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Versions: (draft-winfaa-intarea-broadcast-consider) 00 01 02 03 04 05 06 07 08 09 RFC 8386

Internet Engineering Task Force                                R. Winter
Internet-Draft                   University of Applied Sciences Augsburg
Intended status: Informational                                  M. Faath
Expires: July 19, 2018                                      Conntac GmbH
                                                            F. Weisshaar
                                 University of Applied Sciences Augsburg
                                                        January 15, 2018

    Privacy considerations for protocols relying on IP broadcast and


   A number of application-layer protocols make use of IP broadcasts or
   multicast messages for functions like local service discovery or name
   resolution.  Some of these functions can only be implemented
   efficiently using such mechanisms.  When using broadcasts or
   multicast messages, a passive observer in the same broadcast/
   multicast domain can trivially record these messages and analyze
   their content.  Therefore, designers of protocols that make use
   broadcast/multicast messages need to take special care when designing
   their protocols.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   Internet-Drafts are draft documents valid for a maximum of six months
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on July 19, 2018.

Copyright Notice

   Copyright (c) 2018 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Types and usage of broadcast and multicast  . . . . . . .   4
     1.2.  Requirements Language . . . . . . . . . . . . . . . . . .   4
   2.  Privacy considerations  . . . . . . . . . . . . . . . . . . .   4
     2.1.  Message frequency . . . . . . . . . . . . . . . . . . . .   5
     2.2.  Persistent identifiers  . . . . . . . . . . . . . . . . .   5
     2.3.  Anticipate user behavior  . . . . . . . . . . . . . . . .   6
     2.4.  Consider potential correlation  . . . . . . . . . . . . .   7
     2.5.  Configurability . . . . . . . . . . . . . . . . . . . . .   7
   3.  Operational considerations  . . . . . . . . . . . . . . . . .   8
   4.  Summary . . . . . . . . . . . . . . . . . . . . . . . . . . .   8
   5.  Other considerations  . . . . . . . . . . . . . . . . . . . .   9
   6.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   9
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  10
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  10
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12

1.  Introduction

   Broadcast and multicast messages have a large (and to the sender
   unknown) receiver group by design.  Because of that, these two
   mechanisms are vital for a number of basic network functions such as
   auto-configuration or link-layer address lookup.  Also application
   developers use broadcast/multicast messages to implement things like
   local service or peer discovery.  It appears that an increasing
   number of applications make use of it as suggested by experimental
   results obtained on campus networks including the IETF meeting
   network [TRAC2016].  This trend is not entirely surprising.  As RFC
   919 [RFC0919] puts it, "The use of broadcasts [...] is a good base
   for many applications".  Broadcast and multicast functionality in a
   subnetwork are therefore important as a lack thereof renders the
   protocols relying on these mechanisms inoperable [RFC3819].

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   Using broadcast/multicast can become problematic if the information
   that is being distributed can be regarded as sensitive or when the
   information that is distributed by multiple of these protocols can be
   correlated in a way that sensitive data can be derived.  This is
   clearly true for any protocol, but broadcast/multicast is special in
   at least two respects:

   (a)  The aforementioned large receiver group, consisting of receivers
        unknown to the sender.  This makes eavesdropping without special
        privileges or a special location in the network trivial for
        anybody in the broadcast/multicast domain.

   (b)  Encryption is more difficult when broadcast/multicast messages,
        leaving content of these messages in the clear and making it
        easier to spoof and replay them.

   Given the above, privacy protection for protocols based on broadcast
   or multicast communication is significantly more difficult compared
   to unicast communication and at the same time invading the privacy is
   much easier.

   Privacy considerations of IETF-specified protocols have received some
   attention in the recent past (e.g.  RFC 7721 [RFC7721] or RFC 7819
   [RFC7819]).  There is also general guidance available for document
   authors on when and how to include a privacy considerations section
   in their documents and on how to evaluate the privacy implications of
   Internet protocols [RFC6973].  RFC6973 also describes potential
   threats to privacy in great detail and lists terminology that is also
   used in this document.  In contrast to RFC6973, this document
   contains a number of privacy considerations especially for protocols
   that rely on broadcast/multicast, intended to reduce the likelihood
   that a broadcast/multicast protocol can be misused to collect
   sensitive data about devices, users and groups of users on a
   broadcast/multicast domain.

   The above mentioned considerations particularly apply to protocols
   designed outside the IETF - for two reasons.  For one, non-standard
   protocols will likely not receive operational attention and support
   in making them more secure such as e.g.  DHCP snooping does for DHCP
   because they typically are not documented.  The other reason is that
   these protocols have been designed in isolation, where a set of
   considerations to follow is useful in the absence of a larger
   community providing feedback.  In particular, carelessly designed
   protocols that use broadcast/multicast can break privacy efforts at
   different layers of the protocol stack such as MAC address or IP
   address randomization [RFC4941].

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1.1.  Types and usage of broadcast and multicast

   In IPv4, two major types of broadcast addresses exist, the limited
   broadcast which is defined as all-ones (, defined in
   section of [RFC1812]) and the directed broadcast with the
   given network prefix of an IP address and the host part of all-ones
   (defined in section of [RFC1812]).  Broadcast packets are
   received by all nodes in a subnetwork.  Limited broadcasts never
   transit a router.  The same is true for directed broadcasts by
   default, but routers MAY provide an option to do this [RFC2644].
   IPv6 on the other hand does not provide broadcast addresses but
   solely relies on multicast [RFC4291].

   In contrast to broadcast addresses, multicast addresses represent an
   identifier for a set of interfaces that can be a set different from
   all nodes in the subnetwork.  All interfaces that are identified by a
   given multicast address receive packets destined towards that address
   and are called a multicast group.  In both IPv4 and IPv6, multiple
   pre-defined multicast addresses exist.  The ones most relevant for
   this document are the ones with subnet scope.  For IPv4, an IP prefix
   is reserved for this purpose called the Local Network Control Block
   (, defined in section 4 of [RFC5771]).  For IPv6, the
   relevant multicast addresses are the two All Nodes Addresses, which
   every IPv6-capable host is required to recognize as identifying
   itself (see section 2.7.1 of [RFC4291]).

   Typical usage of these addresses include local service discovery
   (e.g. mDNS [RFC6762] and LLMNR [RFC4795] make use of multicast),
   autoconfiguration (e.g.  DHCPv4 [RFC2131] uses broadcasts and DHCPv6
   [RFC3315] uses multicast addresses) and other vital network services
   such as address resolution or duplicate address detection.  But
   besides these core network functions, also applications make use of
   broadcast and multicast functionality, often implementing proprietary
   protocols.  In sum, these protocols distribute a diverse set of
   potentially privacy sensitive information to a large receiver group.

1.2.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

2.  Privacy considerations

   There are a few obvious and a few not necessarily obvious things
   designers of protocols utilizing broadcast/multicast should consider
   in respect to the privacy implications of their protocol.  Most of

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   these items are based on protocol behavior observed as part of
   experiments on operational networks [TRAC2016].

2.1.  Message frequency

   Frequent broadcast/multicast traffic caused by an application can
   give away user behavior and online connection times.  This allows a
   passive observer to potentially deduce a user's current activity
   (e.g. a game) and it allows to create an online profile (i.e. times
   the user is on the network).  The higher the frequency of these
   messages and the duration of time these messages are sent, the more
   accurate this profile will be.  Given that broadcasts/multicasts are
   only visible in the same broadcast/multicast domain, these messages
   also give the rough location of the user away (e.g. a campus or

   This behavior has e.g. been observed by a synchronization mechanism
   of a popular application, where multiple messages have been sent per
   minute via broadcast.  Given this behavior, it is possible to record
   a device's time on the network with a sub-minute accuracy given only
   the traffic of this single application installed on the device.  But
   also services used for local name resolution in modern operating
   systems utilize broadcast/multicast protocols (e.g. mDNS, LLMNR or
   NetBIOS) to announce for example their shares regularly and allow a
   tracking of the online time of a device.

   If a protocol relies on frequent or periodic broadcast/multicast
   messages, the frequency SHOULD be chosen conservatively, in
   particular if the messages contain persistent identifiers (see next
   subsection).  Also, intelligent message suppression mechanisms such
   as the ones employed in mDNS [RFC6762] SHOULD be implemented.  The
   lower the frequency of broadcast messages, the harder passive traffic
   analysis and surveillance becomes.

2.2.  Persistent identifiers

   A few protocols that make use of broadcast/multicast messages
   observed in the wild make use of persistent identifiers.  This
   includes the use of host names or more abstract persistent
   identifiers such as a universally unique identifiers (UUID) or
   similar.  These IDs, which e.g. identify the installation of a
   certain application might not change across updates of the software
   and are therefore extremely long lived.  This allows a passive
   observer to track a user precisely if broadcast/multicast messages
   are frequent.  This is even true in case the IP and/or MAC address
   changes.  Such identifiers also allow two different interfaces (e.g.
   WiFi and Ethernet) to be correlated to the same device.  If the
   application makes use of persistent identifiers for multiple

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   installations of the same application for the same user, this even
   allows to infer that different devices belong to the same user.

   The aforementioned broadcast messages from a synchronization
   mechanism of a popular application also included a persistent
   identifier in every broadcast.  This identifier did never change
   after the application was installed and allowed to track a device
   even when it changed its network interface or when it connected to a
   different network.

   Persistent IDs are considered bad practice in general for broadcast
   and multicast communication, as persistent application layer IDs will
   make efforts on lower layers to randomize identifiers (e.g.
   [I-D.huitema-6man-random-addresses]) useless.  When protocols that
   make use of broadcast/multicast need to make use of IDs, frequent
   rotations of these IDs SHOULD be considered to make user tracking
   more difficult.

2.3.  Anticipate user behavior

   A large number of users name their device after themselves, either
   using their first name, last name or both.  Often a host name
   includes the type, model or maker of a device, its function or
   includes language specific information.  Based on data gathered
   during experiments performed at IETF meetings and at a large campus
   network, this appears currently to be prevalent user behavior
   [TRAC2016].  For protocols using the host name as part of the
   messages, this clearly will reveal personally identifiable
   information to everyone on the local network.  This information can
   also be used to mount more sophisticated attacks, when e.g. the owner
   of a device is identified (as an interesting target) or properties of
   the device are known (e.g. known vulnerabilities).

   Some of the most commonly used operating systems include the name the
   user chooses for the user account during the installation process as
   part of the host name of the device.  The name of the operating
   system can also be included, revealing therefore two pieces of
   information, which can be regarded as private information if the host
   name is used in broadcast/multicast messages.

   Where possible, the use of host names and other user-provided
   information in protocols making use of broadcast/multicast SHOULD be
   avoided.  If all that is required is a persistent identifier (which
   SHOULD be avoided, see Section 2.2), this SHOULD be generated
   randomly.  An application might want to display the information it
   will broadcast on the LAN at install/config time, so the user is at
   least aware of the application's behavior.  More host name

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   considerations can be found in [RFC8117].  More information on user
   participation can be found in RFC 6973 [RFC6973].

2.4.  Consider potential correlation

   A large number of services and applications make use of the
   broadcast/multicast mechanism.  That means there are various sources
   of information that are easily accessible by a passive observer.  In
   isolation, the information these protocols reveal might seem
   harmless, but given multiple such protocols, it might be possible to
   correlate this information.  E.g.  a protocol that uses frequent
   messages including a UUID to identify the particular installation
   does not give the identity of the user away.  But a single message
   including the user's host name might just do that and it can be
   correlated using e.g. the MAC address of the device's interface.

   In the experiments described in [TRAC2016], it was possible to
   correlate frequently sent broadcast messages that included a unique
   identifier with other broadcast/multicast messages containing
   usernames (e.g. mDNS, LLMNR or NetBIOS), but also relationships to
   other users.  This allowed to reveal the real identity of the users
   of many devices but it also gave some information about their social
   environment away.

   A designer of a protocol that makes use of broadcast/multicast needs
   to be aware of the fact that even if - in isolation - the information
   a protocol leaks seems harmless, there might be ways to correlate
   that information with information from other protocols to reveal
   sensitive information about a user.

2.5.  Configurability

   A lot of applications and services relying on broadcast/multicast
   protocols do not include the means to declare "safe" environments
   (e.g. based on the SSID of a WiFi network and the MAC addresses of
   the access points).  E.g. a device connected to a public WiFi will
   likely broadcast the same information as when connected to the home
   network.  It would be beneficial if certain behavior could be
   restricted to "safe" environments.

   A popular operating system e.g. allows the user to specify the trust
   level of the network the device connects to, which for example
   restricts specific system services (using broadcast/multicast
   messages for their normal operation) to be used in trusted networks
   only.  Such functionality could implemented as part of an

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   An application developer making use of broadcasts/multicasts as part
   of the application SHOULD make the broadcast feature, if possible,
   configurable, so that potentially sensitive information does not leak
   on public networks, where the threat to privacy is much larger.

3.  Operational considerations

   Besides changing end-user behavior, choosing sensible defaults as an
   operating system vendor (e.g. for suggesting host names) and the
   considerations for protocol designers mentioned in this document,
   there are things that the network administrators/operators can do to
   limit the above mentioned problems.

   A feature not uncommonly found on access points e.g. is to filter
   broadcast and multicast traffic.  This will potentially break certain
   applications or some of their functionality but will also protect the
   users from potentially leaking sensitive information.

4.  Summary

   Increasingly, applications rely on protocols that send and receive
   broadcast and multicast messages.  For some, broadcasts/multicasts
   are the basis of their application logic, others use broadcasts/
   multicasts to improve certain aspects of the application but are
   fully functional in case broadcasts/multicasts fail.  Irrespective of
   the role of broadcast and multicast messages for the application, the
   designers of protocols that make use of them should be very careful
   in their protocol design because of the special nature of broadcast
   and multicast.

   It is not always possible to implement certain functionality via
   unicast, but in case a protocol designer chooses to rely on
   broadcast/multicast, the following should be carefully considered:

   o  IETF-specified protocols, such as mDNS [RFC6762], SHOULD be used
      if possible as operational support might exist to protect against
      the leakage of private information.  Also, for some protocols
      privacy extensions are being specified, which can be used if
      implemented.  E.g. for DNS-SD privacy extensions are documented in

   o  Using user-specified information inside broadcast/multicast
      messages SHOULD be avoided, as users will often use personal
      information or other information aiding attackers, in particular
      if the user is unaware about how that information is being used

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   o  The use of persistent IDs in messages SHOULD be avoided, as this
      allows user tracking, correlation and potentially has a
      devastating effect on other privacy protection mechanisms

   o  If one really must design a new protocol relying on broadcast/
      multicast and cannot use an IETF-specified protocol, then:

      *  You SHOULD be very conservative in how frequently you send
         messages as an effort in data minimization

      *  You SHOULD seek advice from IETF-specified protocols such as
         message suppression in mDNS

      *  You SHOULD try to design the protocol in a way that information
         sent in broadcast/multicast messages cannot be correlated with
         information from other protocols using broadcast/multicast

      *  You SHOULD let the user configure safe environments if possible
         (e.g. based on the SSID)

5.  Other considerations

   Besides privacy implications, frequent broadcasting also represents a
   performance problem.  In particular in certain wireless technologies
   such as 802.11, broadcast and multicast are transmitted at a much
   lower rate (the lowest common denominator rate) compared to unicast
   and therefore have a much bigger impact on the overall available
   airtime [I-D.perkins-intarea-multicast-ieee802].  Further, it will
   limit the ability for devices to go to sleep if frequent broadcasts
   are being sent.  A similar problem in respect to Router
   Advertisements is addressed in
   [I-D.ietf-v6ops-reducing-ra-energy-consumption].  In that respect
   broadcasts/multicast can be used for another class of attacks that is
   not related to privacy.  The potential impact on network performance
   should nevertheless be considered when designing a protocol that
   makes use of broadcast/multicast.

6.  Acknowledgments

   We would like to thank Eliot Lear, Joe Touch and Stephane Bortzmeyer
   for their valuable input to this document.

   This work was partly supported by the European Commission under grant
   agreement FP7-318627 mPlane.  Support does not imply endorsement.

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7.  IANA Considerations

   This memo includes no request to IANA.

8.  Security Considerations

   This document deals with privacy-related considerations of broadcast-
   and multicast-based protocols.  It contains advice for designers of
   such protocols to minimize the leakage of privacy-sensitive
   information.  The intent of the advice is to make sure that
   identities will remain anonymous and user tracking will be made

   It should be noted that certain applications could make use of
   existing mechanisms to protect multicast traffic such as the ones
   defined in [RFC5374].  Examples of such applications can be found in
   Appendix A. of [RFC5374].  Given the required infrastructure and
   assumptions about these applications and the security infrastructure,
   many applications will not be able to make use of such mechanisms.

9.  References

9.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

9.2.  Informative References

              Huitema, C., "Implications of Randomized Link Layers
              Addresses for IPv6 Address Assignment", draft-huitema-
              6man-random-addresses-03 (work in progress), March 2016.

              Huitema, C. and D. Kaiser, "Privacy Extensions for DNS-
              SD", draft-ietf-dnssd-privacy-00 (work in progress),
              October 2016.

              Yourtchenko, A. and L. Colitti, "Reducing energy
              consumption of Router Advertisements", draft-ietf-v6ops-
              reducing-ra-energy-consumption-03 (work in progress),
              November 2015.

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              Perkins, C., Stanley, D., Kumari, W., and J. Zuniga,
              "Multicast Considerations over IEEE 802 Wireless Media",
              draft-perkins-intarea-multicast-ieee802-03 (work in
              progress), July 2017.

   [RFC0919]  Mogul, J., "Broadcasting Internet Datagrams", STD 5, RFC
              919, DOI 10.17487/RFC0919, October 1984,

   [RFC1812]  Baker, F., Ed., "Requirements for IP Version 4 Routers",
              RFC 1812, DOI 10.17487/RFC1812, June 1995,

   [RFC2131]  Droms, R., "Dynamic Host Configuration Protocol", RFC
              2131, DOI 10.17487/RFC2131, March 1997,

   [RFC2644]  Senie, D., "Changing the Default for Directed Broadcasts
              in Routers", BCP 34, RFC 2644, DOI 10.17487/RFC2644,
              August 1999, <http://www.rfc-editor.org/info/rfc2644>.

   [RFC3315]  Droms, R., Ed., Bound, J., Volz, B., Lemon, T., Perkins,
              C., and M. Carney, "Dynamic Host Configuration Protocol
              for IPv6 (DHCPv6)", RFC 3315, DOI 10.17487/RFC3315, July
              2003, <http://www.rfc-editor.org/info/rfc3315>.

   [RFC3819]  Karn, P., Ed., Bormann, C., Fairhurst, G., Grossman, D.,
              Ludwig, R., Mahdavi, J., Montenegro, G., Touch, J., and L.
              Wood, "Advice for Internet Subnetwork Designers", BCP 89,
              RFC 3819, DOI 10.17487/RFC3819, July 2004,

   [RFC4291]  Hinden, R. and S. Deering, "IP Version 6 Addressing
              Architecture", RFC 4291, DOI 10.17487/RFC4291, February
              2006, <http://www.rfc-editor.org/info/rfc4291>.

   [RFC4795]  Aboba, B., Thaler, D., and L. Esibov, "Link-local
              Multicast Name Resolution (LLMNR)", RFC 4795, DOI
              10.17487/RFC4795, January 2007,

   [RFC4941]  Narten, T., Draves, R., and S. Krishnan, "Privacy
              Extensions for Stateless Address Autoconfiguration in
              IPv6", RFC 4941, DOI 10.17487/RFC4941, September 2007,

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   [RFC5374]  Weis, B., Gross, G., and D. Ignjatic, "Multicast
              Extensions to the Security Architecture for the Internet
              Protocol", RFC 5374, DOI 10.17487/RFC5374, November 2008,

   [RFC5771]  Cotton, M., Vegoda, L., and D. Meyer, "IANA Guidelines for
              IPv4 Multicast Address Assignments", BCP 51, RFC 5771, DOI
              10.17487/RFC5771, March 2010,

   [RFC6762]  Cheshire, S. and M. Krochmal, "Multicast DNS", RFC 6762,
              DOI 10.17487/RFC6762, February 2013,

   [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,

   [RFC7721]  Cooper, A., Gont, F., and D. Thaler, "Security and Privacy
              Considerations for IPv6 Address Generation Mechanisms",
              RFC 7721, DOI 10.17487/RFC7721, March 2016,

   [RFC7819]  Jiang, S., Krishnan, S., and T. Mrugalski, "Privacy
              Considerations for DHCP", RFC 7819, DOI 10.17487/RFC7819,
              April 2016, <http://www.rfc-editor.org/info/rfc7819>.

   [RFC8117]  Huitema, C., Thaler, D., and R. Winter, "Current Hostname
              Practice Considered Harmful", RFC 8117, DOI 10.17487/
              RFC8117, March 2017, <https://www.rfc-editor.org/info/

              Faath, M., Weisshaar, F., and R. Winter, "How Broadcast
              Data Reveals Your Identity and Social Graph", 7th
              International Workshop on TRaffic Analysis and
              Characterization IEEE TRAC 2016, September 2016.

Authors' Addresses

   Rolf Winter
   University of Applied Sciences Augsburg

   Email: rolf.winter@hs-augsburg.de

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   Michael Faath
   Conntac GmbH

   Email: faath@conntac.net

   Fabian Weisshaar
   University of Applied Sciences Augsburg

   Email: fabian.weisshaar@hs-augsburg.de

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