United States v. Maher

23-6181
United States v. Maher
In the
United States Court of Appeals
for the Second Circuit
A UGUST TERM 2023
No. 23-6181-cr
U NITED S TATES OF AMERICA,
Appellee,
v.
RYAN M. MAHER ,
Defendant-Appellant,
__________
On Appeal from the United States District Court
for the Northern District of New York
__________
ARGUED: MAY 15, 2024
D ECIDED: O CTOBER 30, 2024
________________
Before: R AGGI, CHIN, and PÉREZ , Circuit Judges.
________________
Defendant Ryan M. Maher appeals his conviction in the United States
District Court for the Northern District of New York (Suddaby, J.) on charges of
receiving and possessing child pornography. Maher argues that the district court
erred in relying on the “private search” doctrine to deny his motion to suppress
evidence that was obtained by, or that is the fruit of, a warrantless visual police
search of a digital file that Maher uploaded to his Google email account. We agree.

-- 1 of 46 --

2
Without itself ever visually examining the contents of Maher’s uploaded file,
Google reported that it contained child pornography because the hash value for
the image contained therein matched the hash value that Google had assigned an
image previously located in another file, which image a Google employee or
contractor had visually examined and identified as child pornography. In these
circumstances, neither the private search doctrine relied on by the district court
nor the Google Terms of Service agreement cited by the government supports the
challenged warrantless search. That, however, does not mean that Maher is
entitled to relief from conviction. As the district court correctly ruled in the
alternative, the good faith exception to the exclusionary rule supports denial of
Maher’s suppression motion because, at the time authorities opened his uploaded
file, they had a good faith basis to believe that no warrant was required.
A FFIRMED.
_________________
MELISSA A. TUOHEY , Assistant Federal Public Defender,
Office of the Federal Public Defender, Syracuse, NY, for
Defendant-Appellant.
MICHAEL D. G ADARIAN, Assistant United States
Attorney, for Carla B. Freedman, United States Attorney
for the Northern District of New York, Syracuse, NY, for
Appellee.
_________________

-- 2 of 46 --

3
REENA R AGGI, Circuit Judge:
Defendant Ryan M. Maher stands convicted following a guilty plea in
the United States District Court for the Northern District of New York
(Glenn T. Suddaby, Judge) of both receiving and possessing approximately
4,000 images and five videos depicting child pornography. See 18 U.S.C.
§ 2252A(a)(2)(A), (a)(5)(B), (b)(1)-(2). Sentenced to a total 294 months’
incarceration and life supervised release, Maher now appeals from his
February 9, 2023 judgment of conviction, arguing that the district court
erred in relying on the “private search” doctrine to deny his motion to
suppress evidence that was obtained by, or that is the fruit of, a warrantless
visual police search of a digital file that Maher uploaded to an email account
that he maintained with Google (the “Maher file”). See Decision and Order,
United States v. Maher, No. 21 Cr. 275 (N.D.N.Y Aug. 22, 2022), ECF No. 48.
We agree.
No one at Google visually examined the contents of the Maher file
before reporting it to the National Center for Missing and Exploited
Children (the “NCMEC”) as “apparent child pornography.” App’x 29.
Rather, that report was based on a computer-conducted algorithmic search
of the Maher file, which identified a match between the hash value for the
image contained in the Maher file (the “Maher file image”) and the hash
value of an image (the “original file image”) that Google had earlier located
in another file (the “original file”).1 Thus, when law enforcement authorities
1 A “hash” or “hash value” is “(usually) a short string of characters generated from
a much larger string of data (say, an electronic image) using an algorithm—and
calculated in a way that makes it highly unlikely another set of data will produce
the same value.” United States v. Ackerman, 831 F.3d 1292, 1294 (10th Cir. 2016)
(Gorsuch, J.).

-- 3 of 46 --

4
visually examined the contents of the Maher file, they went beyond the
scope of Google’s private algorithmic search in that they learned more than
the hash value for the Maher file image; they learned exactly what was
depicted in that image.2
Nor is a different conclusion warranted because, before assigning a
hash value to the original file image, a Google employee or contractor had
visually examined that file image and determined that it depicted child
pornography. That visual examination was not a private search that
extinguished any of Maher’s Fourth Amendment rights because such rights
are personal, and Maher had no reasonable expectation of privacy in the
original file, which did not belong to him. But just as Maher could not
challenge any search of the original file, so the government cannot argue
that Google’s visual examination of the contents of that file extinguished
Maher’s Fourth Amendment rights as to a file—the Maher file—in which he
did have a privacy interest and whose contents were never visually
examined but only hash matched by Google.
In these circumstances, Google’s hash match may well have
established probable cause for a warrant to allow police to conduct a visual
examination of the Maher file. But, for reasons stated in this opinion, we
conclude that neither the private search doctrine relied on by the district
court nor the Google Terms of Service agreement cited by the government
2 Google apparently does not retain images determined to depict child
pornography after assigning them a hash value. Thus, Google did not—and could
not—report to the NCMEC what specifically was depicted in either the Maher file
image or the original file image based on their matching hash value.

-- 4 of 46 --

5
authorized the police to open the Maher file and to conduct such a visual
examination of its contents without a warrant.
That, however, does not mean that Maher is entitled to relief from
conviction. As the district court correctly recognized in the alternative, the
good faith exception to the exclusionary rule defeated Maher’s suppression
motion because, at the time police opened the Maher file and visually
inspected its contents, they had a good faith basis to believe that no warrant
was required to do so. Accordingly, on that basis, we affirm the judgment
of conviction.
BACKGROUND
I. Google’s Use of Hash Values To Identify Child Pornography
While the facts relevant to this appeal are not disputed, their
discussion requires some understanding of how Google identifies and
reports child pornography found on its platform. In this case, that
understanding derives largely from a declaration filed with the district court
by Claire Lilley, a Google Manager for Child Safety and Abuse Enforcement.
Lilley states that, consistent with Google’s “strong business interest” in
“ensuring its services are free of illegal content,” the company’s “Terms of
Service” prohibit persons from using Google’s services “in violation of law.”
App’x 108.3 These Terms of Service advise users that Google “may review
content” on its platform “to determine whether it is illegal or violates our
policies,” and “may remove or refuse to display content that we reasonably
believe violates our policies or the law.” Id. at 113–14. In the very next
3 Google’s Terms of Service make this point in the affirmative rather than the
negative: “You may use our Services only as permitted by law.” Id. at 113.

-- 5 of 46 --

6
sentence, Google states: “But that does not necessarily mean that we review
content, so please don’t assume that we do.” Id. at 114. The Terms of Service
also state that Google “may . . . report” a detected “violation [of law or its
policies] to appropriate authorities.” Id. at 142. Elsewhere, they state that
Google “will share personal information outside of Google” where
necessary to “[m]eet any applicable law . . . or enforceable governmental
request.” Id. at 131.
As Lilley further explains, Google uses “a proprietary hashing
technology” to monitor its platform for “apparent child sexual abuse
material.” Id. at 109. Toward this end, certain Google employees and
contractors are “train[ed] . . . on how to recognize” child pornography. Id.4
When, based on a visual inspection, such an employee or contractor
identifies material on the company’s platform as child pornography, Google
gives the image “a digital fingerprint” known as a “hash” or “hash value.”
Id. The company then apparently removes the image from its platform but
adds the image’s hash value to a “repository of hashes of apparent child
pornography” maintained by the company. Id. Google’s computers can
then automatically compare the hash values of content later uploaded to its
platform to such stored hash values and thereby digitally “identify exact or
very similar images of apparent child pornography.” Id.
4 On this point, Lilley states that, “[u]nder guidance of its lawyers, Google trains
Google Reviewers on the legal obligation to report such material, on the statutory
definition of child pornography, and on how to recognize it on our products and
services.” Id. She does not state what, if any, particular findings Google requires
reviewers to make in identifying child pornography. Nor does she identify the
Google employee or contractor who identified the original file image in this case
as child pornography.

-- 6 of 46 --

7
Google reports such hash matches to the NCMEC by filing a
CyberTipline Report.5 Before doing so, a Google employee or contractor
will sometimes conduct a “manual, human review” of the hash matched
image to confirm that it depicts child pornography. Id. at 110. But in many
cases—as here—Google “automatically reports” the computer matched
image to the NCMEC as “apparent child pornography” without any person
viewing it. Id. In those cases, Google advises the NCMEC that the report is
based on a hash match to an image previously viewed by a Google
employee or contractor and identified as “apparent child pornography.” Id.
at 29. Because Google apparently does not retain the previously viewed
image, it cannot, based only on a hash match, describe the specific contents
of either matched file, i.e., it cannot describe the age of any child depicted,
the number of children depicted, whether any adults are also depicted, or
the particular circumstances depicted that might be deemed child
pornography. See Richard P. Salgado, Fourth Amendment Search and the
Power of the Hash, 119 H ARV. L. R EV. F. 38, 40 (2005) (explaining that “[o]ne
can calculate a hash value from input, but cannot derive the input from the
hash value,” such that hash value of photograph “cannot be ‘reversed’ to
generate the photo itself”).
5 The NCMEC is an entity “organized as a private nonprofit but established by
Congress,” United States v. Johnson, 93 F.4th 605, 609 (2d Cir. 2024), and “statutorily
obliged to operate the official national clearinghouse for information about
missing and exploited children,” United States v. Ackerman, 831 F.3d at 1296.
Although the law does not mandate that electronic communication service
providers such as Google “affirmatively search, screen, or scan” for images of child
sexual exploitation found on their platforms, see 18 U.S.C. § 2258A(f)(3), it does
require them to report such images to the NCMEC through its CyberTipline once
they have “actual knowledge” that such material resides on their platforms, id.
§ 2258A(a)(1)(A), (B).

-- 7 of 46 --

8
II. Maher Uploads Child Pornography to His Google Account
On January 27, 2020, Ryan Maher uploaded the Maher file to one of
his Google email accounts. Using its hash algorithm, a Google computer
determined that the Maher file contained an image whose hash value
matched the hash value—2eb373380383f50820e648d1c304a3db—that
Google had earlier assigned to an image found in the original file, which a
Google employee or contractor had visually examined and identified as
depicting child pornography.
On January 28, 2020, Google transmitted the Maher file to the
NCMEC, reporting it as “apparent child pornography.” App’x 29. In
response to a question on the NCMEC CyberTipline Report form asking
whether Google had viewed the ”entire contents” of the reported file, the
company answered: “No.” Id. at 31. Google further explained that such a
response “means that while the contents of the [reported] file were not
reviewed concurrently to making the report, historically a person had
reviewed a file whose hash (or digital fingerprint) matched the hash of the
reported image and determined it contained apparent child pornography.”
Id. at 29.
Google further advised the NCMEC that the reported file had been
uploaded to email address newbennings8608@gmail.com, which was
registered to Ryan Maher, and which had a secondary email address of
newbeginnings8608@gmail.com. Google further provided the NCMEC
with a mobile phone number associated with the account, as well as with IP
addresses from which the accounts had recently been accessed and
associated geographical coordinates.

-- 8 of 46 --

9
III. New York State Police Investigation of Maher
After receipt of Google’s report, the NCMEC also did not open the
Maher file or visually examine its contents. Rather, almost two months later,
on March 16, 2020, the NCMEC sent Google’s report and the unopened
Maher file to the New York State Police, describing it as “Apparent Child
Pornography (Unconfirmed).” Id. at 27.
The State Police did not replicate Google’s algorithmic search of the
Maher file. Nor did they replicate Google’s visual search of the original file
image that was hash matched to the Maher file image by the algorithmic
search—something that was no longer possible.6 Rather, without obtaining
a search warrant, State Police Investigator Laura Croneiser opened the
Maher file and visually examined its contents, a search never conducted by
Google. She reported what she saw—“a prepubescent female, who appears
approximately six to seven years old, exposing her vagina,”—in a July 21,
2020 affidavit submitted to a state court judge in support of a warrant to
search Maher’s newbennings8608@gmail.com and
newbeginnings8608@gmail.com email accounts. Id. at 44–46. In explaining
how police came to view the contents of the Maher file, Investigator
Croneiser stated that on “January 28, 2020, Google reported to the
[NCMEC]” that the newbennings8608@gmail.com account had “uploaded
an image of child pornography on January 27, 2020,” which image was
“stored in the Google Gmail infrastructure.” Id. at 44. She did not state that
Google’s report of child pornography was based solely on a computer match
of hash values made by a computer and not any human visual examination
of the Maher file image.
6 See supra note 2.

-- 9 of 46 --

10
Upon issuance of the requested warrant, State Police searched the two
email accounts and confirmed that they belonged to Maher. Also, in
searching the newbennings8608@gmail.com account, police found the same
Maher file reported by Google to the NCMEC, containing the same image
of child pornography visually examined by Investigator Croneiser.
Using the totality of information thus learned, State Police then
sought and obtained a second warrant to search the residence where Maher
was then living with his grandparents, including any electronic devices
found therein. That warranted search resulted in the seizure of the
approximately 4,000 images and five videos of child pornography charged
in the counts of conviction.
IV. Procedural History
Following these seizures, federal authorities filed child pornography
charges against Maher in the Northern District of New York. On August 17,
2021, Maher waived indictment and pleaded guilty to two counts of
receiving and possessing child pornography. 18 U.S.C. § 2252A(a)(2)(A),
(a)(5)(B), (b)(1)-(2). Several months later, the district court granted Maher
leave to withdraw his guilty plea based on defense counsel’s belated
realization that Maher had a colorable basis to move for suppression of the
seized evidence.
In so moving, Maher argued that the State Police had violated the
Fourth Amendment by conducting a warrantless search of the Maher file,
and then relying on the tainted fruits of that search to obtain search warrants
for his email accounts, residence, and computers. The government opposed
the motion on the grounds that (1) Maher lacked a reasonable expectation
of privacy in the Maher file because Google’s Terms of Service expressly

-- 10 of 46 --

11
reserved the right to monitor users’ accounts for illegal material and to
inform law enforcement when such material is found; (2) under the private
search doctrine, State Police did not need a warrant to open the Maher file;
and (3) even if a warrant were required, suppression is properly denied
under the good faith exception to the exclusionary rule.
In a written opinion dated August 22, 2022, the district court denied
Maher’s suppression motion. See Decision and Order, United States v. Maher,
No. 21 Cr. 275, ECF No. 48. Finding it unnecessary to hold a hearing because
the parties did not dispute the relevant facts, id. at 14, and without deciding
whether Maher had a reasonable expectation of privacy in the Maher file,
the district court held that the police’s warrantless search of that file was
lawful under the private search doctrine, which authorizes a government
actor to repeat a search already conducted by a private party without
securing a warrant. See id. at 21–29.
In so ruling, the district court determined that Google had conducted
a private search of the Maher file before turning it over to the NCMEC,
which therefore permitted the New York State Police to conduct a
warrantless review of that same file. Maher had urged otherwise, arguing
that no Google employee had ever opened or visually examined the contents
of the Maher file, as Inspector Croneiser subsequently did. The district court
was not persuaded, observing that the private search doctrine applies so
long as the challenged government search “was ‘of no greater scope or
intensity than’ Google’s review of the image.” Id. at 25 (quoting United States
v. $557,933.89, More or Less, in U.S. Funds, 287 F.3d 66, 87 (2d Cir. 2002)
(Sotomayor, J.)). The district court held that was the case here because a
Google employee or contractor had “viewed the image at issue in this action
in the past” (i.e., when the employee or contractor viewed the contents of

-- 11 of 46 --

12
the original file); determined “the image depicted child pornography”;
“assigned [that image] a unique hash value that was added to Google’s
repository”; and matched that hash value to the Maher file image. Id. at 25–
26. In these circumstances, the district court concluded that, when police
opened the Maher file, there was a “virtual certainty” that what they would
see “would be child pornography.” Id. at 28.7 Nor was the district court
persuaded otherwise by Maher’s argument that the police would
“inevitably learn[] more from opening the image” in the Maher file than
Google had learned from the hash match. Id. at 26. The court reasoned that
“Google had previously viewed the image” to which it assigned the hash
value matching the image in the Maher file, which was sufficient to
“frustrate[]” Maher’s “expectation of privacy in the image.” Id.
Alternatively, the district court held that the good faith exception to
the exclusionary rule defeated Maher’s suppression motion because, when
Investigator Croneiser opened the Maher file, she had an objectively good
faith basis to believe that no warrant was necessary. See id. at 29–31.
Following this ruling, Maher again pleaded guilty to charges of
receiving and possessing child pornography, reserving his right to appeal
7 The district court supported that conclusion by citing out-of-circuit cases
recognizing hash values to be “specific to the makeup of a particular image’s
data,” id. at 29 (quoting United States v. Reddick, 900 F.3d 636, 639 (5th Cir. 2018)),
and “the chance of two [digital] files coincidentally sharing the same hash value is
1 in 9,223,372,036,854,775,808,” id. (quoting United States v. Miller, 982 F.3d 412, 430
(6th Cir. 2020)). In those cases, however, record evidence was offered to support
these conclusions. By contrast, here, the government appears not to have offered
any evidence as to the reliability of Google’s particular hash matching technology.

-- 12 of 46 --

13
the denial of his motion to suppress. See Fed. R. Crim. P. 11(a)(2). After
sentencing and entry of judgment, Maher timely filed this appeal.
DISCUSSION
I. Standard of Review
On appeal from the denial of a motion to suppress, we review a
district court’s findings of fact for clear error and its legal rulings de novo.
See United States v. Haak, 884 F.3d 400, 408 (2d Cir. 2018). Because the parties
here do not dispute relevant facts, but only the lawfulness of police searches,
our review is de novo.
II. Warrantless Search of the Maher File
Maher’s search challenges are based on a common argument: that the
police’s initial warrantless visual examination of the contents of the Maher
file violated the Fourth Amendment. We agree.
The Fourth Amendment guarantees “[t]he right of the people to be
secure in their persons, houses, papers, and effects, against unreasonable
searches and seizures.” U.S. C ONST. amend. IV. As the Supreme Court has
recognized, “warrantless searches” of a person’s papers or effects are
“presumptively unreasonable” and, thus, violative of the Fourth
Amendment, United States v. Jacobsen, 466 U.S. 109, 114 (1984), at least insofar
as the person has a “reasonable expectation of privacy” in his property,
United States v. Jones, 565 U.S. 400, 406 (2012). At the same time, the Court
has recognized certain “well-delineated exceptions” to the warrant
requirement. United States v. Lewis, 386 F.3d 475, 481 (2d Cir. 2004). This
case involves one such exception: the private search doctrine.

-- 13 of 46 --

14
The private search doctrine instructs that where a private party has
already searched property belonging to another person, government
authorities may repeat that search without a warrant so long as they do “not
exceed the scope of the private search.” United States v. Jacobsen, 466 U.S. at
116. Applying that doctrine to the particular circumstances of this case, we
conclude that Google’s use of its proprietary hashing technology to identify
the contents of the Maher file as “apparent child pornography,” App’x 29,
did not permit State Police thereafter to conduct a warrantless visual
examination of that contents. Such an examination did not simply replicate
Google’s own algorithmic search of the Maher file for a hash match, but
expanded on it in a way not employed by Google, i.e., human visual
inspection, which allowed the police to learn more than Google had learned.
Specifically, Google’s use of its hashing technology to search the Maher file
revealed only that the numerical hash value for its contents matched the
numerical hash value for an image previously located in another file, which
image a Google employee or contractor had then visually examined and
identified as child pornography. But a computer’s discovery of a hash
match in the Maher file revealed nothing, either to Google or to those with
whom it shared the match, about what in particular the image depicted (or
even what the original file image depicted). To obtain that specific
information about the Maher file image—which was “more than [police]
already had been told” by Google or the NCMEC—authorities needed to
exceed the scope of Google’s hash value search of the Maher file. United
States v. Jacobsen, 466 U.S. at 119. They needed to open the Maher file, and a
human being had to conduct a visual examination of its contents. Such an
expanded search required a warrant.

-- 14 of 46 --

15
A. Maher’s Reasonable Expectation of Privacy in the Maher File
Was Not Extinguished by Google’s Terms of Service
In explaining that conclusion, we first consider the government’s
argument that Maher lacked a reasonable expectation of privacy in the
contents of the Maher file. See United States v. Jones, 565 U.S. at 406 (holding
that person must have “reasonable expectation of privacy” in property
searched to complain of Fourth Amendment violation). Such an argument,
if successful, would mean that Maher could not complain about the
government’s warrantless search of the Maher file regardless of whether
Google had conducted a private search of that item. In fact, the argument
fails for reasons that we now explain.8
At the outset, we note that the government does not here contend that,
as a general matter, persons lack a reasonable expectation of privacy in their
email communications. On that point, we here hold what this court has
previously assumed, i.e., “that a United States person ordinarily has a
reasonable expectation in the privacy of his e-mails sufficient to trigger a
Fourth Amendment reasonableness inquiry.” United States v. Hasbajrami,
945 F.3d 641, 666 (2d Cir. 2019). As the Sixth Circuit has observed in that
regard, “[g]iven the fundamental similarities between email and traditional
forms of communication, it would defy common sense to afford emails
lesser Fourth Amendment protection.” United States v. Warshak, 631 F.3d
8 Jones instructs that common-law trespass as well as reasonable expectations of
privacy properly inform Fourth Amendment analysis. See id. at 409 (explaining
that Fourth Amendment “reasonable-expectation-of-privacy test has been added
to, not substituted for, the common-law trespassory test” (emphasis in original)).
Because the challenged search here fails the reasonable-expectation-of-privacy
test, we need not consider how it fares under the common-law trespassory test.

-- 15 of 46 --

16
266, 285–86 (6th Cir. 2010); see United States v. Ackerman, 831 F.3d at 1304
(analogizing email to physical mail, search of which requires warrant, in
observing that “[n]o one in this appeal disputes that an email is a ‘paper’ or
‘effect’ for Fourth Amendment purposes, a form of communication capable
of storing all sorts of private and personal details”).9
Rather, the government argues that Maher’s expectation of privacy in
the Maher file that he emailed to his own Google account was extinguished
by Google’s Terms of Service, which advise users that Google (1) “may
review content to determine whether it is illegal or violates our policies,”
App’x 113, (2) “may” report “illegal content” to “appropriate authorities,”
id. at 142, and (3) “will share” users’ information with law enforcement
when necessary to comply with applicable law, id. at 131.
This court has not had occasion to address what effect, if any, a private
company’s terms of service might have on a defendant’s reasonable
expectation of privacy. It may well be that such terms, as parts of “[p]rivate
contracts[,] have little effect in Fourth Amendment law because the nature
of those [constitutional] rights is against the government rather than private
parties.” Orin S. Kerr, Terms of Service and Fourth Amendment Rights, 172 U.
PA. L. R EV. 287, 291 (2024) (summarizing case law). We need not here draw
any categorical conclusions about how terms of service affect a user’s
expectation of privacy as against the government. On this appeal, it suffices
that we conclude that Google’s particular Terms of Service—which advise
9 The Ninth Circuit has suggested that a reasonable expectation of privacy in
emails lasts only until the email is delivered to the recipient. See United States v.
Mohamud, 843 F.3d 420, 442 (9th Cir 2016). We need not pursue that point here
because Maher emailed the Maher file to himself, thereby retaining an expectation
of privacy in its contents.

-- 16 of 46 --

17
that Google “may” review users’ content, App’x 113—did not extinguish
Maher’s reasonable expectation of privacy in that content as against the
government.
In reaching that conclusion, we adopt the reasoning of the Sixth
Circuit in United States v. Warshak, 631 F.3d at 286–87 (holding that
government violated Fourth Amendment when, without warrant, it
compelled internet service provider to surrender contents of user emails).
There too, the government argued that an internet service provider’s
contractual reservation of the right to access user emails extinguished a
defendant’s expectation of privacy in his emails. In rejecting the
argument—at least with respect to a reservation phrased in terms of what
the provider may do, see id. at 287 (quoting Acceptable Use Policy provision
stating that provider “may access and use individual Subscriber information
in the operation of the Service and as necessary to protect the Service”
(emphasis in original))—the Sixth Circuit held that “the mere ability of a
third-party intermediary to access the contents of a communication cannot
be sufficient to extinguish a reasonable expectation of privacy” as against
the government, id. at 286 (emphasis in original). As the court explained,
that conclusion finds support in the seminal Fourth Amendment case, Katz
v. United States, 389 U.S. 347 (1967), where “the Supreme Court found it
reasonable to expect privacy during a telephone call despite the ability of an
operator to listen in.” United States v. Warshak, 631 F.3d at 287 (noting that
telephone companies could then “listen in when reasonably necessary to
protect . . . against the improper and illegal use of their facilities” (internal
quotation marks omitted)). It also finds support in cases recognizing that
hotel guests retain a reasonable expectation of privacy in their rooms, “even
though maids routinely enter hotel rooms.” Id.; see United States v. Stokes,

-- 17 of 46 --

18
733 F.3d 438, 443 n.7 (2d Cir. 2013) (“Hotel guests retain a legitimate
expectation of privacy in the hotel room and in any articles located in their
hotel room for the duration of their rental period.”). We too conclude from
these precedents that Google’s Terms of Service, advising users of what the
company “may review,” App’x 113, did not extinguish Maher’s reasonable
expectation of privacy in his emails as against the government.
Nor is a different conclusion compelled by the fact that Google’s
Terms of Service also warn users that the company “will share personal
information outside of Google if . . . reasonably necessary to[] . . . [m]eet any
applicable law.” Id. at 131 (emphasis added). As noted supra at 7 n.5, federal
law requires electronic service providers such as Google to file a report with
the NCMEC when they have “actual knowledge” of child pornography on
their platforms. 18 U.S.C. § 2258A(a)(1)(A), (B). But the same law
specifically does not require Google “affirmatively [to] search, screen, or
scan” for such material. Id. § 2258A(f)(3). Not surprisingly then, Google
does not tell users that it will engage in the sort of content review for
illegality that could trigger disclosure obligations under § 2258A(a)(1)(A),
(B). Rather, it tells users only that it “may” engage in such review. App’x
113. Indeed, in the next sentence, Google emphasizes that it “does not
necessarily . . . review content,” and tells users, “please don’t assume that we
do.” Id. at 114 (emphasis added). Such qualified language is hardly a per se
signal to Google users that they can have no expectation of privacy in their
emails, even as against the government. Cf. United States v. Rosenow, 50 F.4th
715, 730 (9th Cir. 2022) (stating, with respect to § 2258A, that “[m]andated
reporting is different than mandated searching” (emphasis in original)).
In a different context that is nevertheless instructive here, the
Supreme Court declined to construe even unqualified language in a private

-- 18 of 46 --

19
contract as extinguishing a person’s expectation of privacy as against the
government. See Byrd v. United States, 584 U.S. 395 (2018). There, a car rental
agreement expressly forbade anyone not identified in the contract from
operating the leased vehicle. The government argued that this meant any
driver not so identified had no reasonable expectation of privacy in the
vehicle. The Court, however, declined to derive such a “per se rule” from
the contract’s identified-operator provision. Id. at 405. Recognizing that
“car-rental agreements are filled with long lists of restrictions,” id. at 407,
the Court adhered to the “general rule” that a person “in otherwise lawful
possession and control of a rental car has a reasonable expectation of
privacy” against the government in that vehicle even if he is not authorized
by the rental agreement to be operating the car, id. at 398–99.
Here, we need not decide whether terms of service pertaining to
content review might ever be so broadly and emphatically worded as to
categorically extinguish internet service users’ reasonable expectations of
privacy in the contents of their emails, even as against the government. See
United States v. Warshak, 631 F.3d at 287 (declining to foreclose possibility).
We conclude only that Google’s Terms of Service, repeatedly qualifying the
content review that the company “may” conduct, do not effect such a
complete extinguishment.
Thus, to justify its warrantless search of the Maher file, the
government had to show that it simply repeated the private search of that
file already conducted by Google. We now turn to that point.10
10 Citing United States v. Lewis, 62 F.4th 733, 742 (2d Cir. 2023), the government
argues that Maher failed, in any event, to offer any evidence that he had a

-- 19 of 46 --

20
B. Private Search Doctrine
1. Legal Precedents
The private search doctrine permits government officials, without a
warrant, to repeat a search of personal papers and effects already conducted
by a private party, so long as the government does not expand upon the
prior private search. See United States v. Jacobsen, 466 U.S. at 114–22.
The doctrine is grounded in the longstanding recognition that the
Fourth Amendment proscribes only “governmental action.” Burdeau v.
McDowell, 256 U.S. 465, 475 (1921). In Burdeau, the Supreme Court held that
the government does not violate the Fourth Amendment when it prosecutes
a defendant using evidence obtained in the first instance—even
unlawfully—by a private party. The Court reasoned that so long as no
government official “had anything to do with the [private party’s] wrongful
seizure of the petitioner’s property . . . there was no invasion of the security
afforded by the Fourth Amendment against unreasonable search and
seizure, as whatever wrong was done was the act of individuals in taking
the property.” Id. Similarly, in Coolidge v. New Hampshire, 403 U.S. 443
(1971), the Supreme Court rejected a Fourth Amendment challenge to the
subjective expectation of privacy in the contents of the Maher file. Lewis, however,
required only that a defendant seeking to suppress evidence “respond to the
Government’s argument” that he lacked a reasonable expectation of privacy,
whether by pointing to relevant evidence or making “any arguments pertinent to
his reasonable expectation of privacy . . . in his motion papers.” Id. at 741. Maher
made such an argument below, and the government cites no case requiring him
further to file a declaration attesting to his subjective expectation of privacy. On
this record, we see no basis to question whether Maher—who emailed the Maher
file to himself—subjectively expected the file to be private as against the
government.

-- 20 of 46 --

21
government’s use of incriminating evidence that the defendant’s spouse had
voluntarily given to law enforcement officials, explaining, “it is no part of
the policy underlying the Fourth and Fourteenth Amendments to
discourage citizens from aiding . . . in the apprehension of criminals.” Id. at
487–88.
The principles supporting these precedents, in turn, informed the
Supreme Court’s discussion of the private search doctrine in two cases
challenging warrantless government searches following private searches of
the same or related property: Walter v. United States, 447 U.S. 649 (1980),
which produced no majority opinion, and United States v. Jacobsen, 466 U.S.
109 (1984), which did.
In Walter, boxes containing pornographic films were delivered to the
wrong recipient. See 447 U.S. at 651 (plurality opinion). Employees of the
mistaken recipient opened the boxes and saw on the films’ labels
“suggestive drawings” and “explicit descriptions” of the films’ contents. Id.
at 652. After one employee “attempted without success to view portions”
of one of the films “by holding it up to the light,” the recipient company
contacted the FBI. Id. Taking possession of the boxes and their contents, an
FBI agent, without a warrant, proceeded to view the films contained therein
using a projector. See id.
Five members of the Court concluded that the FBI’s warrantless
viewing of the films violated the Fourth Amendment for varying reasons,
none of which commanded a majority. In the plurality opinion, Justice
Stevens, writing for himself and Justice Stewart, so concluded because the
agent’s viewing of the films exceeded the scope of the private search. He
explained that while “a wrongful search or seizure conducted by a private

-- 21 of 46 --

22
party . . . does not deprive the government of the right to use evidence that
it has acquired lawfully,” the government “may not exceed the scope of the
private search unless it has the right to make an independent search” or has
obtained a warrant. Id. at 656–57 (emphasis added). In these two Justices’
view, the agents’ viewing of the films with a projector “was a significant
expansion of the search that had been conducted previously by a private
party and therefore must be characterized as a separate search” that,
without a warrant, violated the Fourth Amendment. Id. at 657. They did
not think that employees’ viewing of images and descriptions on the outside
of the films—a fact emphasized by the four dissenting justices—warranted
a different conclusion because that private action “frustrated” the
defendants’ expectation of privacy only “in part,” which did not
“automatically justify a total invasion” of privacy by the government. Id. at
659 & n.13.
Concurring in the Court’s identification of a Fourth Amendment
violation, Justice White, joined by Justice Brennan, expressed general
reservations about the private search doctrine: “The notion that private
searches insulate from Fourth Amendment scrutiny subsequent
governmental searches of the same or lesser scope is inconsistent with
traditional Fourth Amendment principles.” Id. at 660.11
The four dissenters—Justice Blackmun, joined by Chief Justice Burger
and Justices Powell and Rehnquist—agreed with Justices Stevens and
Stewart that the private search doctrine could provide an exception to the
Fourth Amendment warrant requirement. See id. at 662 (stating that
11 Justice Marshall also concurred in the judgment, but without authoring or
joining in an opinion. See id.

-- 22 of 46 --

23
plurality opinion “at least preserves the integrity of the rule specifically
recognized long ago in Burdeau v. McDowell”). But they thought the
employees’ review of pictures and descriptions on the films’ labels sufficed
to extinguish any reasonable expectation of privacy that defendants had in
the specific contents of the films, thereby permitting the FBI, without a
warrant, to use a projector to view the films. See id. at 663–66.
Four years later, the Supreme Court revisited the private search
doctrine in United States v. Jacobsen, 466 U.S. 109 (1984). In that case, Federal
Express (“FedEx”) employees, per company policy, opened a package
damaged in transit and therein saw crumpled newspaper cushioning a tube
constructed of duct tape. Inside the tube, employees found four plastic bags
filled with white powder. See id. at 111. FedEx notified the Drug
Enforcement Administration (“DEA”) but, before an agent arrived at the
scene, employees had put the plastic bags back into the tube and the tube
and newspapers back into the box. See id. Upon arrival, a DEA agent
reopened the box; removed the tube from the box, the plastic bags from the
tube, and a small amount of white powder from one of the bags; and
conducted a chemical “field test” on the powder, which reacted positively
for cocaine. Id. at 111–12 & n.1.
This time, Justice Stevens wrote for a six-member majority in
concluding that the private search doctrine supported the agent’s
warrantless removal of the tube from the box and the plastic bags from the
tube because these actions merely duplicated the search previously
conducted by FedEx employees. See id. at 115 (stating that these “initial
invasions of respondents’ package were occasioned by private action,”
which necessarily “did not violate the Fourth Amendment”). Nevertheless,
citing approvingly to the Walter plurality, the Court held that any

-- 23 of 46 --

24
“additional invasions of respondents’ privacy by the government agent
must be tested by the degree to which they exceeded the scope of the private
search.” Id. at 115; see also id. at 117 n.12 (observing that plurality and dissent
in Walter agreed that this was “the standard to be applied”). Explaining the
distinction that could thus arise when applying the private search doctrine,
the Court in Jacobsen stated that “[o]nce frustration of the original
expectation of privacy occurs” by a private party, “the Fourth Amendment
does not prohibit governmental use of the now-nonprivate information.” Id.
at 117. But “[t]he Fourth Amendment is implicated . . . if the authorities use
information with respect to which the expectation of privacy has not already
been frustrated,” because “[i]n such a case the authorities have not relied on
what is in effect a private search, and therefore presumptively violate the
Fourth Amendment if they act without a warrant.” Id. at 117–18. Thus,
when authorities look to exceed the scope of a prior private search, i.e., when
they look to learn more than what had been revealed by the private search,
they must ordinarily obtain a warrant. See id. at 119-20.
Applying this test to Jacobsen’s facts, the Supreme Court held that,
even though FedEx employees had put the plastic bags containing white
powder back into the tube and had placed the tube together with
surrounding sheets of newspaper back into the original package, the DEA
agent’s warrantless removal of the tube from the package and the plastic
bags from the tube did not violate the Fourth Amendment because those
actions simply repeated the private search conducted by FedEx employees
and enabled the agents “to learn nothing that had not previously been
learned during the private search.” Id. (observing that when agent opened
box and removed contents, “there was a virtual certainty that nothing else
of significance was in the package and that a manual inspection of the tube

-- 24 of 46 --

25
and its contents would not tell him anything more than he already had been
told” by the FedEx employees who had conducted the private search).
At the same time, however, the Court ruled that by then conducting
a field test on some of the white powder—something FedEx employees had
not done—the agent effected an “additional intrusion” that “exceeded the
scope of the private search” and, thus, did not fall within that particular
exception to the warrant requirement. Id. at 122. Instead, the Court ruled
that no warrant was required for the field test because it “could disclose
only one fact previously unknown to the agent—whether or not a suspicious
white powder was cocaine.” Id. Specifically, if the test indicated the powder
was not cocaine, it “could tell [the agent] nothing more, not even whether the
substance was sugar or talcum powder.” Id. (emphasis added). Thus, the
Court concluded that, Congress having decided “to treat the interest in
‘privately’ possessing cocaine as illegitimate,” a simple binary field test
“that can reveal whether a substance is cocaine, and no other arguably
‘private’ fact, compromises no legitimate privacy interest.” Id. at 122–23.
In sum, as construed by the Supreme Court in Walter and Jacobsen, the
private search doctrine is properly understood to authorize law
enforcement authorities to conduct a warrantless search only when they
repeat a search already conducted by a private party to the same degree it
“frustrate[s]” a person’s expectation of privacy. Id. at 117. If law
enforcement authorities “exceed[] the scope of the private search,” seeking
to learn “anything more” than the private party had learned from its search,
they must either secure a warrant or have some further legal basis for not
doing so. Id. at 115–22; see United States v. Knoll, 16 F.3d 1313, 1319–21 (2d
Cir. 1994) (citing plurality opinion in Walter for proposition that government

-- 25 of 46 --

26
“may not expand the scope of an ongoing private search unless it has an
independent right to do so”).
2. Application of Private Search Doctrine to This Case
Following the principles articulated in these precedents, we now
consider whether the private search doctrine permitted State Police visually
to examine the contents of the Maher file without a warrant.
In conducting that inquiry, we recognize that this court has applied
the private search doctrine in various cases involving searches of physical
items. See, e.g., United States v. $557,933.89, More or Less, in U.S. Funds, 287
F.3d at 87–88 (holding that officer did not violate Fourth Amendment when,
without warrant, he opened briefcase earlier reviewed by airport security
personnel, so long as officer’s search “was of no greater scope or intensity
than the airport security personnel’s”); United States v. Knoll, 16 F.3d at 1320–
21 (holding no warrant required for law enforcement authorities to read
documents already searched by burglars, while noting that, “[i]f the files
were closed and their contents not apparent from the exterior, the
reasonable expectation of privacy continued so long as the files had not been
searched before contact with the government occurred”).
This court has not, however, had occasion in a published opinion to
apply the doctrine to searches of electronically stored data. We did not do
so in United States v. DiTomasso, 932 F.3d 58, 67–68 (2d Cir. 2019), because
the defendant there appealing his child pornography conviction did not
challenge the district court’s reliance on the private search doctrine to deny
a motion to suppress evidence. Meanwhile, in United States v. Wilbert, 818
F. App’x 113, 114 (2d Cir. 2020), this court decided in a non-precedential
summary order that (1) a law enforcement official did not need a warrant to

-- 26 of 46 --

27
view an image that a defendant uploaded to an online chat service because
an employee of that service had previously reviewed the image; but (2) the
officer violated the Fourth Amendment when, without a warrant, he
expanded upon the private search by also viewing an image that the
employee had not reviewed.12
A number of our sister circuits, however, have issued published
opinions applying Jacobsen to warrantless searches of electronically stored
digital information. Almost uniformly, these courts have held that the
private search doctrine authorizes law enforcement officers to conduct
warrantless examinations of digital files that a private person has already
visually examined.13 Were that the circumstance here, we would readily
12 The algorithm used in Wilbert to flag potentially suspicious images appears to
differ from that used by Google here. See United States v. Wilbert, No. 16 Cr. 6084,
2018 WL 6729659, at *3 (W.D.N.Y. Aug. 20, 2018) (stating that algorithm there relies
on “shapes, colors and . . . [image] features”).
13 See United States v. Rivera-Morales, 961 F.3d 1, 5–6, 8–15 (1st Cir. 2020) (holding
warrant not required for authorities to view image of child pornography on
defendant’s cell phone when defendant’s wife had already discovered image on
phone and showed it to local police); United States v. Runyan, 275 F.3d 449, 463–64
(5th Cir. 2001) (holding that police could conduct warrantless review of computer
disks already viewed by private party, but not of disks that had not been so
viewed); Rann v. Atchison, 689 F.3d 832, 836–37 (7th Cir. 2012) (holding warrant
not required for police to view digital images seen by victim’s mother and turned
over by her to authorities); United States v. Goodale, 738 F.3d 917, 921 (8th Cir. 2013)
(holding warrant not required for authorities to view child pornography websites
on defendant’s laptop computer when victim’s mother had brought laptop to
police station and showed officers websites in viewing history); United States v.
Phillips, 32 F.4th 865, 875 (9th Cir. 2022) (holding warrant not required for law
enforcement authorities to view child pornography images on defendant’s laptop
computer when former fiancée had discovered images on laptop and showed

-- 27 of 46 --

28
reach the same conclusion, which requires nothing more than a
straightforward application of Jacobsen to modern technology. When a
private person has already visually examined a defendant’s digital image,
he has thereby “frustrated” the defendant’s expectation of privacy in that
image, such that when law enforcement authorities also visually examine it,
they learn “nothing more” by doing so than what had “previously been
learned during the private search.” United States v. Jacobsen, 466 U.S. at 120.
Thus, the private search doctrine there relieves the government of the need
to obtain a warrant.14
them to authorities); United States v. Benoit, 713 F.3d 1, 8–11 (10th Cir. 2013)
(holding warrant not required for police to view images of child pornography on
defendant’s computer that girlfriend had discovered and shown to police); United
States v. Castaneda, 997 F.3d 1318, 1326–29 (11th Cir. 2021) (holding warrant not
required for FBI agent to view child pornography file opened by defendant’s
friends who alerted FBI); cf. United States v. Lichtenberger, 786 F.3d 478, 488–89 (6th
Cir. 2015) (holding private search doctrine inapplicable where “there was a very
real possibility” that officer’s warrantless search uncovered information that no
private party had seen). In United States v. Fall, the Fourth Circuit found it
unnecessary to “address[] the private search doctrine in the context of electronic
devices,” because the “good faith exception to the exclusionary rule” supported
affirmance there in any event. 955 F.3d 363, 370–71 (4th Cir. 2020).
14 The conclusion may be more obvious when the search at issue pertains to a
particular digital image rather to an electronic device. See United States v. Phillips,
32 F.4th at 873 (observing “that it may be more difficult to have ‘virtual certainty’
that a search of an electronic device does not reveal more than the private search
had already revealed, given the dynamic nature of such devices”); United States v.
Wilson, 13 F.4th 961, 977 n.13 (9th Cir. 2021) (noting circuit split on question of
whether individual’s expectation of privacy in digital device is “entirely frustrated
whenever any part of the container is searched” or if, instead, device owner retains
expectation of privacy in files on device that have not been searched). We need
not pursue this point here because the warrantless search at issue pertains to a
single digital image.

-- 28 of 46 --

29
This case, however, presents a different scenario raising a more
challenging question, i.e., whether the private search doctrine authorizes
law enforcement authorities to conduct a warrantless visual examination of
the contents of a digital file where a private party has not visually examined
the contents of that file but, rather, has used a computer to match the hash
value of the contents of that file to the hash value of an image previously
located in another file, which image, upon visual examination, was
determined to depict child pornography. Three Courts of Appeals have
considered that question, with the Fifth and Sixth Circuits answering it in
the affirmative and the Ninth Circuit responding in the negative. Compare
United States v. Wilson, 13 F.4th 961 (9th Cir. 2021), with United States v. Miller,
982 F.3d 412 (6th Cir. 2020), and United States v. Reddick, 900 F.3d 636 (5th
Cir. 2018).15 We here join the Ninth Circuit in concluding that such a hash
match may well provide strong probable cause for a warranted visual
examination of the as-yet-unviewed matched image, but the private search
doctrine does not authorize a warrantless visual examination of that
computer-matched image.
We start by explaining why we are not persuaded by the reasoning of
the Fifth and Sixth Circuits. In United States v. Reddick, the Fifth Circuit ruled
that the private search doctrine authorized police to conduct a warrantless
visual examination of an image that a private party found to have the same
hash value as an image that the party had earlier determined to depict child
pornography because “opening the [matched] file merely confirmed that the
15 The Tenth Circuit, in United States v. Ackerman, declined to address this particular
question, there concluding only that where a private party reported that one email
attachment contained known child pornography, the government could not open
other images attached to the same email without a warrant. See 831 F.3d at 1306–07.

-- 29 of 46 --

30
flagged file was indeed child pornography, as suspected.” 900 F.3d at 639.
We do not adopt this reasoning because we do not understand the Fourth
Amendment to permit law enforcement officials to conduct warrantless
searches of unopened property to confirm a private party’s report—
however strong—that the property contains contraband. Indeed, in United
States v. Jacobsen, the Supreme Court stated that police violate the Fourth
Amendment when they “simply learn from a private party that a container
contains contraband . . . and conduct a warrantless search.” 466 U.S. at 120
n.17. This court has also held that the private search doctrine does not
authorize the warrantless opening of a package even when police were told
by the party who packed and mailed the package that it contained stolen
property. United States v. Martin, 157 F.3d 46, 55 (2d Cir. 1994); see generally
Horton v. California, 496 U.S. 128, 137 n.7 (1990) (stating that “no amount of
probable cause can justify a warrantless search or seizure absent exigent
circumstances”(internal quotation marks omitted)). Here, Google’s report
that the unopened Maher file contained an image whose hash value
matched that of an image previously found in another file that, upon visual
inspection, was determined to depict child pornography may well have
provided authorities with strong probable cause to believe that the image in
the Maher file also depicted child pornography and, thus, supported
issuance of a warrant. But the reported hash match did not authorize them
to conduct an unwarranted search of the unopened Maher file to confirm
that belief.
Rather, the private search doctrine authorizes government officials to
conduct a warrantless search only insofar as they effectively duplicate the
search conducted by a private party, thereby frustrating no greater
expectation of privacy and learning nothing more than what had been

-- 30 of 46 --

31
learned during the private search. See United States v. Jacobsen, 466 U.S. at
119–20, 122. That does not appear to have been the case in Reddick. It is
certainly not the case here. A Google computer “searched” the Maher file
only for a hash value, which the computer then matched to a hash value
already in its repository: 2eb373380383f50820e648d1c304a3db. That hash
value search of the Maher file, however, did not reveal the particulars of the
file’s contents. To learn that additional information required a further
search. It required a police officer to open the Maher file and visually to
examine its contents—a more expansive search never conducted by Google
in this case and, thus, not falling within the private search doctrine.
In concluding otherwise, the Fifth Circuit analogized visual
examination of a hash matched file to the “chemical tests on the white
powder in Jacobsen,” also a form of examination not employed by the private
party in that case. United States v. Reddick, 900 F.3d at 639. But the Supreme
Court did not approve the warrantless field test in Jacobsen under the private
search doctrine. To the contrary, the Court observed that no such test
having been conducted by the searching private party, the field test there
“exceeded the scope of the private search.” United States v. Jacobsen, 466 U.S.
at 121. Rather, the Court concluded that a field test did not require a warrant
because its further intrusion was limited to a binary disclosure, i.e., it “could
disclose only . . . whether or not a suspicious white powder [discovered by
the private search] was cocaine,” and “could tell [the agent] nothing more,
not even whether the substance was sugar or talcum powder.” Id. at 122
(emphasis added).
That is not the case here. Unlike a field test, a human visual
examination of a computer hash matched image does not disclose only
whether or not the image depicts child pornography. Visual examination

-- 31 of 46 --

32
necessarily also reveals the particulars supporting either a “yes” or “no”
answer. In the case of an affirmative answer, those particulars would
include the individual children depicted, the number of such children, their
approximate ages, any adults also depicted, whether the defendant is
depicted, the circumstances of depiction indicative of child pornography,
etc.16 Google did not learn any of these particulars in its computer hash
value search of the Maher file. Also, in the case of a negative answer, a
human visual examination would still reveal particulars, ranging from the
innocuous to the embarrassing, that the account holder reasonably expected
were private. Thus, a visual examination’s revelation of particulars is a far
cry from a field test’s disclosure of nothing more than a binary answer.17
In United States v. Miller, the Sixth Circuit recognized that warrantless
visual examination of a hash matched image cannot be analogized to a
16 While we do not here describe any of the disturbing 4,000 images and five videos
at issue in this case, they all too sadly evidence the variety of particulars that can
be depicted in child pornography.
17 For much the same reason, the warrantless visual examination in this case cannot
be analogized to a dog sniff, another sort of binary test invoked by the government.
See United States v. Place, 462 U.S. 696, 707 (1983) (upholding warrantless dog sniff
that “discloses only the presence or absence of narcotics”); Illinois v. Caballes, 543
U.S. 405, 408 (2005) (stating that dog sniff, as “governmental conduct that only
reveals the possession of contraband[,] compromises no legitimate privacy
interest” (emphasis in original and internal quotation marks omitted)). But see
United States v. McKenzie, 13 F.4th 223, 232 (2d Cir. 2021) (holding that when canine
sniff pertains to constitutionally protected areas, such as exterior of home, it can
implicate privacy interests so as to require warrant). In any event, we note that
when authorities want to open and inspect the contents of a closed container to
which a trained dog reacted for contraband, the Supreme Court has held that a
warrant is required. See United States v. Chadwick, 433 U.S. 1, 3–4, 13–15 (1977),
abrogated on other grounds by California v. Acevedo, 500 U.S. 565 (1991).

-- 32 of 46 --

33
binary field test. See 982 F.3d at 429 (rejecting Fifth Circuit reasoning).
Nevertheless, that court also concluded that the private search doctrine
supported a warrantless visual examination of a hash matched image,
reasoning that the high reliability of hash matching technology created the
same “virtual certainty” as in Jacobsen that the warrantless search would
reveal the same evidence uncovered in the private party search. Id. at 429–
30 (stating that private search doctrine applies “if there is a ‘virtual certainty’
that [police] viewing of the [matched] files would disclose the same images
that Google’s employees had already viewed” and identified as child
pornography).18 We are not persuaded.
Jacobsen is distinguishable from Miller and this case in an important
respect. In Jacobsen, a DEA agent conducted a warrantless search of the same
18 In reaching this conclusion, the Sixth Circuit referenced findings by the
magistrate judge, adopted by the district judge, that hash matching technology
was “highly reliable—akin to the reliability of DNA.” Id. at 430 (noting finding
supported by another district court case and publication of Federal Judicial Center,
which indicated that likelihood of different images sharing the same hash value
was one in one billion, or even one in 9.2 quintillion (internal quotation marks and
citations omitted)). We do not pursue this reliability point except to note that the
Sixth Circuit appears to have assigned defendant the burden of disproving the
reliability of hash matching technology. See id. (stating that “Miller, who bore the
burden of proof, never challenged the reliability of hashing in the district court”
(internal quotation marks omitted and alteration adopted)). Because our own
court places the burden on the government to show that a challenged search fell
within an exception to the warrant requirement, see, e.g., United States v. Kiyuyung,
171 F.3d 78, 83 (2d Cir. 1999) (“If the place or object subjected to the warrantless
search is one in which the defendant had a reasonable expectation of privacy, the
burden of showing that the search fell within one of the exceptions to the warrant
requirement is on the government.”), we think that where the government relies
on hash matching or other technology to carry that burden, it assumes the
obligation of demonstrating the technology’s reliability.

-- 33 of 46 --

34
container already privately searched by FedEx employees. It was in that
context that the Court concluded that, even though FedEx employees had
placed all items found in their search back into that container, no warrant
was necessary for a DEA agent to search that same container because there
was a “virtual certainty” that nothing more would be found than what
FedEx employees had already seen. United States v. Jacobsen, 466 U.S. at 119.
By contrast, in Miller and here, police conducted a warrantless visual search
of a digital file (here, the Maher file) that no Google employee or contractor
had ever opened or visually examined. Rather, what a Google employee or
contractor had earlier opened and visually examined was a different file—
i.e., the original file—wherein it identified an image depicting child
pornography.
Maher had no expectation of privacy in the original file that could
have been extinguished by Google’s visual examination of its contents.
Maher did, however, have an expectation of privacy in the Maher file, which
he uploaded to one of his own email accounts. To the extent Google
subsequently “searched” the Maher file, it did so only to the limited degree
of having a computer determine that the hash value derived from the file’s
contents matched the hash value derived from the original file’s contents.
The State Police never replicated that computer search. Rather, they
employed a completely different and more intrusive search method—
human visual examination—to learn more than could be learned from
Google’s hash matching algorithm.
Thus, even if the government in this case had offered evidence that
Google’s hash matching technology made it virtually certain that the images

-- 34 of 46 --

35
contained in two hash matched files were identical,19 the match did not
permit the government to go further than Google had and to examine
visually the contents of the Maher file without a warrant. See United States
v. Jacobsen, 466 U.S. at 120 n.17 (acknowledging that police cannot conduct
warrantless search of unopened container based on private party report that
it contains contraband); United States v. Martin, 157 F.3d at 55 (noting that
police cannot conduct warrantless search of mailed container even though
party who packed and mailed it reported that it contained stolen property).
Google’s hash value search did not tell Google anything about the
particulars depicted in the Maher file—or even the original file image, which
Google apparently had not retained. To learn those additional particulars,
police needed to exceed the scope of Google’s computer search by opening
the Maher file and having an officer visually examine its contents. The
private search doctrine did not permit them to conduct this more intrusive
search without a warrant. See United States v. Jacobsen, 466 U.S. at 120.
In so concluding, we join the Ninth Circuit, which has ruled that the
private search doctrine does not permit police to conduct a warrantless
visual examination of a digital file that a private party has not itself viewed
but only computer hash matched to the contents of another digital file
previously determined to contain child pornography. See United States v.
Wilson, 13 F.4th at 961. In reaching that conclusion, the Ninth Circuit
deemed it “critical” that “no Google employee viewed” the particular
19 As earlier noted, supra at 12 n.7, the government offered no such evidence in this
case. In fact, the affidavit submitted to the district court in this case by Google
Manager Lilley qualified the precision of the company’s hash matches:
“Comparing these hashes to hashes of content uploaded to Google’s services
allows Google to identify exact or very similar images of apparent child
pornography.” App’x 109 (emphasis added).

-- 35 of 46 --

36
contents of defendant’s hash matched files before the government did so.
Id. at 974. We agree. As that court stated on this point: “When the
government views anything other than the specific materials that a private
party saw during the course of a private search, the government search
exceeds the scope of the private search.” Id. (referencing distinction drawn
in Jacobsen between government’s visual search of container contents
already examined by private party and government’s field test of white
powder found in container, which “exceeded the scope of the private
search” (emphasis omitted) (quoting United States v. Jacobsen, 466 U.S. at
122)).
Also, like the Ninth Circuit, we do not think it can be said that
“because Google had already classified the [original file] as child
pornography,” the government could learn “nothing new” by visually
examining an image with the same hash value when it appeared in another
file. Id. at 972 (rejecting argument). Even assuming the high reliability of
Google’s hash matching technology, it could reveal only that two images are
virtually certain to be identical. It could not—and here did not—reveal what
in particular was depicted in the identical images.20
20 As the Ninth Circuit observed in Wilson, “Google does not keep a repository of
child pornography images, so no Google employee or contractor could have
shown the government the images it believed to match Wilson’s. Nor does the
record identify the individual who viewed those images.” 13 F.4th at 972. While
the record in Wilson indicates that Google “tags” images in its child pornography
repository “with one of four generic labels,” id. (noting that image whose hash
value matched Wilson’s was tagged A1, which indicated the depiction of “a sex
act involving a prepubescent minor”), the record in this case contains no
comparable evidence of tagging.

-- 36 of 46 --

37
In these circumstances, Google’s hash matching technology might
better be understood to have labeled the Maher file image as “apparent child
pornography,” App’x 29, much as the pictures and images on the film labels
in Walter v. United States, 447 U.S. at 654, indicated that the films’ content
was pornographic. See United States v. Wilson, 13 F.4th at 973 (drawing
analogy). Such labels can provide the probable cause necessary to secure a
warrant to search the contents of closed containers bearing those labels. But
such a search is certainly going to reveal more than the label itself.21 That is
evident here where Google’s computer hash value search of the Maher file
by a computer supported the company’s labeling the file image as “apparent
child pornography” in its report to the NCMEC. But an entirely different
search, specifically, a human visual examination of the Maher file, was
necessary to learn the exact child pornography depicted: “a prepubescent
21 “Labeled” items should not be confused with those whose containers or
packaging “by their very nature cannot support any reasonable expectation of
privacy because their contents can be inferred from their outward appearance.”
Arkansas v. Sanders, 442 U.S. 753, 765 n.13 (1979), abrogated on other grounds by
California v. Acevedo, 500 U.S. 565 (1991). This exception applies only where an
owner stores his property in a container that makes its content apparent, thus
disclaiming an expectation of privacy. Cf. United States v. Dien, 609 F.2d 1038, 1045
(2d Cir. 1979) (holding defendant retained expectation of privacy in sealed
cardboard box emanating smell of marijuana because fact that box was sealed
“manifested an expectation that the contents would remain free from public
examination”), adhered to on reconsideration, 615 F.2d 10 (2d Cir. 1980). Nothing
about the Maher file or the Google email account to which Maher uploaded it
indicated Maher’s intent to disclaim a privacy interest in the file, the specific
contents of which could not be known until it was visually examined, which
Google never did. See generally United States v. Knoll, 16 F.3d at 1320 (stating, with
respect to non-digital files that if they “were closed and their contents not apparent
from the exterior, the reasonable expectation of privacy continued so long as the
files had not been searched before contact with the government occurred”).

-- 37 of 46 --

38
female, who appears approximately six to seven years old, exposing her
vagina.” App’x 44. In short, Investigator Croneiser’s visual inspection of
the Maher file image was a search of far greater “scope” for purposes of the
Fourth Amendment than Google’s algorithmic search only for matching
hash values “because it allowed the government to learn new, critical
information” that could be used “to prosecute” Maher. United States v.
Wilson, 13 F.4th at 971–72; see United States v. Jacobsen, 466 U.S. at 119–20
(holding private search doctrine authorizes warrantless search doctrine only
when there is “virtual certainty” latter will “not tell [police] anything more
than” already revealed by private search).
In urging otherwise, the government argues that the relevant private
search here is not simply Google’s computer hash match of the Maher file
image to the original file image, but also a Google employee or contractor’s
earlier visual examination of the latter image. We are not persuaded for a
further reason relied on by the Ninth Circuit: Fourth Amendment rights are
personal to an individual. See United States v. Wilson, 13 F.4th at 974; see
generally, e.g., Plumhoff v. Rickard, 572 U.S. 765, 778 (2012); Rakas v. Illinois,
439 U.S. 128, 133–34 (1978). Thus, just as Maher cannot claim any
expectation of privacy in Google’s earlier visual examination of the original
image, which did not belong to him, but which contained the image first
assigned hash value 2eb373380383f50820e648d1c304a3db, so the
government cannot claim that Google’s visual search of that third-party file
somehow defeated Maher’s expectation of privacy in the contents of his own
unopened, unreviewed file as against the government. As the Ninth Circuit
stated on this point, “whether Google had previously reviewed, at some
earlier time, other individuals’ files is not pertinent to whether a private
search eroded Wilson’s expectation of privacy” in his file. United States v.

-- 38 of 46 --

39
Wilson, 13 F.4th at 974–75 (construing precedent to require “focus on the
extent of Google’s private search of Wilson’s effects, not of other individuals’
belongings”) (all emphases in original).
Insofar as Google searched the Maher file for a hash value that
matched the hash value previously assigned to an image identified by a
Google employee or contractor as depicting child pornography, the private
search doctrine likely would have permitted police to rely on that computer
match to demonstrate probable cause to support warrants for their own
searches of Maher’s Google accounts and residence. It might also have
permitted the government—with sufficient foundation—to offer evidence
of the match at trial. But here, the police understandably wanted to obtain
evidence of more than a hash match. They wanted evidence of the
particulars depicted in the matched Maher file image. Because no one at
Google had ever opened or visually examined the contents of the Maher file,
and because such a visual examination would reveal more information than
Google knew at the time it reported the Maher file to the NCMEC, such a
visual examination by the police did not fall within the private search
doctrine’s exception to the warrant requirement.
In sum, we here conclude that the private search doctrine does not
authorize government authorities to conduct a warrantless human visual
examination of the contents of an unopened file attached to an email based
on Google’s computer hash value match of an image in that file to another
image previously identified by a Google employee or contractor as child
pornography. The former search does not duplicate the latter but rather
exceeds its scope, thereby allowing authorities to learn more than had been
revealed by the private search. See United States v. Jacobsen, 466 U.S. at 118–
22. In so holding, we suggest no constitutional limitation on Google’s own

-- 39 of 46 --

40
ability, as a private actor, to search for and remove child pornography on its
platform. See generally Burdeau v. McDowell, 256 U.S. at 475 (stating that
Fourth Amendment is “not intended to be a limitation upon other than
governmental agencies”). Nor do we limit government authorities from
using a private party’s reliable hash matches between an identified image
of child pornography and an unviewed file image to demonstrate probable
cause for a warrant to conduct more expansive searches. See United States v.
Cartier, 543 F.3d 442, 444–46 (8th Cir. 2008) (holding government established
probable cause to search defendant’s home after detecting defendant sent
images whose hash values matched images of known child pornography).
But as the Ninth Circuit has explained, the reliability of a company’s hash
matching technology “is pertinent to whether probable cause could be
shown to obtain a warrant, not to whether the private search doctrine
precludes the need for the warrant.” United States v. Wilson, 13 F.4th at 979.
Thus, we hold that police here violated the Fourth Amendment by
visually examining the contents of the reported Maher file without a
warrant.
III. The Good Faith Exception to the Exclusionary Rule Supports
Affirmance
Our identification of a Fourth Amendment violation in this case does
not afford Maher relief from conviction because, like the district court, we
conclude that Maher’s suppression motion failed in any event under the
good faith exception to the exclusionary rule.
As the Supreme Court has instructed, “[t]he fact that a Fourth
Amendment violation occurred—i.e., that a search or arrest was
unreasonable—does not necessarily mean that the exclusionary rule

-- 40 of 46 --

41
applies.” Herring v. United States, 555 U.S. 135, 140 (2009). Rather, exclusion
of evidence is properly a court’s “last resort, not [its] first impulse.” Id.
(internal quotation marks omitted). Thus, a court will suppress illegally
obtained evidence “only where it results in appreciable deterrence” and not
when an officer acts in an “objectively reasonable” manner. Id. at 141–42
(internal quotation marks omitted and alteration adopted).
That last caveat, the basis for the good faith exception to the
exclusionary rule, most commonly applies when officers act “in objectively
reasonable reliance” on a judge’s issuance of a search warrant that is, in fact,
legally defective. See United States v. Leon, 468 U.S. 897, 922 (1984).
Nevertheless, the exception can also apply where officers “committed a
constitutional violation” by acting without a warrant under circumstances
that “they did not reasonably know, at the time, [were] unconstitutional.”
United States v. Ganias, 824 F.3d 199, 221–22 (2d Cir. 2016) (en banc)
(referencing United States v. Thomas, 757 F.2d 1359, 1368 (2d Cir. 1985); see id.
(holding warrant required for dog sniff conducted outside closed apartment
but recognizing that, at time, officers acted in good faith in thinking no
warrant required)); United States v. Raymonda, 780 F.3d 105, 118 n.5 (2d Cir.
2015) (presuming that “‘basic insight of the Leon line of cases’ that exclusion
should be limited to cases of ‘deliberate, reckless, or grossly negligent
disregard for Fourth Amendment rights,’ applies equally to searches
conducted with or without a warrant” (quoting Davis v. United States, 564
U.S. 229, 238 (2011))); see also United States v. Goldstein, 914 F.3d 200, 204–05
& n.28 (3d Cir. 2019) (holding that good faith exception applied when
“government obtained [cell site location information] without a warrant”
before practice was held unlawful by Supreme Court and collecting cases
from other circuits so holding).

-- 41 of 46 --

42
In assessing the reasonableness of an officer’s mistaken belief that no
warrant was required in a particular circumstance, we consider not only our
own precedents but also those of other courts. See United States v. Felder, 993
F.3d 57, 75–76 (2d Cir. 2021) (relying on good faith exception to deny
suppression in circumstances where, prior to Supreme Court ruling, those
courts of appeals to have considered question had concluded no warrant
required). When we do that here, we conclude that, in or about July 2020,
when Investigator Croneiser opened the Maher file and visually examined
its contents, she had a reasonable basis to believe that she did not need a
warrant to do so.
At that time, neither the Supreme Court nor this court had considered
whether a warrant is required for government authorities to open and
visually examine a digital image that the service provider has reported
depicts child pornography on the basis of a hash match to an image
previously reviewed and labelled as child pornography by the service
provider. The single appellate court to have done so, the Fifth Circuit, had
held that no warrant was required in those circumstances. See United States
v. Reddick, 900 F.3d 636 (5th Cir. 2018). Two years after Reddick was decided,
a second appellate court, the Sixth Circuit, reached the same conclusion. See
United States v. Miller, 982 F.3d 412 (6th Cir. 2020). Not until September
2021—more than a year after the challenged warrantless search here—did
the Ninth Circuit become the first appellate court to hold that a warrant was
required for the government visually to examine a hash matched image in a
file not opened by the service provider. See United States v. Wilson, 13 F.4th
961 (9th Cir. 2021). In such circumstances, we think it was objectively
reasonable for Investigator Croneiser to think that she did not need a
warrant to visually examine the Maher file image reported by Google as

-- 42 of 46 --

43
“apparent child pornography” based on a hash match to a file image
previously “reviewed” by a Google employee or contractor and
“determined [to] contain[] apparent child pornography.” App’x 29.
In urging otherwise, Maher argues that Inspector Croneiser could not
have been acting in good faith “because no binding precedent authorized
the [warrantless] search in this case.” Appellant Br. at 34. He is mistaken.
While the good faith exception certainly applies “when binding appellate
precedent specifically authorizes a particular police practice,” Davis v.
United States, 564 U.S. at 241 (emphasis omitted), it can also apply “[w]here
a relevant legal deficiency was not previously established in precedent,”
such that “the agent’s failure to recognize that deficiency cannot vitiate good
faith,” United States v. Raymonda, 780 F.3d at 119 (internal quotation marks
omitted); see also United States v. Hernandez, No. 22-471, 2024 WL 47666, at *3
(2d Cir. Jan. 4, 2024) (summary order) (holding that where precedent
rendering search impermissible was decided after search occurred,
precedent was not “previously established” and good faith exception
applied), cert. denied, 144 S. Ct. 1470 (2024). That is the case here where, at
the time of the challenged search, the only appellate court to have ruled on
the question had held that no warrant was required to open a digital file
identified as child pornography based on a hash match, and where more
than a year would pass before any appellate court held otherwise.
Maher nevertheless argues that evidence obtained by warranted
searches of his Google accounts and residence should be suppressed
because State Police misled the state court by failing to disclose in their
supporting affidavits “potentially adverse information to the issuing
judge,” specifically, that no one at Google had visually examined the Maher
file image. Appellant Br. at 33 (quoting United States v. Ganias, 824 F.3d at

-- 43 of 46 --

44
221). We disagree. The first search warrant affidavit for Maher’s two
Google accounts states that “Google reported to [the NCMEC]” that one of
the subject accounts “uploaded an image of child pornography on January
27, 2020.” App’x 44. The second search warrant affidavit for Maher’s
residence states that, “[o]n January 27, 2020, Google Inc. reported an
incident of Apparent Child Pornography to the [NCMEC].” Id. at 60. While
neither affidavit states the basis for Google’s report, i.e., that it was a hash
match rather than a visual examination, the omission was not misleading.
Nor do we think its inclusion was required as material adverse information.
While a hash match search of the Maher file did not reveal particulars
depicted in an image contained therein so as to support a warrantless
government search of that file under the private search doctrine, the hash
match of that image to one earlier identified by Google as depicting child
pornography provided strong probable cause to search the Maher file
visually for child pornography. Thus, disclosure that Google’s report of
child pornography in the Maher file was based on a hash match rather than
a visual examination would only have supported, not undercut, probable
cause for issuance of the warrant.
Insofar as the warrant affidavits both state that their respective
affiants—Investigator Croneiser for the first warrant and Investigator Esche
for the second—had personally viewed the Maher file image and detailed
what it depicted, see id. at 44, 60, we identify no basis to think that the issuing
judge was misled to think that such viewing was warranted. Certainly, the
affidavits did not say so. Rather, we think it likely that the issuing judge
(the same for both warrants) would have understood silence on this point
to mean that viewings of the Maher file image were not warranted because,
in seeking warrants, law enforcement authorities generally report when

-- 44 of 46 --

45
evidence already obtained was searched pursuant to a warrant. And, here,
Investigator Esche, in seeking the second warrant, made a point of stating
that the search of Maher’s Google accounts had been pursuant to a warrant.
In any event, for the same reasons we conclude that Investigator Croneiser
had a good faith basis to think that she could conduct a warrantless visual
examination of the Maher file image that Google reported to the NCMEC as
containing apparent child pornography, we also conclude that Investigators
Croneiser and Esche had a good faith basis to think that they did not need
to state in a warrant affidavit that such an examination had been
warrantless.
We therefore hold that while Investigator Croneiser’s warrantless
examination of the Maher file image was not authorized by the private
search doctrine, the good faith exception to the exclusionary rule supports
affirmance of the district court’s denial of Maher’s motion to suppress that
image and the evidence subsequently obtained.
CONCLUSION
To summarize, we conclude that,
1. Google’s Terms of Service did not extinguish Maher’s
reasonable expectation of privacy as against the government in
the contents of his Google emails or images uploaded to such
emails.
2. The private search doctrine did not authorize State Police to
conduct a warrantless visual examination of the Maher file
image. While Google matched the hash value for that image to
the hash value of an image previously located in another file,
which a Google employee or contractor, on visual examination,

-- 45 of 46 --

46
identified to depict child pornography, (a) Google’s
examination of that third-party file did not extinguish Maher’s
reasonable expectation of privacy in his own unopened file,
and (b) Google’s hash value search of Maher’s file did not
reveal the particulars depicted therein that constituted child
pornography. To secure that further evidence, police had to
conduct a visual examination of the unopened Maher file
image. Because that search went beyond Google’s own hash
value search of the Maher file, it required a warrant.
3. The warrantless police search of the Maher file does not entitle
Maher to relief from conviction because, at the time of that
search, police had a good faith basis to believe that no warrant
was required. Thus, the good faith exception to the
exclusionary rule here supports affirmance.
Accordingly, we AFFIRM the February 9, 2023 judgment of
conviction.

-- 46 of 46 --