them. All law enforcement officers need to do is generate the hash value of a file, compare it to a pre-calculated hash value for the same file, and, if the match is correct, identify the file as CSAM.

Can a File Be Reconstructed from its Hash Value?

No. Hashing is a one-way function. As a result, it does not permit a practical way to reconstruct its source file. Once hash values are generated, reconstructing the source file from the hash value itself is simply not feasible.

Can Two Different Files Have the Same Hash Value?

Yes, if using the same hash function, two different files can produce the same hash value, and this is known as a hash “collision.” As the hash value length increases, the probability of a collision decreases.

What is Perceptual Hashing?

Perceptual hashing is a form of perceptual encoding in which images that look similar to one another produce similar hash values. Unlike a standard hash, a perceptual hash takes the content and features of an image into account. As a result, different images with the same overall structure will have similar perceptual hashes. Similar images will have mathematically similar signatures.

How Do Perceptual Systems Know When to Label an Image Match?

Perceptual systems determine when to label an image a match by measuring the “distance” between signatures. If two images are close enough to each other (or the distance is small enough), these images are flagged as having a match. However, unlike standard hash matching, the results are just a similarity score and are not definitive proof of byte identity.

What happens after a platform reports one file

How Do Platforms Automatically Identify CSAM?

Many online platforms utilize hash matching in order to automatically identify illegal content. As a user uploads a file to a platform, the platform can hash the uploaded file, compare its hash value to a hash value database for known child sexual abuse material (CSAM), and immediately determine whether it has matched that material. The platform can then take appropriate action, including blocking access to the content and/or reporting the upload to the CyberTipline.

When are Online Platforms Required to Report Apparent CSAM to the CyberTipline?

While platforms utilize hash matching to identify CSAM, this is not a requirement under federal law. However, if a platform identifies apparent CSAM, federal law then requires it to report that CSAM to the National Center for Missing & Exploited Children (NCMEC). This reporting is required under 18 U.S.C. § 2258A, the federal statute that governs online providers’ reporting obligations. In other words, while platforms are not required to proactively search for illegal content under federal law, once apparent child sexual abuse material (CSAM) is identified, reporting obligations are triggered.

What Obligation Does the Federal Law (i.e., 18 U.S.C. § 2258A) Impose on Online Providers?

18 U.S.C. § 2258A requires online providers to forwardapparent child exploitation (specifically child sexual abuse material (CSAM)) that they discover to the National Center for Missing & Exploited Children (NCMEC) through the NCMEC CyberTipline. More specifically, § 2258A(b) requires providers to forward information about a “file” they obtain or possess, and § 2258A(c) then directs NCMEC to forward each report to the appropriate federal, state, local, tribal, or foreign law enforcement agency. It also imposes preservation obligations on online providers. Specifically, it requires online providers to preserve “the apparent child exploitation” they forward under the statute, as well as certain “account information” related to the transmission or advertisement of child sexual abuse material (CSAM).

What Does NCMEC Do Once a File Has Been Reported via the CyberTipline?

Once reported via the CyberTipline, NCMEC forwards the information to the law enforcement authorities in the proper jurisdiction. From there, investigators work to identify a suspected uploader or other illegal activity and initiate a federal investigation if warranted. This process involves using the available account and IP information provided by the online platform to identify a suspect. Investigators also use 18 U.S.C. § 2703 to obtain the subscriber’s name, address, and billing information from the online platform.

Does Law Enforcement Need a Warrant to Open a File, Video, or Image, After an Automated Hash Match?

This is the core legal issue involved when one file match leads to a federal investigation. If the cybertipline, an internet service provider (ISP), or another entity conducts a private search, government intervention is allowed without a warrant if it falls within the scope of the private search.

The Fourth Amendment and the Private Search Doctrine

The Fourth Amendment to the U.S. Constitution restricts “unreasonable searches and seizures.” However, these restrictions apply to government entities, and not to independent searches conducted by private citizens or entities. While the Supreme Court of the United States officially established the private search doctrine in United States v. Jacobsen, 466 U.S. 109 (1984), this doctrine has long been a cornerstone of federal law.

Under Jacobsen, “the government may conduct a search that is no more intrusive than a prior search conducted by a private party.” As a result, if an online service provider or another entity conducts an initial automated hash match, law enforcement will not need a warrant to open the file. However, they are not permitted to exceed the scope of that private search.

Federal Circuit Disagreement over Warrantless Viewing of a Matched File

While the Fourth Amendment gives the government broad authority to repeat a search conducted by a private party, federal circuits disagree over whether viewing a matched file after a computer has identified the file hash value as CSAM exceeds that scope.

In United States v. Maher, 120 F.4th 297 (2d Cir. 2024), the court found that law enforcement’s warrantless examination of the digital files constituted a search. The court concluded that examining the image did not fall within the scope of the private search by the online company, and thus, that this action was unconstitutional under the Fourth Amendment.

Other federal circuits have reached similar conclusions:

• In United States v. Lowers, 170 F.4th 134 (4th Cir. 2026), the court held that the value of a single file’s hash did not support warrantless viewing of the image file.
• In United States v. Wilson, 13 F.4th 961 (9th Cir. 2021), the Ninth Circuit also found that warrantless viewing of a matching image following the identification of the matched hash value violated the Fourth Amendment.

Interestingly, in United States v. Reddick, 900 F.3d 636 (5th Cir. 2018), the Fifth Circuit found that law enforcement’s warrantless examination of the matching image (following Microsoft SkyDrive’s PhotoDNA matching result) does not exceed the scope of Google’s private search. Consequently, warrantless viewing was found to be constitutionally permitted.

What About Viewing Other Files?

If viewing one file is considered outside the scope of a private search, opening any additional files will almost certainly be considered outside of the private search’s scope. If this is the case, law enforcement will need a warrant.

If you are facing this situation, Spodek Law Group handles federal criminal defense matters nationwide, from offices in New York and Los Angeles.

Is One Matched File Enough to Warrant Federal CSAM Charges?

Yes, if prosecutors believe that there is sufficient proof to prove an individual’s guilt at trial, one matched file can be enough to warrant federal charges. In fact, prosecutors will seek charges for possession, receipt, transmission, or other offenses even where they cannot prove the existence of other illicit material. The relevant federal statute, 18 U.S.C. § 2252A, imposes criminal liability for the receipt, distribution, and possession of CSAM. In addition, simply accessing the material is also a chargeable federal offense. Under § 2252A(a)(5)(B), a person who “knowingly possesses, or knowingly accesses with intent to view,” material containing an image of child pornography that has an interstate or foreign commerce nexus is subject to a criminal penalty.

If convicted, each of these offenses can be pursued as a separate charge, and each carries its own distinct penalty.

What Proof Is Necessary to Establish Guilt in a Federal Case?

To secure a conviction in a federal CSAM case, prosecutors must establish evidence of each of the underlying elements that constitute the offense. These elements can vary depending on the charge at hand, but broadly speaking, they generally include:

• Proof that the defendant acted knowingly and not unintentionally;
• Proof that the material depicted is sexually explicit and that it contains a depiction of a real person under the age of 18;
• Proof that the defendant (or the material in question) had a nexus to interstate or foreign commerce; and
• Proof that the defendant is guilty of the particular form of abuse or exploitation that is charged under the statute (i.e., distribution, receipt, possession, viewing, etc.).

What Prevents a Hash Match from Automatically Being Used to Charge an Individual?

While a hash match is technically sufficient to warrant charges, charging policies, prosecutorial discretion, and other practical constraints will keep a defendant’s prosecution from happening in many cases. Broadly speaking, as noted above, a hash match alone neither proves that a defendant’s conduct was illegal nor that the defendant is criminally liable for the act. Even though most file hash matches result in criminal charges, the prosecutors’ ability to prove the defendant’s guilt beyond a reasonable doubt will largely dictate whether a defendant faces criminal penalties or gets off scot-free.

Does a Matching Account, IP Address, or Device Prove Who Possessed a File?

No, but identifying a matching account, IP address, or device is a foundational step in a federal investigation. From there, investigators work to link that information to a specific individual or device.

While identifying a matching account, IP address, or device is easy for federal agents, attributing stored files to a single user is more difficult. A number of factors can complicate this analysis. Among others, they include:

• Sharing a device with household members, a spouse, or roommates;
• Automatic synchronization of cloud files to a personal device (i.e., automatic syncing);
• Browser caches and thumbnails;
• Compromised accounts and remote access;
• Unsolicited attachments; and
• Deletion of the file.

What Does Cloud Synchronization Mean for Child Sexual Abuse Material (CSAM)?

As noted above, files can synchronize to a user’s device without that user’s permission or knowledge. This can happen in a number of ways. If cloud providers synchronize data to a user’s device in an attempt to make it easy for the user to access their data from any device, then files saved in a cloud account can appear on the user’s device without the user’s knowledge or consent.

Unfortunately, automatic synchronization can also make it possible to accidentally possess CSAM. In other words, if a user’s cloud account has been hacked and someone has used the hacked account to upload CSAM, automatic synchronization will do the rest of the work. Even if the user is unaware of how it got on their device, the fact that the file exists can satisfy the possession element of 18 U.S.C. § 2252A.

What Are Browser Caches and Thumbnails?

Along with automatic cloud synchronization, browser caches and thumbnails can also explain how CSAM can get onto a computer or device without the user’s consent or knowledge. Along with these types of files, unsolicited attachments that can be downloaded automatically are among the most common ways that people accidentally possess CSAM.

Are There Other Explanations for Why Someone Has a Matching File?

Unfortunately, no, and all these explanations for having a matching file do not automatically make it go away. If you have a matching file on your computer, then this can satisfy the possession element of a federal CSAM charge. While this can explain the defendant’s lack of awareness and/or intent, this is only one part of establishing the intent element. Another part of intent involves knowing receipt or possession of child exploitation material.

Do Deleted Files Still Count as Possession?

Yes, because even if a file is deleted, it can remain in unallocated space and may be forensically recoverable. In determining whether the file constitutes possession, courts will examine whether a defendant could still access the file and whether the defendant maintained any control over the file.

If the deleted file still exists in any form, it is generally considered a violation of 18 U.S.C. § 2252A.

How Do I Prove a Hash Value is Not a Match?

If the FBI or another law enforcement agent tells you that your file matches a known hash value for child exploitation, you can fight the allegations. You do not need to prove that the file does not match the hash value. Instead, the government has the burden of showing the file does not match the hash value.

At Spodek Law Group, we can use digital forensic tools to confirm that there are not any matching hash values on your device, and we can use this proof to force a dismiss.

What Can be Challenged About the Hash Value Evidence?

Any piece of evidence that the government uses to sustain a conviction must be admissible under the Federal Rules of Evidence. As a result, prosecutors may also face challenges when attempting to use hash value evidence as one of their points of proof in a federal CSAM case. As a result, even if federal agents have correctly identified a matching hash value, this can still pose challenges to the admissibility of this evidence.

These challenges may include, among others:

1. Challenges to the Proper Authentication of the Evidence

Under Federal Rule of Evidence 901, the government must authenticate any piece of evidence it uses in order to admit evidence at trial. This requires producing evidence sufficient to support a finding that the item is what the proponent claims it is.

2. Challenges to the Admissibility of Expert Testimony

The government may use expert testimony to support the admissibility of hash value evidence, but this testimony must satisfy the requirements of Federal Rule of Evidence 702. As a result, if the testimony fails to satisfy one of the rule’s requirements, the evidence may be ruled inadmissible.

3. Challenges to the Evidentiary Weight of the Hash Match

A hash value match alone proves that a file is a duplicate of the reference file to which it matched. However, the evidentiary weight of a hash match depends on several factors including:

• The identity of the person or entity that classified the reference file
• The date the file was produced and hash matching performed
• The nature of the content that matched

The evidentiary weight of a hash match alone can be challenged, or it can be undermined by evidence showing that the reference hash was derived from an unlawfully or improperly obtained file.

4. Challenges to the Custody and/or Processing of the Files

While the government has the right to search for digital evidence that it obtains through a warrant, the government must also document the chain of custody and verify that any digital files found on a device are not subject to alteration, manipulation, or deletion. Even if the government is using the original files, prosecutors still need to prove that these files have been preserved since their acquisition. In these cases, federal prosecutors may need to seek an independent digital forensic examination or a forensic review to obtain a reliable forensic copy of the device.

5. Challenges to the Lawfulness of a Warrantlessly-Examined Hash Match

If law enforcement agents view a matching image or video following an automated hash match without obtaining a warrant, it could violate the Fourth Amendment. When law enforcement agents obtain a warrant that justifies viewing the files, the affidavit for this warrant must provide information about:

• The origin of the hash match, whether it was based on a similarity match (etc.), and whether the hash value match was produced by an automated machine.
• The extent of the data collected for the reference hash, the data collected as a result of the automated hash match, and the extent to which the defendant’s files are potentially relevant.
• The purpose of the search, whether the warrant has a connection to a specific file, and the extent of the warrant’s requirements for searching additional files.

As a result, if the warrant is defective, the defense has multiple avenues to challenge the government’s evidence.

6. Challenges to the Accuracy of the Reference Hash

When the reference hash is based on the reference hash entry’s classification as child exploitation material, the accuracy of the reference hash entry’s label is fundamental. If the file depicted is not sexually explicit or is not derived from a child, the hash value generated will match a file that does not match the definition of CSAM.

Even if the mathematics of the hash matching system is correct, if the reference hash value entry is mislabeled, it will produce a false flag. Similarly, any change to a reference file, including the file itself or the description associated with the file, can have the same effect.

As a result, challenging the classification of a reference file that produces an automated match could potentially change the nature of the charge. This could lead to the dismissal of the charges if no other evidence is available.

What Penalties Follow in a Federal CSAM Case?

Sentencing for CSAM cases is determined by the federal sentencing guidelines. Along with various mitigating and aggravating factors, the sentencing guidelines largely look at:

• The number of victims, whether the victims are underage, and the nature of the images/videos
• The number of images or videos the defendant accessed, transmitted, or distributed

If the government has any hash match evidence, it may try to use this as proof to justify an increase in the sentencing guidelines. For example, sentencing guideline § 2G2.2 increases offense levels based on the number of images and videos in a defendant’s collection. Additionally, when determining the number of matching images, under § 2G2.2, “each video counts as 75 images, or as a greater number if applicable.”

While criminal penalties are primarily determined by the federal sentencing guidelines, prosecutors may seek a significant enhancement if they can justify that a defendant was involved in distribution or the possession or transmission of images via the internet. When the government can substantiate distribution charges under these conditions, a significant enhancement in the range of potential penalties applies.

What are the Penalties for Receipt, Distribution, and Possession of CSAM?

For offenses involving the transmission and/or the knowing receipt or distribution of CSAM, the federal law imposing criminal liability under 18 U.S.C. § 2252A(a)(1) (which is often in tandem with 18 U.S.C. § 2252A(a)(3)) carries a mandatory minimum sentence of five years of imprisonment. By contrast, a conviction involving simple possession under 18 U.S.C. § 2252A(a)(5)(B) has no mandatory minimum sentence, and the guidelines set out in the chapter discuss aggravating and mitigating factors in determining the length of incarceration.

If any victims of the crime are identified, 18 U.S.C. § 2259 requires restitution paid to these victims. If you are guilty, your conviction will also trigger all sex-offender registration obligations in your state and in the United States as a result of the Sex Offender Registration and Notification Act (SORNA).

For all cases, these criminal offenses are categorized as a “serious offense,” making them eligible for maximum penalties on appeal, including five years to life of supervised release. All charges can lead to lifelong consequences, and so you will want to make informed decisions about your next steps.

Get Advice on Your Situation

If you want someone to look at the specifics of your case, Spodek Law Group handles federal criminal defense nationwide from New York and Los Angeles. The firm has been practicing since 1976 and its motto is simple: we owe loyalty to only you. Call 212-300-5196.