DRIP                                                        R. Moskowitz
Internet-Draft                                            HTT Consulting
Updates: 7401, 7343 (if approved)                                S. Card
Intended status: Standards Track                         A. Wiethuechter
Expires: 31 December 2022                             AX Enterprize, LLC
                                                               A. Gurtov
                                                    Linköping University
                                                            29 June 2022


 DRIP Entity Tag (DET) for Unmanned Aircraft System Remote ID (UAS RID)
                         draft-ietf-drip-rid-29

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 Unmanned Aircraft System Remote
   Identification and tracking (UAS RID).

   This document updates RFC7401 and RFC7343.

   Within the context of RID, HHITs will be called DRIP Entity Tags
   (DETs).  HHITs self-attest to the included explicit hierarchy that
   provides registry (via, e.g., DNS, EPP) 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
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   This Internet-Draft will expire on 31 December 2022.

Copyright Notice

   Copyright (c) 2022 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
     1.1.  HHIT Statistical Uniqueness different from UUID or X.509
           Subject . . . . . . . . . . . . . . . . . . . . . . . . .   4
   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) . . . . . . . . . .   6
     3.1.  HHIT Prefix for RID Purposes  . . . . . . . . . . . . . .   7
     3.2.  HHIT Suite IDs  . . . . . . . . . . . . . . . . . . . . .   7
       3.2.1.  HDA custom HIT Suite IDs  . . . . . . . . . . . . . .   8
     3.3.  The Hierarchy ID (HID)  . . . . . . . . . . . . . . . . .   8
       3.3.1.  The Registered Assigning Authority (RAA)  . . . . . .   8
       3.3.2.  The Hierarchical HIT Domain Authority (HDA) . . . . .   9
     3.4.  Edward-Curve Digital Signature Algorithm for HHITs  . . .  10
       3.4.1.  HOST_ID . . . . . . . . . . . . . . . . . . . . . . .  10
       3.4.2.  HIT_SUITE_LIST  . . . . . . . . . . . . . . . . . . .  11
     3.5.  ORCHIDs for Hierarchical HITs . . . . . . . . . . . . . .  12
       3.5.1.  Adding Additional Information to the ORCHID . . . . .  13
       3.5.2.  ORCHID Encoding . . . . . . . . . . . . . . . . . . .  14
       3.5.3.  ORCHID Decoding . . . . . . . . . . . . . . . . . . .  15
       3.5.4.  Decoding ORCHIDs for HIPv2  . . . . . . . . . . . . .  15
   4.  Hierarchical HITs as DRIP Entity Tags . . . . . . . . . . . .  16
     4.1.  Nontransferablity of DETs . . . . . . . . . . . . . . . .  16
     4.2.  Encoding HHITs in CTA 2063-A Serial Numbers . . . . . . .  16
     4.3.  Remote ID DET as one Class of Hierarchical HITs . . . . .  18
     4.4.  Hierarchy in ORCHID Generation  . . . . . . . . . . . . .  18
     4.5.  DRIP Entity Tag (DET) Registry  . . . . . . . . . . . . .  18
     4.6.  Remote ID Authentication using DETs . . . . . . . . . . .  18
   5.  DRIP Entity Tags (DETs) in DNS  . . . . . . . . . . . . . . .  19
   6.  Other UAS Traffic Management (UTM) Uses of HHITs Beyond
           DET . . . . . . . . . . . . . . . . . . . . . . . . . . .  20
   7.  Summary of Addressed DRIP Requirements  . . . . . . . . . . .  20
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  20
     8.1.  New Well-Known IPv6 prefix for DETs . . . . . . . . . . .  20
     8.2.  New IANA DRIP Registry  . . . . . . . . . . . . . . . . .  21
     8.3.  IANA CGA Registry Update  . . . . . . . . . . . . . . . .  22



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     8.4.  IANA HIP Registry Updates . . . . . . . . . . . . . . . .  22
     8.5.  IANA IPSECKEY Registry Update . . . . . . . . . . . . . .  23
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  24
     9.1.  Post Quantum Computing out of scope . . . . . . . . . . .  25
     9.2.  DET Trust in ASTM messaging . . . . . . . . . . . . . . .  25
     9.3.  DET Revocation  . . . . . . . . . . . . . . . . . . . . .  26
     9.4.  Privacy Considerations  . . . . . . . . . . . . . . . . .  26
     9.5.  Collision Risks with DETs . . . . . . . . . . . . . . . .  27
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  28
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  28
     10.2.  Informative References . . . . . . . . . . . . . . . . .  29
   Appendix A.  EU U-Space RID Privacy Considerations  . . . . . . .  32
   Appendix B.  The 14/14 HID split  . . . . . . . . . . . . . . . .  32
   Appendix C.  Calculating Collision Probabilities  . . . . . . . .  34
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  34
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  35

1.  Introduction

   Drone Remote ID Protocol (DRIP) Requirements [RFC9153] describe an
   Unmanned Aircraft System Remote ID (UAS ID) as unique (ID-4), non-
   spoofable (ID-5), and identify a registry where the ID is listed (ID-
   2); all within a 19-character identifier (ID-1).

   This DRIP foundational document (i.e., all else in DRIP enables or
   uses it) describes (per Section 3 of [drip-architecture]) 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 add explicit hierarchy to the 128-bit HITs,
   enabling DNS HHIT queries (Host ID for authentication, e.g.,
   [drip-authentication]) and for Extensible Provisioning Protocol (EPP)
   Registrar discovery [RFC9224] for 3rd-party identification
   attestation (e.g., [drip-authentication]).

   This addition of hierarchy to HITs is an extension to [RFC7401] and
   requires an update to [RFC7343].  As this document also adds EdDSA
   (Section 3.4) for Host Identities (HIs), a number of Host Identity
   Protocol (HIP) parameters in [RFC7401] are updated, but these should
   not be needed in a DRIP implementation that does not use HIP.

   HHITs as used within the context of Unmanned Aircraft System (UAS)
   are labeled as DRIP Entity Tags (DETs).  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.

   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).



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   Hierarchical HITs are valid, though non-routable, IPv6 addresses
   [RFC8200].  As such, they fit in many ways within various IETF
   technologies.

1.1.  HHIT Statistical Uniqueness different from UUID or X.509 Subject

   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.  If the HHITs cannot be
   looked up with services provided by the registrar identified via the
   embedded hierarchical information or its registration validated by
   registration attestations messages [drip-authentication], then the
   HHIT is either fraudulent or revoked/expired.  In-depth discussion of
   these processes are out of scope for this document.

   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 either case, there can
   be no unique proof of ownership/registration.

   For example, 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.

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 Section 2.2 of [RFC9153].
   The following new terms are used in the document:





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   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 (HHIT Domain Authority):
      The 14-bit field that identifies the HHIT Domain Authority under a
      Registered Assigning Authority (RAA).  See Figure 1.

   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 28-bit field providing the HIT Hierarchy ID.  See Figure 1.

   HIP (Host Identity Protocol)
      The origin [RFC7401] of HI, HIT, and HHIT.

   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.  It refers in particular to 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 14-bit field identifying the business or organization that
      manages a registry of HDAs.  See Figure 1.

   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.





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   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 Host Identity Tag (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.

   The 128-bit HHITs represent the HI in only a 64-bit hash, rather than
   the 96 bits in HITs. 4 of these 32 freed up bits expand the Suite ID
   to 8 bits, and the other 28 bits are used 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 (Figure 1):

   *  p = an IPV6 prefix (max 28 bit)

   *  28-bit Hierarchy ID (HID) which provides the structure to organize
      HITs into administrative domains.  HIDs are further divided into
      two fields:

      -  14-bit Registered Assigning Authority (RAA) (Section 3.3.1)

      -  14-bit Hierarchical HIT Domain Authority (HDA) (Section 3.3.2)

   *  8-bit HHIT Suite ID (HHSI)

   *  ORCHID hash (92 - prefix length, e.g., 64) See Section 3.5 for
      more details.

                  14 bits| 14 bits              8 bits
                 +-------+-------+         +--------------+
                 |  RAA  | HDA   |         |HHIT Suite ID |
                 +-------+-------+         +--------------+
                  \              |    ____/   ___________/
                   \             \  _/    ___/
                    \             \/     /
      |    p bits    |  28 bits   |8bits|      o=92-p bits       |
      +--------------+------------+-----+------------------------+
      | IPV6 Prefix  |    HID     |HHSI |      ORCHID hash       |
      +--------------+------------+-----+------------------------+



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                           Figure 1: HHIT Format

   The Context ID (generated with openssl rand) 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].

3.1.  HHIT Prefix for RID Purposes

   The IPv6 HHIT prefix MUST be distinct from that used in the flat-
   space HIT as allocated in [RFC7343].  Without this distinct prefix,
   the first 4 bits of the RAA would be interpreted as the HIT Suite ID
   per HIPv2 [RFC7401].

   Initially, for DET use, one 28-bit prefix should be assigned out of
   the IANA IPv6 Special Purpose Address Block ([RFC6890]).

        HHIT Use     Bits  Value
        DET          28    TBD6 (suggested value 2001:30::/28)

   Other prefixes may be added in the future either for DET use or other
   applications of HHITs.  For a prefix to be added to the registry in
   Section 8.2, its usage and HID allocation process have to be publicly
   available.

3.2.  HHIT Suite IDs

   The HHIT Suite IDs specify the HI and hash algorithms.  These are a
   superset of the 4/8-bit HIT Suite ID as defined in Section 5.2.10 of
   [RFC7401].

   The HHIT values of 1 - 15 map to the basic 4-bit HIT Suite IDs.  HHIT
   values of 17 - 31 map to the extended 8-bit HIT Suite IDs.  HHIT
   values unique to HHIT will start with value 32.

   As HHIT introduces a new Suite ID, EdDSA/cSHAKE128, and since this is
   of value to HIPv2, it will be allocated out of the 4-bit HIT space
   and result in an update to HIT Suite IDs.  Future HHIT Suite IDs may
   be allocated similarly, or may come out of the additional space made
   available by going to 8 bits.

   The following HHIT Suite IDs are defined:







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        HHIT Suite          Value
        RESERVED            0
        RSA,DSA/SHA-256     1    [RFC7401]
        ECDSA/SHA-384       2    [RFC7401]
        ECDSA_LOW/SHA-1     3    [RFC7401]
        EdDSA/cSHAKE128     TBD3 (suggested value 5)   (RECOMMENDED)

3.2.1.  HDA custom HIT Suite IDs

   Support for 8-bit HHIT Suite IDs allows for HDA custom HIT Suite IDs.
   These will be assigned values greater than 15 as follows:

        HHIT Suite             Value
        HDA Private Use 1      TBD4 (suggested value 254)
        HDA Private Use 2      TBD5 (suggested value 255)

   These custom HIT Suite IDs, 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.

   They should not be used to create a "de facto standardization".
   Section 8.2 states that additional Suite IDs can be made through IETF
   Review.

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:

   *  14-bit Registered Assigning Authority (RAA)

   *  14-bit Hierarchical HIT Domain Authority (HDA)

   The rationale for the 14/14 HID split is described in Appendix B.

   The two levels of hierarchy allows for Civil Aviation Authorities
   (CAAs) to have it least one RAA for their National Air Space (NAS).
   Within its RAA(s), the CAAs can delegate HDAs as needed.  There may
   be other RAAs allowed to operate within a given NAS; this is a policy
   decision of each CAA.

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) or Japan Civil
   Aviation Bureau (JCAB) could be an RAA.




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   The RAA is a 14-bit field (16,384 RAAs).  The management of this
   space is further elaborated in [drip-registries].  An RAA MUST
   provide a set of services to allocate HDAs to organizations.  It
   SHOULD 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 HIP RVS servers for the HID.
   The zone delegation is also covered in [drip-registries].

   As DETs under an administrative control may be used in many different
   domains (e.g., commercial, recreation, military), RAAs should be
   allocated in blocks (e.g. 16-19) with consideration on the likely
   size of a particular usage.  Alternatively, different prefixes can be
   used to separate different domains of use of HHITs.

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

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

   Note that if the zone hhit.arpa is ultimately used, some registrar
   will need to manage this for all HHIT applications.  Thus further
   thought will be needed in the actual zone tree and registration
   process [drip-registries].

3.3.2.  The Hierarchical HIT Domain Authority (HDA)

   An HDA may be an Internet Service Provider (ISP), UAS Service
   Supplier (USS), or any third party that takes on the business to
   provide UAS services management, HIP RVSs or other needed services
   such as those required for HHIT and/or HIP-enabled devices.

   The HDA is a 14-bit field (16,384 HDAs per RAA) assigned by an RAA is
   further elaborated in [drip-registries].  An HDA must maintain public
   and private UAS registration information and 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, though covered in [drip-registries].  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.







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3.4.  Edward-Curve Digital Signature Algorithm for HHITs

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

   The intent in this document is to add EdDSA as a HI algorithm for
   DETs, but doing so impacts the HIP parameters used in a HIP exchange.
   The subsections of this section document the required updates of HIP
   parameters.  Other than the HIP DNS RR (Resource Record), 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 DRIP.

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 5.2.9 of [RFC7401].

        Algorithm
        profiles    Values

        EdDSA       TBD1 (suggested value 13) [RFC8032]    (RECOMMENDED)

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           |             NULL              /
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     /                         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:



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        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 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 uses [RFC8080] for the IPSECKEY RR encoding:

      Value  Description

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

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:

        HIT Suite        Value
        EdDSA/cSHAKE128  TBD3 (suggested value 5)   (RECOMMENDED)

   Table 1 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).












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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

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.

   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 update to ORCHIDv2 [RFC7343], as it can
   produce ORCHIDv2 output.

   The follow sub-sections define the general, new, ORCHID construct
   with the specific application here for HHITs.  Thus items like the
   hash size is only discussed as it impacts HHIT's 64-bit hash.  Other
   hash sizes should be discussed in any other specific use of this new
   ORCHID construct.







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3.5.1.  Adding Additional Information to the ORCHID

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

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

   where:

   Prefix          : A constant 28-bit-long bitstring value
                     (IPV6 prefix)

   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.

   The new ORCHID function is as follows:

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

   where:

   Prefix (p)      : An IPv6 prefix of length p (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.
                     When used for HHIT generation this is the
                     HHIT Suite ID.

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

   Sizeof(p + n + o + m) 128 bits











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   The ORCHID length MUST be 128 bits.  For HHITs with a 28-bit IPv6
   prefix, there are 100 bits remaining to be divided in any manner
   between the additional information ("Info"), OGA ID, and the hash
   output.  Consideration must be given to the size of the hash portion,
   taking into account risks like pre-image attacks. 64 bits, as used
   here for HHITs, may be as small as is acceptable.  The size of 'n',
   for the HID, is then determined as what is left; in the case of the
   8-bit OGA used for HHIT, this is 28 bits.

3.5.2.  ORCHID Encoding

   This update 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 ("Info"), 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 28 bits for the HID and 8 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 cSHAKE function call for this update 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 a /28 prefix 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 HITs use the Context ID defined in Section 3.







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3.5.2.1.  Encoding ORCHIDs for HIPv2

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

   For HIPv2, the Prefix is 2001:20::/28 (Section 6 of [RFC7343]).
   'Info' is zero-length (i.e., not included), and OGA ID is 4-bit.
   Thus, the HI Hash is 96-bit length.  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

   Then, the ORCHID is constructed as follows:

       Prefix | OGA ID | Hash Output

3.5.3.  ORCHID Decoding

   With this update, the decoding of an ORCHID is determined by the
   Prefix and OGA 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 Section 8.2.

3.5.4.  Decoding ORCHIDs for HIPv2

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



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   HITs 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 DRIP Entity Tags

   HHITs for UAS ID (called, DETs) use the new EdDSA/SHAKE128 HIT suite
   defined in Section 3.4 (GEN-2 in [RFC9153]).  This hierarchy,
   cryptographically bound within the HHIT, provides the information for
   finding the UA's HHIT registry (ID-3 in [RFC9153]).

   The 2022 forthcoming updated release of ASTM Standard Specification
   for Remote ID and Tracking [F3411] adds support for DETs.  This is
   within the UAS ID type 4, "Specific Session ID (SSI)".

   Note to RFC Editor: This, and all references to F3411 need to be
   updated to this new version which is in final ASTM editing.  A new
   link and replacement text will be provided when it is published.

   The original UAS ID Types 1 - 3 allow for an UAS ID with a maximum
   length of 20 bytes, this new SSI (Type 4) uses the first byte of the
   ID for the SSI Type, thus restricting the UAS ID of this type to a
   maximum of 19 bytes.  The SSI Types initially assigned are:

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

   ID 2  3GPP - IEEE 1609.2-2016 HashedID8

4.1.  Nontransferablity of DETs

   A HI and its DET 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 follows:







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

   where:

   MFR Code     : 4 character code assigned by ICAO
                    (International Civil Aviation Organization,
                     a UN Agency).

   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 HHIT Suite ID and ORCHID
   hash will take the full 15 characters (as described below) of the MFR
   SN field.

   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
   encoding of the binary HHIT Suite ID and ORCHID hash in 15
   characters.  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 72 bits (HHIT Suite ID | ORCHID hash) of the HHIT SHALL
   be left-padded with 3 bits of zeros.  This 75-bit number will be
   encoded into the 15-character MFR SN field using the digit/letters
   above.  The manufacturer MUST use a Length Code of F (15).

   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:

       8653F02T7B8RA85D19LX

   A mapping service (e.g., DNS) MUST provide a trusted (e.g., via
   DNSSEC [RFC4034]) conversion of the 4-character Manufacturer Code to
   high-order 58 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 (Section 2.2 of [RFC9153]).  The DET is used in the
   Authentication Messages (i.e., the messages that provide framing for
   authentication data only).



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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 8).

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 rationale at the time of
   developing ORCHID was attacks against these fields are Denial-of-
   Service (DoS) attacks against protocols using ORCHIDs and thus up to
   those protocols to address the issue.

   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 (Section 4.5).

4.5.  DRIP Entity Tag (DET) Registry

   DETs are registered to HDAs.  A registration process,
   [drip-registries], ensures DET global uniqueness (ID-4 in [RFC9153]).
   It also provides the mechanism to create UAS public/private data that
   are associated with the DET (REG-1 and REG-2 in [RFC9153]).

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 [RFC9153]).
   In practice, the Wrapper and Manifest authentication formats
   (Sections 6.3.3 and 6.3.4 of [drip-authentication]) implicitly
   provide this self-attestation.  A lookup service like DNS can provide
   the HI and registration proof (GEN-3 in [RFC9153]).

   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
   (Section 2.2 of [RFC9153]) with no room for growth.  In practice,
   [drip-authentication] provides this offline self-attestation in two



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   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 [RFC9153]).  When a
   Location/Vector Message (i.e., a message that provides UA location,
   altitude, heading, speed, and status) 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.

   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.
   The following are examples of how this may be done.  This will serve
   as guidance to the actual deployment of DETs in DNS.  However, this
   document does not intend to provide a recommendation.  Further DNS-
   related considerations are covered in [drip-registries].

   *  As FQDNs, for example, ".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
   [RFC9153]).  For example, the USS for the HHIT could be found via the
   following: assume the RAA is decimal 100 and the HDA is decimal 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.

   The DET reverse lookup can be a standard IPv6 reverse look up, or it
   can leverage off the HHIT structure.  Using the allocated prefix for
   HHITs TBD6 [suggested value 2001:30::/28] (See Section 3.1), the RAA
   is 10 and the HDA is 20, the DET is:

       2001:30:280:1405:a3ad:1952:ad0:a69e




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   A DET reverse lookup could be to:

       a69e.ad0.1952.a3ad.1405.280.30.2001.20.10.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  5.0.4.1.0.8.2.0.0.3.0.0.1.0.0.2.ip6.arpa.
       e.9.6.a.0.d.a.0.2.5.9.1.d.a.3.a    IN   PTR
       a3ad1952ad0a69e.20.10.det.rid.icao.int.

   This DNS entry for the DET can also provide a revocation service.
   For example, instead of returning the HI RR it may return some record
   showing that the HI (and thus DET) has been revoked.  Guidance on
   revocation service will be provided in [drip-registries].

6.  Other UAS Traffic Management (UTM) Uses of HHITs Beyond DET

   HHITs will be used within the UTM architecture beyond DET (and USS in
   UA ID registration and authentication), for example, as a Ground
   Control Station (GCS) HHIT ID.  Some GCS will use its HHIT for
   securing its Network Remote ID (to USS HHIT) and Command and Control
   (C2, Section 2.2.2 of [RFC9153]) transports.

   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.  Details about
   such issues are out of the scope of this document).

7.  Summary of Addressed DRIP Requirements

   This document provides the details to solutions for GEN 1 - 3, ID 1 -
   5, and REG 1 - 2 requirements that are described in [RFC9153].

8.  IANA Considerations

8.1.  New Well-Known IPv6 prefix 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.





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   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] (TBD6, suggested: 2001:30::/28).

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

   RFC:
      This document.

   Allocation Date:
      Date this document published.

   Termination Date:
      Forever.

   Source:
      False.

   Destination:
      False.

   Forwardable:
      False.

   Globally Reachable:
      False.

   Reserved-by-Protocol:
      False.

8.2.  New IANA DRIP Registry

   This document requests IANA to create a new registry titled "Drone
   Remote ID Protocol" registry.  The following two subregistries should
   be created under that registry.

   Hierarchical HIT (HHIT) Prefixes:
      Initially, for DET use, one 28-bit prefix should be assigned out
      of the IANA IPv6 Special Purpose Address Block, namely 2001::/23,
      as per [RFC6890].  Future additions to this subregistry are to be
      made through Expert Review (Section 4.5 of [RFC8126]).  Entries
      with network-specific prefixes may be present in the registry.

        HHIT Use     Bits  Value
        DET          28    TBD6 (suggested value 2001:30::/28)




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   Hierarchical HIT (HHIT) Suite ID:
      This 8-bit valued subregistry is a superset of the 4/8-bit "HIT
      Suite ID" subregistry of the "Host Identity Protocol (HIP)
      Parameters" registry in [IANA-HIP].  Future additions to this
      subregistry are to be made through IETF Review (Section 4.8 of
      [RFC8126]).  The following HHIT Suite IDs are defined:

        HHIT Suite          Value
        RESERVED            0
        RSA,DSA/SHA-256     1    [RFC7401]
        ECDSA/SHA-384       2    [RFC7401]
        ECDSA_LOW/SHA-1     3    [RFC7401]
        EdDSA/cSHAKE128     TBD3 (suggested value 5)   (RECOMMENDED)
        HDA Private Use 1   TBD4 (suggested value 254)
        HDA Private Use 2   TBD5 (suggested value 255)


      The HHIT Suite ID values 1 - 31 are reserved for IDs that MUST be
      replicated as HIT Suite IDs (Section 8.4) as is TBD3 here.  Higher
      values (32 - 255) are for those Suite IDs that need not or cannot
      be accommodated as a HIT Suite ID.

8.3.  IANA CGA Registry Update

   This document requests that this document be added to the reference
   field for the "CGA Extension Type Tags" registry [IANA-CGA], where
   IANA registers the following Context ID:

   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]:

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


8.4.  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.






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        Algorithm
        profiles    Values

        EdDSA       TBD1 (suggested value 13) [RFC8032]    (RECOMMENDED)


   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.  Future additions to this subregistry are to
      be made through IETF Review (Section 4.8 of [RFC8126]).

        Algorithm    Curve            Values

        EdDSA        RESERVED         0
        EdDSA        EdDSA25519       1 [RFC8032]          (RECOMMENDED)
        EdDSA        EdDSA25519ph     2 [RFC8032]
        EdDSA        EdDSA448         3 [RFC8032]          (RECOMMENDED)
        EdDSA        EdDSA448ph       4 [RFC8032]
                                      5-65535               Unassigned


   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        Value
        EdDSA/cSHAKE128  TBD3 (suggested value 5)   (RECOMMENDED)


      The HIT Suite ID 4-bit values 1 - 15 and 8-bit values 0x00 - 0x0F
      MUST be replicated as HHIT Suite IDs (Section 8.2) as is TBD3
      here.

8.5.  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.






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      Value  Description

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


9.  Security Considerations

   The 64-bit hash in HHITs presents a real risk of second pre-image
   cryptographic hash attack Section 9.5.  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); an attacker may well be
   satisfied stealing any one of them.  Then rather than needing to
   satisfy a 64-bit condition on the cSHAKE128 output, an attacker needs
   only to satisfy what is equivalent to a 54-bit condition (since there
   are 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 9.5, it is computationally and economically feasible.
   Therefore, the HHIT registration and HHIT/HI registration validation
   is strongly recommended.

   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 messages containing DETs using DNS to find the HI for the
   DET.  As such, use of DNSSEC by the DET registries is recommended to
   provide trust in HI retrieval.

   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 detailed in [drip-authentication].





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   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.

9.1.  Post Quantum Computing out of scope

   As stated in Section 8.1 of [drip-architecture], there has been no
   effort, at this time, to address post quantum computing cryptography.
   UAs and Broadcast Remote ID communications are so constrained that
   current post quantum computing cryptography is not applicable.  Plus
   since a UA may use a unique DET for each operation, the attack window
   could be limited to the duration of the operation.

   HHITs contain the ID for the cryptographic suite used in its
   creation, a future post quantum computing safe algorithm that fits
   the Remote ID constraints may readily be added.

9.2.  DET Trust in ASTM messaging

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



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   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.  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,
   thus asserting DET trust.

9.3.  DET Revocation

   The DNS entry for the DET can also provide a revocation service.  For
   example, instead of returning the HI RR it may return some record
   showing that the HI (and thus DET) has been revoked.  Guidance on
   revocation service will be provided in [drip-registries].

9.4.  Privacy Considerations

   There is no expectation of privacy for DETs; it is not part of the
   privacy normative requirements listed in, Section 4.3.1, of
   [RFC9153].  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.

















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   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 MAC 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.

   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 DET
   only starts the changes that need to be implemented.

   These privacy realities may present challenges for the EU U-space
   (Appendix A) program.

9.5.  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 C for the collision probability formula.








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   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.

   Thus an adversary trying to generate a collision and 'steal' the DET
   would run afoul of this registration process and associated
   validation process mentioned in Section 1.1.

10.  References

10.1.  Normative References

   [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>.




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   [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>.

   [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>.

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

   [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>.

10.2.  Informative References

   [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>.

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

   [drip-architecture]
              Card, S. W., Wiethuechter, A., Moskowitz, R., Zhao, S.,
              and A. Gurtov, "Drone Remote Identification Protocol
              (DRIP) Architecture", Work in Progress, Internet-Draft,
              draft-ietf-drip-arch-24, 10 June 2022,
              <https://datatracker.ietf.org/doc/html/draft-ietf-drip-
              arch-24>.

   [drip-authentication]
              Wiethuechter, A., Card, S., and R. Moskowitz, "DRIP Entity
              Tag Authentication Formats & Protocols for Broadcast
              Remote ID", Work in Progress, Internet-Draft, draft-ietf-
              drip-auth-14, 21 June 2022,
              <https://datatracker.ietf.org/doc/html/draft-ietf-drip-
              auth-14>.




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   [drip-registries]
              Wiethuechter, A., Card, S., Moskowitz, R., and J. Reid,
              "DRIP Entity Tag Registration & Lookup", Work in Progress,
              Internet-Draft, draft-ietf-drip-registries-04, 24 June
              2022, <https://datatracker.ietf.org/doc/html/draft-ietf-
              drip-registries-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>.

   [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>.

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

   [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>.

   [RFC4034]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "Resource Records for the DNS Security Extensions",
              RFC 4034, DOI 10.17487/RFC4034, March 2005,
              <https://www.rfc-editor.org/info/rfc4034>.






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   [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>.

   [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>.

   [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>.

   [RFC9153]  Card, S., Ed., Wiethuechter, A., Moskowitz, R., and A.
              Gurtov, "Drone Remote Identification Protocol (DRIP)
              Requirements and Terminology", RFC 9153,
              DOI 10.17487/RFC9153, February 2022,
              <https://www.rfc-editor.org/info/rfc9153>.

   [RFC9224]  Blanchet, M., "Finding the Authoritative Registration Data
              Access Protocol (RDAP) Service", STD 95, RFC 9224,
              DOI 10.17487/RFC9224, March 2022,
              <https://www.rfc-editor.org/info/rfc9224>.








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Appendix A.  EU U-Space RID Privacy Considerations

   The 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 the 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.

   This is potentially a contrary expectation as that presented in
   Section 9.4.  U-space will have to deal with this reality within the
   GDPR regulations.  Still, DETs as defined here present a large step
   in the right direction for agnostic IDs.

   The project lists "E-Identification" and "E-Registrations" services
   as to be developed.  These services can use DETs and follow the
   privacy considerations outlined in this document for DETs.

   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 DET (Hierarchical HIT)
   provides 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.  The 14/14 HID split

   The following explains the logic behind selecting to divide the 28
   bits of the HID into 2 14-bit components.

   At this writing ICAO has 273 member "States", each may want to
   control RID assignment within its National Air Space (NAS).  Some
   members may want separate RAAs to use for Civil, general Government,



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   and Military use.  They may also want allowances for competing Civil
   RAA operations.  It is reasonable to plan for 8 RAAs per ICAO member
   (plus regional aviation organizations like in the European Union).
   Thus at a start a 4,096 RAA space is advised.

   There will be requests by commercial entities for their own, RAA
   allotments.  Examples could include international organizations that
   will be using UAS and international delivery service associations.
   These may be smaller than the RAA space needed by ICAO member States
   and could be met with a 2,048 space allotment, but as will be seen,
   might as well be 4,096 as well.

   This may well cover currently understood RAA entities.  There will be
   future new applications, branching off into new areas.  So yet
   another space allocation should be set aside.  If this is equal to
   all that has been reserved, we should allow for 16,384 (2^14) RAAs.

   The HDA allocation follows a different logic from that of RAAs.  Per
   Appendix C, an HDA should be able to easily assign 63M RIDs and even
   manage 663M with a "first come, first assigned" registration process.
   For most HDAs this is more than enough, and a single HDA assignment
   within their RAA will suffice.  Most RAAs will only delegate to a
   couple HDAs for their operational needs.  But there are major
   exceptions that point to some RAAs needing large numbers of HDA
   assignments.

   Delivery service operators like Amazon (est. 30K delivery vans) and
   UPS (est. 500K delivery vans) may choose, for anti-tracking reasons,
   to use unique RIDs per day or even per operation.  30K delivery UA
   could need 11M upwards to 44M RIDs.  Anti-tracking would be hard to
   provide if the HID were the same for a delivery service fleet, so
   such a company may turn to an HDA that provides this service to
   multiple companies so that who's UA is who's is not evident in the
   HID.  A USS providing this service could well use multiple HDA
   assignments per year, depending on strategy.

   Perhaps a single RAA providing HDAs for delivery service (or similar
   behaving) UAS could 'get by' with a 2048 HDA space (11-bits).  So the
   HDA space could well be served with only 12 bits allocated out of the
   28-bit HID space.  But as this is speculation, and it will take years
   of deployment experience, a 14-bit HDA space has been selected.










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   There may also be 'small' ICAO member States that opt for a single
   RAA and allocate their HDAs for all UA that are permitted in their
   NAS.  The HDA space is large enough that some to use part for
   government needs as stated above and for small commercial needs.  Or
   the State may use a separate, consecutive RAA for commercial users.
   Thus it would be 'easy' to recognize State-approved UA by HID high-
   order bits.

Appendix C.  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.

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



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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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