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Versions: 00 01 02 03 04 05 06 07                                       
DRIP                                                        R. Moskowitz
Internet-Draft                                            HTT Consulting
Intended status: Standards Track                                 S. Card
Expires: 2 October 2020                                  A. Wiethuechter
                                                           AX Enterprize
                                                           31 March 2020

                 Operator Privacy for RemoteID Messages


   This document describes a method of providing privacy for Operator
   information specified in the ASTM UAS Remote ID and Tracking
   messages.  This is achieved by encrypting, in place, those fields
   containing Operator sensitive data using a hybrid ECIES.

Status of This Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 2 October 2020.

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
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   Please review these documents carefully, as they describe your rights
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   provided without warranty as described in the Simplified BSD License.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terms and Definitions . . . . . . . . . . . . . . . . . . . .   3
     2.1.  Requirements Terminology  . . . . . . . . . . . . . . . .   3
     2.2.  Definitions . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  The Operator - USS Security Relationship  . . . . . . . . . .   4
   4.  System Message Privacy  . . . . . . . . . . . . . . . . . . .   4
     4.1.  Using AES in the System Message . . . . . . . . . . . . .   5
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
     6.1.  Crypto Agility  . . . . . . . . . . . . . . . . . . . . .   5
   7.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   6
   8.  Normative References  . . . . . . . . . . . . . . . . . . . .   6
   9.  Informative References  . . . . . . . . . . . . . . . . . . .   6
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   6

1.  Introduction

   This document defines a mechanism to provide privacy in the ASTM
   Remote ID and Tracking messages [F3411-19] by encrypting, in place,
   those fields that contain sensitive Operator information.  An example
   of such, and the initial application of this mechanism is the
   Operator longitude and latitude location in the System Message.

   It is assumed that the Operator registers a mission with a USS.
   During this mission registration, the Operator and USS exchange
   public keys to use in the hybrid ECIES.  The USS key may be long
   lived, but the Operator key SHOULD be unique to a specific mission.
   This provides protection if the ECIES secret is exposed from prior

   The actual Tracking message field encryption MUST be an "encrypt in
   place" cipher.  There is rarely any room in the tracking messages for
   a cipher IV or encryption MAC.  There is rarely any data in the
   messages that can be used as an IV.  A cipher that meets this
   requirement is SPECK [Need Reference]; which is an initial
   recommendation.  There are risks with this cipher, only partially
   mitigated by the ephemeral nature of the sensitive Operator
   information in the Tracking messages and the short-lived nature of
   the ECIES secret.  Other ciphers will be investigated.

   Future applications of this mechanism may be provided.  At that time,
   they will be added to this document.

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2.  Terms and Definitions

2.1.  Requirements Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "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

      Broadcast Remote ID.  A method of sending RID messages as 1-way
      transmissions from the UA to any Observers within radio range.

      Civil Aeronautics Administration.  An example is the Federal
      Aviation Administration (FAA) in the United States of America.

      Elliptic Curve Integrated Encryption Scheme.  A hybrid encryption
      scheme which provides semantic security against an adversary who
      is allowed to use chosen-plaintext and chosen-ciphertext attacks.

      Ground Control Station.  The part of the UAS that the remote pilot
      uses to exercise C2 over the UA, whether by remotely exercising UA
      flight controls to fly the UA, by setting GPS waypoints, or
      otherwise directing its flight.

      Referred to in other UAS documents as a "user", but there are also
      other classes of RID users, so we prefer "observer" to denote an
      individual who has observed an UA and wishes to know something
      about it, starting with its RID.

      Network Remote ID.  A method of sending RID messages via the
      Internet connection of the UAS directly to the UTM.

      Remote ID.  A unique identifier found on all UA to be used in
      communication and in regulation of UA operation.

      Unmanned Aircraft.  In this document UA's are typically though of
      as drones of commercial or military variety.  This is a very

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      strict definition which can be relaxed to include any and all
      aircraft that are unmanned.

      Unmanned Aircraft System.  Composed of Unmanned Aircraft and all
      required on-board subsystems, payload, control station, other
      required off-board subsystems, any required launch and recovery
      equipment, all required crew members, and C2 links between UA and
      the control station.

      UAS Service Supplier.  Provide UTM services to support the UAS
      community, to connect Operators and other entities to enable
      information flow across the USS network, and to promote shared
      situational awareness among UTM participants.  (From FAA UTM
      ConOps V1, May 2018).

      UAS Traffic Management.  A "traffic management" ecosystem for
      uncontrolled operations that is separate from, but complementary
      to, the FAA's Air Traffic Management (ATM) system.

3.  The Operator - USS Security Relationship

   All CAAs have rules defining which UAS must be registered to operate
   in their National Airspace.  This includes UAS and Operator
   registration in a USS.  Further, operator's are expected to report
   flight missions to their USS.  This mission reporting provides a
   mechanism for the USS and operator to establish a mission security
   context.  Here it will be used to exchange public keys for use in

   The operator's public key SHOULD be unique for each mission.  The USS
   public key may be unique for each operator and mission, but not
   required.  For best post-compromise security (PCS), even the USS
   public key should be changed over some operational window.

   The public key algorithm should be Curve25519 [RFC7748].
   Correspondingly, the ECIES 128 bit shared secret should be generated
   using KMAC as specified in sec 5 of [new-crypto].

4.  System Message Privacy

   The System Message contains 8 bytes of Operator specific information:
   Longitude and Latitude of the Remote Pilot of the UA.  The GCS can
   encrypt these as follows.

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   The 8 bytes of Operator information are encrypted, using the ECIES
   128 bit shared secret with Speck64/128.

   Bit 2 of the Flags byte is set to "1" to indicate the Operator
   information is encrypted.

   The USS similarly decrypts these 8 bytes and provides the information
   to authorized entities.

4.1.  Using AES in the System Message

   If 2 bytes of the System Message can be set aside to contain a
   counter that is incremented each time the Operator information
   changes, AES-CTR can be used as follows.

   The Operator includes a 64 bit UNIX timestamp for the mission time,
   along with its mission pubic key.  The Operator also includes the UA
   MAC address (or multiple addresses if flying multiple UA).

   The high order bits of an AES-CTR counter is constructed by the
   Operator and USS as: LTRUNC(HASH(MAC|UTCTime), 14).

   AES-CTR would then be used to encrypt the Operator information.

5.  IANA Considerations


6.  Security Considerations

   The use of Speck for the block cipher has risks.  Speck has been
   extensively analyzed.  The risk is mitigated as the key is used to
   protect a limited number of blocks.  In a 4 hour mission with a
   System Message every 10 seconds, there are only 1,440 applications of
   the Speck cipher, provided that the operator reported to the UA a new
   location within those 10 second windows.

   Further, an attacker has no known text after decrypting to determine
   a successful attack.  There is no knowledge of where the operator is
   in relation to the UA.  Only if changing location values "make sense"
   might an attacker assume to have revealed the operator's location.

6.1.  Crypto Agility

   The Remote ID System Message does not provide any space for a crypto
   suite indicator or any other method to manage crypto agility.

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   All crypto agility is left to the USS policy and the relation between
   the USS and operator.  The selection of the ECIES public key
   algorithm, the shared secret key derivation function, and the actual
   symmetric cipher used for on the System Message are set by the USS
   which informs the operator what to do.

7.  Acknowledgments

   The recommendation to use Speck for the block cipher comes after
   discussions on the IRTF CFRG mailing list.  Better known ciphers will
   not work for this situation without changes to the System Message

8.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [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>.

9.  Informative References

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

              Moskowitz, R., Card, S., and A. Wiethuechter, "New
              Cryptographic Algorithms for HIP", Work in Progress,
              Internet-Draft, draft-moskowitz-hip-new-crypto-04, 23
              January 2020, <https://tools.ietf.org/html/draft-

   [RFC7748]  Langley, A., Hamburg, M., and S. Turner, "Elliptic Curves
              for Security", RFC 7748, DOI 10.17487/RFC7748, January
              2016, <https://www.rfc-editor.org/info/rfc7748>.

Authors' Addresses

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

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   Email: rgm@labs.htt-consult.com

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

   Email: stu.card@axenterprize.com

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

   Email: adam.wiethuechter@axenterprize.com

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