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RFC 8002

Proposed STD
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Host Identity Protocol Certificates

Obsoletes:    6253
Updates:    7401


Top       ToC       Page 1 
Internet Engineering Task Force (IETF)                           T. Heer
Request for Comments: 8002               Albstadt-Sigmaringen University
Obsoletes: 6253                                              S. Varjonen
Updates: 7401                                     University of Helsinki
Category: Standards Track                                   October 2016
ISSN: 2070-1721


                  Host Identity Protocol Certificates

Abstract

   The Certificate (CERT) parameter is a container for digital
   certificates.  It is used for carrying these certificates in Host
   Identity Protocol (HIP) control packets.  This document specifies the
   certificate parameter and the error signaling in case of a failed
   verification.  Additionally, this document specifies the
   representations of Host Identity Tags (HITs) in X.509 version 3 (v3).

   The concrete use cases of certificates, including how certificates
   are obtained and requested and which actions are taken upon
   successful or failed verification, are specific to the scenario in
   which the certificates are used.  Hence, the definition of these
   scenario-specific aspects is left to the documents that use the CERT
   parameter.

   This document updates RFC 7401 and obsoletes RFC 6253.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc8002.

[Page 2] 
Copyright Notice

   Copyright (c) 2016 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
   (http://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.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
   2.  CERT Parameter  . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  X.509 v3 Certificate Object and Host Identities . . . . . . .   5
   4.  Revocation of Certificates  . . . . . . . . . . . . . . . . .   6
   5.  Error Signaling . . . . . . . . . . . . . . . . . . . . . . .   7
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   8.  Differences from RFC 6253 . . . . . . . . . . . . . . . . . .   8
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .   9
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  10
   Appendix A.  X.509 v3 Certificate Example . . . . . . . . . . . .  11
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  13
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  13

Top      ToC       Page 3 
1.  Introduction

   Digital certificates bind pieces of information to a public key by
   means of a digital signature and thus enable the holder of a private
   key to generate cryptographically verifiable statements.  The Host
   Identity Protocol (HIP) [RFC7401] defines a new cryptographic
   namespace based on asymmetric cryptography.  The identity of each
   host is derived from a public key, allowing hosts to digitally sign
   data and issue certificates with their private key.  This document
   specifies the CERT parameter, which is used to transmit digital
   certificates in HIP.  It fills the placeholder specified in
   Section 5.2 of [RFC7401] and thus updates [RFC7401].

1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   RFC 2119 [RFC2119].

2.  CERT Parameter

   The CERT parameter is a container for certain types of digital
   certificates.  It does not specify any certificate semantics.
   However, it defines supplementary parameters that help HIP hosts to
   transmit semantically grouped CERT parameters in a more systematic
   way.  The specific use of the CERT parameter for different use cases
   is intentionally not discussed in this document.  Hence, the use of
   the CERT parameter will be defined in the documents that use the CERT
   parameter.

   The CERT parameter is covered and protected, when present, by the HIP
   SIGNATURE field and is a non-critical parameter.

   The CERT parameter can be used in all HIP packets.  However, using it
   in the first Initiator (I1) packet is NOT RECOMMENDED because it can
   increase the processing times of I1s, which can be problematic when
   processing storms of I1s.  Each HIP control packet MAY contain
   multiple CERT parameters, each carrying one certificate.  These
   parameters MAY be related or unrelated.  Related certificates are
   managed in CERT groups.  A CERT group specifies a group of related
   CERT parameters that SHOULD be interpreted in a certain order (e.g.,
   for expressing certificate chains).  Ungrouped certificates exhibit a
   unique CERT group field and set the CERT count to 1.  CERT parameters
   with the same group number in the CERT group field indicate a logical
   grouping.  The CERT count field indicates the number of CERT
   parameters in the group.

Top      ToC       Page 4 
   CERT parameters that belong to the same CERT group MAY be contained
   in multiple sequential HIP control packets.  This is indicated by a
   higher CERT count than the amount of CERT parameters with matching
   CERT group fields in a HIP control packet.  The CERT parameters MUST
   be placed in ascending order, within a HIP control packet, according
   to their CERT group field.  CERT groups MAY only span multiple
   packets if the CERT group does not fit the packet.  A HIP packet MUST
   NOT contain more than one incomplete CERT group that continues in the
   next HIP control packet.

   The CERT ID acts as a sequence number to identify the certificates in
   a CERT group.  The numbers in the CERT ID field MUST start from 1 up
   to CERT count.

   The CERT group and CERT ID namespaces are managed locally by each
   host that sends CERT parameters in HIP control packets.

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |             Type              |             Length            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |  CERT group   |  CERT count   |    CERT ID    |   CERT type   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                          Certificate                          /
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     /                               |   Padding (variable length)   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

     Type          768
     Length        Length in octets, excluding Type, Length, and
                   Padding.
     CERT group    Group ID grouping multiple related CERT parameters.
     CERT count    Total count of certificates that are sent, possibly
                   in several consecutive HIP control packets.
     CERT ID       The sequence number for this certificate.
     CERT Type     Indicates the type of the certificate.
     Padding       Any Padding, if necessary, to make the TLV a multiple
                   of 8 bytes.  Any added padding bytes MUST be zeroed
                   by the sender, and their values SHOULD NOT be checked
                   by the receiver.

   The certificates MUST use the algorithms defined in [RFC7401] as the
   signature and hash algorithms.

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   The following certificate types are defined:

             +--------------------------------+-------------+
             |          CERT format           | Type number |
             +--------------------------------+-------------+
             |            Reserved            |      0      |
             |            X.509 v3            |      1      |
             |           Obsoleted            |      2      |
             |    Hash and URL of X.509 v3    |      3      |
             |           Obsoleted            |      4      |
             |      LDAP URL of X.509 v3      |      5      |
             |           Obsoleted            |      6      |
             | Distinguished Name of X.509 v3 |      7      |
             |           Obsoleted            |      8      |
             +--------------------------------+-------------+

   The next sections outline the use of HITs in X.509 v3.  X.509 v3
   certificates and the handling procedures are defined in [RFC5280].
   The wire format for X.509 v3 is the Distinguished Encoding Rules
   format as defined in [X.690].

   Hash and Uniform Resource Locator (URL) encoding (3) is used as
   defined in Section 3.6 of [RFC7296].  Using hash and URL encodings
   result in smaller HIP control packets than by including the
   certificate(s) but requires the receiver to resolve the URL or check
   a local cache against the hash.

   Lightweight Directory Access Protocol (LDAP) URL encoding (5) is used
   as defined in [RFC4516].  Using LDAP URL encoding results in smaller
   HIP control packets but requires the receiver to retrieve the
   certificate or check a local cache against the URL.

   Distinguished Name (DN) encoding (7) is represented by the string
   representation of the certificate's subject DN as defined in
   [RFC4514].  Using the DN encoding results in smaller HIP control
   packets but requires the receiver to retrieve the certificate or
   check a local cache against the DN.

3.  X.509 v3 Certificate Object and Host Identities

   If needed, HITs can represent an issuer, a subject, or both in X.509
   v3.  HITs are represented as IPv6 addresses as defined in [RFC7343].
   When the Host Identifier (HI) is used to sign the certificate, the
   respective HIT SHOULD be placed into the Issuer Alternative Name
   (IAN) extension using the GeneralName form iPAddress as defined in
   [RFC5280].  When the certificate is issued for a HIP host, identified
   by a HIT and an HI, the respective HIT SHOULD be placed into the

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   Subject Alternative Name (SAN) extension using the GeneralName form
   iPAddress, and the full HI is presented as the subject's public key
   info as defined in [RFC5280].

   The following examples illustrate how HITs are presented as the
   issuer and subject in the X.509 v3 extension alternative names.

       Format of X509v3 extensions:
           X509v3 Issuer Alternative Name:
               IP Address:hit-of-issuer
           X509v3 Subject Alternative Name:
               IP Address:hit-of-subject

       Example X509v3 extensions:
           X509v3 Issuer Alternative Name:
               IP Address:2001:24:6cf:fae7:bb79:bf78:7d64:c056
           X509v3 Subject Alternative Name:
               IP Address:2001:2c:5a14:26de:a07c:385b:de35:60e3

   Appendix A shows a full example X.509 v3 certificate with HIP
   content.

   As another example, consider a managed Public Key Infrastructure
   (PKI) environment in which the peers have certificates that are
   anchored in (potentially different) managed trust chains.  In this
   scenario, the certificates issued to HIP hosts are signed by
   intermediate Certification Authorities (CAs) up to a root CA.  In
   this example, the managed PKI environment is neither HIP aware nor
   can it be configured to compute HITs and include them in the
   certificates.

   When HIP communications are established, the HIP hosts not only need
   to send their identity certificates (or pointers to their
   certificates) but also the chain of intermediate CAs (or pointers to
   the CAs) up to the root CA, or to a CA that is trusted by the remote
   peer.  This chain of certificates SHOULD be sent in a CERT group as
   specified in Section 2.  The HIP peers validate each other's
   certificates and compute peer HITs based on the certificate public
   keys.

4.  Revocation of Certificates

   Revocation of X.509 v3 certificates is handled as defined in
   Section 5 of [RFC5280] with two exceptions.  First, any HIP
   certificate serial number that appears on the Certificate Revocation
   List (CRL) is treated as invalid regardless of the reason code.
   Second, the certificateHold is not supported.

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5.  Error Signaling

   If the Initiator does not send all the certificates that the
   Responder requires, the Responder may take actions (e.g., reject the
   connection).  The Responder MAY signal this to the Initiator by
   sending a HIP NOTIFY message with NOTIFICATION parameter error type
   CREDENTIALS_REQUIRED.

   If the verification of a certificate fails, a verifier MAY signal
   this to the provider of the certificate by sending a HIP NOTIFY
   message with NOTIFICATION parameter error type INVALID_CERTIFICATE.

     NOTIFICATION PARAMETER - ERROR TYPES     Value
     ------------------------------------     -----

     CREDENTIALS_REQUIRED                      48

     The Responder is unwilling to set up an association,
     as the Initiator did not send the needed credentials.

     INVALID_CERTIFICATE                       50

     Sent in response to a failed verification of a certificate.
     Notification Data MAY contain a CERT group and CERT ID octet
     (in this order) of the CERT parameter that caused the
     failure.

6.  IANA Considerations

   This document defines the CERT parameter for HIP [RFC7401].  The CERT
   parameter type number (768) is defined in [RFC7401].

   The CERT parameter has an 8-bit unsigned integer field for different
   certificate types, for which IANA has created and maintains a
   subregistry entitled "HIP Certificate Types" under "Host Identity
   Protocol (HIP) Parameters".  Values for the "HIP Certificate Types"
   registry are given in Section 2.  New values for the Certificate
   types from the unassigned space are assigned through IETF Review.

   In Section 5, this document defines two types for the "NOTIFY Message
   Types" subregistry under "Host Identity Protocol (HIP) Parameters".

   As this document obsoletes [RFC6253], references to [RFC6253] in IANA
   registries have been replaced by references to this document.  This
   document changes the "HIP Certificate Types" registry in Section 2.

Top      ToC       Page 8 
   The following updates to the "HIP Certificate Types" registry have
   been made.

      The references have been updated from [RFC6253] to this document.

      This document obsoleted the type numbers "2", "4", "6", and "8"
      for the Simple Public Key Infrastructure (SPKI) certificates.

7.  Security Considerations

   Certificate grouping allows the certificates to be sent in multiple
   consecutive packets.  This might allow similar attacks, as IP-layer
   fragmentation allows, for example, the sending of fragments in the
   wrong order and skipping some fragments to delay or stall packet
   processing by the victim in order to use resources (e.g., CPU or
   memory).  Hence, hosts SHOULD implement mechanisms to discard
   certificate groups with outstanding certificates if state space is
   scarce.

   Although the CERT parameter is allowed in the I1 packet, it is NOT
   RECOMMENDED because it can increase the processing times of I1s,
   which can be problematic when processing storms of I1s.  Furthermore,
   the Initiator has to take into consideration that the Responder can
   drop the CERT parameter in I1 without processing the parameter.

   Checking of the URL and LDAP entries might allow denial-of-service
   (DoS) attacks, where the target host may be subjected to bogus work.

   Security considerations for X.509 v3 are discussed in [RFC5280].

8.  Differences from RFC 6253

   This section summarizes the technical changes made from [RFC6253].
   This section is informational and is intended to help implementors of
   the previous protocol version.  If any text in this section
   contradicts text in other portions of this specification, the text
   found outside of this section should be considered normative.

   The following change has been made.

   o  Support for SPKI certificates has been removed.

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9.  References

9.1.  Normative References

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

   [RFC4514]  Zeilenga, K., Ed., "Lightweight Directory Access Protocol
              (LDAP): String Representation of Distinguished Names",
              RFC 4514, DOI 10.17487/RFC4514, June 2006,
              <http://www.rfc-editor.org/info/rfc4514>.

   [RFC4516]  Smith, M., Ed. and T. Howes, "Lightweight Directory Access
              Protocol (LDAP): Uniform Resource Locator", RFC 4516,
              DOI 10.17487/RFC4516, June 2006,
              <http://www.rfc-editor.org/info/rfc4516>.

   [RFC5280]  Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
              Housley, R., and W. Polk, "Internet X.509 Public Key
              Infrastructure Certificate and Certificate Revocation List
              (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
              <http://www.rfc-editor.org/info/rfc5280>.

   [RFC7296]  Kaufman, C., Hoffman, P., Nir, Y., Eronen, P., and T.
              Kivinen, "Internet Key Exchange Protocol Version 2
              (IKEv2)", STD 79, RFC 7296, DOI 10.17487/RFC7296, October
              2014, <http://www.rfc-editor.org/info/rfc7296>.

   [RFC7343]  Laganier, J. and F. Dupont, "An IPv6 Prefix for Overlay
              Routable Cryptographic Hash Identifiers Version 2
              (ORCHIDv2)", RFC 7343, DOI 10.17487/RFC7343, September
              2014, <http://www.rfc-editor.org/info/rfc7343>.

   [RFC7401]  Moskowitz, R., Ed., Heer, T., Jokela, P., and T.
              Henderson, "Host Identity Protocol Version 2 (HIPv2)",
              RFC 7401, DOI 10.17487/RFC7401, April 2015,
              <http://www.rfc-editor.org/info/rfc7401>.

   [X.690]    ITU-T, , "Information Technology - ASN.1 encoding rules:
              Specification of Basic Encoding Rules (BER), Canonical
              Encoding Rules (CER) and Distinguished Encoding Rules
              (DER)", ITU-T Recommendation X.690 | ISO/IEC 8825-1,
              August 2015.

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9.2.  Informative References

   [RFC6253]  Heer, T. and S. Varjonen, "Host Identity Protocol
              Certificates", RFC 6253, DOI 10.17487/RFC6253, May 2011,
              <http://www.rfc-editor.org/info/rfc6253>.

Top      ToC       Page 11 
Appendix A.  X.509 v3 Certificate Example

   This section shows an X.509 v3 certificate with encoded HITs.

   Certificate:
       Data:
           Version: 3 (0x2)
           Serial Number: 12705268244493839545 (0xb0522e27291b2cb9)
       Signature Algorithm: sha256WithRSAEncryption
           Issuer: DC=Example, DC=com, CN=Example issuing host
           Validity
               Not Before: Feb 25 11:28:29 2016 GMT
               Not After : Feb 24 11:28:29 2017 GMT
           Subject: DC=Example, DC=com, CN=Example issuing host
           Subject Public Key Info:
               Public Key Algorithm: rsaEncryption
                   Public-Key: (2048 bit)
                   Modulus:
                       00:c9:b0:85:94:af:1f:3a:77:39:c9:d5:81:a5:ee:
                       d2:b5:6b:72:91:5d:22:2c:1e:59:e5:06:29:bd:a2:
                       19:f6:ac:ca:eb:f7:88:d8:54:55:41:01:58:d8:87:
                       64:d8:c8:cf:6e:c2:38:81:22:1a:ae:e9:a6:80:22:
                       03:ee:f3:1b:7e:68:11:e3:f4:7b:98:33:28:bf:40:
                       ec:4f:19:e8:10:8a:8b:07:60:f7:9f:e4:82:f8:a7:
                       58:04:3d:42:07:c8:34:ca:99:6d:11:eb:73:c1:d9:
                       96:93:55:e5:c7:ed:80:4f:8a:f2:1a:6f:83:c8:15:
                       a4:8f:b8:6a:fe:f3:4f:49:1a:5c:1f:89:bb:30:e6:
                       98:bc:ce:a3:a2:37:85:b1:79:1c:26:e6:44:0c:b9:
                       3e:d8:37:81:46:f4:02:25:46:a2:ea:da:25:5c:46:
                       a2:a3:c5:58:80:53:1f:c5:e5:11:a0:da:d8:f2:ad:
                       d6:98:d4:ce:55:35:cc:0b:d3:5b:09:48:ef:57:65:
                       80:cb:65:79:fd:cb:4d:5b:b3:8d:1a:ff:2a:58:3e:
                       96:65:10:3e:04:81:78:2b:d5:ca:89:78:ea:28:5c:
                       bc:02:4a:54:cd:aa:a9:99:8d:d6:39:e9:5e:a9:73:
                       1a:5d:93:55:39:9b:72:1a:c2:a0:1f:e3:4c:b0:41:
                       98:97
                   Exponent: 65537 (0x10001)
           X509v3 extensions:
               X509v3 Subject Alternative Name:
                   IP Address:2001:27:DCFC:CB8:F885:D53F:4E63:48B7
               X509v3 Issuer Alternative Name:
                   IP Address:2001:2D:F878:64C1:67E3:9716:88BD:68E4

Top      ToC       Page 12 
       Signature Algorithm: sha256WithRSAEncryption
            6d:e6:a9:a6:30:c4:ab:3e:86:39:1e:de:76:4d:4e:a4:2d:63:
            4d:bb:41:bf:d3:0c:66:13:8b:4d:b2:50:59:36:fc:ae:42:9e:
            c8:a0:41:1a:1c:94:56:05:28:82:34:4e:63:75:87:31:25:67:
            36:a6:1a:0f:b8:f7:db:03:e7:dd:a6:9a:26:c4:68:e2:cf:59:
            54:e6:ee:cc:a7:ce:fb:56:bf:31:60:f4:cb:e7:f0:0e:50:f8:
            b7:c5:3c:1a:de:74:d0:aa:83:e5:15:25:b1:bf:be:a4:7f:af:
            0a:de:08:09:0e:13:1d:2a:3b:1a:99:d9:af:10:fc:08:92:5f:
            d8:d0:10:d6:b9:0c:86:da:85:3b:44:b5:97:90:10:02:4f:5a:
            1f:ae:07:30:6b:f5:e6:12:93:72:e2:10:c9:8e:2c:00:8b:d6:
            f0:05:c3:ff:91:24:69:6d:5b:5a:0c:40:28:01:f2:5b:45:b8:
            9b:ae:9e:73:e9:dd:83:e0:85:d7:ad:6c:b1:81:ac:a0:30:37:
            9d:60:bd:92:3b:d2:a1:21:87:8b:c4:d9:5a:5c:21:56:3e:02:
            7e:f3:6f:a5:de:40:75:80:f5:41:68:5c:b2:61:fb:1d:9a:a5:
            97:a8:d4:a9:82:45:86:79:3c:63:76:3d:fd:86:a0:f8:14:84:
            55:c1:8c:fa

   -----BEGIN CERTIFICATE-----
   MIIDWTCCAkGgAwIBAgIJALBSLicpGyy5MA0GCSqGSIb3DQEBCwUAME0xFzAVBgoJ
   kiaJk/IsZAEZFgdFeGFtcGxlMRMwEQYKCZImiZPyLGQBGRYDY29tMR0wGwYDVQQD
   ExRFeGFtcGxlIGlzc3VpbmcgaG9zdDAeFw0xNjAyMjUxMTI4MjlaFw0xNzAyMjQx
   MTI4MjlaME0xFzAVBgoJkiaJk/IsZAEZFgdFeGFtcGxlMRMwEQYKCZImiZPyLGQB
   GRYDY29tMR0wGwYDVQQDExRFeGFtcGxlIGlzc3VpbmcgaG9zdDCCASIwDQYJKoZI
   hvcNAQEBBQADggEPADCCAQoCggEBAMmwhZSvHzp3OcnVgaXu0rVrcpFdIiweWeUG
   Kb2iGfasyuv3iNhUVUEBWNiHZNjIz27COIEiGq7ppoAiA+7zG35oEeP0e5gzKL9A
   7E8Z6BCKiwdg95/kgvinWAQ9QgfINMqZbRHrc8HZlpNV5cftgE+K8hpvg8gVpI+4
   av7zT0kaXB+JuzDmmLzOo6I3hbF5HCbmRAy5Ptg3gUb0AiVGouraJVxGoqPFWIBT
   H8XlEaDa2PKt1pjUzlU1zAvTWwlI71dlgMtlef3LTVuzjRr/Klg+lmUQPgSBeCvV
   yol46ihcvAJKVM2qqZmN1jnpXqlzGl2TVTmbchrCoB/jTLBBmJcCAwEAAaM8MDow
   GwYDVR0RBBQwEocQIAEAJ9z8DLj4hdU/TmNItzAbBgNVHRIEFDAShxAgAQAt+Hhk
   wWfjlxaIvWjkMA0GCSqGSIb3DQEBCwUAA4IBAQBt5qmmMMSrPoY5Ht52TU6kLWNN
   u0G/0wxmE4tNslBZNvyuQp7IoEEaHJRWBSiCNE5jdYcxJWc2phoPuPfbA+fdppom
   xGjiz1lU5u7Mp877Vr8xYPTL5/AOUPi3xTwa3nTQqoPlFSWxv76kf68K3ggJDhMd
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   -----END CERTIFICATE-----

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Acknowledgments

   The authors would like to thank A. Keranen, D. Mattes, M. Komu, and
   T. Henderson for the fruitful conversations on the subject.
   D. Mattes most notably contributed the non-HIP-aware use case in
   Section 3.

Authors' Addresses

   Tobias Heer
   Albstadt-Sigmaringen University
   Poststr. 6
   72458 Albstadt
   Germany

   Email: heer@hs-albsig.de


   Samu Varjonen
   University of Helsinki
   Gustaf Haellstroemin katu 2b
   00560 Helsinki
   Finland

   Email: samu.varjonen@helsinki.fi