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STIR                                                           E. Burger
Internet-Draft                                     Georgetown University
Intended status: Standards Track                           March 5, 2018
Expires: September 6, 2018


  Registry for Country-Specific Secure Telephone Identity (STIR) Root
                              Certificates
                     draft-burger-stir-iana-cert-00

Abstract

   This document defines an IANA registry that maps country codes to
   secure telephone identity (STIR) root certificates authorized to
   create signing certificates for telephone numbers under the authority
   of a given country.  Some countries allow carriers to block
   unsolicited, automatically generated nuisance calls commonly known as
   'robocalls.'  The use of signed STIR tokens in the Session Initiation
   Protocol (SIP) may be useful in such scenarios to provide positive
   attestations as to call origin.  Legacy telephone numbering resources
   are administrated by national policy.  Unlike the market-driven use
   case of Web commerce, some nations may restrict the list of STIR root
   certificate authorities acceptable for issuing signing certificates
   for STIR tokens that provide attestations for their local legacy
   telephone numbering resources.  The registry described in this
   document enables call recipients in a first country to validate that
   signaling it receives from a caller with a telephone number claiming
   to be in a second country conforms to the second country's policy of
   (1) having a limited list of STIR root certificate authorities (or
   not) and (2) the certificate that produced the signature over the
   signaling is signed by one of those authorized STIR root certificate
   authorities.

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
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."




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   This Internet-Draft will expire on September 6, 2018.

Copyright Notice

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

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   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.

1.  Introduction

   One problem that plagues some communications applications is where
   the caller deliberately misrepresents their identity with the intent
   to defraud, cause harm, or wrongfully obtain anything of value.  The
   IETF Secure Telephone Identity Revisited (STIR) work group has
   developed a series of RFCs specifying the mechanisms for
   cryptographically signing the asserted identity and other elements in
   Session Initiation Protocol (SIP) [RFC3261] messages.  One kind of
   identity used in SIP is a telephone number [E.164].  A telephone
   number is a string of digits, where the first one to three digits
   indicate a country code.  The International Telecommunications Union
   - Telecommunications Sector (ITU-T) defines country codes and
   delegates the authority for numbers under a country code to the
   respective national communications authority for that country, as
   listed in E.164 Annex D [E.164D].

   Section 7 of Authenticated Identity Management in the Session
   Initiation Protocol [RFC8224] describes the process for signing
   identity tokens.  Correspondingly, the STIR Certificates document
   [RFC8226] describes the format of the signing certificate.  The
   protocol and formats are independent of and can have uses beyond that
   of signing originating telephone numbers.  As well, given that for
   the most part governments are responsible for managing the numbering
   resources within their country code, governmental policy may impact
   who is authorized to issue signing certificates and what constitutes
   a valid signing chain.  As such, the base STIR documents defer
   certificate and validation policy to other documents.  This document
   describes a registry for finding the appropriate STIR root
   certificate authority for a given country code for signed telephone
   numbers.  This document neither implies nor endorses any policies for



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   non-E.164 number identity assertions, such as arbitrary SIP URI's.
   Moreover, while this document describes the STIR root certificate
   registry for various nation's STIR root certificates, it does not
   mandate any particular policy regime.

   Recalling the STIR problem statement [RFC7340], the goal is to
   provide authenticated identity for the caller.  When a SIP endpoint
   receives a message with a signed STIR token, that endpoint needs to
   know whether the signing certificate is, in fact, allowed to make
   assertions for that identity.  It does us no good for a caller with
   ill intent to have a signed assertion that has a valid certificate
   chain to an unauthorized root.  Likewise, it does us no good to use
   self-signed certificates to sign a SIP message, as even with some
   limited verification, if there is the slightest chance of an entity
   with nefarious intent to succeed in either spoofing or taking over
   the identify of a caller, experience has shown they will do so.

   As mentioned above, telephone numbers are assigned by the ITU-T to
   national communications authorities responsible for the number space
   below the numeric country code.  A national regulator can inform
   service providers under its authority which root certificate
   authorities are authoritative for numbers under its control.  This is
   straightforward within a country.  However, this does not work for
   the global, interconnected communications network.  When someone in a
   first country calls someone in a second country, how is the service
   provider or end user in the second country to know who is
   authoritative for signing certificates in the first country?

   To solve this problem, this document establishes an IANA registry of
   STIR root certificate authorities, indexed by country.  This document
   also establishes an IANA registry of numeric country codes to ISO
   3166-1 [ISO.3166-1.2013] alpha-2 country codes.

2.  Data Model

2.1.  Country Code Registry

   The ITU-T publishes a list of assigned numeric country codes in E.164
   Annex D [E.164D].  The International Standards Organization (ISO)
   publishes a list of two-character country codes in ISO 3166-1
   [ISO.3166-1.2013].  The Country Code Registry maps the telephone
   country codes to two-letter country codes.  From here on, this
   document refers to the former as "numeric country codes" and the
   latter as "ISO country codes".

   Applications are expected to do a longest-match search to find the
   ISO country code corresponding to a numeric country code.  This
   enables overlapping numeric country codes such as for +1 and +7.  Let



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   us say an enclosing numeric country code, such as +7 for the Russian
   Federation, will specify the certificates of an enclosed numeric
   country code, such as +76 for Kazakhstan.  It also enables
   overlapping countries to provide their own, distinct set of roots for
   the enclosed numeric country code or to specify they are not
   specifying any STIR root certificates.

2.2.  STIR Root Certificate Registry

   This registry maps ISO country codes to STIR root certificates.
   There can be one or more STIR root certificates per ISO country code.

2.3.  Operation

   If a country is participating, it ensures it has the appropriate
   mapping from numeric country code to ISO country code in the Country
   Code Registry.  Then, if the country does have STIR root
   certificate(s) to list, it places them in the STIR Root Certificate
   Registry.  If the country wants to indicate that it is not specifying
   STIR root certificates, it creates an entry in the Country Code
   Registry but has no entries in the STIR Root Certificate Registry.

   Besides directly indicating non-participation, this model enables
   handling of overlapping country codes.

   Take the case of an overlapping numeric country code where the
   enclosed numbering country uses the same roots as the enclosing
   numbering country.  The enclosed numbering country refrains from
   making an entry in the Country Code Registry.  For example, let us
   say Kazakhstan uses the same STIR root certificates as the Russian
   Federation.  We would expect to see

                             +---------+-----+
                             | Numeric | ISO |
                             +---------+-----+
                             |    7    |  RF |
                             +---------+-----+

   in the Country Code Registry and

                 +-----+--------------------------------+
                 | ISO |          Certificate           |
                 +-----+--------------------------------+
                 |  RF | [STIR public root certificate] |
                 +-----+--------------------------------+






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   in the STIR Root Certificate Registry.  Calls to +76 and +77 will
   match +7 in the Country Codes Registry, which maps to the string RF,
   which maps to the shared STIR root certificate.

   Take the case where Kazakhstan uses a different certificate than the
   Russian Federation.  Then we would expect to see

                             +---------+-----+
                             | Numeric | ISO |
                             +---------+-----+
                             |    7    |  RF |
                             |         |     |
                             |    76   |  KZ |
                             |         |     |
                             |    77   |  KZ |
                             +---------+-----+

   in the Country Code Registry and

               +-----+-------------------------------------+
               | ISO |             Certificate             |
               +-----+-------------------------------------+
               |  RF | [RF's STIR public root certificate] |
               |     |                                     |
               |  KZ | [KZ's STIR public root certificate] |
               +-----+-------------------------------------+

   in the STIR Root Certificate Registry.

   Finally, take the case the Russian Federation specifies authorized
   STIR root certificate authorities, but Kazakhstan does not.  Then we
   would see

                             +---------+-----+
                             | Numeric | ISO |
                             +---------+-----+
                             |    7    |  RF |
                             |         |     |
                             |    76   |  KZ |
                             |         |     |
                             |    77   |  KZ |
                             +---------+-----+

   in the Country Code Registry and







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               +-----+-------------------------------------+
               | ISO |             Certificate             |
               +-----+-------------------------------------+
               |  RF | [RF's STIR public root certificate] |
               +-----+-------------------------------------+

   in the STIR Root Certificate Registry.  Here, calls from Kazakhstan
   would match the +76 mapping, but applications will notice there are
   no KZ STIR root certificate authorities in the STIR Root Certificates
   Registry.

   The registry indicates multiple STIR root certificate authorities by
   having multiple entities with the same ISO country code and different
   STIR root certificates in the STIR Root Certificates Registry.  For
   example,

                             +---------+-----+
                             | Numeric | ISO |
                             +---------+-----+
                             |    1    |  US |
                             +---------+-----+

   in the Country Code Registry and

          +-----+-----------------------------------------------+
          | ISO |                  Certificate                  |
          +-----+-----------------------------------------------+
          |  US | [US STIR public root certificate authority A] |
          |     |                                               |
          |  US | [US STIR public root certificate authority Z] |
          +-----+-----------------------------------------------+

   in the STIR Root Certificate Registry.

3.  Registry Elements

3.1.  Numeric Country Code

   E.164 [E.164] defines the country code as a one- to three-digit
   string.  However, there are some country codes that have different
   country delegations beyond the country code.  For example, footnote b
   of E.164 Annex D [E.164D] shows 25 countries under country code +1
   and two countries under country code +7.  As well, country code +881,
   for satellite services, and codes +882 and +883, for international
   networks, are under the jurisdiction of various national authorities.

   To distinguish the various national authorities under a given country
   code, the country code entry can contain these identity codes.



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   Currently, the longest entry can be seven digits, but this could
   change in the future.

   Applications using this registry to find the ISO country code for a
   given numeric country code (and identity codes) use the longest match
   in the registry.  A potential error condition would be if a country
   has not designated a mapping in the registry and another country with
   a shorter, overlapping numeric country code string does have a
   mapping.  At the time of this writing, this is only possible for the
   overlapping country codes of +1 and +7 as well as the special use
   codes +881, +882, and +883.

   Unfortunately, there is no easy algorithm or pattern to the identity
   digits (area codes) in country code +1.  As of the time of the
   writing this document, the North American Numbering Plan
   Administrator (NANPA) reports that the United States has about 275
   area codes assigned (including free phone and local number
   portability routing), Canada has 65 area codes assigned, and the
   various Caribbean nations have 1-4 area codes assigned each
   [NPAreport].  As a further complication, the freephone number space,
   such as +1800 and +1888, is also shared.  Some countries have
   exclusive responsibility for some 800 number prefixes, such as
   +1800389 for the Bahamas and +1800271 for Trinidad.

3.2.  STIR Root CA Public Key

   Each country can have zero or more STIR root certificate authorities.
   The STIR root certificate authority is the trust anchor for STIR
   (SIP) PKI in the given jurisdiction.  The expectation is the
   authority for signing the identity of a caller will be much stricter
   than the authority for signing the identity of, for example, a Web
   site.  In the common Web browser situation, a Web server operator can
   purchase a certificate issued by one of hundreds of certificate
   authorities from anywhere in the world.  To ensure interoperability,
   browser and operating system manufacturers need to include the STIR
   root certificates from those certificate authorities so when a user
   in one part of the world accesses a Web server in another part of the
   world that has a certificate issued by a certificate authority in yet
   a different part of the world, the site will validate.  In the
   telephone number identity situation, it is expected that for the most
   part the individual national numbering authorities will choose a very
   limited set of STIR root certificate authorities who will be allowed
   to issue signing certificates for numbers assigned to that country.

   Within a single country, it would be a relatively easy matter for the
   national communications regulator to impose and inform their domestic
   service providers who is the designated certificate authority within
   that country.  However, given the large amount of international



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   telephone traffic (as an example, there were over 100,000,000,000
   minutes of traffic between the U.S. and other countries in 2014,
   including VoIP [FCC_intl]), there is a need for service providers and
   users in different countries to validate that one of the proper
   certificate authorities for that country has issued the signing
   certificate.

   The entry for each national STIR root certificate authority is a P7B
   certificate [RFC2315] that contains the public key of the STIR root
   certificate authority, matching the private key the STIR root
   certificate authority uses to sign signing keys used by its
   delegates, such as telecommunications service providers.

   Countries that are not participating in STIR but want to avoid the
   shortest-match issue raised above can create an entry in the Country
   Code registry with no entry in the STIR Root Certificate registry.

4.  Terminology

   This document uses the terms "MUST", "MUST NOT", "REQUIRED", "SHALL",
   "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and
   "OPTIONAL" as RFC 2119 [RFC2119] defines them.

5.  IANA Considerations

   Refer to [RFC8126] for a description of IANA Considerations terms and
   their meanings.

5.1.  Registry Policy: Expert Review

   This registry is Expert Review with registry-based delegation.  The
   integrity of a given nation's numbering system is generally the
   purview of the respective national government.  We do not anticipate
   IANA to intervene in disputes of who has the authority for entering
   and changing STIR root certificates.  In general, IANA SHOULD
   validate the request is related to the recognized national authority
   for the country as specified in [ITU-D.Agencies], unless it is not
   clear who the national authority is.

      TO DO: Instead of using the RAI list, should we setup a dedicated
      list for dispute resolution?

5.2.  Appealing Registry Decisions

   IANA makes decisions based on expertise as well as guidance from the
   community.  If a member of the community has a concern with an
   individual decision made by IANA with regard to the registry, the
   individual shall proceed as follows:



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   1.  Attempt to resolve the concern directly with IANA.

   2.  If a resolution cannot be reached directly with IANA, express the
       concern to the community and attempt to achieve rough consensus
       regarding a resolution on the RAI list.  The Area Directors of
       the IETF Real-time Applications and Infrastructure Areamay, at
       their discretion, attempt to guide the members of the community
       to rough consensus.

   3.  As a last resort, if a resolution cannot be reached on the RAI
       mailing list, appeal to the IESG for a resolution.  The appellant
       must show that the decision made by IANA (a) was materially in
       error and (b) has caused material harm.  In its deliberations
       regarding an appeal, the IESG shall weigh all the evidence
       presented to it and use its best judgment in determining a
       resolution.

5.3.  Registry Elements

   The STIR Root Certificate registry consists of one or more entities
   indicating the public keys of STIR root certificate authorities for a
   given country code.  With around 200 countries, each of which might
   have one to four STIR root certificate authorities, results in a
   registry with a total participation of about one thousand entries.
   The expectation is there would be substantially fewer entries in
   practice.

5.3.1.  Numeric Country Code

   The numeric country code is a one- to eight-digit string indicating
   the numeric country code and optional identity digits.  Identity
   digits are often known as an area code or city code.  [E.164D] lists
   country codes and the identity digits when there are overlapping
   country codes (+1, +7, and some international codes).

   IANA MUST verify the requested mapping includes a valid numeric
   country code as specified in E.164 Annex D.

   NOTE: The conventional leading + to indicate the string identifies a
   country code is NOT part of the Country Code element in the registry.

5.3.2.  ISO Country Code

   The ISO country code is a two-character string drawn from ISO 3166-1
   alpha-2 [ISO.3166-1.2013].

   IANA should verify the requested mapping includes a valid two-digit
   country code appropriate for the requested numeric country code,



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   subject to the understanding that a country's numeric country code
   may map to an enclosing ISO country code if there is no longer match
   in the Country Code Registry.  IANA MAY verify whether there is a
   need to place entries for enclosed numeric country codes if an
   enclosing Country Code mapping is established.  This is only an issue
   for numeric country codes in +1, +7, +881, +882, and +883 at the time
   of this writing.

5.3.3.  STIR Root Certificate

   The STIR root certificate is a P7B file [RFC2315] that contains the
   public key of the authorized STIR root certificate that signs the
   certificates authorized to sign STIR signaling in the given country.
   There can be one or more entries in the registry for a given ISO
   country code to allow for multiple STIR root certificate authorities
   for a given country.

   IANA MUST verify the certificate is valid.

5.4.  Other IANA Considerations

   The expectation is the relevant national authorities or their
   designates will keep IANA informed on updates to things such as
   numbering plans.  This is most prominently an issue in numeric
   country code +1, where the numbering administrator often assigns new
   area codes, which could end up in different countries.  Specifically,
   IANA has no obligation to monitor the ITU-T, North American Numbering
   Plan Administrator (NANPA), or other entity to keep the Country Code
   Registry up to date.  It should be noted there is a single NANPA for
   the entire +1 numeric country code.

   At the time of this writing, we expect both the United States and
   Canada to be specifying a limited set of STIR root certificate
   authorities.  The most difficult overlap set is the overlap between
   Canada and the United States in the numeric country code list.  As a
   convenience to the community we request IANA pre-populate the Country
   Code Registry with +1 mapped to the string US and to pre-populate the
   Country Code Registry with the area codes assigned to Canada with the
   string CA, as found in the authoritative listing of +1 area code
   assignments [NPAreport].  As an example, but not necessarily the
   normative entries:










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                             +---------+-----+
                             | Numeric | ISO |
                             +---------+-----+
                             |    1    |  US |
                             |         |     |
                             |   1204  |  CA |
                             |         |     |
                             |   1226  |  CA |
                             |         |     |
                             |   1236  |  CA |
                             |         |     |
                             |   ...   | ... |
                             +---------+-----+

6.  Security Considerations

   The choice of having the STIR root certificate stored by IANA means
   that users accessing the certificates MUST use a source-authenticated
   retrieval mechanism, such as HTTPS [RFC7231].  It almost goes without
   saying implementers should be using the most up-to-date TLS
   implementation (or its successor) when retrieving registry elements
   from IANA.  Likewise, the application resolving the URI MUST verify
   the domain in the certificate matches the IANA domain.  The
   application resolving the URI MUST use DNSSEC [RFC4035] if it is
   available to the client.  Finally, during TLS negotiation the
   application MUST verify the authority signing IANA's certificate
   matches the application's understanding of who is expected to sign
   IANA's certificate.  At the time of this writing, that root
   certificate would be the DigiCert High Assurance EV Root CA.

7.  Acknowledgements

   Russ Housley and Sean Turner helped with the decision of registering
   certificates instead of URIs.  Ken Carlberg and Padma Krishnaswamy of
   the United States Federal Communications Commission provided useful
   feedback in an incredibly short time period.  Finally, a huge thank-
   you to Michelle Cotton and Kim Davies for helping normalize the
   registries and the procedures for populating them.

8.  References

8.1.  Normative References

   [E.164D]   International Telecommunications Union, "List of ITU-T
              Recommendation E.164 Assigned Country Codes",
              ITU-T Recommendation E.164 Annex D, 11 2011,
              <https://www.itu.int/dms_pub/itu-t/opb/sp/
              T-SP-E.164D-2016-PDF-E.pdf>.



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   [ISO.3166-1.2013]
              International Organization for Standardization, "Codes for
              the representation of names of countries and their
              subdivisions -- Part 1: Country codes, 3rd edition",
              ISO Standard 3166-1, 11 2013.

   [ITU-D.Agencies]
              International Telecommunications Union - Development
              Sector, "National Telecommunication Agencies", 12 2017,
              <http://www.itu.int/en/ITU-D/Statistics/Pages/links/
              nta.aspx>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC2315]  Kaliski, B., "PKCS #7: Cryptographic Message Syntax
              Version 1.5", RFC 2315, DOI 10.17487/RFC2315, March 1998,
              <https://www.rfc-editor.org/info/rfc2315>.

   [RFC4035]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "Protocol Modifications for the DNS Security
              Extensions", RFC 4035, DOI 10.17487/RFC4035, March 2005,
              <https://www.rfc-editor.org/info/rfc4035>.

   [RFC7231]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
              DOI 10.17487/RFC7231, June 2014,
              <https://www.rfc-editor.org/info/rfc7231>.

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/info/rfc8126>.

8.2.  Informative References

   [E.164]    International Telecommunications Union, "The International
              Public Telecommunication Numbering Plan",
              ITU-T Recommendation E.164, 11 2010,
              <https://www.itu.int/rec/dologin_pub.asp?lang=e&id=T-REC-
              E.164-201011-I!!PDF-E&type=items>.








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   [FCC_intl]
              Ashton, S. and L. Blake, "2014 U.S. International
              Telecommunications Traffic and Revenue Data", 7 2016, <htt
              p://transition.fcc.gov/Daily_Releases/Daily_Business/2016/
              db0701/DOC-340121A1.pdf>.

   [NPAreport]
              North American Numbering Plan Administrator, "NPA
              Database", 12 2017,
              <https://www.nationalnanpa.com/nanp1/npa_report.csv>.

   [RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
              A., Peterson, J., Sparks, R., Handley, M., and E.
              Schooler, "SIP: Session Initiation Protocol", RFC 3261,
              DOI 10.17487/RFC3261, June 2002,
              <https://www.rfc-editor.org/info/rfc3261>.

   [RFC7340]  Peterson, J., Schulzrinne, H., and H. Tschofenig, "Secure
              Telephone Identity Problem Statement and Requirements",
              RFC 7340, DOI 10.17487/RFC7340, September 2014,
              <https://www.rfc-editor.org/info/rfc7340>.

   [RFC8224]  Peterson, J., Jennings, C., Rescorla, E., and C. Wendt,
              "Authenticated Identity Management in the Session
              Initiation Protocol (SIP)", RFC 8224,
              DOI 10.17487/RFC8224, February 2018,
              <https://www.rfc-editor.org/info/rfc8224>.

   [RFC8226]  Peterson, J. and S. Turner, "Secure Telephone Identity
              Credentials: Certificates", RFC 8226,
              DOI 10.17487/RFC8226, February 2018,
              <https://www.rfc-editor.org/info/rfc8226>.

Author's Address

   Eric W. Burger
   Georgetown University
   37th & O St, NW
   Washington, DC  20057
   USA

   Email: eburger@standardstrack.com









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