UAS Operator Privacy for RemoteID Messages
draft-moskowitz-drip-operator-privacy-03

DRIP                                                        R. Moskowitz
Internet-Draft                                            HTT Consulting
Intended status: Standards Track                                 S. Card
Expires: 9 November 2020                                 A. Wiethuechter
                                                           AX Enterprize
                                                              8 May 2020

               UAS Operator Privacy for RemoteID Messages
                draft-moskowitz-drip-operator-privacy-03

Abstract

   This document describes a method of providing privacy for UAS
   Operator/Pilot 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

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   This Internet-Draft will expire on 9 November 2020.

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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  . . . . . . . . . .   5
   4.  System Message Privacy  . . . . . . . . . . . . . . . . . . .   5
     4.1.  Rules for encrypting System Message content . . . . . . .   5
     4.2.  Rules for decrypting System Message content . . . . . . .   6
   5.  Operator ID Message Privacy . . . . . . . . . . . . . . . . .   6
     5.1.  Rules for encrypting Operator ID Message content  . . . .   7
     5.2.  Rules for decrypting Operator ID Message content  . . . .   7
   6.  Cipher choices for Operator PII encryption  . . . . . . . . .   7
     6.1.  Using AES-CFB16 . . . . . . . . . . . . . . . . . . . . .   7
     6.2.  Using a Feistel scheme  . . . . . . . . . . . . . . . . .   8
     6.3.  Using AES-CTR . . . . . . . . . . . . . . . . . . . . . .   8
   7.  DRIP Requirements addressed . . . . . . . . . . . . . . . . .   8
   8.  ASTM Considerations . . . . . . . . . . . . . . . . . . . . .   8
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   10. Security Considerations . . . . . . . . . . . . . . . . . . .   9
     10.1.  CFB16 Risks  . . . . . . . . . . . . . . . . . . . . . .   9
     10.2.  Crypto Agility . . . . . . . . . . . . . . . . . . . . .   9
   11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   9
   12. Normative References  . . . . . . . . . . . . . . . . . . . .   9
   13. Informative References  . . . . . . . . . . . . . . . . . . .  10
   Appendix A.  Feistel Scheme . . . . . . . . . . . . . . . . . . .  10
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  11

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 UAS Operator/Pilot information.
   Encrypting in place means that the ciphertext is exactly the same
   length as the cleartext, and directly replaces it.

   An example of and an initial application of this mechanism is the 8
   bytes of UAS Operator/Pilot (hereafter called simply Operator)
   longitude and latitude location in the System Message.  This meets
   the Drip Requirements [drip-requirements], Priv-01.

   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
   missions.

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   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 some AES modes of operation
   are proposed here that can encrypt a multiple of 4 bytes.

   The System Message is not a simple, one-time, encrypt the PII with
   the ECIES derived key.  The Operator may move during a mission and
   these fields change, correspondingly.  Further, not all messages will
   be received by the USS, so each message's encryption must stand on
   its own and not be at risk of attack by the content of other
   messages.

   Another candidate message is the optional Operator ID Message with
   its 20 character Operator ID field.  The Operator ID does not change
   during a mission, so this is a one-time encryption operation for the
   mission.  The same cipher SHOULD be used for all messages from the
   UAS and this will influence the cipher selection.

   Future applications of this mechanism may be provided.  The content
   of the System Message may change to meet CAA requirements, requiring
   encrypting a different amount of data.  At that time, they will be
   added to this document.

2.  Terms and Definitions

2.1.  Requirements Terminology

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

2.2.  Definitions

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

   CAA
      Civil Aeronautics Administration.  Two CAAs are the US Federal
      Aviation Administration (FAA) and European Union Aviation Safety
      Agency (EASA).

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   ECIES
      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.

   GCS
      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.

   Observer
      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.

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

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

   UA
      Unmanned Aircraft.  In this document UA's are typically though of
      as drones of commercial or military variety.  This is a very
      strict definition which can be relaxed to include any and all
      aircraft that are unmanned.

   UAS
      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.

   USS
      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).

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   UTM
      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
   ECIES.

   The operator's ECIES public key SHOULD be unique for each mission.
   The USS ECIES public key may be unique for each operator and mission,
   but not required.  For best post-compromise security (PCS), even the
   USS ECIES 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 Operator (Pilot in the field
   description) of the UA.  The GCS MAY encrypt these as follows.

   The 8 bytes of Operator information are encrypted, using the ECIES
   derived 128 bit shared secret, with one of the cipher's specified
   below.  The choice of cipher is based on USS policy and is agreed to
   as part of the mission registration.  AES-CFB16 is the recommended
   default cipher.

   ASTM Remote ID and Tracking messages [F3411-19] SHOULD be updated to
   allow Bit 2 of the Flags byte in the System Message 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.  Rules for encrypting System Message content

   If the Operator location is encrypted the encrypted bit flag MUST be
   set to 1.

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   The Operator MAY be notified by the USS that the mission has entered
   a location or time where privacy of Operator location is not allowed.
   In this case the Operator MUST disable this privacy feature and send
   the location unencrypted or land the UA or route around the
   restricted area.

   If the Operator looses connectivity to the USS, the privacy feature
   SHOULD be disabled or land the UA.

   If the mission is in an area or time with no Internet Connectivity,
   the privacy feature MUST NOT be used.

4.2.  Rules for decrypting System Message content

   An Observer receives a System Message with the encrypt bit set to 1.
   The Observer sends a query to its USS Display Provider containing the
   UA's ID and the encrypted fields.

   The USS Display Provider MAY deny the request if the Observer does
   not have the proper authorization.

   The USS Display Provider MAY reply to the request with the decrypted
   fields if the Observer has the proper authorization.

   The USS Display Provider MAY reply to the request with the decrypting
   key if the Observer has the proper authorization.

   The Observer MAY notify the USS through its USS Display Provider that
   content privacy for a UAS in this location/time is not allowed.  If
   the Observer has the proper authorization for this action, the USS
   notifies the Operator to disable this privacy feature.

5.  Operator ID Message Privacy

   The Operator ID Message contains 20 bytes for Operator the ID.  The
   GCS MAY encrypt these as follows.

   The 20 bytes Operator ID is encrypted, using the ECIES derived 128
   bit shared secret, with one of the cipher's specified below.  The
   choice of cipher is based on USS policy and is agreed to as part of
   the mission registration.  AES-CFB16 is the recommended default
   cipher.

   ASTM Remote ID and Tracking messages [F3411-19] SHOULD be updated to
   allow Operator ID Type in the Operator ID Message set to "1" to
   indicate the Operator ID is encrypted.

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   The USS similarly decrypts these 20 bytes and provides the
   information to authorized entities.

5.1.  Rules for encrypting Operator ID Message content

   If the Operator ID is encrypted the Operator ID Type field MUST be
   set to 1.

   The Operator MAY be notified by the USS that the mission has entered
   a location or time where privacy of Operator ID is not allowed.  In
   this case the Operator MUST disable this privacy feature and send the
   ID unencrypted or land the UA or route around the restricted area.

   If the Operator looses connectivity to the USS, the privacy feature
   SHOULD be disabled or land the UA.

   If the mission is in an area or time with no Internet Connectivity,
   the privacy feature MUST NOT be used.

5.2.  Rules for decrypting Operator ID Message content

   An Observer receives a Operator ID Message with the Operator ID Type
   field set to 1.  The Observer sends a query to its USS Display
   Provider containing the UA's ID and the encrypted fields.

   The USS Display Provider MAY deny the request if the Observer does
   not have the proper authorization.

   The USS Display Provider MAY reply to the request with the decrypted
   fields if the Observer has the proper authorization.

   The USS Display Provider MAY reply to the request with the decrypting
   key if the Observer has the proper authorization.

   The Observer MAY notify the USS through its USS Display Provider that
   content privacy for a UAS in this location/time is not allowed.  If
   the Observer has the proper authorization for this action, the USS
   notifies the Operator to disable this privacy feature.

6.  Cipher choices for Operator PII encryption

6.1.  Using AES-CFB16

   CFB16 is defined in [NIST.SP.800-38A], Section 6.3.  This is the
   Cipher Feedback (CFB) mode operating on 16 bits at a time.  This
   variant of CFB can be used to encrypt any multiple of 2 bytes of
   cleartext.

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   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 128 bit IV for AES-CFB16 is constructed by the Operator and USS
   as: SHAKE128(MAC|UTCTime|Message_Type, 128).  Inclusion of the ASTM
   Message_Type ensures a unique IV for each Message type that contains
   PII to encrypt.

   AES-CFB16 would then be used to encrypt the Operator information.

6.2.  Using a Feistel scheme

   If the encryption speed doesn't matter, we can use the following
   approach based on the Feistel scheme.  This approach is already being
   used in format-preserving encryption (e.g. credit card numbers).  The
   Feistal scheme is explained in Appendix A.

6.3.  Using AES-CTR

   If 2 bytes of the 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: SHAKE128(MAC|UTCTime|Message_Type, 112).
   Inclusion of the ASTM Message_Type ensures a unique IV for each
   Message type that contains PII to encrypt.

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

7.  DRIP Requirements addressed

   This document provides solution to PRIV-1 for PII in the ASTM System
   Message.

8.  ASTM Considerations

   ASTM will need to make the following changes to the "Flags" in the
   System Message:

   Bit 2:
      Value 1 for encrypted; 0 for cleartext (see Section 4).

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   ASTM will need to make the following changes to the "Operator ID
   Type" in the Operator ID Message:

   Operator ID Type
      Value 1 for encrypted Operator ID (see Section 5).

9.  IANA Considerations

   TBD

10.  Security Considerations

   An attacker has no known text after decrypting to determine a
   successful attack.  An attacker can make assumptions about the high
   order byte values for Operator Longitude and Latitude that may
   substitute for known cleartext.  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.

10.1.  CFB16 Risks

   Using the same IV for different Operator information values with
   CFB16 presents a cyptoanalysis risk.  Typically only the low order
   bits would change as the Operators position changes.  Thus the first
   2 encrypted bytes would not change, and only subsequent bytes would.
   The risk is mitigated due to the short-term value of the data.
   Further analysis is need to properly place risk.

10.2.  Crypto Agility

   The ASTM Remote ID Messages do not provide any space for a crypto
   suite indicator or any other method to manage crypto agility.

   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.

11.  Acknowledgments

   TBD

12.  Normative References

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   [NIST.SP.800-38A]
              Barker, E., Chen, L., and R. Davis, "Recommendation for
              key-derivation methods in key-establishment schemes",
              National Institute of Standards and Technology report,
              DOI 10.6028/nist.sp.800-56cr1, April 2018,
              <https://doi.org/10.6028/nist.sp.800-56cr1>.

   [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>.

13.  Informative References

   [drip-requirements]
              Card, S., Wiethuechter, A., and R. Moskowitz, "Drone
              Remote Identification Protocol (DRIP) Requirements", Work
              in Progress, Internet-Draft, draft-card-drip-reqs-02, 20
              April 2020,
              <https://tools.ietf.org/html/draft-card-drip-reqs-02>.

   [F3411-19] ASTM International, "Standard Specification for Remote ID
              and Tracking", February 2020,
              <http://www.astm.org/cgi-bin/resolver.cgi?F3411>.

   [new-crypto]
              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-
              moskowitz-hip-new-crypto-04>.

   [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>.

Appendix A.  Feistel Scheme

   This approach is already being used in format-preserving encryption.

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   According to the theory, to provide CCA security guarantees (CCA =
   Chosen Ciphertext Attacks) for m-bit encryption X |-> Y, we should
   choose d >= 6.  It seems very ineffective that when shortening the
   block length, we have to use 6 times more block encryptions.  On the
   other hand, we preserve both the block cipher interface and security
   guarantees in a simple way.

   How to encrypt an m-bit plaintext X using an n-bit block cipher
       E = {E_K} for n > m?

       Enc(X, K):
         1. Y <- X.
         2. Split Y into 2 equal parts: Y = Y1 || Y2
         (let us assume for simplicity that m is even).
         3. For i = 1, 2, ..., d do:
           Y <- Y2 || (Y1 ^ first_m/2_bits(E_K(Y2 || Ci)),
         where Ci is a (n - m/2)-bit round constant.
         4. Y <- Y2 || Y1.
         5. Return Y.

       Dec(Y, K):
         1. X <- Y.
         2. Split X into 2 equal parts: X = X1 || X2.
         3. For i = d, ..., 2, 1 do:
           X <- X2 || (X1 ^ first_m/2_bits(E_K(X2 || Ci)).
         4. X <- X2 || X1.
         5. Return X.

Authors' Addresses

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

   Email: rgm@labs.htt-consult.com

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

   Email: stu.card@axenterprize.com

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

   Email: adam.wiethuechter@axenterprize.com

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