UAS Remote ID
draft-ietf-drip-rid-04

DRIP                                                        R. Moskowitz
Internet-Draft                                            HTT Consulting
Updates: 7401, 7343 (if approved)                                S. Card
Intended status: Standards Track                         A. Wiethuechter
Expires: May 5, 2021                                  AX Enterprize, LLC
                                                               A. Gurtov
                                                    Linköping University
                                                        November 1, 2020

                             UAS Remote ID
                         draft-ietf-drip-rid-04

Abstract

   This document describes the use of Hierarchical Host Identity Tags
   (HHITs) as self-asserting IPv6 addresses and thereby a trustable
   Identifier for use as the UAS Remote ID.  HHITs self-attest to the
   included explicit hierarchy that provides Registrar discovery for
   3rd-party ID attestation.

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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   This Internet-Draft will expire on May 5, 2021.

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   extracted from this document must include Simplified BSD License text
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Nontransferablity of HHITs  . . . . . . . . . . . . . . .   3
   2.  Terms and Definitions . . . . . . . . . . . . . . . . . . . .   3
     2.1.  Requirements Terminology  . . . . . . . . . . . . . . . .   3
     2.2.  Notation  . . . . . . . . . . . . . . . . . . . . . . . .   4
     2.3.  Definitions . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Hierarchical HITs as Remote ID  . . . . . . . . . . . . . . .   5
     3.1.  Remote ID as one class of Hierarchical HITs . . . . . . .   5
     3.2.  Hierarchy in ORCHID Generation  . . . . . . . . . . . . .   5
     3.3.  Hierarchical HIT Registry . . . . . . . . . . . . . . . .   6
     3.4.  Remote ID Authentication using HHITs  . . . . . . . . . .   6
   4.  UAS ID HHIT in DNS  . . . . . . . . . . . . . . . . . . . . .   6
   5.  Other UTM uses of HHITs . . . . . . . . . . . . . . . . . . .   7
   6.  DRIP Requirements addressed . . . . . . . . . . . . . . . . .   7
   7.  ASTM Considerations . . . . . . . . . . . . . . . . . . . . .   8
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
     9.1.  Hierarchical HIT Trust  . . . . . . . . . . . . . . . . .   9
     9.2.  Collision risks with Hierarchical HITs  . . . . . . . . .  10
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  10
     10.2.  Informative References . . . . . . . . . . . . . . . . .  11
   Appendix A.  EU U-Space RID Privacy Considerations  . . . . . . .  12
   Appendix B.  The Hierarchical Host Identity Tag (HHIT)  . . . . .  13
     B.1.  HHIT prefix . . . . . . . . . . . . . . . . . . . . . . .  13
     B.2.  HHIT Suite IDs  . . . . . . . . . . . . . . . . . . . . .  13
       B.2.1.  8 bit HIT Suite IDs . . . . . . . . . . . . . . . . .  14
     B.3.  The Hierarchy ID (HID)  . . . . . . . . . . . . . . . . .  14
       B.3.1.  The Registered Assigning Authority (RAA)  . . . . . .  14
       B.3.2.  The Hierarchical HIT Domain Authority (HDA) . . . . .  15
   Appendix C.  ORCHIDs for Hierarchical HITs  . . . . . . . . . . .  15
     C.1.  Adding additional information to the ORCHID . . . . . . .  15
     C.2.  ORCHID Encoding . . . . . . . . . . . . . . . . . . . . .  17
       C.2.1.  Encoding ORCHIDs for HITv2  . . . . . . . . . . . . .  17
     C.3.  ORCHID Decoding . . . . . . . . . . . . . . . . . . . . .  18
     C.4.  Decoding ORCHIDs for HITv2  . . . . . . . . . . . . . . .  18
   Appendix D.  Edward Digital Signature Algorithm for HITs  . . . .  18
     D.1.  HOST_ID . . . . . . . . . . . . . . . . . . . . . . . . .  19
     D.2.  HIT_SUITE_LIST  . . . . . . . . . . . . . . . . . . . . .  19
   Appendix E.  Example HHIT Self Attestation  . . . . . . . . . . .  20
     E.1.  HHIT Offline Self Attestation . . . . . . . . . . . . . .  21
   Appendix F.  Calculating Collision Probabilities  . . . . . . . .  21

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   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  22
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  22

1.  Introduction

   [drip-requirements] describes a UAS ID as a "unique (ID-4), non-
   spoofable (ID-5), and identify a registry where the ID is listed (ID-
   2)"; all within a 20 character Identifier (ID-1).

   This document describes the use of Hierarchical HITs (HHITs)
   (Appendix B) as self-asserting IPv6 addresses and thereby a trustable
   Identifier for use as the UAS Remote ID.  HHITs include explicit
   hierarchy to provide Registrar discovery for 3rd-party ID
   attestation.

   HITs are statistically unique through the cryptographic hash feature
   of second-preimage resistance.  The cryptographically-bound addition
   of the Hierarchy and a HHIT registration process (TBD; e.g. based on
   Extensible Provisioning Protocol, [RFC5730]) provide complete, global
   HHIT uniqueness.  This is in contrast to general IDs (e.g. a UUID or
   device serial number) as the subject in an X.509 certificate.

   In a multi-CA PKI, a subject can occur in multiple CAs, possibly
   fraudulently.  CAs within the PKI would need to implement an approach
   to enforce assurance of uniqueness.

   Hierarchical HITs provide self attestation of the HHIT registry.  A
   HHIT can only be in a single registry within a registry system (e.g.
   EPP and DNS).

   Hierarchical HITs are valid, though non-routable, IPv6 addresses.  As
   such, they fit in many ways within various IETF technologies.

1.1.  Nontransferablity of HHITs

   HIs and its HHITs SHOULD NOT be transferable between UA or even
   between replacement electronics for a UA.  The private key for the HI
   SHOULD be held in a cryptographically secure component.

2.  Terms and Definitions

2.1.  Requirements Terminology

   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 BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

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2.2.  Notation

   |  Signifies concatenation of information - e.g., X | Y is the
      concatenation of X and Y.

2.3.  Definitions

   See [drip-requirements] for common DRIP terms.

   cSHAKE (The customizable SHAKE function):
      Extends the SHAKE scheme to allow users to customize their use of
      the function.

   HDA (Hierarchical HIT Domain Authority):
      The 16 bit field identifying the HHIT Domain Authority under an
      RAA.

   HHIT
      Hierarchical Host Identity Tag.  A HIT with extra hierarchical
      information not found in a standard HIT.

   HI
      Host Identity.  The public key portion of an asymmetric keypair
      used in HIP.

   HID (Hierarchy ID):
      The 32 bit field providing the HIT Hierarchy ID.

   HIP
      Host Identity Protocol.  The origin of HI, HIT, and HHIT, required
      for DRIP.  Optional full use of HIP enables additional DRIP
      functionality.

   HIT
      Host Identity Tag.  A 128 bit handle on the HI.  HITs are valid
      IPv6 addresses.

   Keccak (KECCAK Message Authentication Code):
      The family of all sponge functions with a KECCAK-f permutation as
      the underlying function and multi-rate padding as the padding
      rule.

   RAA (Registered Assigning Authority):
      The 16 bit field identifying the business or organization that
      manages a registry of HDAs.

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   RVS (Rendezvous Server):
      The HIP Rendezvous Server for enabling mobility, as defined in
      [RFC8004].

   SHAKE (Secure Hash Algorithm KECCAK):
      A secure hash that allows for an arbitrary output length.

   XOF (eXtendable-Output Function):
      A function on bit strings (also called messages) in which the
      output can be extended to any desired length.

3.  Hierarchical HITs as Remote ID

   Hierarchical HITs are a refinement on the Host Identity Tag (HIT) of
   HIPv2 [RFC7401].  HHITs require a new ORCHID mechanism as described
   in Appendix C.  HHITs for UAS ID also use the new EdDSA/SHAKE128 HIT
   suite defined in Appendix D (requirements GEN-2).  This hierarchy,
   cryptographically embedded within the HHIT, provides the information
   for finding the UA's HHIT registry (ID-3).

   The current ASTM [F3411-19] specifies three UAS ID types:

   TYPE-1  A static, manufacturer assigned, hardware serial number per
           ANSI/CTA-2063-A "Small Unmanned Aerial System Serial Numbers"
           [CTA2063A].

   TYPE-2  A CAA assigned (presumably static) ID.

   TYPE-3  A UTM system assigned UUID [RFC4122], which can but need not
           be dynamic.

   For HHITs to be used effectively as UAS IDs, F3411-19 SHOULD add UAS
   ID type 4 as HHIT.

3.1.  Remote ID as one class of Hierarchical HITs

   UAS Remote ID may be one of a number of uses of HHITs.  As such these
   follow-on uses need to be considered in allocating the RAAs
   Appendix B.3.1 or HHIT prefix assignments Section 8.

3.2.  Hierarchy in ORCHID Generation

   ORCHIDS, as defined in [RFC7343], do not cryptographically bind the
   IPv6 prefix nor the Orchid Generation Algorithm (OGA) ID (the HIT
   Suite ID) to the hash of the HI.  The justification then was attacks
   against these fields are DoS attacks against protocols using them.

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   HHITs, as defined in Appendix C, cryptographically bind all content
   in the ORCHID through the hashing function.  Thus a recipient of a
   HHIT that has the underlying HI can directly act on all content in
   the HHIT.  This provides a strong, self attestation for using the
   hierarchy to find the HHIT Registry.

3.3.  Hierarchical HIT Registry

   HHITs are registered to Hierarchical HIT Domain Authorities (HDAs).
   A registration process (TBD) ensures UAS ID global uniqueness (ID-4).
   It also provides the mechanism to create UAS Public/Private data
   associated with the HHIT UAS ID (REG-1 and REG-2).

   The 2 levels of hierarchy within the HHIT allows for CAAs to have
   their own Registered Assigning Authority (RAA) for their National Air
   Space (NAS).  Within the RAA, the CAAs can delegate HDAs as needed.
   There may be other RAAs allowed to operate within a given NAS; this
   is a policy decision by the CAA.

3.4.  Remote ID Authentication using HHITs

   The EdDSA25519 Host Identity (HI) [Appendix D] underlying the HHIT
   can be used in an 84 byte self proof attestation as shown in
   Appendix E to provide proof of Remote ID ownership (requirements GEN-
   1).  An Internet lookup service like DNS can provide the HI and
   registration proof (requirements GEN-3).

   Similarly the 200 byte offline self attestation shown in Appendix E.1
   provide the same proofs without Internet access and with a small
   cache that contains the HDA's HI/HHIT and HDA meta-data.  These self
   attestations are carried in the ASTM Authentication Message (Msg Type
   0x2).

   Hashes of previously sent ASTM messages can be placed in a signed
   "Manifest" Authentication Message (requirements GEN-2).  This can be
   either a standalone Authentication Message, or an enhanced self
   attestation Authentication Message.  Alternatively the ASTM Message
   Pack (Msg Type 0xF) can provide this feature, but only over Bluetooth
   5 or WiFi NAN broadcasts.

4.  UAS ID HHIT in DNS

   There are 2 approaches for storing and retrieving the HHIT from DNS.
   These are:

   *  As FQDNs in the .aero TLD.

   *  Reverse DNS lookups as IPv6 addresses per [RFC8005].

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   The HHIT can be used to construct an FQDN that points to the USS that
   has the Public/Private information for the UA (REG-1 and REG-2).  For
   example the USS for the HHIT could be found via the following.
   Assume the RAA is 100 and the HDA is 50.  The PTR record is
   constructed as:

       100.50.hhit.uas.aero   IN PTR      foo.uss.aero.

   The individual HHITs are potentially too numerous (e.g. 60 - 600M)
   and dynamic to actually store in a signed, DNS zone.  The HDA SHOULD
   provide DNS service for its zone and provide the HHIT detail
   response.

   The HHIT reverse lookup can be a standard IPv6 reverse look up, or it
   can leverage off the HHIT structure.  Assume a Prefix of
   2001:30::/28, the RAA is 10 and the HDA is 20 and the HHIT is:

       2001:30:a0:145:a3ad:1952:ad0:a69e

   An HHIT reverse lookup could be to:

       a69e.ad0.1952.a3ad.145.a0.30.2001.20.10.hhit.arpa.

   A 'standard' ip6.arpa RR has the advantage of only one Registry
   service supported.

       $ORIGIN  5.4.1.0.0.a.0.0.0.3.0.0.1.0.0.2.ip6.arpa.
       e.9.6.a.0.d.a.0.2.5.9.1.d.a.3.a    IN   PTR

5.  Other UTM uses of HHITs

   HHITs can be used extensively within the UTM architecture beyond UA
   ID (and USS in UA ID registration and authentication).  This includes
   a GCS HHIT ID.  The GCS could use its HIIT if it is the source of
   Network Remote ID for securing the transport and for secure C2
   transport [drip-secure-nrid-c2].

   Observers SHOULD have HHITs to facilitate UAS information retrieval
   (e.g., for authorization to private UAS data).  They could also use
   their HHIT for establishing a HIP connection with the UA Pilot for
   direct communications per authorization.  Further, they can be used
   by FINDER observers, [crowd-sourced-rid].

6.  DRIP Requirements addressed

   This document provides solutions to GEN 1 - 3, ID 1 - 5, and REG 1 -
   2.

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7.  ASTM Considerations

   ASTM will need to make the following changes to the "UA ID" in the
   Basic Message (Msg Type 0x0):

   Type 4:
      This document UA ID of Hierarchical HITs (see Section 3).

8.  IANA Considerations

   IANA will need to make the following changes to the "Host Identity
   Protocol (HIP) Parameters" registries:

   Host ID:
      This document defines the new EdDSA Host ID (see Appendix D.1).

   HIT Suite ID:
      This document defines the new HIT Suite of EdDSA/cSHAKE (see
      Appendix D.2).

   HIT Suite ID:
      This document defines two new HDA domain HIT Suites (see
      Appendix B.2.1).

   Because HHIT format is not compatible with [RFC7343], IANA is
   requested to allocated a new 28-bit prefix out of the IANA IPv6
   Special Purpose Address Block, namely 2001:0000::/23, as per
   [RFC6890].

9.  Security Considerations

   A 64 bit hash space presents a real risk of second pre-image attacks
   Section 9.2.  The HHIT Registry services effectively block attempts
   to "take over" a HHIT.  It does not stop a rogue attempting to
   impersonate a known HHIT.  This attack can be mitigated by the
   receiver of the HHIT using DNS to find the HI for the HHIT.

   Another mitigation of HHIT hijacking is if the HI owner (UA) supplies
   an object containing the HHIT and signed by the HI private key of the
   HDA such as Appendix E.1 as shown in Section 3.4.

   The two risks with hierarchical HITs are the use of an invalid HID
   and forced HIT collisions.  The use of a DNS zone (e.g.
   "hhit.arpa.") is a strong protection against invalid HIDs.  Querying
   an HDA's RVS for a HIT under the HDA protects against talking to
   unregistered clients.  The Registry service has direct protection
   against forced or accidental HIT hash collisions.

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   Cryptographically Generated Addresses (CGAs) provide a unique
   assurance of uniqueness.  This is two-fold.  The address (in this
   case the UAS ID) is a hash of a public key and a Registry hierarchy
   naming.  Collision resistance (more important that it implied second-
   preimage resistance) makes it statistically challenging to attacks.
   A registration process (TBD) within the HDA provides a level of
   assured uniqueness unattainable without mirroring this approach.

   The second aspect of assured uniqueness is the digital signing
   (attestation) process of the HHIT by the HI private key and the
   further signing (attestation) of the HI public key by the Registry's
   key.  This completes the ownership process.  The observer at this
   point does not know WHAT owns the HHIT, but is assured, other than
   the risk of theft of the HI private key, that this UAS ID is owned by
   something and is properly registered.

9.1.  Hierarchical HIT Trust

   The HHIT UAS RID in the ASTM Basic Message (Msg Type 0x0, the actual
   Remote ID message) does not provide any assertion of trust.  The best
   that might be done within this Basic Message is 4 bytes truncated
   from a HI signing of the HHIT (the UA ID field is 20 bytes and a HHIT
   is 16).  This is not trustable.  Minimally, it takes 84 bytes,
   Appendix E, to prove ownership of a HHIT.

   The ASTM Authentication Messages (Msg Type 0x2) as shown in
   Section 3.4 can provide practical actual ownership proofs.  These
   attestations include timestamps to defend against replay attacks.
   But in themselves, they do not prove which UA actually sent the
   message.  They could have been sent by a dog running down the street
   with a Broadcast Remote ID device strapped to its back.

   Proof of UA transmission comes when the Authentication Message
   includes proofs for the ASTM Location/Vector Message (Msg Type 0x1)
   and the observer can see the UA or that information is validated by
   ground multilateration [crowd-sourced-rid].  Only then does an
   observer gain full trust in the HHIT Remote ID.

   HHIT Remote IDs obtained via the Network Remote ID path provides a
   different approach to trust.  Here the UAS SHOULD be securely
   communicating to the USS (see [drip-secure-nrid-c2]), thus asserting
   HHIT RID trust.

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9.2.  Collision risks with Hierarchical HITs

   The 64 bit hash size does have an increased risk of collisions over
   the 96 bit hash size used for the other HIT Suites.  There is a 0.01%
   probability of a collision in a population of 66 million.  The
   probability goes up to 1% for a population of 663 million.  See
   Appendix F for the collision probability formula.

   However, this risk of collision is within a single "Additional
   Information" value, i.e. a RAA/HDA domain.  The UAS/USS registration
   process should include registering the HHIT and MUST reject a
   collision, forcing the UAS to generate a new HI and thus HHIT and
   reapplying to the registration process.

10.  References

10.1.  Normative References

   [F3411-19] ASTM International, "Standard Specification for Remote ID
              and Tracking", February 2020,
              <http://www.astm.org/cgi-bin/resolver.cgi?F3411>.

   [NIST.SP.800-185]
              Kelsey, J., Change, S., and R. Perlner, "SHA-3 derived
              functions: cSHAKE, KMAC, TupleHash and ParallelHash",
              National Institute of Standards and Technology report,
              DOI 10.6028/nist.sp.800-185, December 2016,
              <https://doi.org/10.6028/nist.sp.800-185>.

   [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>.

   [RFC6890]  Cotton, M., Vegoda, L., Bonica, R., Ed., and B. Haberman,
              "Special-Purpose IP Address Registries", BCP 153,
              RFC 6890, DOI 10.17487/RFC6890, April 2013,
              <https://www.rfc-editor.org/info/rfc6890>.

   [RFC8032]  Josefsson, S. and I. Liusvaara, "Edwards-Curve Digital
              Signature Algorithm (EdDSA)", RFC 8032,
              DOI 10.17487/RFC8032, January 2017,
              <https://www.rfc-editor.org/info/rfc8032>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

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10.2.  Informative References

   [corus]    CORUS, "U-space Concept of Operations", September 2019,
              <https://www.sesarju.eu/node/3411>.

   [crowd-sourced-rid]
              Moskowitz, R., Card, S., Wiethuechter, A., Zhao, S., and
              H. Birkholz, "Crowd Sourced Remote ID", Work in Progress,
              Internet-Draft, draft-moskowitz-drip-crowd-sourced-rid-04,
              May 20, 2020, <https://tools.ietf.org/html/draft-
              moskowitz-drip-crowd-sourced-rid-04>.

   [CTA2063A] ANSI, "Small Unmanned Aerial Systems Serial Numbers",
              September 2019.

   [drip-requirements]
              Card, S., Wiethuechter, A., Moskowitz, R., and A. Gurtov,
              "Drone Remote Identification Protocol (DRIP)
              Requirements", Work in Progress, Internet-Draft, draft-
              ietf-drip-reqs-05, October 16, 2020,
              <https://tools.ietf.org/html/draft-ietf-drip-reqs-05>.

   [drip-secure-nrid-c2]
              Moskowitz, R., Card, S., Wiethuechter, A., and A. Gurtov,
              "Secure UAS Network RID and C2 Transport", Work in
              Progress, Internet-Draft, draft-moskowitz-drip-secure-
              nrid-c2-01, September 27, 2020,
              <https://tools.ietf.org/html/draft-moskowitz-drip-secure-
              nrid-c2-01>.

   [Keccak]   Bertoni, G., Daemen, J., Peeters, M., Van Assche, G., and
              R. Van Keer, "The Keccak Function",
              <https://keccak.team/index.html>.

   [RFC4122]  Leach, P., Mealling, M., and R. Salz, "A Universally
              Unique IDentifier (UUID) URN Namespace", RFC 4122,
              DOI 10.17487/RFC4122, July 2005,
              <https://www.rfc-editor.org/info/rfc4122>.

   [RFC5730]  Hollenbeck, S., "Extensible Provisioning Protocol (EPP)",
              STD 69, RFC 5730, DOI 10.17487/RFC5730, August 2009,
              <https://www.rfc-editor.org/info/rfc5730>.

   [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, <https://www.rfc-editor.org/info/rfc7343>.

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   [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,
              <https://www.rfc-editor.org/info/rfc7401>.

   [RFC8004]  Laganier, J. and L. Eggert, "Host Identity Protocol (HIP)
              Rendezvous Extension", RFC 8004, DOI 10.17487/RFC8004,
              October 2016, <https://www.rfc-editor.org/info/rfc8004>.

   [RFC8005]  Laganier, J., "Host Identity Protocol (HIP) Domain Name
              System (DNS) Extension", RFC 8005, DOI 10.17487/RFC8005,
              October 2016, <https://www.rfc-editor.org/info/rfc8005>.

Appendix A.  EU U-Space RID Privacy Considerations

   EU is defining a future of airspace management known as U-space
   within the Single European Sky ATM Research (SESAR) undertaking.
   Concept of Operation for EuRopean UTM Systems (CORUS) project
   proposed low-level Concept of Operations [corus] for UAS in EU.  It
   introduces strong requirements for UAS privacy based on European GDPR
   regulations.  It suggests that UAs are identified with agnostic IDs,
   with no information about UA type, the operators or flight
   trajectory.  Only authorized persons should be able to query the
   details of the flight with a record of access.

   Due to the high privacy requirements, a casual observer can only
   query U-space if it is aware of a UA seen in a certain area.  A
   general observer can use a public U-space portal to query UA details
   based on the UA transmitted "Remote identification" signal.  Direct
   remote identification (DRID) is based on a signal transmitted by the
   UA directly.  Network remote identification (NRID) is only possible
   for UAs being tracked by U-Space and is based on the matching the
   current UA position to one of the tracks.

   The project lists "E-Identification" and "E-Registrations" services
   as to be developed.  These services can follow the privacy mechanism
   proposed in this document.  If an "agnostic ID" above refers to a
   completely random identifier, it creates a problem with identity
   resolution and detection of misuse.  On the other hand, a classical
   HIT has a flat structure which makes its resolution difficult.  The
   Hierarchical HITs provide a balanced solution by associating a
   registry with the UA identifier.  This is not likely to cause a major
   conflict with U-space privacy requirements, as the registries are
   typically few at a country level (e.g. civil personal, military, law
   enforcement, or commercial).

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Appendix B.  The Hierarchical Host Identity Tag (HHIT)

   The Hierarchical HIT (HHIT) is a small but important enhancement over
   the flat HIT space.  By adding two levels of hierarchical
   administration control, the HHIT provides for device registration/
   ownership, thereby enhancing the trust framework for HITs.

   HHITs represent the HI in only a 64 bit hash and uses the other 32
   bits to create a hierarchical administration organization for HIT
   domains.  Hierarchical HIT construction is defined in Appendix C.
   The input values for the Encoding rules are in Appendix C.1.

   A HHIT is built from the following fields:

   *  IANA prefix (max 28 bit)

   *  32 bit Hierarchy ID (HID)

   *  4 (or 8) bit HIT Suite ID

   *  ORCHID hash (96 - prefix length - Suite ID length bits, e.g. 64)
      See Appendix C

   The Context ID for the ORCHID hash is:

       Context ID :=  0x00B5 A69C 795D F5D5 F008 7F56 843F 2C40

B.1.  HHIT prefix

   A unique IANA IPv6 prefix, no larger than 28 bit, for HHITs is
   recommended.  It clearly separates the flat-space HIT processing from
   HHIT processing per Appendix C.

   Without a unique prefix, the first 4 bits of the RRA would be
   interpreted as the HIT Suite ID per HIPv2 [RFC7401].

B.2.  HHIT Suite IDs

   The HIT Suite IDs specifies the HI and hash algorithms.  Any HIT
   Suite ID can be used for HHITs.  The 8 bit format is supported (only
   when the first 4 bits are ZERO), but this reduces the ORCHID hash
   length.

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B.2.1.  8 bit HIT Suite IDs

   Support for 8 bit HIT Suite IDs is allowed in Sec 5.2.10, [RFC7401],
   but not specified in how ORCHIDs are generated with these longer
   OGAs.  Appendix C provides the algorithmic flexiblity, allowing for
   HDA custom HIT Suite IDs as follows:

        HIT Suite       Four-bit ID    Eight-bit encoding
        HDA Assigned 1            NA             0x0E
        HDA Assigned 2            NA             0x0F

   This feature may be used for large-scale experimenting with post
   quantum computing hashes or similar domain specific needs.  Note that
   currently there is no support for domain specific HI algorithms.

B.3.  The Hierarchy ID (HID)

   The Hierarchy ID (HID) provides the structure to organize HITs into
   administrative domains.  HIDs are further divided into 2 fields:

   *  16 bit Registered Assigning Authority (RAA)

   *  16 bit Hierarchical HIT Domain Authority (HDA)

B.3.1.  The Registered Assigning Authority (RAA)

   An RAA is a business or organization that manages a registry of HDAs.
   For example, the Federal Aviation Authority (FAA) could be an RAA.

   The RAA is a 16 bit field (65,536 RAAs) assigned by a numbers
   management organization, perhaps ICANN's IANA service.  An RAA must
   provide a set of services to allocate HDAs to organizations.  It must
   have a public policy on what is necessary to obtain an HDA.  The RAA
   need not maintain any HIP related services.  It must maintain a DNS
   zone minimally for discovering HID RVS servers.

   As HHITs may be used in many different domains, RAA should be
   allocated in blocks with consideration on the likely size of a
   particular usage.  Alternatively, different Prefixes can be used to
   separate different domains of use of HHTs.

   This DNS zone may be a PTR for its RAA.  It may be a zone in a HHIT
   specific DNS zone.  Assume that the RAA is 100.  The PTR record could
   be constructed:

   100.hhit.arpa   IN PTR      raa.bar.com.

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B.3.2.  The Hierarchical HIT Domain Authority (HDA)

   An HDA may be an ISP or any third party that takes on the business to
   provide RVS and other needed services for HIP enabled devices.

   The HDA is an 16 bit field (65,536 HDAs per RAA) assigned by an RAA.
   An HDA should maintain a set of RVS servers that its client HIP-
   enabled customers use.  How this is done and scales to the
   potentially millions of customers is outside the scope of this
   document.  This service should be discoverable through the DNS zone
   maintained by the HDA's RAA.

   An RAA may assign a block of values to an individual organization.
   This is completely up to the individual RAA's published policy for
   delegation.

Appendix C.  ORCHIDs for Hierarchical HITs

   This section improves on ORCHIDv2 [RFC7343] with three enhancements:

   *  Optional Info field between the Prefix and OGA ID.

   *  Increased flexibility on the length of each component in the
      ORCHID construction, provided the resulting ORCHID is 128 bits.

   *  Use of cSHAKE, NIST SP 800-185 [NIST.SP.800-185], for the hashing
      function.

   The [Keccak] based cSHAKE XOF hash function is a variable output
   length hash function.  As such it does not use the truncation
   operation that other hashes need.  The invocation of cSHAKE specifies
   the desired number of bits in the hash output.  Further, cSHAKE has a
   parameter 'S' as a customization bit string.  This parameter will be
   used for including the ORCHID Context Identifier in a standard
   fashion.

   This ORCHID construction includes the fields in the ORCHID in the
   hash to protect them against substitution attacks.  It also provides
   for inclusion of additional information, in particular the
   hierarchical bits of the Hierarchical HIT, in the ORCHID generation.
   This should be viewed as an addendum to ORCHIDv2 [RFC7343], as it can
   produce ORCHIDv2 output.

C.1.  Adding additional information to the ORCHID

   ORCHIDv2 [RFC7343] is currently defined as consisting of three
   components:

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   ORCHID     :=  Prefix | OGA ID | Encode_96( Hash )

   where:

   Prefix          : A constant 28-bit-long bitstring value
                     (IANA IPv6 assigned).

   OGA ID          : A 4-bit long identifier for the Hash_function
                     in use within the specific usage context.  When
                     used for HIT generation this is the HIT Suite ID.

   Encode_96( )    : An extraction function in which output is obtained
                     by extracting the middle 96-bit-long bitstring
                     from the argument bitstring.

   This addendum will be constructed as follows:

   ORCHID     :=  Prefix (p) | Info (n) | OGA ID (o) | Hash (m)

   where:

   Prefix (p)      : An IANA IPv6 assigned prefix (max 28-bit-long).

   Info (n)        : n bits of information that define a use of the
                     ORCHID.  n can be zero, that is no additional
                     information.

   OGA ID (o)      : A 4 or 8 bit long identifier for the Hash_function
                     in use within the specific usage context.  When
                     used for HIT generation this is the HIT Suite ID.

   Hash (m)        : An extraction function in which output is m bits.

   p + n + o + m = 128 bits

   With a 28 bit IPv6 Prefix, the remaining 100 bits can be divided in
   any manner between the additional information, OGA ID, and the hash
   output.  Care must be taken in determining the size of the hash
   portion, taking into account risks like pre-image attacks.  Thus 64
   bits as used in Hierarchical HITs may be as small as is acceptable.

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C.2.  ORCHID Encoding

   This addendum adds a different encoding process to that currently
   used in ORCHIDv2.  The input to the hash function explicitly includes
   all the header content plus the Context ID.  The header content
   consists of the Prefix, the Additional Information, and OGA ID (HIT
   Suite ID).  Secondly, the length of the resulting hash is set by sum
   of the length of the ORCHID header fields.  For example, a 28 bit
   Prefix with 32 bits for the HID and 4 bits for the OGA ID leaves 64
   bits for the hash length.

   To achieve the variable length output in a consistent manner, the
   cSHAKE hash is used.  For this purpose, cSHAKE128 is appropriate.
   The the cSHAKE function call for this addendum is:

       cSHAKE128(Input, L, "", Context ID)

       Input      :=  Prefix | Additional Information | OGA ID | HOST_ID
       L          :=  Length in bits of hash portion of ORCHID

   For full Suite ID support (those that use fixed length hashes like
   SHA256), the following hashing can be used (Note: this does NOT
   produce output Identical to ORCHIDv2 for Prefix of /28 and Additional
   Information of ZERO length):

       Hash[L](Context ID | Input)

       Input      :=  Prefix | Additional Information | OGA ID | HOST_ID
       L          :=  Length in bits of hash portion of ORCHID

       Hash[L]    :=  An extraction function in which output is obtained
                      by extracting the middle L-bit-long bitstring
                      from the argument bitstring.

   Hierarchical HIT uses the same context as all other HIPv2 HIT Suites
   as they are clearly separated by the distinct HIT Suite ID.

C.2.1.  Encoding ORCHIDs for HITv2

   This section is included to provide backwards compatibility for
   ORCHIDv2 [RFC7343] as used for HITv2 [RFC7401].

   For HITv2s, the Prefix MUST be 2001:20::/28.  Info is length ZERO
   (not included), and OGA ID is length 4.  Thus the HI Hash is length
   96.  Further the Prefix and OGA ID are NOT included in the hash
   calculation.  Thus the following ORCHID calculations for fixed output
   length hashes are used:

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       Hash[L](Context ID | Input)

       Input      :=  HOST_ID
       L          :=  96
       Context ID :=  0xF0EF F02F BFF4 3D0F E793 0C3C 6E61 74EA

       Hash[L]    :=  An extraction function in which output is obtained
                      by extracting the middle L-bit-long bitstring
                      from the argument bitstring.

   For variable output length hashes use:

       Hash[L](Context ID | Input)

       Input      :=  HOST_ID
       L          :=  96
       Context ID :=  0xF0EF F02F BFF4 3D0F E793 0C3C 6E61 74EA

       Hash[L]    :=  The L bit output from the hash function

   Then the ORCHID is constructed as follows:

       Prefix | OGA ID | Hash Output

C.3.  ORCHID Decoding

   With this addendum, the decoding of an ORCHID is determined by the
   Prefix and OGA ID (HIT Suite ID).  ORCHIDv2 [RFC7343] decoding is
   selected when the Prefix is: 2001:20::/28.

   For Hierarchical HITs, the decoding is determined by the presence of
   the HHIT Prefix as specified in the HHIT document.

C.4.  Decoding ORCHIDs for HITv2

   This section is included to provide backwards compatibility for
   ORCHIDv2 [RFC7343] as used for HITv2 [RFC7401].

   HITv2s are identified by a Prefix of 2001:20::/28.  The next 4 bits
   are the OGA ID. is length 4.  The remaining 96 bits are the HI Hash.

Appendix D.  Edward Digital Signature Algorithm for HITs

   Edwards-Curve Digital Signature Algorithm (EdDSA) [RFC8032] are
   specified here for use as Host Identities (HIs) per HIPv2 [RFC7401].
   Further the HIT_SUITE_LIST is specified as used in [RFC7343].

   See Appendix B.2 for use of the HIT Suite for this document.

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D.1.  HOST_ID

   The HOST_ID parameter specifies the public key algorithm, and for
   elliptic curves, a name.  The HOST_ID parameter is defined in
   Section 5.2.19 of [RFC7401].

        Algorithm
        profiles         Values

        EdDSA            13 [RFC8032]       (RECOMMENDED)

   For hosts that implement EdDSA as the algorithm, the following ECC
   curves are available:

        Algorithm    Curve            Values

        EdDSA        RESERVED         0
        EdDSA        EdDSA25519       1 [RFC8032]
        EdDSA        EdDSA25519ph     2 [RFC8032]
        EdDSA        EdDSA448         3 [RFC8032]
        EdDSA        EdDSA448ph       4 [RFC8032]

D.2.  HIT_SUITE_LIST

   The HIT_SUITE_LIST parameter contains a list of the supported HIT
   suite IDs of the Responder.  Based on the HIT_SUITE_LIST, the
   Initiator can determine which source HIT Suite IDs are supported by
   the Responder.  The HIT_SUITE_LIST parameter is defined in
   Section 5.2.10 of [RFC7401].

   The following HIT Suite ID is defined, and the relationship between
   the four-bit ID value used in the OGA ID field and the eight-bit
   encoding within the HIT_SUITE_LIST ID field is clarified:

        HIT Suite       Four-bit ID    Eight-bit encoding
        RESERVED            0             0x00
        EdDSA/cSHAKE128     5             0x50           (RECOMMENDED)

   The following table provides more detail on the above HIT Suite
   combinations.  The input for each generation algorithm is the
   encoding of the HI as defined in this Appendix.

   The output of cSHAKE128 is variable per the needs of a specific
   ORCHID construction.  It is at most 96 bits long and is directly used
   in the ORCHID (without truncation).

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     +=======+===========+=========+===========+====================+
     | Index | Hash      | HMAC    | Signature | Description        |
     |       | function  |         | algorithm |                    |
     |       |           |         | family    |                    |
     +=======+===========+=========+===========+====================+
     |     5 | cSHAKE128 | KMAC128 | EdDSA     | EdDSA HI hashed    |
     |       |           |         |           | with cSHAKE128,    |
     |       |           |         |           | output is variable |
     +-------+-----------+---------+-----------+--------------------+

                           Table 1: HIT Suites

Appendix E.  Example HHIT Self Attestation

   This section shows example uses of HHIT RID to prove trustworthiness
   of the RID and attestation of registration to the RAA|HDA.  These are
   examples only and other documents will provide fully specified
   attestations.  Care has been taken in the example design to minimize
   the risk of replay attacks.

   This ownership/attestation of a HHIT can be proved in 84 bytes via
   the following HHIT Self Attestation:

   *  4 byte Signing Timestamp

   *  16 byte HHIT

   *  64 byte Signature (EdDSA25519 signature)

   The Timestamp MAY be the standard UNIX time at the time of signing.
   A protocol specific timestamp may be used to avoid programming
   complexities.  For example, [F3411-19] uses a 00:00:00 01/01/2019
   offset.

   To minimize the risk of replay, the UA SHOULD create a new Self
   Attestation, with a new timestamp, at least once a minute.  The UA
   MAY precompute these attestations and transmit during the appropriate
   1 minute window.  1 minute is chosen as a balance between attestation
   compute time against risk.  A shorter window of use lessens the risk
   of replay.

   The signature is over the 20 byte Timestamp + HHIT.

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   The receiver of such an attestation would need access to the
   underlying public key (HI) to validate the signature.  This may be
   obtained via a DNS query using the HHIT.  A larger (116 bytes) Self
   Attestation could include the EdDSA25519 HI.  This larger 116
   attestation allows for signature validation before HHIT lookup to
   prove registration attestation.

E.1.  HHIT Offline Self Attestation

   Ownership and RAA|HDA registration of a HHIT can be proved in 200
   bytes without Internet access and a small cache via the following
   HHIT Offline Self Attestation:

   *  16 byte UA HHIT

   *  32 byte UA EdDSA25519 HI

   *  4 byte HDA Signing Expiry Timestamp

   *  16 byte HDA HHIT

   *  64 byte HDA Signature (EdDSA25519 signature)

   *  4 byte UA Signing Timestamp

   *  64 byte UA Signature (EdDSA25519 signature)

   The Timestamps MAY be the standard UNIX time at the time of signing.
   A protocol specific timestamp may be used to avoid programming
   complexities.  For example, [F3411-19] uses a 00:00:00 01/01/2019
   offset.

   The HDA signature is over the 68 byte UA HHIT + UA HI + HDA Expiry
   Timestamp + HDA HHIT.  During the UA Registration process, the UA
   would provide a Self Attestation to the HDA.  The HDA would construct
   its attestation of registry with an Expiry Timestamp, its own HHIT,
   and its signature, returning a 132 byte HDA Registry Attestation to
   the UA.  The UA would use this much the same way as its HHIT only in
   the Self Attestation above, creating a 200 byte Offline Self
   Attestation.

   The receiver of such an attestation would need a cache of RAA ID, HDA
   ID, HDA HHIT, and HDA HI (min 80 bytes per RAA/HDA).

Appendix F.  Calculating Collision Probabilities

   The accepted formula for calculating the probability of a collision
   is:

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       p = 1 - e^{-k^2/(2n)}

       P   Collision Probability
       n   Total possible population
       k   Actual population

   The following table provides the approximate population size for a
   collision for a given total population.

                          Deployed Population
        Total            With Collision Risk of
        Population         .01%            1%

        2^96                 4T           42T
        2^72                 1B           10B
        2^68               250M          2.5B
        2^64                66M          663M
        2^60                16M          160M

Acknowledgments

   Dr. Gurtov is an adviser on Cybersecurity to the Swedish Civil
   Aviation Administration.

   Quynh Dang of NIST gave considerable guidance on using Keccak and the
   NIST supporting documents.  Joan Deamen of the Keccak team was
   especially helpful in many aspects of using Keccak.

Authors' Addresses

   Robert Moskowitz
   HTT Consulting
   Oak Park, MI 48237
   United States of America

   Email: rgm@labs.htt-consult.com

   Stuart W. Card
   AX Enterprize, LLC
   4947 Commercial Drive
   Yorkville, NY 13495
   United States of America

   Email: stu.card@axenterprize.com

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   Adam Wiethuechter
   AX Enterprize, LLC
   4947 Commercial Drive
   Yorkville, NY 13495
   United States of America

   Email: adam.wiethuechter@axenterprize.com

   Andrei Gurtov
   Linköping University
   IDA
   SE-58183 Linköping
   Sweden

   Email: gurtov@acm.org

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