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DRIP Authentication Formats for Broadcast RID
draft-ietf-drip-auth-02

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Authors Adam Wiethuechter , Stuart W. Card , Robert Moskowitz
Last updated 2021-10-21
Replaces draft-wiethuechter-drip-auth
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draft-ietf-drip-auth-02
DRIP Working Group                                       A. Wiethuechter
Internet-Draft                                                   S. Card
Intended status: Standards Track                      AX Enterprize, LLC
Expires: 24 April 2022                                      R. Moskowitz
                                                          HTT Consulting
                                                         21 October 2021

             DRIP Authentication Formats for Broadcast RID
                        draft-ietf-drip-auth-02

Abstract

   This document describes how to include trust into the ASTM Remote ID
   specification defined in ASTM F3411-19 under a Broadcast Remote ID
   (RID) scenario.  It defines a few different message schemes (based on
   the Authentication Message) that can be used to assure past messages
   sent by a UA and also act as an assurance for UA trustworthiness in
   the absence of Internet connectivity at the receiving node.

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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   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 24 April 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 Simplified BSD License text
   as described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  DRIP Requirements Addressed . . . . . . . . . . . . . . .   4
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
     2.1.  Required Terminology  . . . . . . . . . . . . . . . . . .   4
     2.2.  Definitions . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Background  . . . . . . . . . . . . . . . . . . . . . . . . .   4
     3.1.  Problem Space and Focus . . . . . . . . . . . . . . . . .   4
     3.2.  Reasoning for IETF DRIP Authentication  . . . . . . . . .   5
     3.3.  ASTM Authentication Message . . . . . . . . . . . . . . .   5
   4.  DRIP Authentication Formats . . . . . . . . . . . . . . . . .   7
     4.1.  UAS ID Signature  . . . . . . . . . . . . . . . . . . . .   7
     4.2.  Operator ID Signature . . . . . . . . . . . . . . . . . .   8
     4.3.  Message Set Signature . . . . . . . . . . . . . . . . . .   9
     4.4.  Specific Method . . . . . . . . . . . . . . . . . . . . .  10
       4.4.1.  DRIP Frame  . . . . . . . . . . . . . . . . . . . . .  10
       4.4.2.  DRIP Wrapper Format . . . . . . . . . . . . . . . . .  12
       4.4.3.  DRIP Manifest Format  . . . . . . . . . . . . . . . .  12
       4.4.4.  DRIP Link Format  . . . . . . . . . . . . . . . . . .  13
   5.  Transport Methods & Recommendations . . . . . . . . . . . . .  14
     5.1.  Legacy Advertisements (Bluetooth 4.X) . . . . . . . . . .  14
     5.2.  Extended Advertisements (Bluetooth 5.X, Wi-Fi NaN, Wi-Fi
           Beacon) . . . . . . . . . . . . . . . . . . . . . . . . .  14
     5.3.  DRIP Recommendations  . . . . . . . . . . . . . . . . . .  14
       5.3.1.  DRIP Wrapper vs. DRIP Manifest  . . . . . . . . . . .  15
   6.  ICAO Considerations . . . . . . . . . . . . . . . . . . . . .  15
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  15
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  15
     8.1.  Manifest Hash Length  . . . . . . . . . . . . . . . . . .  15
     8.2.  Replay Attacks  . . . . . . . . . . . . . . . . . . . . .  16
     8.3.  Trust Timestamp Offsets . . . . . . . . . . . . . . . . .  17
   9.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  17
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  17
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  17
     10.2.  Informative References . . . . . . . . . . . . . . . . .  18
   Appendix A.  Thoughts on ASTM Authentication Message  . . . . . .  18
   Appendix B.  DRIP Attestations  . . . . . . . . . . . . . . . . .  19
     B.1.  Self-Attestation (SA-xx)  . . . . . . . . . . . . . . . .  19

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     B.2.  Attestation (A-xy)  . . . . . . . . . . . . . . . . . . .  20
     B.3.  Concise Attestation (CA-xy) . . . . . . . . . . . . . . .  21
     B.4.  Mutual Attestation (MA-xy)  . . . . . . . . . . . . . . .  22
     B.5.  Link Attestation (LA-xy)  . . . . . . . . . . . . . . . .  23
     B.6.  Broadcast Attestation (BA-xy) . . . . . . . . . . . . . .  24
     B.7.  Attestation Certificate (AC-zxy)  . . . . . . . . . . . .  26
     B.8.  Concise Certificate (CC-zxy)  . . . . . . . . . . . . . .  26
     B.9.  Link Certificate (LC-zxy) . . . . . . . . . . . . . . . .  27
     B.10. Mutual Certificate (MC-zxy) . . . . . . . . . . . . . . .  28
     B.11. Example Registration with Attestation . . . . . . . . . .  29
       B.11.1.  Operator to Registry . . . . . . . . . . . . . . . .  30
       B.11.2.  Aircraft to Operator . . . . . . . . . . . . . . . .  30
       B.11.3.  Aircraft to Registry . . . . . . . . . . . . . . . .  31
   Appendix C.  DRIP Broadcast Attestation Structure . . . . . . . .  31
     C.1.  Attestor Hierarchical Host Identity Tag . . . . . . . . .  33
     C.2.  Attestation Data  . . . . . . . . . . . . . . . . . . . .  33
     C.3.  Trust Timestamp . . . . . . . . . . . . . . . . . . . . .  33
     C.4.  Signing Timestamp . . . . . . . . . . . . . . . . . . . .  33
     C.5.  Attestor Signature  . . . . . . . . . . . . . . . . . . .  33
   Appendix D.  Forward Error Correction . . . . . . . . . . . . . .  33
     D.1.  Encoding  . . . . . . . . . . . . . . . . . . . . . . . .  34
       D.1.1.  Single Page FEC . . . . . . . . . . . . . . . . . . .  34
       D.1.2.  Multi Page FEC  . . . . . . . . . . . . . . . . . . .  34
     D.2.  Decoding  . . . . . . . . . . . . . . . . . . . . . . . .  34
       D.2.1.  Single Page FEC . . . . . . . . . . . . . . . . . . .  35
       D.2.2.  Multi Page FEC  . . . . . . . . . . . . . . . . . . .  35
     D.3.  FEC Limitations . . . . . . . . . . . . . . . . . . . . .  35
   Appendix E.  Example Authentication Messages  . . . . . . . . . .  35
     E.1.  Authentication Data Only  . . . . . . . . . . . . . . . .  35
     E.2.  Authentication Data & Additional Data . . . . . . . . . .  36
     E.3.  DRIP Link Example . . . . . . . . . . . . . . . . . . . .  38
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  40

1.  Introduction

   UA Systems (UAS) are usually in a volatile environment when it comes
   to communication.  UA are generally small with little computational
   (or flying) horsepower to carry standard communication equipment.
   This limits the mediums of communication to few viable options.

   Observer systems (e.g. smartphones and tablets) place further
   constraints on the communication options.  The Remote ID Broadcast
   messages MUST be available to applications on these platforms without
   modifying the devices.

   The ASTM standard [F3411-19] focuses on two ways of communicating to
   a UAS for RID: Broadcast and Network.

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   This document will focus on adding trust to Broadcast RID in the
   current (and an expanded) Authentication Message format.

1.1.  DRIP Requirements Addressed

   The following [drip-requirements] will be addressed:

   GEN 1: Provable Ownership  This will be addressed using the DRIP Link
      and DRIP Wrapper or DRIP Manifest.

   GEN 2: Provable Binding  This requirement is addressed using the DRIP
      Wrapper or DRIP Manifest.

   GEN 3: Provable Registration  This requirement is addressed using the
      DRIP Link.

   See Section 5.3 for further clarification.

2.  Terminology

2.1.  Required 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.  Definitions

   See [drip-requirements] for common DRIP terms.

   Aircraft:  In this document whenever the word Aircraft is used it is
      referring to an Unmanned Aircraft (UA) not a Manned Aircraft.

3.  Background

3.1.  Problem Space and Focus

   The current standard for Remote ID (RID) does not, in any meaningful
   capacity, address the concerns of trust in the UA space with
   communication in the Broadcast RID environment.  This is a
   requirement that will need to be addressed eventually for various
   different parties that have a stake in the UA industry.

   The following subsections will provide a high level reference to the
   ASTM standard for Authentication Messages and how their current
   limitations effect trust in the Broadcast RID environment.

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3.2.  Reasoning for IETF DRIP Authentication

   The ASTM Authentication Message has provisions in [F3411-19] to allow
   for other organizations to define (and standardize) Authentication
   formats.  The standardization of special formats to support the DRIP
   requirements in UAS RID for trustworthy communications over Broadcast
   RID is an important part of the chain of trust for a UAS ID.  No
   existing formats (defined by ASTM or others) was flexible enough to
   satisfy this goal resulting in the work reflected in this document.

3.3.  ASTM Authentication Message

   The ASTM Authentication Message is a unique message in the Broadcast
   F3411 standard as it is the only one that is paged.

      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
     +---------------+---------------+---------------+---------------+
     | Auth. Header  |                                               |
     +---------------+                                               |
     |                                                               |
     |                                                               |
     |                     Authentication Payload                    |
     |                                                               |
     |                                                               |
     +---------------+---------------+---------------+---------------+

     Auth. Header: (1 byte)
         Contains Authentication Type (AuthType) and Page Number.

     Authentication Payload: (23 bytes per page)
         Authentication Payload, including headers. Null padded.

            Figure 1: Standard ASTM Authentication Message Page

   Figure 1 is an abstracted view of a single Authentication Page used
   in an Authentication Message.  There is a max of 16 pages (indexed 0
   to 15 in the first byte; "Auth.  Header") that can be sent for a
   single Authentication Message, with each page carrying a max 23-byte
   "Authentication Payload".

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      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
     +---------------+---------------+---------------+---------------+
     |                   ASTM Authentication Headers                 |
     |                               +---------------+---------------+
     |                               |                               |
     +---------------+---------------+                               |
     .                                                               .
     .                Authentication Data / Signature                .
     .                                                               .
     |                                                               |
     +---------------+---------------+---------------+---------------+
     |      ADL      |                                               |
     +---------------+                                               |
     .                                                               .
     .                       Additional Data                         .
     .                                                               .
     |                                                               |
     +---------------+---------------+---------------+---------------+

     ASTM Authentication Headers: (6 bytes)
         Contains other header information for the Authentication
         Message from ASTM UAS RID Standard.

     Authentication Data / Signature: (0 to 255 bytes)
         Opaque authentication data.

     Additional Data Length (ADL): (1 byte - unsigned)
         Length in bytes of Additional Data.

     Additional Data: (0 to 255 bytes):
         Data that follows the Authentication Data / Signature but
         is not considered part of the Authentication Data.

                Figure 2: ASTM Authentication Message Fields

   Figure 2 is the abstract view of the data fields found in the
   Authentication Message as defined by [F3411-19].  This data is placed
   into Figure 1's "Authentication Payload", potentially spanning
   multiple pages.

   When "Additional Data" is being sent, a single unsigned byte
   ("Additional Data Length") directly follows the "Authentication Data
   / Signature" and has the length, in bytes, of the following
   "Additional Data".

   Full examples of Authentication Messages (fully paginated; both with
   and without Additional Data) can be found in Appendix E.

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4.  DRIP Authentication Formats

   To keep consistent formatting across the different mediums (Bluetooth
   4, Bluetooth 5 and Wi-Fi NaN) and their independent restrictions the
   authentication data being sent is REQUIRED to fit within the first 9
   pages (Page 0 through Page 8) of the Authentication Message.  Under
   DRIP, the "Length" field in the "ASTM Authentication Headers" (which
   denotes the length in bytes of Authentication Data only) MUST NOT
   exceed the value of 201.

   All formats defined in this section fill the "Authentication Data /
   Signature" field in Figure 2.

   When sending data over a medium that does not have underlying Forward
   Error Correction (FEC), for example Bluetooth 4, then Appendix D MUST
   be used.

4.1.  UAS ID Signature

   The existing ASTM [F3411-19] Authentication Type 0x1 can be used to
   send a fresh Self-Attestation of the UA over 7 pages.

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      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
     +---------------+---------------+---------------+---------------+
     |                                                               |
     |                        UA Hierarchical                        |
     |                       Host Identity Tag                       |
     |                                                               |
     +---------------+---------------+---------------+---------------+
     |                                                               |
     |                                                               |
     |                                                               |
     |                        UA Host Identity                       |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     +---------------+---------------+---------------+---------------+
     |                         Trust Timestamp                       |
     +---------------+---------------+---------------+---------------+
     |                        Signing Timestamp                      |
     +---------------+---------------+---------------+---------------+
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                          UA Signature                         |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     +---------------+---------------+---------------+---------------+

                      Figure 3: DRIP UAS ID Signature

4.2.  Operator ID Signature

   The existing ASTM [F3411-19] Authentication Type 0x2 can be used to
   send a static Self-Attestation of the Operator over 7 pages.

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      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
     +---------------+---------------+---------------+---------------+
     |                                                               |
     |                     Operator Hierarchical                     |
     |                       Host Identity Tag                       |
     |                                                               |
     +---------------+---------------+---------------+---------------+
     |                                                               |
     |                                                               |
     |                                                               |
     |                     Operator Host Identity                    |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     +---------------+---------------+---------------+---------------+
     |                         Trust Timestamp                       |
     +---------------+---------------+---------------+---------------+
     |                        Signing Timestamp                      |
     +---------------+---------------+---------------+---------------+
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                       Operator Signature                      |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     +---------------+---------------+---------------+---------------+

                    Figure 4: DRIP Operator ID Signature

4.3.  Message Set Signature

   When running under Extended Advertisements, the existing ASTM
   [F3411-19] Authentication Type 0x3 can be used to sign over the
   adjacent ASTM Messages in the Message Pack (0xF).

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   The concatenation of all messages in the Message Pack (excluding
   Authentication) before signing MUST be in Message Type order and be
   placed between the UA HHIT and Trust Timestamp field.

      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
     +---------------+---------------+---------------+---------------+
     |                                                               |
     |                        UA Hierarchical                        |
     |                       Host Identity Tag                       |
     |                                                               |
     +---------------+---------------+---------------+---------------+
     |                         Trust Timestamp                       |
     +---------------+---------------+---------------+---------------+
     |                        Signing Timestamp                      |
     +---------------+---------------+---------------+---------------+
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                          UA Signature                         |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     +---------------+---------------+---------------+---------------+

                    Figure 5: DRIP Message Set Signature

4.4.  Specific Method

   For ASTM Specific Method (Authentication Type 0x5) a special Specific
   Authentication Method (SAM) Type is used to multiplex between various
   formats.

4.4.1.  DRIP Frame

   This is the general frame to hold DRIP Authentication for Specific
   Method; containing the SAM Type and laying out the major data fields.

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      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
     +---------------+---------------+---------------+---------------+
     |   SAM Type    |                                               |
     +---------------+                                               |
     .                                                               .
     .                    DRIP Authentication Data                   .
     .                                                               .
     |                                                               |
     +---------------+---------------+---------------+---------------+

     SAM Type (1 byte):
         For DRIP there are four SAM Type's allocated:

             SAM Type                     | Value
             -----------------------------+-------
             DRIP Frame                   | 1
             DRIP Wrapper                 | 2
             DRIP Manifest                | 3m
             DRIP Link                    | 4

     DRIP Authentication Data (0 to 200 bytes):
         DRIP Authentication data.

                        Figure 6: DRIP Frame Format

4.4.1.1.  Specific Authentication Method (SAM) Type

   The SAM Type field is maintained by the International Civil Aviation
   Organization (ICAO) and for DRIP four are allocated; DRIP Frame
   (0x01) (Section 4.4.1), DRIP Wrapper (0x02), DRIP Manifest (0x03) and
   DRIP Link (0x04).

4.4.1.2.  DRIP Authentication Data

   This field has a maximum size of 200 bytes.  When possible the DRIP
   Broadcast Attestation Structure (Appendix C) should be used in this
   space.

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4.4.2.  DRIP Wrapper Format

   This is specified when the SAM Type is DRIP Wrapper.  It is
   encapsulated by the DRIP Frame (Section 4.4.1) and Broadcast
   Attestation Structure (Appendix C); filling the Attestation Data
   (Appendix C.2) field with full (25-byte) ASTM Messages.  The minimum
   number of ASTM Messages being 1 and the max being 4.  The
   encapsulated ASTM Messages MUST be in Message Type order as defined
   by ASTM.  All message types except Authentication (0x2) and Message
   Pack (0xF) are allowed.

   To determine the number of messages wrapped the receiver can check
   that the length of the Attestation Data (Appendix C.2) field of the
   DRIP Broadcast Attestation (Appendix C) is a multiple of 25-bytes.

4.4.2.1.  Wrapper Limitations

   See Section 5.3.1 for a discussion on a comparison between DRIP
   Wrapper and DRIP Manifest.

4.4.3.  DRIP Manifest Format

   This format is specified when SAM Type is set to DRIP Manifest.  It
   is encapsulated by the DRIP Frame (Section 4.4.1) and Broadcast
   Attestation Structure (Appendix C); filling the Attestation Data
   (Appendix C.2) field with 8-byte hashes of previous ASTM Messages.

   By hashing previously sent messages and signing them we gain trust in
   UAs previous reports.  An observer who has been listening for any
   considerable length of time can hash received messages and cross-
   check against listed hashes.  This is a way to evade the limitation
   of a maximum of 4 messages in the Wrapper Format and reduce overhead.

   (Editors Note: Manifests MUST NOT be of a length multiple of 25-bytes
   or 48-bytes.)

4.4.3.1.  Hash Algorithms and Operation

   The hash algorithm used for the Manifest Message is the same hash
   algorithm used in creation of the HHIT that is signing the Manifest.

   A standard HHIT would be using cSHAKE128 from [NIST.SP.800-185].
   With cSHAKE128, the hash is computed as follows:

   cSHAKE128(Message, 128, "", "Remote ID Auth Hash")

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4.4.3.2.  Pseudo-Blockchain Hashes

   Two special hashes are included in all Manifest messages; a previous
   manifest hash, which links to the previous manifest message, as well
   as a current manifest hash.  This gives a pseudo-blockchain
   provenance to the manifest message that could be traced back if the
   observer was present for extended periods of time.

   Creation:  During creation and signing of this message format this
      field MUST be set to 0.  So the signature will be based on this
      field being 0, as well as its own hash.  It is an open question of
      if we compute the hash, then sign or sign then compute.

   Cycling:  There a few different ways to cycle this message.  We can
      "roll up" the hash of 'current' to 'previous' when needed or to
      completely recompute the hash.  This mostly depends on the
      previous note.

4.4.3.3.  Manifest Limitations

   A potential limitation to this format is dwell time of the UA.  If
   the UA is not sticking to a general area then most likely the
   Observer will not obtain many (if not all) of the messages in the
   manifest.  Without the original messages received no verification can
   be done.  Examples of such scenarios include delivery or survey UA.

   See Section 5.3.1 for a discussion on a comparison between DRIP
   Wrapper and DRIP Manifest.

   Another limitation is the length of hash, which is discussed in
   Section 8.1.

4.4.4.  DRIP Link Format

   This format is specified when SAM Type is set to DRIP Link.  It is
   encapsulated by the DRIP Frame (Section 4.4.1) and Broadcast
   Attestation Structure (Appendix C) but the attestation has already
   taken place, thus the UA need not dynamically sign the structure.

   For details on the Broadcast Attestation see [drip-rid] and
   Appendix B.6.

4.4.4.1.  Link Limitations

   See Section 8.2 for details on why this structure is not dynamically
   signed.

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5.  Transport Methods & Recommendations

5.1.  Legacy Advertisements (Bluetooth 4.X)

   With Legacy Advertisements the goal is to attempt to bring reliable
   receipt of the paged Authentication Message.  Forward Error
   Correction (Appendix D) MUST be used when using Legacy Advertising
   methods (such as Bluetooth 4.X).

   Under ASTM Bluetooth 4.X rules, transmission of dynamic messages are
   at least every 1 second while static messages (which is what
   Authentication is classified under) are sent at least every 3
   seconds.

5.2.  Extended Advertisements (Bluetooth 5.X, Wi-Fi NaN, Wi-Fi Beacon)

   Under the ASTM specification, Bluetooth 5 or Wi-Fi NaN transport of
   Remote ID is to use the Message Pack (Type 0xF) format for all
   transmissions.  Under Message Pack messages are sent together (in
   Message Type order) in a single Bluetooth frame (up to 9 single frame
   equivalent messages).  Message Packs are required by ASTM to be sent
   at a rate of 1 per second (like dynamic messages).

   Without any fragmentation or loss of pages with transmission Forward
   Error Correction (Appendix D) MUST NOT be used as it is impractical.

5.3.  DRIP Recommendations

   It is REQUIRED that an aircraft send the following Authentication
   Formats to fulfill the [drip-requirements]:

   1.  DRIP Link using the Broadcast Attestation of USS and the UA
       (satisfying GEN-1 and GEN-3)

   2.  Any other DRIP Authentication Format (RECOMMENDED: DRIP Manifest
       or DRIP Wrapper) where the UA is dynamically signing data
       (satisfying GEN-1 and GEN-2)

   It is RECOMMENDED the following set of Authentication Formats are
   sent for support of offline Observers:

   1.  DRIP Link using the Broadcast Attestation of HID Root and the CAA
       (satisfies GEN-3)

   2.  DRIP Link using the Broadcast Attestation of CAA and the USS
       (satisfies GEN-3)

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   3.  DRIP Link using the Broadcast Attestation of USS and the UA
       (satisfies GEN-1 and GEN-3)

   4.  Any other DRIP Authentication Format (RECOMMENDED: DRIP Manifest
       or DRIP Wrapper) where the UA is dynamically signing data
       (satisfies GEN-1 and GEN-2)

5.3.1.  DRIP Wrapper vs. DRIP Manifest

   (Editors Note: review this to confirm numbers are still accurate.)

   A single Manifest can carry at most (using the full 9-page limit and
   8 byte hashes) 12 unique hashes of previously sent messages (of any
   type).  This results in a total of 22 (12 + 10) frames of Bluetooth
   data being transmitted over Bluetooth.

   In comparison, the Message Wrapper sends 6 pages (each a single
   frame) for each wrapped message.  For backwards compatibility the
   implementation should also send the standard ASTM message that was
   wrapped for non-DRIP compliant receivers to obtain.  This method
   results in 84 total Bluetooth frames (12 + (12 * 6)) sent.

   The question of which is better suited is up to the implementation.

6.  ICAO Considerations

   DRIP requests the following SAM Type's to be allocated:

   1.  DRIP Frame

   2.  DRIP Wrapper

   3.  DRIP Manifest

   4.  DRIP Link

7.  IANA Considerations

   This document does not require any actions by IANA.

8.  Security Considerations

8.1.  Manifest Hash Length

   For DRIP Manifest an 8-byte hash length has been selected by the
   authors for a number of reasons.

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   1.  Hash lengths smaller than 8-bytes (for example 4-bytes) were
       originally contemplated but ruled out by comments by various
       cryptographers.  The main concern raised in this forum was that
       the length of hash would not provide strong resistance against
       collision rate.  The authors also after further review agreed
       with this and also realized operationally it was not necessarily
       viable.  While 4-byte hashes would allow more messages to be
       filled into a single DRIP Manifest payload (up to 22 individual
       hashes) the length of time for the UA to stay in a single place
       where the Observer would receive all the originally messages to
       rehash to verify such a message was impractical.

   2.  Hash lengths larger than 8-bytes (for example 16-bytes) were also
       considered by the authors.  These got the approval of the
       cryptographers but the number of hashes to send became much lower
       (only 5 individual hashes).  While this lower number is a more
       reasonable number of original messages the Observer would have to
       capture it would also mean that potentially more DRIP Manifests
       would need to be sent.  Overall the increase length of the hash
       did not operationally justify the cost.

   3.  Simplifying the current design and locking it into using the same
       hash as the HHIT instead of allowing for agility in either hash
       algorithm or length seemed more realistic to the authors today.

8.2.  Replay Attacks

   The astute reader may note that the DRIP Link messages, which are
   recommended to be sent, are static in nature and contain various
   timestamps.  These Attestation Link messages can easily be replayed
   by an attacker who has copied them from previous broadcasts.  There
   are two things to mitigate this in DRIP:

   1.  If an attacker (who is smart and spoofs more than just the UAS
       ID/data payloads) willing replays an Attestation Link message
       they have in principle actually helped by ensuring the message is
       sent more frequently and be received by potential Observers.

   2.  It is RECOMMENDED to send more than just DRIP Link messages,
       specifically those that sign over changing data using the current
       session keypair, and those messages are sent more frequently.  An
       aircraft beaconing these messages then actually signing other
       messages using the keypair validates the data receiver by an
       Observer.  An UA who does not either run DRIP themselves or does
       not have possession of the same private key, would be clearly
       exposed upon signature verification.

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8.3.  Trust Timestamp Offsets

   Note the discussion of Trust Timestamp Offsets here is in context of
   the DRIP Wrapper (Section 4.4.2) and DRIP Manifest (Section 4.4.3)
   messages.  For DRIP Link (Section 4.4.4) messages these offsets are
   set by the Attestor (typically a registry) and have their own set of
   considerations as seen in (Editors Note: link to registry draft
   security considerations here).

   The offset of the Trust Timestamp (defined as a very short Expiration
   Timestamp) is one that needs careful consideration for any
   implementation.  The offset should be shorter than any given flight
   duration (typically less than an hour) but be long enough to be
   received and processed by Observers (larger than a few seconds).  It
   recommended that 3-5 minutes should be sufficient to serve this
   purpose in any scenario, but is not limited by design.

9.  Acknowledgments

   Ryan Quigley and James Mussi of AX Enterprize, LLC for early
   prototyping to find holes in the draft specifications.

   Soren Friis for pointing out that WiFi protocols would not give
   access to the MAC Address, originally used in calculation of the
   hashes for DRIP Manifest.  Also, for confirming that Message Packs
   (0xF) can only carry up to 9 ASTM frames worth of data (9
   Authentication pages) - this drove the requirement for max page
   length of Authentication Data itself.

10.  References

10.1.  Normative References

   [F3411-19] "Standard Specification for Remote ID and Tracking",
              February 2020.

   [NIST.SP.800-185]
              Kelsey, J., Change, S., and R. Perlner, "SHA-3 Derived
              Functions: cSHAKE, KMAC, TupleHash and ParallelHash", NIST
              Special Publication SP 800-185,
              DOI 10.6028/nist.sp.800-185, December 2016,
              <http://nvlpubs.nist.gov/nistpubs/SpecialPublications/
              NIST.SP.800-185.pdf>.

   [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>.

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   [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

   [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://www.ietf.org/archive/id/draft-ietf-drip-reqs-
              18.txt>.

   [drip-rid] Moskowitz, R., Card, S. W., Wiethuechter, A., and A.
              Gurtov, "UAS Remote ID", Work in Progress, Internet-Draft,
              draft-ietf-drip-uas-rid-01, 9 September 2020,
              <https://www.ietf.org/archive/id/draft-ietf-drip-uas-rid-
              01.txt>.

   [identity-claims]
              Wiethuechter, A., Card, S., and R. Moskowitz, "DRIP
              Identity Claims", Work in Progress, Internet-Draft, draft-
              wiethuechter-drip-identity-claims-03, 2 November 2020,
              <https://www.ietf.org/archive/id/draft-wiethuechter-drip-
              identity-claims-03.txt>.

   [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>.

Appendix A.  Thoughts on ASTM Authentication Message

   (Editor Note: is this valid anymore to keep?)

   The format standardized by the ASTM is designed with a few major
   considerations in mind, which the authors of this document feel put
   significant limitations on the expansion of the standard.

   The primary consideration (in this context) is the use of the
   Bluetooth 5.X Extended Frame format.  This method allows for a 255
   byte payload to be sent in what the ASTM refers to as a "Message
   Pack".

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   The idea is to include up to five standard ASTM Broadcast RID
   messages (each of which are 25 bytes) plus a single authentication
   message (5 pages of 25 bytes each) in the Message Pack.  The
   reasoning is then the Authentication Message is for the entire
   Message Pack.

   The authors have no issues with this proposed approach; this is a
   valid format to use for the Authentication Message provided by the
   ASTM.  However, by limiting the Authentication Message to ONLY five
   pages in the standard it ignores the possibility of other formatting
   options to be created and used.

   Another issue with this format, not fully addressed in this document
   is fragmentation.  Under Bluetooth 4.X, each page is sent separately
   which can result in lose of pages on the receiver.  This is
   disastrous as the loss of even a single page means any signature is
   incomplete.

   With the current limitation of 5 pages, Forward Error Correction
   (FEC) is nearly impossible without sacrificing the amount of data
   sent.  More pages would allow FEC to be performed on the
   Authentication Message pages so loss of pages can be mitigated.

   All these problems are further amplified by the speed at which UA fly
   and the Observer's position to receive transmissions.  There is no
   guarantee that the Observer will receive all the pages of even a 5
   page Authentication Message in the time it takes a UA to traverse
   across their line of sight.  Worse still is that is not including
   other UA in the area, which congests the spectrum and could cause
   further confusion attempting to collate messages from various UA.
   This specific problem is out of scope for this document and our
   solutions in general, but should be noted as a design consideration.

Appendix B.  DRIP Attestations

B.1.  Self-Attestation (SA-xx)

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      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
     +---------------+---------------+---------------+---------------+
     |                                                               |
     |                          Hierarchical                         |
     |                       Host Identity Tag                       |
     |                                                               |
     +---------------+---------------+---------------+---------------+
     |                                                               |
     |                                                               |
     |                                                               |
     |                          Host Identity                        |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     +---------------+---------------+---------------+---------------+
     |                         Trust Timestamp                       |
     +---------------+---------------+---------------+---------------+
     |                        Signing Timestamp                      |
     +---------------+---------------+---------------+---------------+
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                            Signature                          |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     +---------------+---------------+---------------+---------------+

     Length = 120-bytes

                      Figure 7: DRIP Self-Attestation

B.2.  Attestation (A-xy)

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      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
     +---------------+---------------+---------------+---------------+
     |                                                               |
     .                                                               .
     .                             SA-xx                             .
     .                                                               .
     |                                                               |
     +---------------+---------------+---------------+---------------+
     |                                                               |
     .                                                               .
     .                             SA-yy                             .
     .                                                               .
     |                                                               |
     +---------------+---------------+---------------+---------------+
     |                      Trust Timestamp by X                     |
     +---------------+---------------+---------------+---------------+
     |                     Signing Timestamp by X                    |
     +---------------+---------------+---------------+---------------+
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                         Signature by X                        |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     +---------------+---------------+---------------+---------------+

     Length = 312-bytes

                         Figure 8: DRIP Attestation

B.3.  Concise Attestation (CA-xy)

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      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
     +---------------+---------------+---------------+---------------+
     |                                                               |
     |                          Hierarchical                         |
     |                    Host Identity Tag of X                     |
     |                                                               |
     +---------------+---------------+---------------+---------------+
     |                                                               |
     |                          Hierarchical                         |
     |                    Host Identity Tag of Y                     |
     |                                                               |
     +---------------+---------------+---------------+---------------+
     |                      Trust Timestamp by X                     |
     +---------------+---------------+---------------+---------------+
     |                     Signing Timestamp by X                    |
     +---------------+---------------+---------------+---------------+
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                         Signature by X                        |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     +---------------+---------------+---------------+---------------+

     Length = 104-bytes

                     Figure 9: DRIP Concise Attestation

B.4.  Mutual Attestation (MA-xy)

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      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
     +---------------+---------------+---------------+---------------+
     |                                                               |
     .                                                               .
     .                              A-xy                             .
     .                                                               .
     |                                                               |
     +---------------+---------------+---------------+---------------+
     |                      Trust Timestamp by Y                     |
     +---------------+---------------+---------------+---------------+
     |                     Signing Timestamp by Y                    |
     +---------------+---------------+---------------+---------------+
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                         Signature by Y                        |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     +---------------+---------------+---------------+---------------+

     Length = 384-bytes

                     Figure 10: DRIP Mutual Attestation

B.5.  Link Attestation (LA-xy)

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      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
     +---------------+---------------+---------------+---------------+
     |                                                               |
     .                                                               .
     .                             CA-xy                             .
     .                                                               .
     |                                                               |
     +---------------+---------------+---------------+---------------+
     |                      Trust Timestamp by Y                     |
     +---------------+---------------+---------------+---------------+
     |                     Signing Timestamp by Y                    |
     +---------------+---------------+---------------+---------------+
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                         Signature by Y                        |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     +---------------+---------------+---------------+---------------+

     Length = 176-bytes

                      Figure 11: DRIP Link Attestation

B.6.  Broadcast Attestation (BA-xy)

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      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
     +---------------+---------------+---------------+---------------+
     |                                                               |
     |                          Hierarchical                         |
     |                    Host Identity Tag of X                     |
     |                                                               |
     +---------------+---------------+---------------+---------------+
     |                                                               |
     |                          Hierarchical                         |
     |                    Host Identity Tag of Y                     |
     |                                                               |
     +---------------+---------------+---------------+---------------+
     |                                                               |
     |                                                               |
     |                                                               |
     |                       Host Identity of Y                      |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     +---------------+---------------+---------------+---------------+
     |                      Trust Timestamp by X                     |
     +---------------+---------------+---------------+---------------+
     |                     Signing Timestamp by X                    |
     +---------------+---------------+---------------+---------------+
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                         Signature by X                        |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     +---------------+---------------+---------------+---------------+

     Length = 136-bytes

                   Figure 12: DRIP Broadcast Attestation

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B.7.  Attestation Certificate (AC-zxy)

      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
     +---------------+---------------+---------------+---------------+
     |                                                               |
     .                                                               .
     .                             SA-zz                             .
     .                                                               .
     |                                                               |
     +---------------+---------------+---------------+---------------+
     |                                                               |
     .                                                               .
     .                              A-xy                             .
     .                                                               .
     |                                                               |
     +---------------+---------------+---------------+---------------+
     |                      Trust Timestamp by Z                     |
     +---------------+---------------+---------------+---------------+
     |                     Signing Timestamp by Z                    |
     +---------------+---------------+---------------+---------------+
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                         Signature by Z                        |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     +---------------+---------------+---------------+---------------+

     Length = 504-bytes

                  Figure 13: DRIP Attestation Certificate

B.8.  Concise Certificate (CC-zxy)

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      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
     +---------------+---------------+---------------+---------------+
     |                                                               |
     |                          Hierarchical                         |
     |                    Host Identity Tag of Z                     |
     |                                                               |
     +---------------+---------------+---------------+---------------+
     |                                                               |
     .                                                               .
     .                             CA-xy                             .
     .                                                               .
     |                                                               |
     +---------------+---------------+---------------+---------------+
     |                      Trust Timestamp by Z                     |
     +---------------+---------------+---------------+---------------+
     |                     Signing Timestamp by Z                    |
     +---------------+---------------+---------------+---------------+
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                         Signature by Z                        |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     +---------------+---------------+---------------+---------------+

     Length = 192-bytes

                    Figure 14: DRIP Concise Certificate

B.9.  Link Certificate (LC-zxy)

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      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
     +---------------+---------------+---------------+---------------+
     |                                                               |
     |                          Hierarchical                         |
     |                    Host Identity Tag of Z                     |
     |                                                               |
     +---------------+---------------+---------------+---------------+
     |                                                               |
     .                                                               .
     .                             LA-xy                             .
     .                                                               .
     |                                                               |
     +---------------+---------------+---------------+---------------+
     |                      Trust Timestamp by Z                     |
     +---------------+---------------+---------------+---------------+
     |                     Signing Timestamp by Z                    |
     +---------------+---------------+---------------+---------------+
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                         Signature by Z                        |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     +---------------+---------------+---------------+---------------+

     Length = 300-bytes

                      Figure 15: DRIP Link Certificate

B.10.  Mutual Certificate (MC-zxy)

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      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
     +---------------+---------------+---------------+---------------+
     |                                                               |
     .                                                               .
     .                             SA-zz                             .
     .                                                               .
     |                                                               |
     +---------------+---------------+---------------+---------------+
     |                                                               |
     .                                                               .
     .                             MA-xy                             .
     .                                                               .
     |                                                               |
     +---------------+---------------+---------------+---------------+
     |                      Trust Timestamp by Z                     |
     +---------------+---------------+---------------+---------------+
     |                     Signing Timestamp by Z                    |
     +---------------+---------------+---------------+---------------+
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                         Signature by Z                        |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     +---------------+---------------+---------------+---------------+

     Length = 576-bytes

                     Figure 16: DRIP Mutual Certificate

B.11.  Example Registration with Attestation

   A typical registration of a DET consists of at least three parties;
   an Aircraft (Party A), an Operator (Party O) and a Registry (Party
   R).  A fourth party (Party Z) can be present as well but is not
   required.

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   All parties generate their own SelfAttestation (SA-aa, SA-oo, SA-rr
   and if present SA-zz) to be used during the registration process.

B.11.1.  Operator to Registry

   To register the Operator sends to their selected Registry their
   SelfAttestation (SA-oo).

   The Registry, upon acceptance, returns the following items:

   1.  Attestation (A-ro) - using SA-rr and SA-oo [Mandatory]

   2.  ConciseAttestation (CA-ro) - using SA-oo [Optional]

   3.  BroadcastAttestation (BA-ro) - using SA-oo [Optional]

   The Operator may choose to generate the following items after
   successful registration:

   1.  MutualAttestation (MA-ro) - using A-ro

   2.  LinkAttestation (LA-ro) - using CA-ro

   The Operator may further choose to send A-ro, CA-ro, MA-ro and LA-ro
   to Party Z to obtain the following:

   1.  AttestationCertificate (AC-zro) - using SA-zz and A-ro

   2.  MutualCertificate (MC-zro) - using SA-aa and MA-ro

   3.  ConciseCertificate (CC-zro) - using CA-ro

   4.  LinkCertificate (LC-zro) - using LA-ro

B.11.2.  Aircraft to Operator

   After registering with a Registry the Operator can now start to
   provision an Aircraft.  Extracting SA-aa from the Aircraft they can
   create the following items:

   1.  Attestation (A-oa) - using SA-oo and SA-aa [Mandatory]

   2.  ConciseAttestation (CA-oa) - using SA-aa [Optional]

   3.  BroadcastAttestation (BA-oa) - using SA-aa [Optional]

   A-oa can be returned to the Aircraft to produce the following:

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   1.  MutualAttestation (MA-oa) - using A-oa

   2.  LinkAttestation (LA-oa) - using CA-oa

B.11.3.  Aircraft to Registry

   Using the existing A-ro and new A-oa (and if present CA-oa, MA-oa and
   LA-oa) the Operator can attempt registration with the previous
   Registry.

   The Registry upon acceptance can return the following set of items:

   1.  BroadcastAttestation (BA-ra) - generated using the embedded SA-aa
       from A-oa [Mandatory]

   2.  AttestationCertificate (AC-roa) - using A-oa [Mandatory]

   3.  MutualCertificate (MC-roa) - using MA-oa [Optional]

   4.  ConciseCertificate (CC-roa) - using CA-oa [Optional]

   5.  LinkCertificate (LC-roa) - using LA-oa [Optional]

   6.  BroadcastAttestation's of parent Registries in chain
       [Recommended]

   The BroadcastAttestation's are used during the flight of the Aircraft
   being sent via DRIP Link Authentication Messages.

Appendix C.  DRIP Broadcast Attestation Structure

   (Editors Note: move into main document?  Between Section 3 and 4 and
   after FEC section?)

   When possible the following format SHOULD be used in the DRIP
   Authentication Data (Section 4.4.1.2) field under Authentication Type
   0x5 or the "Authentication Data / Signature" of Figure 2.

      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
     +---------------+---------------+---------------+---------------+
     |                                                               |
     |                      Attestor Hierarchical                    |
     |                       Host Identity Tag                       |
     |                                                               |
     +---------------+---------------+---------------+---------------+
     |                                                               |
     .                                                               .

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     .                        Attestation Data                       .
     .                                                               .
     |                                                               |
     +---------------+---------------+---------------+---------------+
     |                         Trust Timestamp                       |
     +---------------+---------------+---------------+---------------+
     |                        Signing Timestamp                      |
     +---------------+---------------+---------------+---------------+
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                        Attestor Signature                     |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     +---------------+---------------+---------------+---------------+

     Attestor Hierarchial Host Identity Tag (16 bytes):
         The Attestors HHIT in byte form (network byte order).

     Attestation Data (0 to 112 bytes):
         Opaque attestation data.

     Trust Timestamp (4 bytes):
         Timestamp denoting recommended time to trust data to.

     Signing Timestamp (4 bytes):
         Current time at signing.

     Attestor Signature (64 bytes):
         Signature over preceding fields using the keypair of
         the Attestor.

            Figure 17: DRIP Broadcast Attestation Data Structure

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C.1.  Attestor Hierarchical Host Identity Tag

   The HHIT is an enhancement of the Host Identity Tag (HIT) [RFC7401]
   introducing hierarchy and how they are used in UAS RID as defined in
   [drip-rid].  Nominally the Aircraft HHIT is used here when signing
   over dynamic formats such as DRIP Wrapper and DRIP Manifest.

C.2.  Attestation Data

   This field has a maximum of 112 bytes in length.  It is nominally
   filled with data as defined by the SAM Type being set or other sub-
   multiplexer in the authentication payload.

C.3.  Trust Timestamp

   Follows the format defined in [F3411-19].  That is a UNIX timestamp
   offset by 01/01/2019 00:00:00.  An additional offset is then added to
   push the timestamp a short time into the future to avoid replay
   attacks.

   The offset used against the UNIX timestamp is not defined in this
   document.  Best practice identifying an acceptable offset should be
   used taking into consideration the UA environment, and propagation
   characteristics of the messages being sent.

C.4.  Signing Timestamp

   Follows the format defined in [F3411-19].  That is a UNIX timestamp
   offset by 01/01/2019 00:00:00.

C.5.  Attestor Signature

   The signature is generated over all the preceding data.  Only
   information within the Broadcast Attestation Structure (Appendix C)
   is signed.

Appendix D.  Forward Error Correction

   (Editors Note: move into main document?  Between Section 3 and 4
   (preferably becoming Section 4)? or move specifics of FEC (everything
   below) into its own draft for titled Integrity Protection?)

   Remote ID data can be sent across different broadcast link media, all
   with different characteristics.  To enable robustness in Remote ID
   transmission media a Forward Error Correction capability SHOULD be
   used.

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   In cases where FEC is not available below the equivalent of the
   transport layer (known as Legacy Advertisements) DRIP Authentication
   REQUIRES that an application level FEC scheme is used.  In cases
   where FEC is available below the equivalent of the transport layer
   (known as Extended Advertisements) DRIP MUST NOT use any application
   level FEC and instead SHALL rely on the lower layers FEC
   functionality.

   For current Remote ID the media options are the following:

   Legacy Advertisements:  Bluetooth 4.X

   Extended Advertisements:  Wi-Fi NAN, Wi-Fi Beacon, Bluetooth 5.X

   (Editors Note: add in self-protecting and more-than-self-protecting
   options, with their justifications)

D.1.  Encoding

D.1.1.  Single Page FEC

   To generate the parity a simple XOR operation using the previous and
   current page is used.  For Page 0, a 25-byte null pad is used for the
   previous page.  The resulting parity fills the "Additional Data"
   field of [F3411-19] with the "Additional Data Length" field being set
   to 25.

D.1.2.  Multi Page FEC

   TODO (Reed Solomon)

   (Editors Note: probably need a table to check against ADLs to which
   parameters of Reed Solomon are being used?)  (Editors Note: this is
   the place to define if we are self-protecting or global-protecting
   with FEC...another multiplex byte here directly after the ADL?)

D.2.  Decoding

   Due to the nature of Bluetooth 4 and the existing ASTM paging
   structure an optimization can be used.  If a Bluetooth frame fails
   its CRC check, then the frame is dropped without notification to the
   upper protocol layers.  From the Remote ID perspective this means the
   loss of a complete frame/message/page.  In Authentication Messages,
   each page is already numbered so the loss of a page allows the
   receiving application to build a "dummy" page filling the entire
   pages with nulls.

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   If Page 0 is being reconstructed an additional check of the "Last
   Page Index" to check against how many pages are actually present,
   MUST be performed for sanity.  An additional check on the Data Length
   field SHOULD also be performed.

   To determine if Single Page FEC or Multi-Page FEC has been used a
   simple check of the "Additional Data Length (ADL)" field can be used.
   If the "ADL" is equal to 25, then Single Page FEC is present,
   anything larger signals Multi-Page FEC.

D.2.1.  Single Page FEC

   Using the same methods as encoding, an XOR operation is used between
   the previous and current page (a 25-byte null pad is used as the
   start).  The resulting 25-bytes should be the missing page.

D.2.2.  Multi Page FEC

   TODO (Reed Solomon)

   (Editors Note: probably need a table to check against ADLs to which
   parameters of Reed Solomon are being used?)  (Editors Note: this is
   the place to define if we are self-protecting or global-protecting
   with FEC...another multiplex byte here directly after the ADL?)

D.3.  FEC Limitations

   If more than one page is lost (>1/5 for 5-page messages, >1/10 for
   10-page messages) than the error rate of the link is already beyond
   saving and the application has more issues to deal with.

   (Editors Note: Is this valid anymore, for XOR yes but for multi-page
   FEC?)

Appendix E.  Example Authentication Messages

E.1.  Authentication Data Only

   This is an example of an Authentication Message with 52-bytes of
   Authentication Data.

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   Page 0:
    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
   +---------------+---------------+---------------+---------------+
   | Auth. Header  |                                               |
   +---------------+  ASTM Authentication Headers  +---------------+
   |                                               |               |
   +---------------+---------------+---------------+               |
   |                                                               |
   |                Authentication Data / Signature                |
   |                                                               |
   |                                                               |
   +---------------+---------------+---------------+---------------+

   Page 1:
    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
   +---------------+---------------+---------------+---------------+
   | Auth. Header  |                                               |
   +---------------+                                               |
   |                                                               |
   |                Authentication Data / Signature                |
   |                                                               |
   |                                                               |
   |                                                               |
   +---------------+---------------+---------------+---------------+

   Page 2:
    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
   +---------------+---------------+---------------+---------------+
   | Auth. Header  |                                               |
   +---------------+                                               |
   |                Authentication Data / Signature                |
   |                                                               |
   |               +---------------+---------------+---------------+
   |               |                                               |
   +---------------+                                               |
   |                        Null Padding                           |
   |                                                               |
   +---------------+---------------+---------------+---------------+

E.2.  Authentication Data & Additional Data

   This example has 52-bytes of Authentication Data and 20-bytes of
   Additional Data.

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   Page 0:
    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
   +---------------+---------------+---------------+---------------+
   | Auth. Header  |                                               |
   +---------------+  ASTM Authentication Headers  +---------------+
   |                                               |               |
   +---------------+---------------+---------------+               |
   |                                                               |
   |                Authentication Data / Signature                |
   |                                                               |
   |                                                               |
   +---------------+---------------+---------------+---------------+

   Page 1:
    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
   +---------------+---------------+---------------+---------------+
   | Auth. Header  |                                               |
   +---------------+                                               |
   |                                                               |
   |                                                               |
   |                Authentication Data / Signature                |
   |                                                               |
   |                                                               |
   +---------------+---------------+---------------+---------------+

   Page 2:
    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
   +---------------+---------------+---------------+---------------+
   | Auth. Header  |                                               |
   +---------------+                                               |
   |                Authentication Data / Signature                |
   |                                                               |
   |               +---------------+---------------+---------------+
   |               |      ADL      |                               |
   +---------------+---------------+                               |
   |                       Additional Data                         |
   |                                                               |
   +---------------+---------------+---------------+---------------+

   Page 3:
    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
   +---------------+---------------+---------------+---------------+
   | Auth. Header  |                                               |
   +---------------+                                               |

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   |                       Additional Data                         |
   |                                               +---------------+
   |                                               |               |
   +---------------+---------------+---------------+               |
   |                                                               |
   |                         Null Padding                          |
   |                                                               |
   +---------------+---------------+---------------+---------------+

E.3.  DRIP Link Example

   This DRIP Link example includes FEC for a single page.

   Page 0:
    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
   +---------------+---------------+---------------+---------------+
   | Auth. Header  |                                               |
   +---------------+  ASTM Authentication Headers  +---------------+
   |                                               |   SAM Type    |
   +---------------+---------------+---------------+---------------+
   |                                                               |
   |                     Broadcast Attestation                     |
   |                                                               |
   |                                                               |
   +---------------+---------------+---------------+---------------+

   Page 1:
    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
   +---------------+---------------+---------------+---------------+
   | Auth. Header  |                                               |
   +---------------+                                               |
   |                                                               |
   |                                                               |
   |                     Broadcast Attestation                     |
   |                                                               |
   |                                                               |
   +---------------+---------------+---------------+---------------+

   Page 2:
    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
   +---------------+---------------+---------------+---------------+
   | Auth. Header  |                                               |
   +---------------+                                               |
   |                                                               |
   |                                                               |

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   |                     Broadcast Attestation                     |
   |                                                               |
   |                                                               |
   +---------------+---------------+---------------+---------------+

   Page 3:
    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
   +---------------+---------------+---------------+---------------+
   | Auth. Header  |                                               |
   +---------------+                                               |
   |                                                               |
   |                                                               |
   |                     Broadcast Attestation                     |
   |                                                               |
   |                                                               |
   +---------------+---------------+---------------+---------------+

   Page 4:
    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
   +---------------+---------------+---------------+---------------+
   | Auth. Header  |                                               |
   +---------------+                                               |
   |                                                               |
   |                                                               |
   |                     Broadcast Attestation                     |
   |                                                               |
   |                                                               |
   +---------------+---------------+---------------+---------------+

   Page 5:
    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
   +---------------+---------------+---------------+---------------+
   | Auth. Header  |                                               |
   +---------------+                                               |
   |                                                               |
   |                                                               |
   |                     Broadcast Attestation                     |
   |                                                               |
   |                                                               |
   +---------------+---------------+---------------+---------------+

   Page 6:
    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
   +---------------+---------------+---------------+---------------+

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   | Auth. Header  |             Broadcast Attestation             /
   +---------------+---------------+---------------+---------------+
   /               |      ADL      |                               |
   +---------------+---------------+                               |
   |                                                               |
   |                   Forward Error Correction                    |
   |                                                               |
   |                                                               |
   +---------------+---------------+---------------+---------------+

   Page 7:
    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
   +---------------+---------------+---------------+---------------+
   | Auth. Header  |                                               |
   +---------------+            Forward Error Correction           |
   |                                                               |
   +---------------+---------------+---------------+---------------+
   |                                                               |
   |                                                               |
   |                         Null Padding                          |
   |                                                               |
   +---------------+---------------+---------------+---------------+

Authors' Addresses

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

   Email: adam.wiethuechter@axenterprize.com

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

   Email: stu.card@axenterprize.com

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

   Email: rgm@labs.htt-consult.com

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