IP Address Privacy Considerations

Document Type Active Internet-Draft (individual)
Authors Matthew Finkel  , Bradford Lassey  , Luigi Iannone  , Brad Chen 
Last updated 2021-11-09
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Network Working Group                                          M. Finkel
Internet-Draft                                           The Tor Project
Intended status: Informational                                 B. Lassey
Expires: 14 May 2022                                              Google
                                                              L. Iannone
                                                               J.B. Chen
                                                        10 November 2021

                   IP Address Privacy Considerations


   This document provides an overview of privacy considerations related
   to user IP addresses.  It includes an analysis of some current use
   cases for tracking of user IP addresses, mainly in the context of
   anti-abuse.  It discusses the privacy issues associated with such
   tracking and provides input on mechanisms to improve the privacy of
   this existing model.  It then captures requirements for proposed
   'replacement signals' for IP addresses from this analysis.  In
   addition, existing and under-development techniques are evaluated for
   fulfilling these requirements.

Discussion Venues

   This note is to be removed before publishing as an RFC.

   Discussion of this document takes place on the mailing list (), which
   is archived at .

   Source for this draft and an issue tracker can be found at

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   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   Drafts is at https://datatracker.ietf.org/drafts/current/.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
     2.1.  Categories of Interaction . . . . . . . . . . . . . . . .   4
   3.  IP address tracking . . . . . . . . . . . . . . . . . . . . .   5
     3.1.  IP address use cases  . . . . . . . . . . . . . . . . . .   5
       3.1.1.  Anti-abuse  . . . . . . . . . . . . . . . . . . . . .   5
       3.1.2.  DDoS and Botnets  . . . . . . . . . . . . . . . . . .   6
       3.1.3.  Multi-platform threat models  . . . . . . . . . . . .   6
       3.1.4.  Rough Geolocation . . . . . . . . . . . . . . . . . .   6
     3.2.  Implications of IP addresses  . . . . . . . . . . . . . .   7
       3.2.1.  Next-User Implications  . . . . . . . . . . . . . . .   7
       3.2.2.  Privacy Implications  . . . . . . . . . . . . . . . .   7
     3.3.  IP Privacy Protection and Law . . . . . . . . . . . . . .   8
     3.4.  Mitigations for IP address tracking . . . . . . . . . . .   9
   4.  Replacement signals for IP addresses  . . . . . . . . . . . .   9
     4.1.  Signals . . . . . . . . . . . . . . . . . . . . . . . . .  10
       4.1.1.  Adoption  . . . . . . . . . . . . . . . . . . . . . .  11
       4.1.2.  Privacy Considerations  . . . . . . . . . . . . . . .  11
       4.1.3.  Provenance  . . . . . . . . . . . . . . . . . . . . .  12
       4.1.4.  Applying Appropriate Signals  . . . . . . . . . . . .  12
     4.2.  Evaluation of existing technologies . . . . . . . . . . .  13
     4.3.  Potential new technologies  . . . . . . . . . . . . . . .  14
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  14
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  14
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  14

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     7.1.  Normative References  . . . . . . . . . . . . . . . . . .  14
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  14
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  17
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  17

1.  Introduction

   The initial intention of this draft is to capture an overview of the
   problem space and research on proposed solutions concerning privacy
   considerations related to user IP addresses (informally, IP privacy).
   The draft is likely to evolve significantly over time and may well
   split into multiple drafts as content is added.

   Tracking of IP addresses is common place on the Internet today, and
   is particularly widely used in the context of anti-abuse, e.g. anti-
   fraud, DDoS management, and child protection activities.  IP
   addresses are currently used in determining "reputation" [RFC5782] in
   conjunction with other signals to protect against malicious traffic,
   since these addresses are usually a relatively stable identifier of a
   request's origin.  Servers use these reputations in determining
   whether or not a given packet, connection, or flow likely corresponds
   to malicious traffic.  In addition, IP addresses are used in
   investigating past events and attributing responsibility.

   However, identifying the activity of users based on IP addresses has
   clear privacy implications ([WEBTRACKING1], [WEBTRACKING2]), e.g.
   user fingerprinting and cross-site identity linking.  Many
   technologies exist today that allow users to obfuscate their external
   IP address to avoid such tracking, e.g.  VPNs ([VPNCMP1], [VPNCMP2])
   and Tor ([TOR], [VPNTOR]).  Several new technologies are emerging, as
   well, in the landscape, e.g.  Apple iCloud Private Relay [APPLEPRIV],
   Gnatcatcher [GNATCATCHER], and Oblivious technologies (ODoH
   [I-D.pauly-dprive-oblivious-doh], OHTTP [I-D.thomson-ohai-ohttp]).

   General consideration about privacy for Internet protocols can be
   found in [RFC6973].  This document builds upon [RFC6973] and more
   specifically attempts to capture the following aspects of the tension
   between valid use cases for user identification and the related
   privacy concerns, including:

   *  An analysis of the current use cases, attempting to categorize/
      group such use cases where commonalities exist.

   *  Find ways to enhance the privacy of existing uses of IP addresses.

   *  Generating requirements for proposed 'replacement signals' from
      this analysis (these could be different for each category/group of
      use cases).

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   *  Research to evaluate existing technologies or propose new
      mechanisms for such signals.

   With the goal of replacing IP addresses as a fundemental signal, the
   following sections enumerate existing use cases and describe
   applicable substitution signals.  This description may not be
   exhaustive due to the breadth of IP address usage.

2.  Terminology

   (Work in progress)

   This section defines basic terms used in this document, with
   references to pre-existing definitions as appropriate.  As in
   [RFC4949] and [RFC6973], each entry is preceded by a dollar sign ($)
   and a space for automated searching.

   *  $ Identity: Extending [RFC6973], an individual's attributes may
      only identify an individual up to an anonymity set within a given

   *  $ Reputation: A random variable with some distribution.  A
      reputation can either be "bad" or "good" with some probability
      according to the distribution.

   *  $ Reputation context: The context in which a given reputation

   *  $ Reputation proof: A non-interactive zero knowledge proof of a
      reputation signal.

   *  $ Reputation signal: A representative of a reputation.

   *  $ Service provider: An entity that provides a service on the
      Internet; examples services include hosted e-mail, e-commerce
      sites, and cloud computing platforms.

2.1.  Categories of Interaction

   Interactions between parties on the Internet may be classified into
   one (or more) of three categories:

   *  $ Private Interaction: An interaction occuring between mutually
      consenting parties, with a mutual expectation of privacy.

   *  $ Public Interaction: An interaction occuring between multiple
      parties that are not engaged in a Private Interaction.

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   *  $ Consumption: An interaction where one party primarily receives
      information from other parties.

3.  IP address tracking

3.1.  IP address use cases

3.1.1.  Anti-abuse

   IP addresses are a passive identifier used in defensive operations.
   They allow correlating requests, attribution, and recognizing
   numerous attacks, including:

   *  account takeover

   *  advertising fraud (e.g., click-fraud)

   *  disinformation operations (e.g., detecting scaled and/or
      coordinated attacks)

   *  financial fraud (e.g., stolen credit cards, email account

   *  malware/ransomware (e.g., detecting C2 connections)

   *  phishing

   *  real-world harm (e.g., child abuse)

   *  scraping (e.g., e-commerce, search)

   *  spam (e.g., email, comments)

   *  vulnerability exploitation (e.g., "hacking")

   Malicious activity recognized by one service provider may be shared
   with other services [RFC5782] as a way of limiting harm.

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3.1.2.  DDoS and Botnets

   Cyber-attackers can leverage the good reputation of an IP address to
   carry out specific attacks that wouldn't work otherwise.  Main
   examples are Distributed Denial of Service (DDoS) attacks carried out
   by spoofing a trusted (i.e., having good reputation) IP address
   (which may or may not be the victim of the attack) so that the
   servers used to generate the DDoS traffic actually respond to the
   attackers trigger (i.e., spoofed packets).  Similarly botnets may use
   spoofed addresses in order to gain access and attack services that
   otherwise would not be reachable.

3.1.3.  Multi-platform threat models

   As siloed (single-platform) abuse defenses improve, abusers have
   moved to multi-platform threat models.  For example, a public
   discussion platform with a culture of anonymity may redirect traffic
   to YouTube as a video library, bypassing YouTube defenses that
   otherwise reduce exposure of potentially harmful content.  Similarly,
   a minor could be solicited by an adult impersonating a child on a
   popular social media platform, then redirected to a smaller, less
   established and less defended platform where illegal activity could
   occur.  Phishing attacks are also common.  There are many such cross-
   platform abuse models and they cause significant public harm.  IP
   addresses are commonly used to investigate, understand and
   communicate these cross-platform threats.  There are very few
   alternatives for cross-platform signals.

3.1.4.  Rough Geolocation

   A rough geolocation can be inferred from a client's IP address, which
   is commonly known as either IP-Geo or Geo-IP.  This information can
   have several useful implications.  When abuse extends beyond attacks
   in the digital space, IP addresses may help identify the physical
   location of real-world harm, such as child exploitation.  Legal compliance

   Legal and regulatory compliance often needs to take the jurisdiction
   of the client into account.  This is especially important in cases
   where regulations are mutually contradictory (i.e. there is no way to
   be in legal compliance universally).  Because Geo-IP is often bound
   to the IP addresses a given ISP uses, and ISPs tend to operate within
   national borders, Geo-IP tends to be a good fit for server operators
   to comply with local laws and regulations

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   Similar to legal compliance, some content and media has licensing
   terms that are valid only for certain locations.  The rough
   geolocation derived from IP addresses allow this content to be hosted
   on the web.  Locally relevant content

   Rough geolocation can also be useful to tailor content to the
   client's location simply to improve their experience.  A search for
   "coffee shop" can include results of coffee shops within reasonable
   travel distance from a user rather than generic information about
   coffee shops, a merchant's website could show brick and mortar stores
   near the user and a news site can surface locally relevant news
   stories that wouldn't be as interesting to visitors from other

3.2.  Implications of IP addresses

3.2.1.  Next-User Implications

   When an attacker uses IP addresses with "good" reputations, the
   collateral damage poses a serious risk to legitimate service
   providers, developers, and end users.  IP addresses may become
   assocaited with a "bad" reputation from temporal abuse, and
   legitimate users may be affected by blocklists as a result.  This
   unintended impact may hurt the reputation of a service or an end user

3.2.2.  Privacy Implications

   IP addresses are sent in the clear throughout the packet journey over
   the Internet.  As such, any observer along the path can pick it up
   and use it for various tracking purposes.  Beside basic information
   about the network or the device, it is possible to associate an IP
   address to an end user, hence, the relevance of IP addresses for user
   privacy.  A very short list of information about user, device, and
   network that can be obtained via the IP address.

   *  Determine who owns and operates the network.  Searching the WHOIS
      database using an IP address can provide a range of information
      about the organization to which the address is assigned, including
      a name, phone number, and civic address;

   *  Through a reverse DNS lookup and/or traceroute the computer name
      can be obtained, which often contains clues to logical and
      physical location;

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   *  Geo-localisation of the device (hence the user) through various
      techniques [GEOIP].  Depending on the lookup tool used, this could
      include country, region/state, city, latitude/longitude, telephone
      area code and a location-specific map;

   *  Search the Internet using the IP address or computer names.  The
      results of these searches might reveal peer-to-peer (P2P)
      activities (e.g., file sharing), records in web server log files,
      or glimpses of the individual's web activities (e.g., Wikipedia
      edits).  These bits of individuals' online history may reveal
      their political inclinations, state of health, sexuality,
      religious sentiments and a range of other personal
      characteristics, preoccupations and individual interests;

   *  Seek information on any e-mail addresses used from a particular IP
      address which, in turn, could be the subject of further requests
      for subscriber information.

3.3.  IP Privacy Protection and Law

   This section aim at providing some basic information about main
   example of laws adopted worldwide and related to IP address privacy
   (usually these laws area by product of the broader user privacy

   Possible content (to focus only on technical IP address related

   *  GDPR (General Data Protection Regulation) - EUROPE: Europe
      considers IP addresses as personal identification information that
      should be treated like any other personal information e.g. social
      security number.

   *  The United States has opted for a different approach to data
      protection.  Instead of formulating one all-encompassing
      regulation such as the EU's GDPR, the US chose to implement
      sector-specific privacy and data protection regulations that work
      together with state laws to safeguard American citizens' data.

   *  In 2020, China released the first draft of Personal Information
      Protection Law (PIPL).  The PIPL is the equivalent of European
      GDPR and will have significant influence.

   *  Japan Protection of Personal Information (APPI) Act (recent
      changes put the act close to the GDPR model).

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3.4.  Mitigations for IP address tracking

   The ability to track individual people by IP address has been well
   understood for decades.  Commercial VPNs and Tor are the most common
   methods of mitigating IP address-based tracking.

   *  Commerical VPNs offer a layer of indirection between the user and
      the destination, however if the VPN endpoint's IP address is
      static then this simply substitutes one address for another.  In
      addition, commerial VPNs replace tracking across sites with a
      single company that may track their users' activities.

   *  Tor is another mitigation option due to its dynamic path selection
      and distributed network of relays, however its current design
      suffers from degraded performance.  In addition, correct
      application integration is difficult and not common.

   *  Address anonymization (e.g.  [GNATCATCHER] and similar):

      -  [GNATCATCHER] is a single-hop proxy system providing more
         protection against third-party tracking than a traditional
         commercial VPN.  However, its design maintains the industry-
         standard reliance on IP addresses for anti-abuse purposes and
         it provides near backwards compatibility for select services
         that submit to periodic audits.

      -  [APPLEPRIV] iCloud Private Relay is described as using two
         proxies between the client and server, and it would provide a
         level of protection somewhere between a commercial VPN and Tor.

   *  Recent interest has resulted in new protocols such as Oblivious
      DNS (ODoH ({{I-D.pauly-oblivious-doh-02.html}})) and Oblivious
      HTTP (OHTTP ({{I-D.thomson-http-oblivious}})).  While they both
      prevent tracking by individual parties, they are not intended for
      the general-purpose web browsing use case.

   *  Temporary addresses

4.  Replacement signals for IP addresses

   Fundamentally, the current ecosystem operates by making the immediate
   peer of a connection accountable for bad traffic, rather than the
   source of the traffic itself.  This is problematic because in some
   network architectures the peer node of the connection is simply
   routing traffic for other clients, and any client's use of that node
   may be only temporary.  Ideally, clients could present appropriate
   identification end-to-end that is separate from the IP address, and
   uniquely bound to a given connection.

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4.1.  Signals

   There are 7 classes of signals identified in this document that may
   be used in place of IP addresses.  A signal's provenance is a
   critical property and will be discussed in Section 4.1.3.

   *  $ ADDRESS_ESCROW: Provides sufficient information for
      retroactively obtaining a client's IP address.

   *  $ IDENTITY_TRANSPARENCY: Reveals a person's identity within a

   *  $ IS_HUMAN: Informs the recipient that, most likely, a human
      recently proved their presence on the opposite end of the

   *  $ PEER_INTEGRITY: Provides a secure, remote attestation of
      hardware and/or software state.

   *  $ REIDENTIFICATION: Provides a mechanism for identifying the same
      user across different connections within a time period.

   *  $ REPUTATION: Provides the recipient with a proof of reputation
      from a reputation provider.

   *  $ SOURCE_ASN: Reveals the ASN from which the client is connecting.

   In some situations one of the above signals may be a sufficient
   replacement signal in isolation, or more than one signal may be
   needed in combination.

   Separately, there are three signal categories that are out-of-scope
   for this document but are important improvements for mitigating abuse
   on platforms.

   *  $ publisher norms: Standard expections of publishers including
      identity transparency and conflicts of interest.

   *  $ protocol improvements: Increasing security of existing

   *  $ ecosystem improvements: Reducing reliance on less secure
      systems, for example, migrating user authentication from password-
      based to WebAuthn [WEBAUTHN] and relying on multiple factors

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4.1.1.  Adoption

   Adoption of replacement signals requires coordination between user
   agents, service providers, and proxy services.  Some user agents and
   proxy services may support only a subset of these signals, while
   service providers may require additional signals.  A mechanism of
   negotiation may be needed for communicating these requirements.

   In addition, service providers should only require a signal within
   the scope it will be used.  In the same way that service provides
   only require user authentication when the user requests access to a
   non-public resource, a signal should not be pre-emptively requested
   before it is needed.  The categories of interaction described above
   may help define scopes within a service, and they may help
   communicate to the user the reasoning for requiring a signal.

4.1.2.  Privacy Considerations

   A signal should not be required without clear justification, service
   providers should practice data minimization [RFC6973] wherever
   possible.  Requiring excessive signals may be more harmful to user
   privacy than requiring IP address transparency.  This section
   provides a more details analysis of some signals.

   ADDRESS_ESCROW gives service providers a time period within which
   they may obtain the client's IP address, but the information-in-
   escrow is not immediately available.  Service providers should not
   gain access to the information in secret.  A service provider may
   misuse the information-in-escrow for tracking and privacy-invasion

   PEER_INTEGRITY partitions users into two groups with valid and
   invalid hardware/software state, at a minimum.  If the signal reveals
   more information, then it may allow more granular tracking of small
   sets of devices.

   IDENTITY_TRANSPARENCY may expose significant information about a user
   to a service provider; the resulting privacy invasion may be
   significantly worse than IP address transparency causes.

   IS_HUMAN depends on the mechanism used for proving humanness.

   REIDENTIFICATION explicitly allows a service provider to associate
   requests across unlinkable connections.  This signal allows for
   profiling user behavior and tracking user activity without requesting
   more identifying information.  First-party reidentification is a use
   case for this signal.

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   REPUTATION partitions users into a set based on their reputation.
   The privacy invasion associated with this signal is intentionally

   SOURCE_ASN allows for identifying request patterns originating from
   an ASN without providing IP address transparency.  However, ASNs are
   not guaranteed to serve large populations, therefore revealing the
   source ASN of a request may reveal more information about the user
   than intended.

4.1.3.  Provenance

   Replacement signals are only useful if they are trustworthy.


4.1.4.  Applying Appropriate Signals

   As previous discussed, IP addresses are used for various reasons;
   therefore, describing a one-size-fits-all replacement signal is not
   appropriate.  In addition, the quality and quantity of replacement
   signals needed by a service depends on the category of interaction of
   its users and potential attacks on the service.

   As an example, the attacks listed above in Section 3.1.1 can be
   organized into six groups based on the signals that may sufficiently
   replace IP addresses:


       *  advertising fraud (e.g., click-fraud)

       *  phishing

       *  scraping (e.g., e-commerce, search)

       *  spam (e.g., email, comments)

   2.  IS_HUMAN, REPUTATION, REIDENTIFICATION, ecosystem improvements

       *  account takeover


       *  influence (e.g., brigading, astroturfing)

   4.  publisher norms, (publisher) IDENTITY_TRANSPARENCY,

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       *  disinformation operations (e.g., detecting scaled and/or
          coordinated attacks)

   5.  publisher norms, (publisher) IDENTITY_TRANSPARENCY,

       *  real-world harm (e.g., child abuse)

   6.  IDENTITY_TRANSPARENCY, protocol improvements

       *  financial fraud (e.g., stolen credit cards, email account

   The remaining two attack categories fall outside of the scope of this

   *  malware/ransomware (e.g., detecting C2 connections)

   *  vulnerability exploitation (e.g., "hacking")

   Note, IP addresses do not provide a perfect signal in their existing
   usage, and the above replacement signals do not provide a better
   signal in all cases.

4.2.  Evaluation of existing technologies

   Technologies exist that are designed to solve some of the problems
   described in this document.

   Privacy Pass [I-D.ietf-privacypass-protocol] is a useful building
   block for solving numerous problems.  Its design involves an
   interaction between a client and server where, at the end, the client
   is issued a set of anonymous tokens.  These tokens may be redeemed at
   a later time, and this redemption should not be linkable with the
   initial issuance interaction.  One existing use case is substituting
   a CAPTCHA challenge with a token, where successfully solving a
   CAPTCHA challenge results in a client being issued a set of anonymous
   tokens, and these tokens may be used in the future to bypass solving
   another CAPTCHA challenge.  Therefore, Privacy Pass may be acceptable
   as an IS_HUMAN signal by some service providers.  The current token
   design can't carry additional metadata like a user's reputation or an
   expiration date, and the tokens are not bound to an identity.  The
   unlinkability property of the tokens is dependent on the
   implementation of key consistency [I-D.wood-key-consistency].

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   Trust Token [TRUSTTOKEN] is an extension of Privacy Pass where the
   issuance and redemption functionality are provided in the browser
   setting.  The tokens are allowed to carry public and private metadata
   as extensions.

   Private Access Tokens [I-D.private-access-tokens] provide a technique
   for partitioning clients based on a per-origin policy within a time
   period.  Its use cases include rate-limiting access to content and
   geo-location.  PATs could be used as a REIDENTIFICATION signal or a
   replacement signal for GeoIP, depending on requirements.

4.3.  Potential new technologies

5.  Security Considerations


6.  IANA Considerations

   This document has no IANA actions.

7.  References

7.1.  Normative References

   [RFC4949]  Shirey, R., "Internet Security Glossary, Version 2",
              FYI 36, RFC 4949, DOI 10.17487/RFC4949, August 2007,

   [RFC5782]  Levine, J., "DNS Blacklists and Whitelists", RFC 5782,
              DOI 10.17487/RFC5782, February 2010,

   [RFC6269]  Ford, M., Ed., Boucadair, M., Durand, A., Levis, P., and
              P. Roberts, "Issues with IP Address Sharing", RFC 6269,
              DOI 10.17487/RFC6269, June 2011,

   [RFC6973]  Cooper, A., Tschofenig, H., Aboba, B., Peterson, J.,
              Morris, J., Hansen, M., and R. Smith, "Privacy
              Considerations for Internet Protocols", RFC 6973,
              DOI 10.17487/RFC6973, July 2013,

7.2.  Informative References

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              "Apple iCloud Private Relay", n.d.,

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              "Global Network Address Translation Combined with Audited
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              Celi, S., Davidson, A., and A. Faz-Hernandez, "Privacy
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              Kinnear, E., McManus, P., Pauly, T., Verma, T., and C. A.
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              Hendrickson, S., Iyengar, J., Pauly, T., Valdez, S., and
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              Thomson, M. and C. A. Wood, "Oblivious HTTP", Work in
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              Bujlow, T., Carela-Espanol, V., Lee, B., and P. Barlet-
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Authors' Addresses

   Matthew Finkel
   The Tor Project

   Email: sysrqb@torproject.org

   Bradford Lassey

   Email: lassey@chromium.org

   Luigi Iannone
   Huawei Technologies France S.A.S.U

   Email: luigi.iannone@huawei.com

   J. Bradley Chen

   Email: bradchen@google.com

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