DRIP Working Group                                       A. Wiethuechter
Internet-Draft                                                   S. Card
Intended status: Standards Track                      AX Enterprize, LLC
Expires: 21 June 2021                                       R. Moskowitz
                                                          HTT Consulting
                                                        18 December 2020


                      DRIP Authentication Formats
                        draft-ietf-drip-auth-00

Abstract

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

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   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 21 June 2021.

Copyright Notice

   Copyright (c) 2020 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
   extracted from this document must include Simplified BSD License text



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

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  DRIP Requirements Addressed . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  Required Terminology  . . . . . . . . . . . . . . . . . .   3
     2.2.  Definitions . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Background  . . . . . . . . . . . . . . . . . . . . . . . . .   3
     3.1.  Problem Space and Focus . . . . . . . . . . . . . . . . .   4
     3.2.  ASTM Authentication Message . . . . . . . . . . . . . . .   4
   4.  DRIP Authentication Framing Formats . . . . . . . . . . . . .   6
     4.1.  DRIP General Frame  . . . . . . . . . . . . . . . . . . .   6
       4.1.1.  DRIP Header . . . . . . . . . . . . . . . . . . . . .   8
       4.1.2.  DRIP Authentication Data  . . . . . . . . . . . . . .   8
       4.1.3.  Forward Error Correction  . . . . . . . . . . . . . .   9
     4.2.  DRIP Wrapper Frame  . . . . . . . . . . . . . . . . . . .  10
       4.2.1.  UA Hierarchical Host Identity Tag . . . . . . . . . .  11
       4.2.2.  Trust Timestamp . . . . . . . . . . . . . . . . . . .  11
       4.2.3.  Wrapped Authentication Data . . . . . . . . . . . . .  12
       4.2.4.  Wrapper Signature . . . . . . . . . . . . . . . . . .  16
     4.3.  DRIP Attestation Frame  . . . . . . . . . . . . . . . . .  16
       4.3.1.  Attestation Data  . . . . . . . . . . . . . . . . . .  17
       4.3.2.  Expiration Timestamp  . . . . . . . . . . . . . . . .  18
       4.3.3.  Attestation Signature . . . . . . . . . . . . . . . .  18
   5.  Transport Methods & Recommendations . . . . . . . . . . . . .  18
     5.1.  Legacy Advertisements (Bluetooth 4.X) . . . . . . . . . .  18
     5.2.  Extended Advertisements (Bluetooth 5.X and Wifi NaN)  . .  19
   6.  ASTM Considerations . . . . . . . . . . . . . . . . . . . . .  19
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  20
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  20
   9.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  20
   10. Appendix A: Thoughts on ASTM Authentication Message . . . . .  20
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  21
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  21
     11.2.  Informative References . . . . . . . . . . . . . . . . .  21
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  22

1.  Introduction

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





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

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

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

1.1.  DRIP Requirements Addressed

   The following [drip-requirements] will be addressed:

   GEN 1: Provable Ownership  This will be addressed using the
      Certificate Message type (Section 4.3.1.1).

   GEN 2: Provable Binding  This requirement is addressed using the
      Wrapped ASTM Message (Section 4.2.3.1.2), Manifest Message
      (Section 4.2.3.2) and Message Pack Signature (Section 4.2.3.1.1)
      types.

   GEN 3: Provable Registration  This requirement is addressed using the
      Certificate Message type (Section 4.3.1.1).

2.  Terminology

2.1.  Required Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

2.2.  Definitions

   See [drip-requirements] for common DRIP terms.

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

3.  Background







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3.1.  Problem Space and Focus

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

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

3.2.  ASTM Authentication Message






































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     Page 0:
      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +---------------+-----------------------------------------------+
     |  Auth Header  |                                               |
     +---------------+  ASTM Authentication Headers  +---------------+
     |                                               |               |
     +-----------------------------------------------+               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                Authentication Data / Signature                |
     |                                                               |
     |                                                               |
     |                                                               |
     +---------------------------------------------------------------+

     Page 1 - 4:
      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +---------------+-----------------------------------------------+
     |  Auth Header  |                                               |
     +---------------+                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                Authentication Data / Signature                |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     +---------------------------------------------------------------+

     Auth Header (1 byte):
         Contains Authentication Type (AuthType) and Page Number. For
         DRIP Authentication AuthType is a value of 0x5.

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

     Authentication Data / Signature: (109 bytes: 17+23*4)
         Opaque authentication data.

           Figure 1: Standard ASTM Authentication Message format





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   The above diagram is the format defined by ASTM [F3411-19] that is
   the frame which everything this document fits into.  The specific
   details of the ASTM headers are abstracted away as they are not
   necessarily required for this document.

   There is a 25th byte exclude in the diagrams that comes before the
   Auth Header.  This is the ASTM Header and consists of the Protocol
   Version and Message Type of the given message frame/page.

4.  DRIP Authentication Framing Formats

   Currently the ASTM AuthType of 0x5 should be used to denote DRIP
   based Authentication.  The max page count of the Authentication
   Message is increased to 10, instead of being capped at 5.

   To keep consistent formatting across the different mediums (Bluetooth
   4, Bluetooth 5 and Wifi NaN) and their independent restrictions the
   authentication data being sent is REQUIRED to fit within the first 9
   pages of the Authentication Message.  The final (10th) page of the
   message is reserved exclusively for Forward Error Correction bytes
   and is only present on Bluetooth 4.

4.1.  DRIP General Frame

     Page 0:
      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +---------------+-----------------------------------------------+
     |  Auth Header  |                                               |
     +---------------+  ASTM Authentication Headers  +---------------+
     |                                               |  DRIP Header  |
     +-----------------------------------------------+---------------+
     |                                                               |
     |                                                               |
     |                                                               |
     |                    DRIP Authentication Data                   |
     |                                                               |
     |                                                               |
     |                                                               |
     +---------------------------------------------------------------+

     Page 1 - 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
     +---------------+-----------------------------------------------+
     |  Auth Header  |                                               |
     +---------------+                                               |
     |                                                               |



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     |                                                               |
     |                                                               |
     |                    DRIP Authentication Data                   |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     +---------------------------------------------------------------+

     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
     +---------------+-----------------------------------------------+
     |  Auth Header  |                                               |
     +---------------+                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                    Forward Error Correction                   |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     +---------------------------------------------------------------+

     DRIP Header (1 byte):
            7     6     5     4     3     2     1     0
         +-----+-----+-----+-----+-----+-----+-----+-----+
         | FEC |              DRIP AuthType              |
         +-----+-----+-----+-----+-----+-----+-----+-----+

         FEC (1 bit):
             Enabled [1] or Disabled [0]. Signals if Page N is
             filled with XOR FEC.

         DRIP AuthType (7 bits):
             DRIP AuthType                        Values
             -------------                        ------
             0 Wrapped ASTM Message(s)            0
             1 Wrapped ASTM Message(s)            1
             2 Wrapped ASTM Message(s)            2
             3 Wrapped ASTM Message(s)            3
             4 Wrapped ASTM Message(s)            4
             5 Wrapped ASTM Message(s)            5
             8 Byte Manifest                      6
             4 Byte Manifest                      7



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             Reserved (Wrapped Messages)          8-15
             Certificate: Registry on Aircraft    16
             Reserved (Certificates)              17-31
             Private Use                          32-63
             Reserved                             64-111
             Experimental Use                     112-127

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

     Forward Error Correction (23 bytes):
         Optional and signaled using DRIP Header. Always last
         Authentication page.

                    Figure 2: DRIP General Frame Format

4.1.1.  DRIP Header

   The DRIP Header is used to signal what kind of Authentication under
   DRIP that the message is using and consists of two fields.

4.1.1.1.  Forward Error Correction (Bit 8)

   The Most Significant Bit is used to signal if FEC is present in the
   final page of the Authentication Message.  It MUST be set to 1 if FEC
   is being used.  This is only enabled under Bluetooth 4 and MUST be
   set to 0 on Bluetooth 5 or Wifi NaN.

4.1.1.2.  DRIP AuthType (Bits 1-7)

   The lower 7 bits are used as the DRIP AuthType field denoting what
   Authentication type is being used.  There are 5 major areas carved
   out of the DRIP AuthType defined by the following bitmaps:

                000 xxxx (0x00-0x0F): Wrapped Messages (16)
                001 xxxx (0x10-0x1F): Certificates (16)
                01x xxxx (0x20-0x3F): Private Use (32)
                1xx xxxx (0x40-0x6F): Reserved (48)
                111 xxxx (0x70-0x7F): Experimental Use (16)

                       Figure 3: DRIP Header Bitmasks

4.1.2.  DRIP Authentication Data

   This field has a maximum size of 200 bytes.  If the data is less than
   the max and a page is only partially filled then the rest of the
   partially filled page must be null padded.




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   This section is generally filled with either the Wrapper Frame
   (Section 4.2) or the Attestation Frame (Section 4.3).

4.1.3.  Forward Error Correction

   To help Bluetooth (specifically Bluetooth 4) achieve the goal of
   reliable receipt of paged messages a Forward Error Correction (FEC)
   scheme is introduced and MUST be used for Legacy Advertising
   (Bluetooth 4) and MUST NOT be used for Extended Advertising
   (Bluetooth 5, Wifi NaN) under DRIP.

4.1.3.1.  Encoding

   A compliant implementation of this standard MUST use XOR for the FEC.
   When generating the parity the first byte of every Authentication
   Page MUST be exclude from the XOR operation.  For pages 1 through N
   this leaves the data portion of the page while page 0 will include a
   number of headers along with 17 bytes of data.

   To generate the parity a simple XOR operation using the previous and
   current page is used.  For page 0, a 23 byte null pad is used for the
   previous page.  The resulting 23 bytes of parity is appended in one
   full page (always the last) allowing for recovery when any single
   page is lost in transmission.

4.1.3.2.  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
   Authentication Data field (and ASTM Authentication Headers fields if
   page 0) with nulls.

   Using the same methods as encoding, an XOR operation is used between
   the previous and current page (a 23 byte null pad is used when page 0
   is the current page).  The resulting 23 bytes is the data of the
   missing page.

   If page 0 is being reconstructed an additional check of the Page
   Count, to check against how many pages are actually present, MUST be
   performed for sanity.  An additional check on the Data Length field
   can also be performed, but is not required.





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4.1.3.3.  Limitations & Recommendations

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

   In theory under Bluetooth 4 up to 15 pages Authentication could be
   sent (9 pages reserved to Authentication and 6 pages reserved for
   Forward Error Correction).  It is currently recommended however for a
   max of 10 pages total.

4.2.  DRIP Wrapper Frame

   This format MUST be encapsulated by the General Frame (Section 4.1)
   and reside in its data field (Section 4.1.2).

   Typically the DRIP Header is set in the range of 0x00 through 0x0F
   (FEC disabled) or 0x80 through 0x8F (FEC enabled).

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +---------------+---------------+---------------+---------------+
     |                                                               |
     |                        UA Hierarchical                        |
     |                       Host Identity Tag                       |
     |                                                               |
     +---------------+---------------+---------------+---------------+
     |                         Trust Timestamp                       |
     +---------------+---------------+---------------+---------------+
     |                                                               |
     .                                                               .
     .                      Authentication Data                      .
     .                                                               .
     |                                                               |
     +---------------+---------------+---------------+---------------+
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                            Signature                          |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |



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     |                                                               |
     |                                                               |
     |                                                               |
     +---------------+---------------+---------------+---------------+

     UA Hierarchial Host Identity Tag (16 bytes):
         The UAs HHIT in byte form. Hashed from the EdDSA25519
         public key.

     Trust Timestamp (4 bytes):
         Timestamp denoting current time plus an offset to trust
         message to.

     Authentication Data (116 bytes):
         Opaque authentication data using DRIP format specified in
         the DRIP Header. 0 to 116 bytes.

     Signature (64 bytes):
         Signature over preceding fields using the EdDSA25519
         keypair of the UA.

                    Figure 4: DRIP Wrapper Frame Format

4.2.1.  UA Hierarchical Host Identity Tag

   To avoid needing the UAs HHIT via the ASTM Basic ID in a detached
   fashion the 16 byte HHIT of the UA is included in the wrapper frame.

   The HHIT for the UA (and other entities in the RID and greater UTM
   system under DRIP) is an enhancement of the Host Identity Tag (HIT)
   [RFC7401] introducing hierarchy (and how they are used in UAS RID) as
   defined in [drip-rid].

4.2.2.  Trust Timestamp

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

   When wrapping a Vector (Position/Location) Message the payload WILL
   contain (by ASTM rules) constantly changing data, this includes its
   own timestamp.  This timestamp is only 2 bytes, which is easily
   attacked and only expresses the 1/10th of seconds since the last
   hour.

   Other ASTM message types, such as Basic ID and Self-ID are static
   messages with no changing data.  To protect a replay of these signed



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   messages the Trust Timestamp is the field during signing to be
   guaranteed to change.

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

4.2.3.  Wrapped Authentication Data

   This field has a maximum of 116 bytes in length.

4.2.3.1.  Wrapped ASTM Message Formats

   When wrapping any ASTM Messages and filling the Wrapped
   Authentication Data field under DRIP the messages MUST be in Message
   Type order as defined by ASTM.  All message types except
   Authentication (0x2) and Message Pack (0xF) are allowed.

4.2.3.1.1.  0 Wrapped ASTM Message(s)

   This payload type MUST only be used under Extended Advertisement
   (Bluetooth 5.X and Wifi NaN).

   The Wrapped Authentication Data is the concatenation of all messages
   in the Message Pack (excluding Authentication) in Message Type order.
   No actual data payload is present in this format as the data is found
   outside the Authentication Message in the same Message Pack.

   The DRIP Header is set to 0x00 (0).

4.2.3.1.2.  1 to 4 Wrapped ASTM Message(s)

   This payload type can be used on either Legacy or Extended
   Advertisements.

   The DRIP Header is set to 0x81-0x84 (129-134) when using Legacy
   Advertisements (FEC is enabled) and 0x01-0x04 (1-4) when using
   Extended Advertisements (FEC is disabled).

4.2.3.1.3.  5 Wrapped ASTM Message(s)

   Editors Note: This payload type does not currently fit in the 116
   byte limit of the Wrapper Frame.  If the ASTM relaxes the Max Page
   Count limit for Legacy Advertisements to use all 15 pages then this
   is possible.





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   This payload type MUST only be used on Legacy Advertisements
   (Bluetooth 4.X).  It requires 11 pages to complete.

   The DRIP Header is set to 0x85 (133).

   This payload type allows in Legacy Advertisements to have a pseudo-
   Message Pack like what is found in Extended Advertisements.

4.2.3.1.4.  Limitations

   When wrapping a single ASTM Message the 25 byte payload actually
   causes an inefficiency in the framing format, create a whole page
   unused except for a single byte.  This can be optimized by removing a
   single byte out of the wrapped message but creates an issue on the
   receiver of knowing which byte was removed.

   When sending a Location Message (Message Type 0x1) a single byte can
   be removed at the end of the message as it is currently unused.  Many
   other messages in the ASTM Message set however do not have this
   ability.  The first byte can not be removed as it is the key to know
   how to decode the message.

4.2.3.2.  Manifests

   Manifests fill the Wrapped Authentication Data field with hashes of
   previously send messages.

   By hashing previously sent messages and signing them we gain trust in
   UAs previous reports.  An observer who has been listening for any
   considerable length of time can hash received messages and cross
   check against listed hashes.

4.2.3.2.1.  Hash Algorithm and Operation

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

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

   cSHAKE128(MAC Address|Message, 8*H-Len, "", "RemoteID Auth Hash")

   The message MAC Address of the transmitter is prepended to the
   message, as the MAC Address is the only information that links UA
   messages from a specific UA.

   Editors Note: It should be noted that for Bluetooth mediums this is
   valid - however Wifi NaN does not give the receiver device the



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   transmitters MAC Address - making this impossible.  Either MAC
   Address should be removed entirely or something different be used in
   its place to link to a given UA.  Thanks Soren Friis for pointing
   this out.

4.2.3.2.2.  8 Byte

      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
     +---------------+---------------+---------------+---------------+
     |                   Hash of Previous Manifest                   |
     |                                                               |
     +---------------+---------------+---------------+---------------+
     |                    Hash of Current Manifest                   |
     |                                                               |
     +---------------+---------------+---------------+---------------+
     |                         Message Hash 1                        |
     |                                                               |
     +---------------+---------------+---------------+---------------+
     |                         Message Hash 2                        |
     |                                                               |
     +---------------+---------------+---------------+---------------+
     .                                                               .
     .                                                               .
     .                                                               .
     +---------------+---------------+---------------+---------------+
     |                        Message Hash 12                        |
     |                                                               |
     +---------------+---------------+---------------+---------------+

     DRIP Header:
         With FEC: 0x87 [135] (RECOMMENDED)
         Without FEC: 0x07 [7]

     Hash of Previous Manifest: (8 bytes)
         A hash of the previously sent Authentication message.

     Hash of Current Manifest: (8 bytes)
         A hash of the current Authentication message.

     Message Hash: (8 bytes)
         A hash of a previously sent message. 12 max.

                         Figure 5: 4 Byte Manifest

4.2.3.2.3.  4 Byte





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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
     +---------------+---------------+---------------+---------------+
     |                   Hash of Previous Manifest                   |
     +---------------+---------------+---------------+---------------+
     |                    Hash of Current Manifest                   |
     +---------------+---------------+---------------+---------------+
     |                         Message Hash 1                        |
     +---------------+---------------+---------------+---------------+
     |                         Message Hash 2                        |
     +---------------+---------------+---------------+---------------+
     .                                                               .
     .                                                               .
     .                                                               .
     +---------------+---------------+---------------+---------------+
     |                        Message Hash 27                        |
     +---------------+---------------+---------------+---------------+

     DRIP Header:
         With FEC: 0x86 [132] (RECOMMENDED)
         Without FEC: 0x06 [6]

     Hash of Previous Manifest: (4 bytes)
         A hash of the previously sent Authentication message.

     Hash of Current Manifest: (4 bytes)
         A hash of the current Authentication message.

     Message Hash: (4 bytes)
         A hash of a previously sent message. 27 max.

                         Figure 6: 4 Byte Manifest

4.2.3.2.4.  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



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      completely recompute the hash.  This mostly depends on the
      previous note.

4.2.3.2.5.  Manifest Limitation

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

4.2.4.  Wrapper Signature

   The wrapper signature is generated using the private key half of the
   the UAs Host Identity (HI) and is done over all preceding data.
   ASTM/DRIP Headers are exclude from this operation only information
   within the Wrapper Fame (Section 4.2) is signed.

4.3.  DRIP Attestation Frame

   This format MUST be encapsulated by the General Frame (Section 4.1)
   and reside in its data field (Section 4.1.2).

   This format is typically used to form a complete certificate using
   attestation data from a Registry defined in [identity-claims].  The
   DRIP Header is normally in the range of 0x10 through 0x1F (FEC
   disable) or 0x90 through 0x9F (FEC enabled).
























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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
     +---------------+---------------+---------------+---------------+
     |                                                               |
     .                                                               .
     .                        Attestation Data                       .
     .                                                               .
     |                                                               |
     +---------------+---------------+---------------+---------------+
     |                      Expiration Timestamp                     |
     +---------------+---------------+---------------+---------------+
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                            Signature                          |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     +---------------+---------------+---------------+---------------+

     Attestation Data: (up to 132 bytes):
         Data the UA asserts claim to.
         Up to 132 bytes in length.

     Expiration Timestamp (4 bytes):
         Generated by the UA to protect against replay attacks.

     Signature (64 bytes):
         Signature over preceding fields using the EdDSA25519
         keypair of the UA.

                     Figure 7: DRIP Attestation Format

4.3.1.  Attestation Data

   Any data up to 132 bytes in length that the UA wishes to assert truth
   to.





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4.3.1.1.  DRIP Certificate

   This payload type can be used in either Legacy or Extended
   Advertising.  It is used to grant the ability to authenticate UA
   Remote ID when the receiving device of the observer (e.g. a
   smartphone with a dedicated RID application) has no Internet service
   (e.g.  LTE signal).

   The DRIP Header is set to 0x90 (144) when used for Legacy
   Advertisements and 0x10 (16) for Extended Advertisements.

   The Attestation Data field is filled with the Attestation: Registry
   on Aircraft (Section 3.2.2 Attestation: X on Y (Offline Form) from
   [identity-claims]).  This is binding claim between the Registry and
   the Aircraft, asserting the relationship between the two entities.
   It also provides the UA Host Identity to allow signature verification
   of messages signed by the UA.  Also included in its structure is the
   HHIT of the Registry to check the local shortlist of Registries that
   the Observer device trusts (mapping HHITs to HIs).

   More details about this Attestation and other certificates and the
   provisioning process can be found in [identity-claims].

4.3.2.  Expiration Timestamp

   Generated by the UA during the creation of the Authentication
   message.  It is set a short time into the future to protect against
   replay attacks of this DRIP format.

   It shares the same format as the Trust Timestamp (Section 4.2.2).

4.3.3.  Attestation Signature

   Performed by the UA using the onboard keypair which matches the HHIT
   in the Basic ID Message (0x0).

5.  Transport Methods & Recommendations

5.1.  Legacy Advertisements (Bluetooth 4.X)

   With Legacy Advertisements the goal is to attempt to bring reliable
   receipt of the paged Authentication Message.  Forward Error
   Correction (Section 4.1.3) MUST be enabled when using Legacy
   Advertising methods (such as Bluetooth 4.X).

   Under ASTM Bluetooth 4.X rules, transmission of dynamic messages are
   at least every 1 second while static messages (which is what




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   Authentication is classified under) are sent at least every 3
   seconds.

   Under DRIP the Certificate Message MUST be transmitted to properly
   meet the GEN 1 and GEN 3 requirement.

   The ASTM Message Wrapper and Manifest both satisfy the GEN 2
   requirement.  At least one MUST be implemented to comply with the GEN
   2 requirement.

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

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

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

5.2.  Extended Advertisements (Bluetooth 5.X and Wifi NaN)

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

   Without any fragmentation or loss of pages with transmission Forward
   Error Correction (Section 4.1.3) MUST NOT be used as it is
   impractical.

6.  ASTM Considerations

   *  Increase Authentication Max Page Count from 5 to 10.  Legacy
      Advertising can use all 10 while Extended Advertising has a
      maximum of 9 due to Bluetooth 5 limitations.

   *  Allocate Authentication Type 0x5 for DRIP from ASTM AuthType
      field.







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7.  IANA Considerations

   This document does not require any actions by IANA.

8.  Security Considerations

   TODO

   (Ed.  Note: Hash lengths (length vs strength/collision rate); replay
   attacks with timestamps; static Cra (issue but nulled if UA signing
   other stuff dynamically meaning signatures will fail as HI won't
   match - this is probably a deeper discussion topic for provisioning
   security considerations when we get to there))

9.  Acknowledgments

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

10.  Appendix A: Thoughts on ASTM Authentication Message

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

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

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

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

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




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

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

11.  References

11.1.  Normative References

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

   [NIST.SP.800-185]
              Kelsey, J., Change, S., and R. Perlner, "SHA-3 Derived
              Functions: cSHAKE, KMAC, TupleHash and ParallelHash",
              DOI 10.6028/nist.sp.800-185, NIST Special Publication 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-requirements]
              Card, S., Wiethuechter, A., Moskowitz, R., and A. Gurtov,
              "Drone Remote Identification Protocol (DRIP)
              Requirements", Work in Progress, Internet-Draft, draft-
              ietf-drip-reqs-06, 1 November 2020, <http://www.ietf.org/
              internet-drafts/draft-ietf-drip-reqs-06.txt>.




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

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

   [RFC7401]  Moskowitz, R., Ed., Heer, T., Jokela, P., and T.
              Henderson, "Host Identity Protocol Version 2 (HIPv2)",
              RFC 7401, DOI 10.17487/RFC7401, April 2015,
              <https://www.rfc-editor.org/info/rfc7401>.

Authors' Addresses

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

   Email: adam.wiethuechter@axenterprize.com


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

   Email: stu.card@axenterprize.com


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

   Email: rgm@labs.htt-consult.com







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