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DRIP Entity Tag Authentication Formats & Protocols for Broadcast Remote ID
draft-ietf-drip-auth-12

Document Type Active Internet-Draft (drip WG)
Authors Adam Wiethuechter , Stuart W. Card , Robert Moskowitz
Last updated 2022-05-25
Replaces draft-wiethuechter-drip-auth
Stream Internet Engineering Task Force (IETF)
Intended RFC status Proposed Standard
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Stream WG state WG Document
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Sep 2020
Solution space documents adopted by the WG
Jun 2022
Submit DRIP Authentication Formats to the IESG
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draft-ietf-drip-auth-12
DRIP Working Group                              A. Wiethuechter (Editor)
Internet-Draft                                                   S. Card
Intended status: Standards Track                      AX Enterprize, LLC
Expires: 26 November 2022                                   R. Moskowitz
                                                          HTT Consulting
                                                             25 May 2022

DRIP Entity Tag Authentication Formats & Protocols for Broadcast Remote
                                   ID
                        draft-ietf-drip-auth-12

Abstract

   This document describes how to include trust into the ASTM Remote ID
   specification defined in ASTM F3411 under Broadcast Remote ID (RID).
   It defines a few message schemes (sent within the Authentication
   Message) that can be used to authenticate past messages sent by a
   unmanned aircraft (UA) and provide proof of UA trustworthiness even
   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
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 26 November 2022.

Copyright Notice

   Copyright (c) 2022 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (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

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   extracted from this document must include Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  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
       3.1.1.  Broadcast RID RF Options  . . . . . . . . . . . . . .   5
     3.2.  Reasoning for IETF DRIP Authentication  . . . . . . . . .   5
     3.3.  ASTM Authentication Message . . . . . . . . . . . . . . .   5
       3.3.1.  Authentication Page . . . . . . . . . . . . . . . . .   5
       3.3.2.  ASTM Constraints  . . . . . . . . . . . . . . . . . .   9
   4.  Forward Error Correction  . . . . . . . . . . . . . . . . . .   9
     4.1.  Encoding  . . . . . . . . . . . . . . . . . . . . . . . .   9
       4.1.1.  Single Page FEC . . . . . . . . . . . . . . . . . . .  10
       4.1.2.  Multiple Page FEC . . . . . . . . . . . . . . . . . .  10
     4.2.  Decoding  . . . . . . . . . . . . . . . . . . . . . . . .  13
       4.2.1.  Single Page FEC . . . . . . . . . . . . . . . . . . .  13
       4.2.2.  Multiple Page FEC . . . . . . . . . . . . . . . . . .  13
     4.3.  FEC Limitations . . . . . . . . . . . . . . . . . . . . .  14
   5.  DRIP Authentication Formats . . . . . . . . . . . . . . . . .  14
     5.1.  DRIP Authentication Field Definitions . . . . . . . . . .  14
       5.1.1.  Broadcast Attestation Structure . . . . . . . . . . .  15
       5.1.2.  SAM Data Format . . . . . . . . . . . . . . . . . . .  16
     5.2.  DRIP Link . . . . . . . . . . . . . . . . . . . . . . . .  18
     5.3.  DRIP Wrapper  . . . . . . . . . . . . . . . . . . . . . .  18
       5.3.1.  Wrapper over Extended Transports  . . . . . . . . . .  20
       5.3.2.  Wrapper Limitations . . . . . . . . . . . . . . . . .  21
     5.4.  DRIP Manifest . . . . . . . . . . . . . . . . . . . . . .  22
       5.4.1.  Hash Count  . . . . . . . . . . . . . . . . . . . . .  23
       5.4.2.  Message Hash Algorithms and Operation . . . . . . . .  24
       5.4.3.  Pseudo-Blockchain Hashes  . . . . . . . . . . . . . .  24
       5.4.4.  Manifest Limitations  . . . . . . . . . . . . . . . .  25
     5.5.  DRIP Frame  . . . . . . . . . . . . . . . . . . . . . . .  25
       5.5.1.  Frame Type  . . . . . . . . . . . . . . . . . . . . .  26
   6.  Requirements & Recommendations  . . . . . . . . . . . . . . .  27
     6.1.  Legacy Transports . . . . . . . . . . . . . . . . . . . .  27
     6.2.  Extended Transports . . . . . . . . . . . . . . . . . . .  27
     6.3.  Authentication  . . . . . . . . . . . . . . . . . . . . .  27
     6.4.  Operational . . . . . . . . . . . . . . . . . . . . . . .  28
       6.4.1.  DRIP Wrapper  . . . . . . . . . . . . . . . . . . . .  29
   7.  ICAO Considerations . . . . . . . . . . . . . . . . . . . . .  29

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   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  29
     8.1.  Update IANA DRIP Registry . . . . . . . . . . . . . . . .  29
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  30
     9.1.  Manifest Hash Length  . . . . . . . . . . . . . . . . . .  30
     9.2.  Replay Attacks  . . . . . . . . . . . . . . . . . . . . .  31
     9.3.  Trust Timestamp Offsets . . . . . . . . . . . . . . . . .  31
   10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  31
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  32
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  32
     11.2.  Informative References . . . . . . . . . . . . . . . . .  32
   Appendix A.  Authentication State Diagrams & Color Scheme . . . .  33
     A.1.  State Table . . . . . . . . . . . . . . . . . . . . . . .  33
     A.2.  State Diagrams  . . . . . . . . . . . . . . . . . . . . .  34
       A.2.1.  Notations . . . . . . . . . . . . . . . . . . . . . .  34
       A.2.2.  General . . . . . . . . . . . . . . . . . . . . . . .  35
       A.2.3.  DRIP SAM  . . . . . . . . . . . . . . . . . . . . . .  36
       A.2.4.  DRIP Link . . . . . . . . . . . . . . . . . . . . . .  37
       A.2.5.  DRIP Wrapper/Manifest/Frame . . . . . . . . . . . . .  38
   Appendix B.  HDA-UA Broadcast Attestation . . . . . . . . . . . .  40
   Appendix C.  Example TX/RX Flow . . . . . . . . . . . . . . . . .  42
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  42

1.  Introduction

   Unmanned Aircraft 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 [F3411] standard focuses on two ways of communicating to a
   UAS for Remote ID (RID): Broadcast and Network.

   This document will focus on adding trust to Broadcast RID via the
   Authentication Message by combining dynamically signed data with an
   Attestation of the UA's identity from a Registry.

   This authentication methodology also provides the missing, but US FAA
   mandated, Error Correction for the Bluetooth 4 transmissions (see
   Section 4).  This is error correction not only for the authentication
   message itself, but indirectly, to other messages authenticated via
   the Manifest method (see Section 5.4).

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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 6.3 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.

   Legacy Transports:  uses broadcast frames (Bluetooth 4).

   Extended Transports:  uses the extended advertisements (Bluetooth 5),
      service info (Wi-Fi NaN) or vendor specific element information
      (Wi-Fi BEACON).  Must use ASTM [F3411] Message Pack (Message Type
      0xF).

3.  Background

3.1.  Problem Space and Focus

   The current standard for Remote ID 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.

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3.1.1.  Broadcast RID RF Options

   A UA has the option of broadcasting using Bluetooth (4 and 5) or Wi-
   Fi (BEACON or NAN), see Section 6.  With Bluetooth, FAA and other CAA
   mandate transmitting simultaneously over both 4 and 5.  With Wi-Fi,
   use of BEACON is recommended.  Wi-Fi NAN is another option, depending
   on CAA.

   Bluetooth 4 presents a payload size challenge in that it can only
   transmit 25 bytes of payload where the others all can support 252
   byte payloads.

3.2.  Reasoning for IETF DRIP Authentication

   The ASTM Authentication Message has provisions in [F3411] to allow
   for other organizations to standardize additional Authentication
   formats beyond those explicitly in [F3411].  The standardization of
   specific 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 in
   [F3411] or other organizations leveraging this feature) provide the
   functionality to satisfy this goal resulting in the work reflected in
   this document.

3.3.  ASTM Authentication Message

   The ASTM Authentication Message (Message Type 0x2) is a unique
   message in the Broadcast [F3411] standard as it is the only one that
   is larger than the Bluetooth 4 frame size.  To address this, it is
   defined as a set of "pages" that each fits into a single Bluetooth 4
   broadcast frame.  For other media these pages are still used but all
   in a single frame.

3.3.1.  Authentication Page

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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
     +---------------+---------------+---------------+---------------+
     |  Page Header  |                                               |
     +---------------+                                               |
     |                                                               |
     |                                                               |
     |                     Authentication Payload                    |
     |                                                               |
     |                                                               |
     +---------------+---------------+---------------+---------------+

     Page Header: (1 byte)
         Authentication Type (4 bits)
         Page Number (4 bits)

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

            Figure 1: Standard ASTM Authentication Message Page

   A single Authentication Message is akin to a UDP packet.  The
   Authentication Message is structured as a set of up to 16 pages.
   Over Bluetooth 4, these pages are "fragmented" into separate
   Bluetooth 4 broadcast frames.

   Either as a single Authentication Message or a set of fragmented
   Authentication Message Pages the structure(s) is further wrapped by
   outer ASTM framing and the specific link framing (Bluetooth or Wi-
   Fi).

3.3.1.1.  Authentication Type

   [F3411] has the following example subset of Authentication Type's
   defined and that can be used in the Page Header:

         +=====================+================================+
         | Authentication Type | Description                    |
         +=====================+================================+
         | 0x2                 | Operator ID Signature          |
         +---------------------+--------------------------------+
         | 0x3                 | Message Set Signature          |
         +---------------------+--------------------------------+
         | 0x5                 | Specific Authentication Method |
         +---------------------+--------------------------------+

                                 Table 1

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3.3.1.1.1.  Specific Authentication Method (SAM)

   This document leverages Authentication Type 0x5, Specific
   Authentication Method (SAM), defining a set of SAM Types in
   Section 5.

3.3.1.2.  Page Number

   There is a technical maximum of 16 pages (indexed 0 to 15 in the Page
   Header) that can be sent for a single Authentication Message, with
   each page carrying a max 23-byte Authentication Payload.  See
   Section 3.3.2 for more details.

3.3.1.3.  Authentication Payload Field

   The following is shown in its complete format.

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

     Authentication Headers: (6-bytes)
         As defined in F3411.

     Authentication Data / Signature: (255-bytes max)
         Opaque authentication data.

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

     Additional Data: (255-bytes max):
         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 source data view of the data fields found in the
   Authentication Message as defined by [F3411].  This data is placed
   into Figure 1's Authentication Payload, 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.  For DRIP, this field is used to carry Forward Error Correction
   as defined in Section 4.

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3.3.2.  ASTM Constraints

   To keep consistent formatting across the different transports (Legacy
   and Extended) and their independent restrictions the authentication
   data being sent is REQUIRED to fit within the page limit of the most
   constrained existing transport can support.  Under Broadcast RID the
   transport that can hold the least amount of authentication data is
   Bluetooth 5 and Wi-Fi BEACON at 9-pages.

   As such DRIP transmitters are REQUIRED to adhere to the following
   when using the Authentication Message:

   1.  Authentication Data / Signature data MUST fit in the first 9
       pages (Page Numbers 0 through 8).

   2.  The Length field in the Authentication Headers (which denotes the
       length in bytes of Authentication Data / Signature only) MUST NOT
       exceed the value of 201.

4.  Forward Error Correction

   For Broadcast RID, Forward Error Correction (FEC) is provided by the
   lower layers in Extended Transports (Bluetooth 5, Wi-Fi NaN, and Wi-
   Fi BEACON).  The Bluetooth 4 Legacy Transport does not have
   supporting FEC so with DRIP Authentication the following application
   level FEC scheme is used to add FEC.  This section is only used for
   Bluetooth 4 transmission/reception.

   The data added during FEC is not included in the Authentication Data
   / Signature but instead in the Additional Data field of Figure 2.
   This may cause the Authentication Message to exceed 9-pages, up to a
   max of 16-pages.

4.1.  Encoding

   For any encoding the FEC data MUST start on a new ASTM Authentication
   Page.  To do this, null padding is added before the actual FEC data
   starts and the length of the whole blob (null padding and FEC) is
   used as the Additional Data Length.  To properly fit FEC data into an
   Authentication Page the number of parity-bytes is limited to 23 or a
   multiple thereof (size of Authentication data per page).  That is,
   the Page Header (and anything before it) is omitted in the FEC
   process.

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4.1.1.  Single Page FEC

   To generate the parity a simple XOR operation using the previous and
   current page is used.  Only the 23-byte Authentication Page data is
   used in the XOR operation.  For Page 0, a 23-byte null pad is used
   for the previous page.  The resulting parity fills the last 23 bytes
   of the Additional Data field of Figure 2 with the Additional Data
   Length field being set to 23 or greater (depending on number of null
   pad bytes are needed to get onto the next page).

     Page N-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
     +---------------+---------------+---------------+---------------+
     |  Page Header  |                                               |
     +---------------+                                               |
     |                Authentication Data / Signature                |
     |                                                               |
     |               +---------------+---------------+---------------+
     |               |    ADL=33     |                               |
     +---------------+---------------+                               |
     |                          Null Padding                         |
     |                                                               |
     +---------------+---------------+---------------+---------------+

     Page N:
      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
     +---------------+---------------+---------------+---------------+
     |  Page Header  |                                               |
     +---------------+                                               |
     |                                                               |
     |                     Forward Error Correction                  |
     |                                                               |
     |                                                               |
     |                                                               |
     +---------------+---------------+---------------+---------------+

                 Figure 3: Example Single Page FEC Encoding

4.1.2.  Multiple Page FEC

   For Multiple Page FEC there are two flavors: Frame Recovery and Page
   Recovery.  Both follow a similar process, but are offset at what data
   is actually protected.

   (Editor Note: to improve interop we MUST explicitly select a
   polynomial for Reed Solomon for DRIP - need suggestions)

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4.1.2.1.  Page Recovery

   Take the following example of an Authentication Message with 7 pages
   that 3 pages of parity are to be generated for.  The first column is
   just the Page Header with a visual space here to show the boundary.

   50 098960bf8c05042001001000a00145aac6b00abba268b7
   51 2001001000a0014579d8a404d48f2ef9bb9a4470ada5b4
   52 ff1352c7402af9d9ebd20034e8d7a12920f4d7e91c1a73
   53 dca7d04e776150825863c512c6eb075a206a95c59b297e
   54 f2935fd416f27b1b42fd5d9dfaa0dec79f32287f41b454
   55 7101415def153a770d3e6c0b17ae560809bc634a822c1f
   56 3b1064b80a000000000000000000000000000000000000

   For Page Recovery the first column is ignored and the last 23-bytes
   of each page are extracted to have Reed Solomon performed on them in
   a column wise fashion to produce parity bytes.  For the example the
   following 3-bytes of parity are generated with the first byte of each
   page:

   dc6c2b = ReedSolomon.encoder(0920ffdcf2713b)

   Each set of parity is the placed into a pseudo-frame as follows (each
   byte in its own message in the same column).  Below is an example of
   the full parity generated and each 23-bytes of parity added into the
   additional pages as Additional Data:

   57 dc6657acd30b2ec4aa582049f52adf9f922e62c469563a
   58 6c636a59145a55417a3895fd543f19e94200be4abc5e94
   59 02bba5e28f5896d754caf50016a983993b149b5c9e6eeb

4.1.2.2.  Frame Recovery

   Frame Recovery uses the full ASTM Message and performs Reed Solomon
   over each byte.  Up to 240 (255 minus 15 pages max of FEC data)
   messages can be protected using Frame Recovery.

   Below is an example of a number of messages.  Here the first column
   is an additional ASTM Header that contain the Message Type; with a
   visual space for clarity.  The last 24-bytes are the actual message
   contents; be it location information or an Authentication Page.

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   10 42012001001000a0014579d8a404d48f2ef9000000000000
   11 249600006efeb019ee111ed37a097a0948081c10ffff0000
   12 50098960bf8c05042001001000a00145aac6b00abba268b7
   12 512001001000a0014579d8a404d48f2ef9bb9a4470ada5b4
   12 52ff1352c7402af9d9ebd20034e8d7a12920f4d7e91c1a73
   12 53dca7d04e776150825863c512c6eb075a206a95c59b297e
   12 54f2935fd416f27b1b42fd5d9dfaa0dec79f32287f41b454
   12 557101415def153a770d3e6c0b17ae560809bc634a822c1f
   12 563b1064b80a000000000000000000000000000000000000
   13 0052656372656174696f6e616c2054657374000000000000
   14 02c2ffb019322d1ed3010000c008e40700fc080000000000
   15 004e2e4f5031323334353600000000000000000000000000

   A similar process is followed as in Section 4.1.2.1.  Here every
   column of bytes has parity generated for it (even the ASTM Header).
   In the below example 5-bytes of parity are generated using the ASTM
   Header column:

   6c3f42b8a8 = ReedSolomon.encoder(101112121212121212131415)

   After doing this to all columns the following pseudo-frames would
   have been generated:

   6c86337bf7ab746f5d62bb7f8de954104b121585d3975f6e92
   3f06c1bce165b0e25930d57a63c24f751145e1dd8dc115029b
   42e9979580327a6a14d421c12a33aa2e1a2e517daaee581016
   b8012a7b3964f7b2720d387bfa77e945556f1831cd477ef3a3
   a85bb403aada89926fb8fc2a14a9caacb4ec2f3a6ed2d8e9f9

   These 25-byte chunks are now concatenated together and are placed in
   Authentication Pages, using the Additional Data, 23-bytes at a time.
   In the below figure the first column is the ASTM Header as before,
   the second column is the Page Header for each Authentication Page and
   then last column is the 23-bytes of data for each page.

   12 57 6c86337bf7ab746f5d62bb7f8de954104b121585d3975f
   12 58 6e923f06c1bce165b0e25930d57a63c24f751145e1dd8d
   12 59 c115029b42e9979580327a6a14d421c12a33aa2e1a2e51
   12 5a 7daaee581016b8012a7b3964f7b2720d387bfa77e94555
   12 5b 6f1831cd477ef3a3a85bb403aada89926fb8fc2a14a9ca
   12 5c acb4ec2f3a6ed2d8e9f900000000000000000000000000

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4.2.  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 page
   with nulls.

   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 Length field SHOULD
   also be performed.

   To determine if Single Page FEC or Multiple Page FEC has been used a
   simple check of the Last Page Index can be used.  If the number of
   pages left after the Length of Authentication Data is exhausted than
   it is clear that the remaining pages are all FEC.  The Additional
   Data Length byte can further confirm this; taking into account any
   null padding needed for page alignment.

4.2.1.  Single Page FEC

   Using the same methods as encoding, an XOR operation is used between
   the previous and current page (a 23-byte null pad is used as the
   start).  The resulting 23-bytes should be data of the missing page.

4.2.2.  Multiple Page FEC

   To determine if Page Recovery or Frame Recovery is used two modulo
   checks with the ADL after the length of the null-pad is removed are
   needed.  One against the value of 23, and the other against the value
   of 25.  If 23 comes back with a value of 0 then Page Recovery is
   being used.  If 25 comes back with 0 then Frame Recovery is used.
   Any other combination indicates an error.

4.2.2.1.  Page Recovery

   To decode Page Recovery, dummy pages (pages with nulls as the data)
   are needed in the places no page was received.  Then Reed Solomon can
   decode across the columns of the 23-bytes of each page.  Erasures can
   be used as it is known which pages are missing and can improve the
   Reed Solomon results by specifying them.

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4.2.2.2.  Frame Recovery

   To decode Frame Recovery, the receiver must first extract all FEC
   data from the pages; concatenate them and then break into 25-byte
   chunks.  This will produce the pseudo-frames.  Now Reed Solomon can
   be used to decode columns, with dummy frames inserted where needed.

4.3.  FEC Limitations

   The worst case scenario is when the Authentication Data / Signature
   ends perfectly on a page (Page N-1).  This means the Additional Data
   Length would start the next page (Page N) and have 22-bytes worth of
   null padding to align the FEC in to the next page (Page N+1).  In
   this scenario an entire page (Page N) is being wasted just to carry
   the Additional Data Length.  This should be be avoided at all costs -
   in an effort to maintain efficiency.

5.  DRIP Authentication Formats

   All formats defined in this section are the content for the
   Authentication Data / Signature field in Figure 2 and uses the
   Specific Authentication Method (SAM, Authentication Type 0x5).  The
   first byte of the Authentication Data / Signature of Figure 2, is
   used to multiplex between these various formats.

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

5.1.  DRIP Authentication Field Definitions

   ASTM Message (25-bytes):  Full ASTM Message as defined in [F3411]
      specifically Message Types 0x0, 0x1, 0x3, 0x4, and 0x5

   ASTM Message Hash (12-bytes):  Hash of a single full ASTM Message
      using hash operations described in (Section 5.4.2).  Multiple
      hashes MUST be in Message Type order.

   Attestation Data (0 to 112 bytes):  Opaque attestation data that the
      UA is attesting during its flight in Figure 4.

   Broadcast Attestation (136-bytes):  HDA HI over UA DET/HI.  Generated
      by a DRIP Registry during Session ID registration.  Used in
      Section 5.2.

   Current Manifest Hash (12-bytes):  See Section 5.4.3.

   Frame Type (1-byte):  Sub-type for future different DRIP Frame

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      formats.  See Section 5.5.1.

   Not Before Timestamp by UA (4-bytes):  Timestamp denoting recommended
      time to start trusting data in Figure 4.  MUST follow the format
      defined in [F3411].  That is a Unix-style timestamp but with an
      epoch of 01/01/2019 00:00:00.  MUST be set to the time the
      signature is generated.

   Not After Timestamp by UA (4-bytes):  Timestamp denoting recommended
      time to stop trusting data in Figure 4.  MUST follow the format
      defined in [F3411].  That is a Unix-style timestamp but with an
      epoch of 01/01/2019 00:00:00 with an additional offset is then
      added to push a short time into the future (relative to Not Before
      Timestamp) to avoid replay attacks.  The offset used against the
      Unix-style 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 and clock differences
      between the UA and Observers.  A reasonable time would be to set
      Not After Timestamp 2 minutes ahead of Not Before Timestamp.

   Previous Manifest Hash (12-bytes):  See Section 5.4.3.

   UA DRIP Entity Tag (16-bytes):  The UA DET in byte form (network byte
      order) and is part of Figure 4.

   UA Signature (64-bytes):  Signature over preceding fields of Figure 4
      using the HI of the UA.

5.1.1.  Broadcast Attestation Structure

   To directly support Broadcast RID a variation of the Attestation
   Structure format of [drip-registries] SHOULD be used when running
   DRIP under the various SAM Types (filling the SAM Authentication Data
   field (Section 5.1.2.2)).  The notable changes of the structure is
   that the timestamps are set by the UA and the Attestor Identity
   Information is set to the DET of the UA.

   When using this structure the UA is always self-attesting its DRIP
   Entity Tag (DET).  The Host Identity of the UA DET can be looked up
   by mechanisms described in [drip-registries] or by extracting it from
   Broadcast Attestation (see Section 5.2 and Section 6.3).

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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                               |
     |                        DRIP Entity Tag                        |
     |                                                               |
     +---------------+---------------+---------------+---------------+
     |                                                               |
     .                                                               .
     .                        Attestation Data                       .
     .                                                               .
     |                                                               |
     +---------------+---------------+---------------+---------------+
     |                   Not Before Timestamp by UA                  |
     +---------------+---------------+---------------+---------------+
     |                    Not After Timestamp by UA                  |
     +---------------+---------------+---------------+---------------+
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                          UA Signature                         |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     +---------------+---------------+---------------+---------------+

                 Figure 4: Broadcast Attestation Structure

5.1.2.  SAM Data Format

   Figure 5 is the general format to hold authentication data when using
   SAM and is placed inside the Authentication Data / Signature field in
   Figure 2.

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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    |                                               |
     +---------------+                                               |
     .                                                               .
     .                     SAM Authentication Data                   .
     .                                                               .
     |                                                               |
     +---------------+---------------+---------------+---------------+

     SAM Type (1 byte):
         Byte defined by F3411 to multiplex SAMs

     SAM Authentication Data (0 to 200 bytes):
         Opaque SAM authentication data.

                         Figure 5: SAM Data Format

5.1.2.1.  SAM Type

   The SAM Type field is maintained by the International Civil Aviation
   Organization (ICAO) and for DRIP four are planned to be allocated:

                +==========+=============================+
                | SAM Type | Description                 |
                +==========+=============================+
                | 0x01     | DRIP Link (Section 5.2)     |
                +----------+-----------------------------+
                | 0x02     | DRIP Wrapper (Section 5.3)  |
                +----------+-----------------------------+
                | 0x03     | DRIP Manifest (Section 5.4) |
                +----------+-----------------------------+
                | 0x04     | DRIP Frame (Section 5.5)    |
                +----------+-----------------------------+

                                 Table 2

5.1.2.2.  SAM Authentication Data

   This field has a maximum size of 200-bytes, as defined by
   Section 3.3.2.  The Broadcast Attestation Structure (Section 5.1.1)
   SHOULD be used in this space.

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5.2.  DRIP Link

   This SAM Type is used to transmit Broadcast Attestation's.  For
   example, the Broadcast Attestation of the Registry (HDA) over the UA
   is sent (see Section 6.3) as a DRIP Link message.  Its structure is
   defined in [drip-registries] and an example of it can be found in
   Appendix B.

   DRIP Link is important as its contents are used to provide trust in
   the DET/HI that the UA is currently broadcasting.  This message does
   not require internet connectivity to perform signature validations of
   the contents when the registry DET/HI is in the receivers cache.  It
   also provides the UA HI so that connectivity is not required when
   performing validation of other DRIP Authentication Messages.

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

                            Figure 6: DRIP Link

   This DRIP Authentication Message is used in conjunction with other
   DRIP SAM Types (such as Manifest or Wrapper) that contain data that
   is guaranteed to be unique and easily cross checked by the receiving
   device.  A good candidate for this is using the DRIP Wrapper to
   encapsulate a Location Message (Message Type 0x2).

5.3.  DRIP Wrapper

   This SAM Type is used to wrap and sign over a list of other [F3411]
   Broadcast RID messages.  It MUST use the Broadcast Attestation
   Structure (Section 5.1.1).

   The Attestation Data field is filled with full (25-byte) [F3411]
   Broadcast RID messages.  The minimum number being 1 and the maximum
   being 4.  The encapsulated messages MUST be in Message Type order as
   defined by [F3411].  All message types except Authentication (Message
   Type 0x2) and Message Pack (Message Type 0xF) are allowed.

   To determine the number of messages wrapped the receiver can check
   that the length of the Attestation Data field of the DRIP Broadcast
   Attestation (Section 5.1.1) is a multiple of 25-bytes.

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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                               |
     |                        DRIP Entity Tag                        |
     |                                                               |
     +---------------+---------------+---------------+---------------+
     |                                                               |
     |                                                               |
     |                          ASTM Message                         |
     |                                                               |
     |                                                               |
     |                                                               |
     +               +---------------+---------------+---------------+
     |               |                                               |
     +---------------+                                               |
     |                                                               |
     |                          ASTM Message                         |
     |                                                               |
     |                                                               |
     |                                                               |
     +                               +---------------+---------------+
     |                               |                               |
     +---------------+---------------+                               |
     |                                                               |
     |                                                               |
     |                          ASTM Message                         |
     |                                                               |
     |                                                               |
     +                                               +---------------+
     |                                               |               |
     +---------------+---------------+---------------+               |
     |                                                               |
     |                                                               |
     |                          ASTM Message                         |
     |                                                               |
     |                                                               |
     |                                                               |
     +---------------+---------------+---------------+---------------+
     |                   Not Before Timestamp by UA                  |
     +---------------+---------------+---------------+---------------+
     |                    Not After Timestamp by UA                  |
     +---------------+---------------+---------------+---------------+
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |

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     |                                                               |
     |                                                               |
     |                                                               |
     |                          UA Signature                         |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     +---------------+---------------+---------------+---------------+

                  Figure 7: Example 4-Message DRIP Wrapper

5.3.1.  Wrapper over Extended Transports

   To send the DRIP Wrapper over Extended Transports the messages being
   wrapped are co-located with the Authentication Message in a Message
   Pack (0xF).  The ASTM Messages are removed from the DRIP Wrapper
   after signing (as they are redundant) leaving the following structure
   that is placed into the SAM Authentication Data of Figure 5 and sent
   in the same Message Pack.

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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                               |
     |                        DRIP Entity Tag                        |
     |                                                               |
     +---------------+---------------+---------------+---------------+
     |                   Not Before Timestamp by UA                  |
     +---------------+---------------+---------------+---------------+
     |                    Not After Timestamp by UA                  |
     +---------------+---------------+---------------+---------------+
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                          UA Signature                         |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     +---------------+---------------+---------------+---------------+

              Figure 8: DRIP Wrapper under Extended Transports

   To verify the signature the receiver must concatenate all of the
   messages in the Message Pack (excluding Authentication Message found
   in the same Message Pack) in Message Type order and place the blob
   between the UA DRIP Entity Tag and Not Before Timestamp before
   performing signature verification.

5.3.2.  Wrapper Limitations

   The primary limitation of the Wrapper format is the bounding of up to
   4 ASTM Messages that can be sent within it.  Another limitation is
   that the format can not be used as a surrogate for messages it is
   wrapping.  This is due to high potential a receiver on the ground
   does not support DRIP.  Thus when Wrapper is being used the wrapper
   data must effectively be sent twice; once as a single framed message
   (as specified in [F3411]) and then again wrapped within the Wrapper
   format.

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5.4.  DRIP Manifest

   This SAM Type is used to create message manifests.  It MUST use the
   Broadcast Attestation Structure (Section 5.1.1).

   By hashing previously sent messages and signing them we gain trust in
   UAs previous reports.  An observer who has been listening for any
   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.

   The Attestation Data field is filled with 12-byte hashes of previous
   [F3411] Broadcast messages.  A receiver does not need to have
   received every message in the manifest to verify it.  A manifest
   SHOULD typically encompass a single transmission cycle of messages
   being sent, see Section 6.4.

      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
     +---------------+---------------+---------------+---------------+
     |                                                               |
     |                              UA                               |
     |                        DRIP Entity Tag                        |
     |                                                               |
     +---------------+---------------+---------------+---------------+
     |                                                               |
     |                     Previous Manifest Hash                    |
     |                                                               |
     +---------------+---------------+---------------+---------------+
     |                                                               |
     |                     Current Manifest Hash                     |
     |                                                               |
     +---------------+---------------+---------------+---------------+
     |                                                               |
     |                       ASTM Message Hash                       |
     |                                                               |
     +---------------+---------------+---------------+---------------+
     |                                                               |
     |                       ASTM Message Hash                       |
     |                                                               |
     +---------------+---------------+---------------+---------------+
     |                                                               |
     |                       ASTM Message Hash                       |
     |                                                               |
     +---------------+---------------+---------------+---------------+
     |                                                               |
     |                       ASTM Message Hash                       |
     |                                                               |

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     +---------------+---------------+---------------+---------------+
     |                                                               |
     |                       ASTM Message Hash                       |
     |                                                               |
     +---------------+---------------+---------------+---------------+
     |                                                               |
     |                       ASTM Message Hash                       |
     |                                                               |
     +---------------+---------------+---------------+---------------+
     |                                                               |
     |                       ASTM Message Hash                       |
     |                                                               |
     +---------------+---------------+---------------+---------------+
     |                   Not Before Timestamp by UA                  |
     +---------------+---------------+---------------+---------------+
     |                    Not After Timestamp by UA                  |
     +---------------+---------------+---------------+---------------+
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                          UA Signature                         |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     +---------------+---------------+---------------+---------------+

                      Figure 9: Example DRIP Manifest

5.4.1.  Hash Count

   The number of hashes in the Manifest can be variable (3-9).  An easy
   way to determine the number of hashes is to take the length of the
   data between the end of the UA DRIP Entity Tag and Not Before
   Timestamp by UA and divide it by the hash length (12).  If this value
   is not rational, the message is invalid.

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5.4.2.  Message Hash Algorithms and Operation

   The hash algorithm used for the Manifest Message is the same hash
   algorithm used in creation of the DET [drip-rid] that is signing the
   Manifest.

   An DET using cSHAKE128 [NIST.SP.800-185] computes the hash as
   follows:

   cSHAKE128(ASTM Message, 96, "", "Remote ID Auth Hash")

         Note: [drip-rid] specifies cSHAKE128 but is open for the
         expansion of other OGAs.

5.4.2.1.  Legacy Transport Hashing

   Under this transport DRIP hashes the full ASTM Message being sent
   over the Bluetooth Advertising frame.  For Authentication Messages
   all the Authentication Message Pages are concatenated together and
   hashed as one object.  For all other Message Types the 25-byte
   message is hashed.

5.4.2.2.  Extended Transport Hashing

   Under this transport DRIP hashes the full ASTM Message Pack (Message
   Type 0xF) - regardless of its content.

5.4.3.  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.

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5.4.4.  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.  Examples of such scenarios include delivery or survey UA.

   Another limitation is the length of hash, which is discussed in
   Section 9.1.

5.5.  DRIP Frame

   This SAM Type is for when the authentication data does not fit in
   other defined formats under DRIP and is reserved for future expansion
   under DRIP if required.  This SAM Type MUST use the Broadcast
   Attestation Structure (Section 5.1.1).

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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                               |
     |                        DRIP Entity Tag                        |
     |                                                               |
     +---------------+---------------+---------------+---------------+
     |  Frame Type   |                                               |
     +---------------+                                               .
     .                     Frame Attestation Data                    .
     .                                                               .
     |                                                               |
     +---------------+---------------+---------------+---------------+
     |                   Not Before Timestamp by UA                  |
     +---------------+---------------+---------------+---------------+
     |                    Not After Timestamp by UA                  |
     +---------------+---------------+---------------+---------------+
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                          UA Signature                         |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     +---------------+---------------+---------------+---------------+

                       Figure 10: Example DRIP Frame

5.5.1.  Frame Type

   Byte to sub-type for future different DRIP Frame formats.  It takes
   the first byte of Attestation Data in Section 5.1.1 leaving 111-bytes
   for Frame Attestation Data.

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             +============+==============+==================+
             | Frame Type | Name         | Description      |
             +============+==============+==================+
             | 0x00       | Reserved     | Reserved         |
             +------------+--------------+------------------+
             | 0xC0-0xFF  | Experimental | Experimental Use |
             +------------+--------------+------------------+

                                 Table 3

6.  Requirements & Recommendations

6.1.  Legacy Transports

   With Legacy Advertisements the goal is to attempt to bring reliable
   receipt of the paged Authentication Message.  FEC (Section 4) MUST be
   used, per mandated Remote ID rules (for example the US FAA Remote ID
   Rule [faa-rid]), when using Legacy Advertising methods (such as
   Bluetooth 4).

   Under ASTM Bluetooth 4 rules, transmission of dynamic messages are at
   least every 1 second.  DRIP Authentication Messages typically contain
   dynamic data (such as the DRIP Manifest or DRIP Wrapper) and must be
   sent at the dynamic rate of 1 per second.

6.2.  Extended Transports

   Under the ASTM specification, Bluetooth 5, Wi-Fi NaN, and Wi-Fi
   BEACON transport of Remote ID is to use the Message Pack (Message
   Type 0xF) format for all transmissions.  Under Message Pack messages
   are sent together (in Message Type order) in a single Bluetooth 5
   extended frame (up to 9 single frame equivalent messages under
   Bluetooth 4).  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 (Section 4) MUST NOT be used as it is impractical.

6.3.  Authentication

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

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

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   2.  Any other DRIP Authentication Format (RECOMMENDED: DRIP Manifest
       or DRIP Wrapper) where the UA is dynamically signing data that is
       guaranteed to be unique and easily cross checked by the receiving
       device (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 RAA
       (CAA) (satisfies GEN-3)

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

   3.  DRIP Link using the Broadcast Attestation of HDA (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 that is
       guaranteed to be unique and easily cross checked by the receiving
       device (satisfying GEN-1 and GEN-2)

6.4.  Operational

   UAS operation may impact the frequency of sending DRIP Authentication
   messages.  Where a UA is dwelling in one location, and the channel is
   heavily used by other devices, "occasional" message authentication
   may be sufficient for an observer.  Contrast this with a UA
   traversing an area, and then every message should be authenticated as
   soon as possible for greatest success as viewed by the receiver.

   Thus how/when these DRIP authentication messages are sent is up to
   each implementation.  Further complication comes in contrasting
   Legacy and Extended Transports.  In Legacy, each message is a
   separate hash within the Manifest.  So, again in dwelling, may lean
   toward occasional message authentication.  In Extended Transports,
   the hash is over the Message Pack so only few hashes need to be in a
   Manifest.  A single Manifest can handle a potential two Message Packs
   (for a full set of messages) and a DRIP Link Authentication Message
   for the HDA UA assertion.

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   A separate issue is the frequency of transmitting the DRIP Link
   Authentication Message for the HDA UA assertion when using a Manifest
   Message.  This message content is static; its hash never changes
   radically.  The only change is the 4-byte timestamp in the
   Authentication Message headers.  Thus, potentially, in a dwelling
   operation it can be sent once per minute, where its hash is in every
   Manifest.  A receiver can cache all DRIP Link Authentication Message
   for the HDA UA assertion to mitigate potential packet loss.

   The preferred mode of operation is to send the HDA UA assertion every
   3 seconds and Manifest messages immediately after a set of UA
   operation messages (e.g.  Basic, Location, and System messages).

6.4.1.  DRIP Wrapper

   The DRIP Wrapper MUST NOT be used in place of sending the ASTM
   messages as is.  All receivers MUST be able to process all the
   messages specified in [F3411].  Sending them within the DRIP Wrapper
   makes them opaque to receivers lacking support for DRIP
   authentication messages.  Thus messages within a Wrapper are sent
   twice: in the clear, and authenticated within the Wrapper.  The DRIP
   Manifest format would seem to be a more efficient use of the
   transport channel.

   The DRIP Wrapper has a specific use case for DRIP aware receivers.
   For receiver plotting received Location Messages (Message Type 0x2)
   on a map display an embedded Location Message in a DRIP Wrapper can
   be colored differently to signify trust in the Location data - be it
   current or previous Location reports that are wrapped.

7.  ICAO Considerations

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

   1.  DRIP Link

   2.  DRIP Wrapper

   3.  DRIP Manifest

   4.  DRIP Frame

8.  IANA Considerations

8.1.  Update IANA DRIP Registry

   This document requests a new subregistry for Frame Type.

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   DRIP Frame Type:  This 8-bit valued subregistry is for Frame Types to
      be later allocated.  Future additions to this subregistry are to
      be made through Expert Review (Section 4.5 of [RFC8126]).  The
      following values are defined:

             +============+==============+==================+
             | Frame Type | Name         | Description      |
             +============+==============+==================+
             | 0x00       | Reserved     | Reserved         |
             +------------+--------------+------------------+
             | 0xC0-0xFF  | Experimental | Experimental Use |
             +------------+--------------+------------------+

                                 Table 4

9.  Security Considerations

9.1.  Manifest Hash Length

   For DRIP Manifest an 12-byte hash length has been selected by the
   authors for a number of reasons.

   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 12 or 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.

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9.2.  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
       UA 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.

9.3.  Trust Timestamp Offsets

   Note the discussion of Trust Timestamp Offsets here is in context of
   the DRIP Wrapper (Section 5.3) and DRIP Manifest (Section 5.4)
   messages.  For DRIP Link (Section 5.2) messages these offsets are set
   by the Attestor (typically a registry) and have their own set of
   considerations as seen in [drip-registries].

   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.

10.  Acknowledgments

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

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   Soren Friis for pointing out that Wi-Fi implementations would not
   always 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.

11.  References

11.1.  Normative References

   [F3411]    "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>.

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

11.2.  Informative References

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

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

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   [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>.

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

Appendix A.  Authentication State Diagrams & Color Scheme

   ASTM Authentication has only 3 states: None, Invalid or Valid.  This
   is because under ASTM the idea is that Authentication is done by an
   external service hosted somewhere on the Internet so it is assumed
   you will always get some sort of answer back.  With DRIP this
   classification becomes more complex with the support of "offline"
   scenarios where the receiver does not have Internet connectivity.
   With the use of asymmetric keys this means the public key (PK) must
   somehow be obtained - [drip-registries] gets more into detail how
   these keys are stored on DNS and one reason for DRIP Authentication
   is to send PK's over Broadcast RID.

   There are two keys of interest: the PK of the UA and the PK of the
   HDA (or Registry).  This document gives a clear way to send the PK of
   the UA over the Broadcast RID messages - however the PK of the
   Registry is not.  It can be using the same mechanism but is not
   required to do so due to potential operational constraints and
   implementation of a given UA transmitter.  As such there are
   scenarios where you may have part of the key-chain but not all of it.

   The intent of this appendix is to give some kind of recommended way
   to classify these various states and convey it to the user through
   colors and state names/text.

A.1.  State Table

   The table below lays out the RECOMMENDED colors to associate with
   state.

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       +==============+========+===================================+
       | State        | Color  | Details                           |
       +==============+========+===================================+
       | None         | Black  | No Authentication being received  |
       +--------------+--------+-----------------------------------+
       | Partial      | Gray   | Authentication being received but |
       |              |        | missing pages                     |
       +--------------+--------+-----------------------------------+
       | Unsupported  | Brown  | Authentication Type/SAM Type of   |
       |              |        | received message not supported    |
       +--------------+--------+-----------------------------------+
       | Unverifiable | Yellow | Data needed for verification      |
       |              |        | missing                           |
       +--------------+--------+-----------------------------------+
       | Verified     | Green  | Valid verification results        |
       +--------------+--------+-----------------------------------+
       | Trusted      | Blue   | Valid verification results and    |
       |              |        | HDA is marked as trusted          |
       +--------------+--------+-----------------------------------+
       | Questionable | Orange | Inconsistent verification results |
       +--------------+--------+-----------------------------------+
       | Unverified   | Red    | Invalid verification results      |
       +--------------+--------+-----------------------------------+
       | Conflicting  | Purple | Inconsistent verification results |
       |              |        | and HDA is marked as trusted      |
       +--------------+--------+-----------------------------------+

                                  Table 5

A.2.  State Diagrams

   This section gives some RECOMMENDED state flows that DRIP should
   follow.

A.2.1.  Notations

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                   o--------------o
                   |   PROCESS    |
                   o--------------o

                   +--------------+
                   |    STATE     |
                   +--------------+

                    ooooo
                   o  N  o    Transition N
                    ooooo

                   +----->    Transition Option False/No

                   ----->     Transition Option True/Yes

                        Figure 11: Diagram Notations

A.2.2.  General

 o---------------------o      ooooo        +------+
 |        Start        |---->o  1  o+----->| None |
 o---------------------o      ooooo        +------+
                                |
                                v
                              ooooo        +-------------+
                             o  2  o+----->| Unsupported |
                              ooooo        +-------------+
                                |             ^
                                v             |
           +---------+        ooooo           |
           | Partial |<-----+o  3  o          |
           +---------+        ooooo           |
                                |             |
                                v             +
                              ooooo         ooooo        o-------------o
                             o  4  o------>o  5  o------>| SAM Decoder |
                              ooooo         ooooo        o-------------o
                                +
                                |
                                v
                         o------------------o
                         | AuthType Decoder |
                         o------------------o

            Figure 12: Standard Authentication Colors/State

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    +============+=============================+======================+
    | Transition | Transition Query            | Next State/Process/  |
    |            |                             | Transition (Yes, No) |
    +============+=============================+======================+
    | 1          | Receiving Authentication    | 2, None              |
    |            | Pages?                      |                      |
    +------------+-----------------------------+----------------------+
    | 2          | Authentication Type         | 3, Unsupported       |
    |            | Supported?                  |                      |
    +------------+-----------------------------+----------------------+
    | 3          | All Pages of Authentication | 4, Partial           |
    |            | Message Received?           |                      |
    +------------+-----------------------------+----------------------+
    | 4          | Is Authentication Type      | 5, AuthType Decoder  |
    |            | received 5?                 |                      |
    +------------+-----------------------------+----------------------+
    | 5          | Is SAM Type Supported?      | SAM Decoder,         |
    |            |                             | Unsupported          |
    +------------+-----------------------------+----------------------+

                                  Table 6

A.2.3.  DRIP SAM

   o-------------o      ooooo        o-----------------------------o
   | SAM Decoder |---->o  6  o------>| DRIP Wrapper/Manifest/Frame |
   o-------------o      ooooo        o-----------------------------o
                          +                 |              ^
                          |                 |              |
                          v                 v              |
                   o-----------o    o--------------------o |
                   | DRIP Link |--->| Update State Cache | |
                   o-----------o    o--------------------o |
                                      |                    |
                                      v                    |
           o--------------o         ooooo       o----------------------o
           | NOP / Return |<------+o  7  o----->| Extract Message from |
           o--------------o         ooooo       | Verification Queue   |
                                                o----------------------o

                        Figure 13: DRIP SAM Decoder

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       +============+=====================+========================+
       | Transition | Transition Query    | Next State/Process/    |
       |            |                     | Transition (Yes, No)   |
       +============+=====================+========================+
       | 6          | Is SAM Type DRIP    | DRIP Link, DRIP        |
       |            | Link?               | Wrapper/Manifest/Frame |
       +------------+---------------------+------------------------+
       | 7          | Messages in         | Extract Message from   |
       |            | Verification Queue? | Verification Queue,    |
       |            |                     | NOP / Return           |
       +------------+---------------------+------------------------+

                                  Table 7

A.2.4.  DRIP Link

      o-----------o       ooooo         ooooo        +--------------+
      | DRIP Link |----->o  8  o+----->o  9  o+----->| Unverifiable |
      o-----------o       ooooo         ooooo        +--------------+
                            |             |
                            |-------------'
                            v
                          ooooo        +------------+
                         o  10 o+----->| Unverified |
                          ooooo        +------------+
                            |
                            v
                      o---------------------o
                      | Add UA DET/PK       |
                      | to Key Cache        |
                      o---------------------o
                            |
                            v
                          ooooo         +----------+
                         o  11 o+------>| Verified |
                          ooooo         +----------+
                            |              ^
                            v              |
                      o-------------------------o
                      | Mark UA DET/PK          |
                      | as Trusted in Key Cache |
                      o-------------------------o

                     Figure 14: DRIP Link State Decoder

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   +============+==========================+===========================+
   | Transition | Transition Query         | Next State/Process/       |
   |            |                          | Transition (Yes, No)      |
   +============+==========================+===========================+
   | 8          | Registry DET/PK in Key   | 10, 9                     |
   |            | Cache?                   |                           |
   +------------+--------------------------+---------------------------+
   | 9          | Registry PK found        | 10, Unverifiable          |
   |            | Online?                  |                           |
   +------------+--------------------------+---------------------------+
   | 10         | Registry Signature       | Add UA DET/PK to Key      |
   |            | Verified?                | Cache, Unverified         |
   +------------+--------------------------+---------------------------+
   | 11         | Registry DET/PK marked   | Mark UA DET/PK as         |
   |            | as Trusted in Key Cache? | Trusted in Key            |
   |            |                          | Cache, Verified           |
   +------------+--------------------------+---------------------------+

                                  Table 8

A.2.5.  DRIP Wrapper/Manifest/Frame

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     o-----------------------------o         +--------------+
     | DRIP Wrapper/Manifest/Frame |         | Unverifiable |
     o-----------------------------o         +--------------+
                |                                   ^
                v                                   |
              ooooo         ooooo        o--------------------o
             o  12 o+----->o  13 o+----->| Add Message to     |
              ooooo         ooooo        | Verification Queue |
                |             |          o--------------------o
                |             |
                |-------------'
                v
              ooooo         ooooo         ooooo        +------------+
             o  14 o+----->o  15 o+----->o  16 o+----->| Unverified |
              ooooo         ooooo         ooooo        +------------+
                |             |             |
                v             v             |
              ooooo        +-------------+  |
             o  17 o+----->| Conflicting |  |
              ooooo        +-------------+  |
                |                           |
                v                           v
              ooooo                  +--------------+
             o  18 o---------------->| Questionable |
              ooooo                  +--------------+
                +
                |
                v
              ooooo        +----------+
             o  19 o+----->| Verified |
              ooooo        +----------+
                |
                v
             +---------+
             | Trusted |
             +---------+

            Figure 15: DRIP Wrapper/Manifest/Frame State Decoder

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   +============+==============================+======================+
   | Transition | Transition Query             | Next State/Process/  |
   |            |                              | Transition (Yes, No) |
   +============+==============================+======================+
   | 12         | UA DET/PK in Key Cache?      | 14, 13               |
   +------------+------------------------------+----------------------+
   | 13         | UA PK found Online?          | 14, Add Message to   |
   |            |                              | Verification Queue   |
   +------------+------------------------------+----------------------+
   | 14         | UA Signature Verified?       | 17, 15               |
   +------------+------------------------------+----------------------+
   | 15         | Has past Messages of this    | Conflicting, 16      |
   |            | type been marked as Trusted? |                      |
   +------------+------------------------------+----------------------+
   | 16         | Has past Messages of this    | Questionable,        |
   |            | type been marked as          | Unverified           |
   |            | Questionable or Verified?    |                      |
   +------------+------------------------------+----------------------+
   | 17         | Has past Messages of this    | Conflicting, 18      |
   |            | type been marked as          |                      |
   |            | Conflicting?                 |                      |
   +------------+------------------------------+----------------------+
   | 18         | Has past Messages of this    | Questionable, 19     |
   |            | type been marked as          |                      |
   |            | Questionable or Unverified?  |                      |
   +------------+------------------------------+----------------------+
   | 19         | Is UA DET/PK marked as       | Trusted, Verified    |
   |            | Trusted in Key Cache?        |                      |
   +------------+------------------------------+----------------------+

                                 Table 9

Appendix B.  HDA-UA Broadcast Attestation

      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
     +---------------+---------------+---------------+---------------+
     |                                                               |
     |                             DRIP                              |
     |                       Entity Tag of HDA                       |
     |                                                               |
     +---------------+---------------+---------------+---------------+
     |                                                               |
     |                             DRIP                              |
     |                       Entity Tag of UA                        |
     |                                                               |
     +---------------+---------------+---------------+---------------+
     |                                                               |

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     |                                                               |
     |                                                               |
     |                      Host Identity of UA                      |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     +---------------+---------------+---------------+---------------+
     |                   Not Before Timestamp by HDA                 |
     +---------------+---------------+---------------+---------------+
     |                    Not After Timestamp by HDA                 |
     +---------------+---------------+---------------+---------------+
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                       Signature by HDA                        |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     +---------------+---------------+---------------+---------------+

     DRIP Entity Tag of HDA: (16-bytes)
         DET of HDA.

     DRIP Entity Tag of UA: (16-bytes)
         DET of UA.

     Host Identity of UA: (32-bytes)
         HI of UA

     Expiration Timestamp by HDA (4 bytes):
         Timestamp denoting recommended time to trust data to.

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

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

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            Figure 16: Example DRIP HDA-UA Broadcast Attestation

Appendix C.  Example TX/RX Flow

   In this example the UA is sending all DRIP Authentication Message
   formats (DRIP Link, DRIP Wrapper and DRIP Manifest) during flight,
   along with standard ASTM Messages.  The objective is to show the
   combinations of messages that must be received to properly validate a
   DRIP equipped UA and examples of their various states (Appendix A).

           +-------------------+
     .-----| Unmanned Aircraft |-----.
     |     +-------------------+     |
     | 1       | 2         | 3       | 4
     |         |           |         |

     O         O           O         O
   --|--     --|--       --|--     --|--
    / \       / \         / \       / \
     A         B           C         D

   Broadcast Paths: Messages Received
   1: DRIP Link
   2: DRIP Link and DRIP Wrapper or DRIP Manifest
   3: DRIP Wrapper or DRIP Manifest
   4: None

   Observers: Authentication State
   A: Unverifiable
   B: Verified, Trusted, Unverified, Questionable, or Conflicting
   C: Unverifiable
   D: None

   As the above example shows to properly authenticate both a DRIP Link
   and a DRIP Wrapper or DRIP Manifest are required.

Authors' Addresses

   Adam Wiethuechter
   AX Enterprize, LLC
   4947 Commercial Drive
   Yorkville, NY 13495
   United States of America
   Email: adam.wiethuechter@axenterprize.com

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   Stuart Card
   AX Enterprize, LLC
   4947 Commercial Drive
   Yorkville, NY 13495
   United States of America
   Email: stu.card@axenterprize.com

   Robert Moskowitz
   HTT Consulting
   Oak Park, MI 48237
   United States of America
   Email: rgm@labs.htt-consult.com

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