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DRIP Entity Tag (DET) for Unmanned Aircraft System Remote Identification (UAS RID)
draft-ietf-drip-rid-15

The information below is for an old version of the document.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 9374.
Authors Robert Moskowitz , Stuart W. Card , Adam Wiethuechter , Andrei Gurtov
Last updated 2021-12-08
Replaces draft-ietf-drip-uas-rid
RFC stream Internet Engineering Task Force (IETF)
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Stream WG state In WG Last Call
Revised I-D Needed - Issue raised by WGLC
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Sep 2020
Solution space documents adopted by the WG
Dec 2021
Submit DRIP UAS Remote ID to the IESG
Document shepherd Mohamed Boucadair
Shepherd write-up Show Last changed 2021-12-02
IESG IESG state Became RFC 9374 (Proposed Standard)
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Send notices to mohamed.boucadair@orange.com
draft-ietf-drip-rid-15
DRIP                                                        R. Moskowitz
Internet-Draft                                            HTT Consulting
Updates: 7401, 7343 (if approved)                                S. Card
Intended status: Standards Track                         A. Wiethuechter
Expires: 11 June 2022                                 AX Enterprize, LLC
                                                               A. Gurtov
                                                    Linköping University
                                                         8 December 2021

DRIP Entity Tag (DET) for Unmanned Aircraft System Remote Identification
                               (UAS RID)
                         draft-ietf-drip-rid-15

Abstract

   This document describes the use of Hierarchical Host Identity Tags
   (HHITs), updating both [RFC7401] and [RFC7343], as self-asserting
   IPv6 addresses and thereby a trustable identifier for use as the
   Unmanned Aircraft System Remote Identification and tracking (UAS
   RID).  Within the context of RID, HHITs will be called DRIP Entity
   Tags (DET).  HHITs self-attest to the included explicit hierarchy
   that provides Registrar discovery for 3rd-party identifier
   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
   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."

   This Internet-Draft will expire on 11 June 2022.

Copyright Notice

   Copyright (c) 2021 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (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 Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terms and Definitions . . . . . . . . . . . . . . . . . . . .   4
     2.1.  Requirements Terminology  . . . . . . . . . . . . . . . .   4
     2.2.  Notations . . . . . . . . . . . . . . . . . . . . . . . .   4
     2.3.  Definitions . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  The Hierarchical Host Identity Tag (HHIT) . . . . . . . . . .   5
     3.1.  HHIT Prefix for RID Purposes  . . . . . . . . . . . . . .   6
     3.2.  HHIT Suite IDs  . . . . . . . . . . . . . . . . . . . . .   6
       3.2.1.  8-bit HIT Suite IDs . . . . . . . . . . . . . . . . .   7
     3.3.  The Hierarchy ID (HID)  . . . . . . . . . . . . . . . . .   7
       3.3.1.  The Registered Assigning Authority (RAA)  . . . . . .   7
       3.3.2.  The Hierarchical HIT Domain Authority (HDA) . . . . .   8
     3.4.  Edward Digital Signature Algorithm for HITs . . . . . . .   8
       3.4.1.  HOST_ID . . . . . . . . . . . . . . . . . . . . . . .   8
       3.4.2.  HIT_SUITE_LIST  . . . . . . . . . . . . . . . . . . .  10
     3.5.  ORCHIDs for Hierarchical HITs . . . . . . . . . . . . . .  10
       3.5.1.  Adding additional information to the ORCHID . . . . .  11
       3.5.2.  ORCHID Encoding . . . . . . . . . . . . . . . . . . .  12
       3.5.3.  ORCHID Decoding . . . . . . . . . . . . . . . . . . .  14
       3.5.4.  Decoding ORCHIDs for HIPv2  . . . . . . . . . . . . .  14
   4.  Hierarchical HITs as Remote ID DRIP Entity Tags (DET) . . . .  14
     4.1.  Nontransferablity of DETs . . . . . . . . . . . . . . . .  15
     4.2.  Encoding HHITs in CTA 2063-A Serial Numbers . . . . . . .  15
     4.3.  Remote ID DET as one class of Hierarchical HITs . . . . .  16
     4.4.  Hierarchy in ORCHID Generation  . . . . . . . . . . . . .  16
     4.5.  DRIP Entity Tag (DET) Registry  . . . . . . . . . . . . .  17
     4.6.  Remote ID Authentication using DETs . . . . . . . . . . .  17
   5.  DRIP Entity Tags (DETs) in DNS  . . . . . . . . . . . . . . .  18
   6.  Other UTM uses of HHITs beyond DET  . . . . . . . . . . . . .  19
   7.  DRIP Requirements addressed . . . . . . . . . . . . . . . . .  19
   8.  DET Privacy . . . . . . . . . . . . . . . . . . . . . . . . .  19
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  20
     9.1.  IANA CGA registry update  . . . . . . . . . . . . . . . .  20
     9.2.  IANA HIP registry updates . . . . . . . . . . . . . . . .  20
     9.3.  IANA IPSECKEY registry update . . . . . . . . . . . . . .  21
     9.4.  New IPv6 prefix needed for DETs . . . . . . . . . . . . .  21
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  22

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     10.1.  DET Trust  . . . . . . . . . . . . . . . . . . . . . . .  23
     10.2.  Collision risks with DETs  . . . . . . . . . . . . . . .  24
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  24
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  24
     11.2.  Informative References . . . . . . . . . . . . . . . . .  25
   Appendix A.  EU U-Space RID Privacy Considerations  . . . . . . .  28
   Appendix B.  Calculating Collision Probabilities  . . . . . . . .  29
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  29
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  30

1.  Introduction

   [drip-requirements] describes an Unmanned Aircraft System Remote
   Identification and tracking (UAS ID) as 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 Host Identity Tags
   (HHITs) (Section 3) as self-asserting IPv6 addresses and thereby a
   trustable identifier for use as the UAS Remote ID.  HHITs include
   explicit hierarchy to enable DNS HHIT queries (Host ID for
   authentication, e.g., [drip-authentication]) and for Extensible
   Provisioning Protocol (EPP) Registrar discovery [RFC7484] for 3rd-
   party identification attestation (e.g., [drip-authentication]).

   This addition of hierarchy to HITs requires updates to both [RFC7401]
   and [RFC7343].

   HHITs as used within the context of UAS will be labeled as DRIP
   Entity Tags (DET).  Throughout this document HHIT and DET will be
   used appropriately.  HHIT will be used when covering the technology,
   and DET for their context within UAS RID.

   HHITs are statistically unique through the cryptographic hash feature
   of second-preimage resistance.  The cryptographically-bound addition
   of the hierarchy and a HHIT registration process [drip-registries]
   provide complete, global HHIT uniqueness.  This contrasts with using
   general identifiers (e.g., a Universally Unique IDentifiers (UUID)
   [RFC4122] or device serial numbers) as the subject in an X.509
   [RFC5280] certificate.

   In a multi Certificate Authority (multi-CA) PKI alternative to HHITs,
   a Remote ID as the Subject (Section 4.1.2.6 of [RFC5280]) can occur
   in multiple CAs, possibly fraudulently.  CAs within the PKI would
   need to implement an approach to enforce assurance of the uniqueness
   achieved with HHITs.

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   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
   [RFC8200].  As such, they fit in many ways within various IETF
   technologies.

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.

2.2.  Notations

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

2.3.  Definitions

   This document uses the terms defined in [drip-requirements].  The
   following new terms are used in the document:

   cSHAKE (The customizable SHAKE function [NIST.SP.800-185]):
      Extends the SHAKE [NIST.FIPS.202] scheme to allow users to
      customize their use of the SHAKE function.

   HDA (Hierarchical HIT Domain Authority):
      The 16-bit field that identifies the HHIT Domain Authority under a
      Registered Assigning Authority (RAA).

   HHIT
      Hierarchical Host Identity Tag.  A HIT with extra hierarchical
      information not found in a standard HIT [RFC7401].

   HI
      Host Identity.  The public key portion of an asymmetric key pair
      as defined in [RFC9063].

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

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   HIP (Host Identity Protocol)
      The origin of HI, HIT, and HHIT, required for DRIP.

   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.  In particular all the functions referenced from
      [NIST.FIPS.202] and [NIST.SP.800-185].

   KMAC (KECCAK Message Authentication Code [NIST.SP.800-185]):
      A Pseudo Random Function (PRF) and keyed hash function based on
      KECCAK.

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

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

   SHAKE (Secure Hash Algorithm KECCAK [NIST.FIPS.202]):
      A secure hash that allows for an arbitrary output length.

   XOF (eXtendable-Output Function [NIST.FIPS.202]):
      A function on bit strings (also called messages) in which the
      output can be extended to any desired length.

3.  The Hierarchical Host Identity Tag (HHIT)

   The Hierarchical HIT (HHIT) is a small but important enhancement over
   the flat HIT space, constructed as an Overlay Routable Cryptographic
   Hash IDentifier (ORCHID) [RFC7343].  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 use the other 32
   bits to create a hierarchical administration organization for HIT
   domains.  Hierarchical HIT construction is defined in Section 3.5.
   The input values for the Encoding rules are described in
   Section 3.5.1.

   A HHIT is built from the following fields:

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   *  p = IANA prefix (max 28 bit)

   *  32 bit Hierarchy ID (HID)

   *  s = 4 (or 8) bit HIT Suite ID

   *  ORCHID hash (96 - prefix length - Suite ID length bits, e.g., 64)
      See Section 3.5

                  16 bits| 16 bits           4 or 8 bits
                 +-------+-------+         +--------------+
                 |  RAA  | HDA   |         | HIT Suite ID |
                 +-------+-------+         +--------------+
                  \              |    ____/   ___________/
                   \             \  _/  _____/
                    \             \/   /
      |    p bits    |  32 bits   |s  |   o=96-p-s bits        |
      +--------------+------------+---+------------------------+
      | IANA Prefix  |    HID     |HSI|      ORCHID hash       |
      +--------------+------------+---+------------------------+

                           Figure 1: HHIT Format

   The Context ID for the ORCHID hash is:

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

   Context IDs are allocated out of the namespace introduced for
   Cryptographically Generated Addresses (CGA) Type Tags [RFC3972].

   A python script is available for generating HHITs [hhit-gen].

3.1.  HHIT Prefix for RID Purposes

   A unique IANA IPv6 prefix, no larger than 28 bits, for HHITs is
   recommended.  It clearly separates the flat-space HIT processing from
   HHIT processing per Section 3.5.

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

3.2.  HHIT Suite IDs

   The HIT Suite IDs specify 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 set to zeros), but this reduces the ORCHID hash
   length.

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3.2.1.  8-bit HIT Suite IDs

   Support for 8 bit HIT Suite IDs is allowed in Section 5.2.10 of
   [RFC7401], but not specified in how ORCHIDs are generated with these
   longer ORCHID Generation Algorithms (OGAs).  Section 3.5 provides the
   algorithmic flexibility, allowing for HDA custom HIT Suite IDs as
   follows:

        HIT Suite             Four-bit ID    Eight-bit encoding
        HDA Assigned 1            NA         TBD4 (suggested value 0x0E)
        HDA Assigned 2            NA         TBD5 (suggested value 0x0F)

   This feature, for example, 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.

3.3.  The Hierarchy ID (HID)

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

   *  16-bit Registered Assigning Authority (RAA)

   *  16-bit Hierarchical HIT Domain Authority (HDA)

3.3.1.  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 ICAO.  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.

   The ICAO registration process will be available from ICAO.  Once ICAO
   accepts an RAA, it will assign a number and create a zone delegation
   under the uas.icao.int.  DNS zone for the RAA.

   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.

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

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

3.3.2.  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 such as those required 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 for UAS clients that may
   use HIP.  How this is done and scales to the potentially millions of
   customers are 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.  Such policy is out of scope.

3.4.  Edward Digital Signature Algorithm for HITs

   The Edwards-Curve Digital Signature Algorithm (EdDSA) [RFC8032] is
   specified here for use as Host Identities (HIs) per HIPv2 [RFC7401].

   The intent in this document is to add EdDSA as a HI algortihm for
   DETs, but doing so impacts the HIP parameters used in a HIP exchange.
   As such the following update HIP parameters.  Other than the HIP DNS
   RR, these should not be needed in a DRIP implementation that does not
   use HIP.

   See Section 3.2 for use of the HIT Suite in the context of this
   document.

3.4.1.  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       TBD1 (suggested value 13) [RFC8032]    (RECOMMENDED)

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3.4.1.1.  HIP Parameter support for EdDSA

   The addition of EdDSA as a HI algorithm requires a subfield in the
   HIP HOST_ID parameter Section 5.2.9 of [RFC7401] as was done for
   ECDSA when used in a HIP exchange.

   For HIP hosts that implement EdDSA as the algorithm, the following
   EdDSA curves are represented by the following fields:

      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |         EdDSA Curve           |                               /
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     /                         Public Key                            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

     EdDSA Curve   Curve label
     Public Key    Represented in Octet-string format      [RFC8032]

                                  Figure 2

   For hosts that implement EdDSA as a HIP algorithm the following EdDSA
   curves are required:

        Algorithm    Curve            Values

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

3.4.1.2.  HIP DNS RR support for EdDSA

   The HIP DNS RR (Resource Record) is defined in [RFC8005].  It uses
   the values defined for the 'Algorithm Type' of the IPSECKEY RR
   [RFC4025] for its PK Algorithm field.

   The new EdDSA HI will use [RFC8080] for the IPSECKEY RR encoding:

      Value  Description

      TBD2 (suggested value 4)
             An EdDSA key is present, in the format defined in [RFC8080]

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3.4.2.  HIT_SUITE_LIST

   The HIT_SUITE_LIST parameter contains a list of the supported HIT
   suite IDs of the HIP Responder.  Based on the HIT_SUITE_LIST, the HIP
   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        4-bit ID                   8-bit encoding
        RESERVED         0                          0x00
        EdDSA/cSHAKE128  TBD3 (suggested value 5)   0x50   (RECOMMENDED)

   The following table provides more detail on the above HIT Suite
   combination.

   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).

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

                           Table 1: HIT Suites

3.5.  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.

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   The Keccak [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.

3.5.1.  Adding additional information to the ORCHID

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

   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:

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   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.
   Note that if a 8-bit OGA is used, the hash will be 4 bits shorter.
   This will result in a greater risk of pre-image attacks and a
   corresponding greater need to manage HHIT registration and require
   look up of the HI from a trusted source.

3.5.2.  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

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   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.

3.5.2.1.  Encoding ORCHIDs for HIPv2

   This section is included to provide backwards compatibility for
   ORCHIDv2 [RFC7343] as used in HIPv2 [RFC7401].

   For HIPv2, the Prefix is 2001:20::/28.  Info is zero-length (i.e.,
   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:

       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

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   Then the ORCHID is constructed as follows:

       Prefix | OGA ID | Hash Output

3.5.3.  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 Section 9.4.

3.5.4.  Decoding ORCHIDs for HIPv2

   This section is included to provide backwards compatibility for
   ORCHIDv2 [RFC7343] as used for HIPv2 [RFC7401].

   HIPv2s are identified by a Prefix of 2001:20::/28.  The next 4 bits
   are the OGA ID.  The remaining 96 bits are the HI Hash.

4.  Hierarchical HITs as Remote ID DRIP Entity Tags (DET)

   Hierarchical HITs are a refinement on the Host Identity Tag (HIT) of
   HIPv2.  HHITs require a new ORCHID mechanism as described in
   Section 3.5.

   HHITs for UAS ID (called, DETs) also use the new EdDSA/SHAKE128 HIT
   suite defined in Section 3.4 (GEN-2 in [drip-requirements]).  This
   hierarchy, cryptographically embedded within the HHIT, provides the
   information for finding the UA's HHIT registry (ID-3 in
   [drip-requirements]).

   ASTM Standard Specification for Remote ID and Tracking [F3411]
   specifies four 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].  These can be dynamic,
           but do not need to be.

   TYPE-4  Specific Session ID (SSI)

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   Note that Types 1 - 3 allow for an UAS ID with a maximum length of 20
   bytes, the SSI (Type 4) uses the first byte of the ID for the SSI
   value, thus restricting the UAS ID to a maximum of 19 bytes.  The SSI
   values initially assigned (as per 2021) are:

   ID 1  IETF - DRIP Drone Remote Identification Protocol (DRIP) entity
         ID.

   ID 2  3GPP - IEEE 1609.2-2016 HashedID8

4.1.  Nontransferablity of DETs

   A HI and its HHIT SHOULD NOT be transferable between UA or even
   between replacement electronics (e.g., replacement of damaged
   controller CPU) for a UA.  The private key for the HI SHOULD be held
   in a cryptographically secure component.

4.2.  Encoding HHITs in CTA 2063-A Serial Numbers

   In some cases, it is advantageous to encode HHITs as a CTA 2063-A
   Serial Number [CTA2063A].  For example, the FAA Remote ID Rules
   [FAA_RID] state that a Remote ID Module (i.e., not integrated with UA
   controller) must only use "the serial number of the unmanned
   aircraft"; CTA 2063-A meets this requirement.

   Encoding an HHIT within the CTA 2063-A format is not simple.  The CTA
   2063-A format is defined as:

   Serial Number   :=  MFR Code | Length Code | MFR SN

   where:

   MFR Code        : 4 character code assigned by ICAO.

   Length Code     : 1 character Hex encoding of MFR SN length (1-F).

   MFR SN          : Alphanumeric code (0-9, A-Z except O and I).
                     Maximum length of 15 characters.

   There is no place for the HID; there will need to be a mapping
   service from Manufacturer Code to HID.  The HIT Suite ID and ORCHID
   hash will take 14 characters (as described below), leaving 1
   character to distinguish encoded DETs from other manufacturer use of
   CTA 2063-A Serial Numbers.

   A character in a CTA 2063-A Serial Number "shall include any
   combination of digits and uppercase letters, except the letters O and
   I, but may include all digits".  This would allow for a Base34

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   encoding of the binary HIT Suite ID and ORCHID hash.  Although,
   programmatically, such a conversion is not hard, other technologies
   (e.g., credit card payment systems) that have used such odd base
   encoding have had performance challenges.  Thus here a Base32
   encoding will be used by also excluding the letters Z and S (too
   similar to the digits 2 and 5).

   The low-order 68 bits (HIT Suite ID | ORCHID hash) of the HHIT SHALL
   be left-padded with 2 bits of zeros.  This 70-bit number will be
   encoded into 14 characters using the digit/letters above.  The
   manufacturer MUST use a Length Code of F (15).  The first character
   after the Length Code MUST be 'Z', followed by the 14 characters of
   the encoded HIT Suite ID and ORCHID hash.  This construct allows the
   manufacturer to construct other MFR SN of length 15 by avoiding
   starting them with 'Z'.

   Using the sample DET from Section 5 that is for HDA=20 under RAA=10
   and having the ICAO CTA MFR Code of 8653, the 20-character CTA 2063-A
   Serial Number would be:

       8653FZ2T7B8RA85D19LX

   A mapping service (e.g., DNS) MUST provide a trusted (e.g., via
   DNSSEC) conversion of the 4-character Manufacturer Code to high-order
   60 bits (Prefix | HID) of the HHIT.  Definition of this mapping
   service is currently out of scope of this document.

   It should be noted that this encoding would only be used in the Basic
   ID Message.  The HHIT DET will still be used in the Authentication
   Messages.

4.3.  Remote ID DET as one class of Hierarchical HITs

   UAS Remote ID DET may be one of a number of uses of HHITs.  However,
   it is out of the scope of the document to elaborate on other uses of
   HHITs.  As such these follow-on uses need to be considered in
   allocating the RAAs Section 3.3.1 or HHIT prefix assignments
   Section 9.

4.4.  Hierarchy in ORCHID Generation

   ORCHIDS, as defined in [RFC7343], do not cryptographically bind an
   IPv6 prefix nor the Orchid Generation Algorithm (OGA) ID (the HIT
   Suite ID) to the hash of the HI.  The rational at the time of
   developing ORCHID was attacks against these fields are DoS attacks
   against protocols using ORCHIDs and thus up to those protocols to
   address the issue.

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   HHITs, as defined in Section 3.5, cryptographically bind all content
   in the ORCHID through the hashing function.  A recipient of a DET
   that has the underlying HI can directly trust and act on all content
   in the HHIT.  This provides a strong, self-attestation for using the
   hierarchy to find the DET Registry based on the HID.

4.5.  DRIP Entity Tag (DET) Registry

   DETs are registered to HDAs.  A registration process,
   [drip-registries], ensures DET global uniqueness (ID-4 in
   [drip-requirements]).  It also provides the mechanism to create UAS
   public/private data that are associated with the DET (REG-1 and REG-2
   in [drip-requirements]).

   The two levels of hierarchy within the DET 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.

4.6.  Remote ID Authentication using DETs

   The EdDSA25519 HI (Section 3.4) underlying the DET can be used in an
   84-byte self-proof attestation (timestamp, HHIT, and signature of
   these) to provide proof of Remote ID ownership (GEN-1 in
   [drip-requirements]).  In practice, the Wrapper and Manifest
   authentication formats in the ASTM Authentication Message (Msg Type
   0x2) [drip-authentication] implicitly provide this self-attestation.
   A lookup service like DNS can provide the HI and registration proof
   (GEN-3 in [drip-requirements]).

   Similarly, for Observers without Internet access, a 200-byte offline
   self-attestation could provide the same Remote ID ownership proof.
   This attestation would contain the HDA's signing of the UA's HHIT,
   itself signed by the UA's HI.  Only a small cache that contains the
   HDA's HI/HHIT and HDA meta-data is needed by the Observer.  However,
   such an object would just fit in the ASTM Authentication Message with
   no room for growth.  In practice [drip-authentication] provides this
   offline self-attestation in two authentication messages: the HDA's
   certification of the UA's HHIT registration in a Link authentication
   message whose hash is sent in a Manifest authentication message.

   Hashes of any previously sent ASTM messages can be placed in a
   Manifest authentication message (GEN-2 in [drip-requirements]).  When
   a Location/Vector Message (Msg Type 0x1) hash along with the hash of
   the HDA's UA HHIT attestation are sent in a Manifest authentication
   message and the Observer can visually see a UA at the claimed
   location, the Observer has a very strong proof of the UA's Remote ID.

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   All this behavior and how to mix these authentication messages into
   the flow of UA operation messages are detailed in
   [drip-authentication].

5.  DRIP Entity Tags (DETs) in DNS

   There are two approaches for storing and retrieving DETs using DNS.

   *  As FQDNs in ".icao.int.".

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

   A DET 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 in
   [drip-requirements]).  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 follows:

       100.50.det.uas.icao.int   IN PTR      foo.uss.icao.int.

   The individual DETs may be potentially too numerous (e.g., 60 - 600M)
   and dynamic (e.g., new DETs every minute for some HDAs) to store in a
   signed, DNS zone.  The HDA SHOULD provide DNS service for its zone
   and provide the HHIT detail response.  A secure connection (e.g., DNS
   over TLS) to the authoritative zone may be a viable alternative to
   DNSSEC.

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

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

   A DET reverse lookup could be to:

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

   or:

       a3ad1952ad0a69e.5.20.10.30.2001.det.remoteid.icao.int.

   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

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6.  Other UTM uses of HHITs beyond DET

   HHITs might be used within the UTM architecture beyond DET (and USS
   in UA ID registration and authentication).  For example as a GCS HHIT
   ID.  The GCS may use its HIIT if it is the source of Network Remote
   ID for securing the transport and for secure C2 transport (e.g.,
   [drip-secure-nrid-c2]).

   Observers may have their own 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 (this use is
   currently outside the scope of this document).  Further, they can be
   used by FINDER observers, (e.g., [crowd-sourced-rid]).

7.  DRIP Requirements addressed

   This document in the previous sections provides the details to
   solutions for GEN 1 - 3, ID 1 - 5, and REG 1 - 2 as described in
   [drip-requirements].

8.  DET Privacy

   There is no expectation of privacy for DETs; it is not part of the
   Privacy Normative Requirements, Section 4.3.1, of
   [drip-requirements].  DETs are broadcast in the clear over the open
   air via Bluetooth and Wi-Fi.  They will be collected and collated
   with other public information about the UAS.  This will include DET
   registration information and location and times of operations for a
   DET.  A DET can be for the life of a UA if there is no concern about
   DET/UA activity harvesting.

   Further, the MAC address of the wireless interface used for Remote ID
   broadcasts are a target for UA operation aggregation that may not be
   mitigated through address randomization.  For Bluetooth 4 Remote ID
   messaging, the MAC address is used by observers to link the Basic ID
   Message that contains the RID with other Remote ID messages, thus
   must be constant for a UA operation.  This message linkage use of MAC
   addresses may not be needed with the Bluetooth 5 or Wi-Fi PHYs.
   These PHYs provide for a larger message payload and can use the
   Message Pack (Msg Type 0xF) and the Authentication Message to
   transmit the RID with other Remote ID messages.  However, it is not
   mandatory to send the RID in a Message Pack or Authentication
   Message, so allowance for using the MAC address for UA message
   linking must be maintained.  That is, the MAC address should be
   stable for at least a UA operation.

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   Finally, it is not adequate to simply change the DET and MAC for a UA
   per operation to defeat historically tracking a UA's activity.

   Any changes to the UA MAC may have impacts to C2 setup and use.  A
   constant GCS MAC may well defeat any privacy gains in UA MAC and RID
   changes.  UA/GCS binding is complicated with changing MAC addresses;
   historically UAS design assumed these to be "forever" and made setup
   a one-time process.  Additionally, if IP is used for C2, a changing
   MAC may mean a changing IP address to further impact the UAS
   bindings.  Finally, an encryption wrapper's identifier (such as ESP
   [RFC4303] SPI) would need to change per operation to insure operation
   tracking separation.

   Creating and maintaining UAS operational privacy is a multifaceted
   problem.  Many communication pieces need to be considered to truly
   create a separation between UA operations.  Simply changing the UAS
   RID only starts the changes that need to be implemented.

9.  IANA Considerations

9.1.  IANA CGA registry update

   This document requests IANA to make the following change to the IANA
   "CGA Extension Type Tags registry [IANA-CGA] registry:

   Context ID:
      The Context ID (Section 3) shares the namespace introduced for CGA
      Type Tags.  Defining new Context IDs follow the rules in Section 8
      of [RFC3972].

9.2.  IANA HIP registry updates

   This document requests IANA to make the following changes to the IANA
   "Host Identity Protocol (HIP) Parameters" [IANA-HIP] registry:

   Host ID:
      This document defines the new EdDSA Host ID with value TBD1
      (suggested: 13) (Section 3.4.1) in the "HI Algorithm" subregistry
      of the "Host Identity Protocol (HIP) Parameters" registry.

   EdDSA Curve Label:
      This document specifies a new algorithm-specific subregistry named
      "EdDSA Curve Label".  The values for this subregistry are defined
      in Section 3.4.1.1.

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   HIT Suite ID:
      This document defines the new HIT Suite of EdDSA/cSHAKE with value
      TBD3 (suggested: 5) (Section 3.4.2) in the "HIT Suite ID"
      subregistry of the "Host Identity Protocol (HIP) Parameters"
      registry.

   HIT Suite ID eight-bit encoding:
      This document defines the first eight-bit encoded HIT Suite IDs as
      defined in Section 5.2.10 of [RFC7401].  These are the new HDA
      domain HIT Suites with values TBD4 and TBD5 (suggested values:
      0x0E and 0x0F) (Section 3.2.1).  IANA is requested to expand the
      "HIT Suite ID" subregistry of the "Host Identity Protocol (HIP)
      Parameters" registry to show both the four-bit and eight-bit
      values as shown in Section 5.2.10 of [RFC7401] and add these new
      values that only have 8-bit representations.

9.3.  IANA IPSECKEY registry update

   This document requests IANA to make the following change to the
   "IPSECKEY Resource Record Parameters" [IANA-IPSECKEY] registry:

   IPSECKEY:
      This document defines the new IPSECKEY value TBD2 (suggested: 4)
      (Section 3.4.1.2) in the "Algorithm Type Field" subregistry of the
      "IPSECKEY Resource Record Parameters" registry.

9.4.  New IPv6 prefix needed for DETs

   Since the DET format is not compatible with [RFC7343], IANA is
   requested to allocate a new prefix following this template for the
   IPv6 Special-Purpose Address Registry.

   Address Block:
      IANA is requested to allocate a new 28-bit prefix out of the IANA
      IPv6 Special Purpose Address Block, namely 2001::/23, as per
      [RFC6890] (suggested: 2001:30::/28).

   Name:
      This block should be named "DRIP Device Entity Tags (DET) Prefix".

   RFC:
      This document.

   Allocation Date:
      Date this document published.

   Termination Date:
      Forever.

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   Source:
      False.

   Destination:
      False.

   Forwardable:
      False.

   Globally Reachable:
      False.

   Reserved-by-Protocol:
      False?

10.  Security Considerations

   The 64-bit hash in HHITs presents a real risk of second pre-image
   cryptographic hash attack Section 10.2.  There are no known (to the
   authors) studies of hash size to cryptographic hash attacks.  A
   Python script is available to randomly generate 1M HHITs that did not
   produce a hash collision which is a simpler attack than a first or
   second pre-image attack.

   However, with today's computing power, producing 2^64 EdDSA keypairs
   and then generating the corresponding HHIT is economically feasible.
   Consider that a *single* bitcoin mining ASIC can do on the order of
   2^46 sha256 hashes a second or about 2^62 hashes in a single day.
   The point being, 2^64 is not prohibitive, especially as this can be
   done in parallel.

   Now it should be noted that the 2^64 attempts is for stealing a
   specific HHIT.  Consider a scenario of a street photography company
   with 1,024 UAs (each with its own HHIT); you'd be happy stealing any
   one of them.  Then rather than needing to satisfy a 64-bit condition
   on the cSHAKE128 output, you need only satisfy what is equivalent to
   a 54-bit condition (since you have 2^10 more opportunities for
   success).

   Thus, although the probability of a collision or pre-image attack is
   low in a collection of 1,024 HHITs out of a total population of 2^64,
   per Section 10.2, it is computationally and economically feasible.
   Thus the HHIT registration and HHIT/HI registration validation is
   strongly recommended.

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   The DET Registry services effectively block attempts to "take over"
   or "hijack" a DET.  It does not stop a rogue attempting to
   impersonate a known DET.  This attack can be mitigated by the
   receiver of the DET using DNS to find the HI for the DET.  As such,
   use of DNSSEC and DNS over TLS by the DET registries is recommended.

   The 60-bit hash for DETs with 8-bit OGAs have a greater hash attack
   risk.  As such its use should be restricted to testing and to small,
   well managed UAS/USS.

   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 discussed in Section 4.6.

   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., "det.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 [drip-registries], through its HHIT
   uniqueness enforcement, provides against forced or accidental HHIT
   hash collisions.

   Cryptographically Generated Addresses (CGAs) provide an 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 ([drip-registries]) 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 DET 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 DET, 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.

10.1.  DET Trust

   The DET in the ASTM Basic ID 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 ID 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; that is, too open to a hash attack.
   Minimally, it takes 84 bytes, Section 4.6, to prove ownership of a
   DET with a full EdDSA signature.  Thus, no attempt has been made to

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   add DET trust directly within the very small Basic ID Message.

   The ASTM Authentication Message (Msg Type 0x2) as shown in
   Section 4.6 can provide practical actual ownership proofs.  These
   attestations include timestamps to defend against replay attacks.
   But in themselves, they do not prove which UA sent the message.  They
   could have been sent by a dog running down the street with a
   Broadcast Remote ID module 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 DET of the UA.

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

10.2.  Collision risks with DETs

   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 B 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 DET and MUST reject a
   collision, forcing the UAS to generate a new HI and thus HHIT and
   reapplying to the DET registration process.

11.  References

11.1.  Normative References

   [IANA-CGA] IANA, "Cryptographically Generated Addresses (CGA) Message
              Type Name Space", <https://www.iana.org/assignments/cga-
              message-types/cga-message-types.xhtml>.

   [IANA-HIP] IANA, "Host Identity Protocol (HIP) Parameters",
              <https://www.iana.org/assignments/hip-parameters/hip-
              parameters.xhtml>.

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   [IANA-IPSECKEY]
              IANA, "IPSECKEY Resource Record Parameters",
              <https://www.iana.org/assignments/ipseckey-rr-parameters/
              ipseckey-rr-parameters.xhtml>.

   [NIST.FIPS.202]
              Dworkin, M., "SHA-3 Standard: Permutation-Based Hash and
              Extendable-Output Functions", National Institute of
              Standards and Technology report,
              DOI 10.6028/nist.fips.202, July 2015,
              <https://doi.org/10.6028/nist.fips.202>.

   [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>.

   [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>.

   [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>.

   [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>.

   [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>.

11.2.  Informative References

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   [cfrg-comment]
              "A CFRG review of draft-ietf-drip-rid", September 2021,
              <https://mailarchive.ietf.org/arch/msg/cfrg/
              tAJJq60W6TlUv7_pde5cw5TDTCU/>.

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

   [crowd-sourced-rid]
              Moskowitz, R., Card, S. W., Wiethuechter, A., Zhao, S.,
              and H. Birkholz, "Crowd Sourced Remote ID", Work in
              Progress, Internet-Draft, draft-moskowitz-drip-crowd-
              sourced-rid-06, 26 May 2021,
              <https://datatracker.ietf.org/doc/html/draft-moskowitz-
              drip-crowd-sourced-rid-06>.

   [CTA2063A] ANSI/CTA, "Small Unmanned Aerial Systems Serial Numbers",
              September 2019, <https://shop.cta.tech/products/small-
              unmanned-aerial-systems-serial-numbers>.

   [drip-authentication]
              Wiethuechter, A., Card, S., and R. Moskowitz, "DRIP
              Authentication Formats for Broadcast Remote ID", Work in
              Progress, Internet-Draft, draft-ietf-drip-auth-03, 8
              November 2021, <https://datatracker.ietf.org/doc/html/
              draft-ietf-drip-auth-03>.

   [drip-registries]
              Wiethuechter, A., Card, S., and R. Moskowitz, "DRIP
              Registries", Work in Progress, Internet-Draft, draft-
              wiethuechter-drip-registries-01, 22 October 2021,
              <https://datatracker.ietf.org/doc/html/draft-wiethuechter-
              drip-registries-01>.

   [drip-requirements]
              Card, S. W., Wiethuechter, A., Moskowitz, R., and A.
              Gurtov, "Drone Remote Identification Protocol (DRIP)
              Requirements", Work in Progress, Internet-Draft, draft-
              ietf-drip-reqs-18, 8 September 2021,
              <https://datatracker.ietf.org/doc/html/draft-ietf-drip-
              reqs-18>.

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   [drip-secure-nrid-c2]
              Moskowitz, R., Card, S. W., Wiethuechter, A., and A.
              Gurtov, "Secure UAS Network RID and C2 Transport", Work in
              Progress, Internet-Draft, draft-moskowitz-drip-secure-
              nrid-c2-04, 21 October 2021,
              <https://datatracker.ietf.org/doc/html/draft-moskowitz-
              drip-secure-nrid-c2-04>.

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

   [FAA_RID]  United States Federal Aviation Administration (FAA),
              "Remote Identification of Unmanned Aircraft", 2021,
              <https://www.govinfo.gov/content/pkg/FR-2021-01-15/
              pdf/2020-28948.pdf>.

   [hhit-gen] "Python script to generate HHITs", September 2021,
              <https://github.com/ietf-wg-drip/draft-ietf-drip-
              rid/blob/master/hhit-gen.py>.

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

   [RFC3972]  Aura, T., "Cryptographically Generated Addresses (CGA)",
              RFC 3972, DOI 10.17487/RFC3972, March 2005,
              <https://www.rfc-editor.org/info/rfc3972>.

   [RFC4025]  Richardson, M., "A Method for Storing IPsec Keying
              Material in DNS", RFC 4025, DOI 10.17487/RFC4025, March
              2005, <https://www.rfc-editor.org/info/rfc4025>.

   [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>.

   [RFC4303]  Kent, S., "IP Encapsulating Security Payload (ESP)",
              RFC 4303, DOI 10.17487/RFC4303, December 2005,
              <https://www.rfc-editor.org/info/rfc4303>.

   [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,
              <https://www.rfc-editor.org/info/rfc5280>.

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   [RFC7484]  Blanchet, M., "Finding the Authoritative Registration Data
              (RDAP) Service", RFC 7484, DOI 10.17487/RFC7484, March
              2015, <https://www.rfc-editor.org/info/rfc7484>.

   [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>.

   [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>.

   [RFC8080]  Sury, O. and R. Edmonds, "Edwards-Curve Digital Security
              Algorithm (EdDSA) for DNSSEC", RFC 8080,
              DOI 10.17487/RFC8080, February 2017,
              <https://www.rfc-editor.org/info/rfc8080>.

   [RFC8200]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", STD 86, RFC 8200,
              DOI 10.17487/RFC8200, July 2017,
              <https://www.rfc-editor.org/info/rfc8200>.

   [RFC9063]  Moskowitz, R., Ed. and M. Komu, "Host Identity Protocol
              Architecture", RFC 9063, DOI 10.17487/RFC9063, July 2021,
              <https://www.rfc-editor.org/info/rfc9063>.

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.

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   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).

Appendix B.  Calculating Collision Probabilities

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

       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.  Nicholas
   Gajcowski [cfrg-comment] provided a concise hash pre-image security
   assessment via the CFRG list.

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   Many thanks to Michael Richardson for the iotdir review, Magnus
   Nystrom for the secdir review and DRIP co-chair and draft shepherd,
   Mohamed Boucadair for his extensive comments and help on document
   clarity.

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

   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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