UAS Operator Privacy for RemoteID Messages
draft-moskowitz-drip-operator-privacy-09

Document Type Active Internet-Draft (individual)
Authors Robert Moskowitz  , Stuart Card  , Adam Wiethuechter 
Last updated 2021-10-07
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DRIP                                                        R. Moskowitz
Internet-Draft                                            HTT Consulting
Intended status: Standards Track                                 S. Card
Expires: 10 April 2022                                   A. Wiethuechter
                                                           AX Enterprize
                                                          7 October 2021

               UAS Operator Privacy for RemoteID Messages
                draft-moskowitz-drip-operator-privacy-09

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

   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
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   This Internet-Draft will expire on 10 April 2022.

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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
     3.1.  ECIES Shared Secret Generation  . . . . . . . . . . . . .   4
   4.  System Message Privacy  . . . . . . . . . . . . . . . . . . .   5
     4.1.  Rules for encrypting System Message content . . . . . . .   6
     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 . . . . . . . . . . . . . . . . . . . . .   8
     6.2.  Using a Feistel scheme  . . . . . . . . . . . . . . . . .   8
     6.3.  Using AES-CTR . . . . . . . . . . . . . . . . . . . . . .   8
   7.  DRIP Requirements addressed . . . . . . . . . . . . . . . . .   8
   8.  ASTM Considerations . . . . . . . . . . . . . . . . . . . . .   9
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   10. Security Considerations . . . . . . . . . . . . . . . . . . .   9
     10.1.  CFB16 Risks  . . . . . . . . . . . . . . . . . . . . . .   9
     10.2.  Crypto Agility . . . . . . . . . . . . . . . . . . . . .   9
     10.3.  Key Derivation vulnerabilities . . . . . . . . . . . . .  10
     10.4.  KMAC Security as a KDF . . . . . . . . . . . . . . . . .  10
   11. Normative References  . . . . . . . . . . . . . . . . . . . .  10
   12. Informative References  . . . . . . . . . . . . . . . . . . .  11
   Appendix A.  Feistel Scheme . . . . . . . . . . . . . . . . . . .  12
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  12
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12

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 ASTM System Message (Msg Type
   0x4).  This meets the Drip Requirements [drip-requirements], Priv-01.

   It is assumed that the Operator, via the UAS, registers an operation
   with its USS.  During this operation registration, the UAS and USS
   exchange public keys to use in the hybrid ECIES.  The USS key may be

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   long lived, but the UAS key SHOULD be unique to a specific operation.
   This provides protection if the ECIES secret is exposed from prior
   operations.

   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 (AEAD tag).  There is rarely any data
   in the messages that can be used as an IV.  The AES-CFB16 mode of
   operation proposed here can encrypt a multiple of 2 bytes.

   The System Message is not a simple, one-time, encrypt the PII with
   the ECIES derived key.  The Operator may move during a operation 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 ASTM Operator ID Message
   (Msg Type 0x5) with its 20 character Operator ID field.  The Operator
   ID does not change during an operation, so this is a one-time
   encryption operation for the operation.  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.

   Editor note: The Rules for allowing encryption need to be updated to
   handle the UA operating in Broadcast Remote ID only mode.  That is
   conditions where the USS cannot notify the UAS to stop encrypting.

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

   See Drip Requirements [drip-requirements] for common DRIP terms.

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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.

   Keccak (KECCAK Message Authentication Code):
      The family of all sponge functions with a KECCAK-f permutation as
      the underlying function and multi-rate padding as the padding
      rule.

   KMAC (KECCAK Message Authentication Code):
      A PRF and keyed hash function based on KECCAK.

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 operations to their USS.  This operation reporting provides a
   mechanism for the USS and operator to establish an operation 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 operation.
   The USS ECIES public key may be unique for each operator and
   operation, but not required.  For best post-compromise security
   (PCS), 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 [NIST.SP.800-185].

3.1.  ECIES Shared Secret Generation

   The KMAC function provides a new, more efficient, key derivation
   function over HKDF [RFC5869].  This will be referred to as KKDF.

   HKDF needs a minimum of 4 hash functions (e.g.  SHA256).  KKDF does
   an equivalent shared secret generation in a single Keccak Sponge
   operation.

   When the USS - UAS Operation Security Context is established, the UAS
   provides its UAS ID (null padded to 20 characters per [F3411-19]) and
   a 256 bit random nonce to the USS.  These are inputs, along with the
   ECDH keys to produce the shared secret as follows.

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   A 64 bit UNIX timestamp for the operation time is also included in
   the Operation Security Context.  This will be used in the IV
   construction.

   Per [NIST.SP.800-56Cr1], Section 4.1, Option 3:

        Shared Secret = KMAC128(salt, IKM, L, S)

   L is the derived key bit length.  Since only a single key is needed,
   L=128.

   S is the byte string 01001011 || 01000100 || 01000110, which
   represents the sequence of characters "K", "D", and "F" in 8-bit
   ASCII.

        salt = Nonce-USS |  Nonce-UAS

   There are special security considerations for IKM per [RFC7748].  The
   IKM as follows:

        IKM = Diffie-Hellman secret | USS-ID | RID

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.

   Editors Note: The next version of [F3411-19], currently in ballot, is
   adding a 2 byte Operator Altitude field, thus increasing the Operator
   specific information to 10 bytes.  This change will be delineated via
   Protocol Version field.  It is this future shift from a multiple of 4
   bytes to a multiple of 2 bytes that is the reason to change from
   CFB32 in earlier drafts to CFB16 used now.

   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 operation registration.  AES-CFB16 is the recommended
   default cipher.

   ASTM Remote ID and Tracking messages [F3411-19] SHOULD be updated to
   allow Bit 5 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.

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4.1.  Rules for encrypting System Message content

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

   The Operator MAY be notified by the USS that the operation 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 UAS looses connectivity to the USS, the privacy feature SHOULD
   be disabled or land the UA.

   If the operation 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 the 20 byte Operator 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 operation registration.  AES-CFB16 is the recommended default
   cipher.

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   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.

   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 operation 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 UAS looses connectivity to the USS, the privacy feature SHOULD
   be disabled or land the UA.

   If the operation 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

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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.

   The Operator includes a 64 bit UNIX timestamp for the operation time,
   along with its operation 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 operation time,
   along with its operation 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.

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8.  ASTM Considerations

   ASTM will need to make the following changes to the "Flags" in the
   System Message (Msg Type 0x4):

   Bit 5:
      Value 1 for encrypted; 0 for cleartext (see Section 4).

   ASTM will need to make the following changes to the "Operator ID
   Type" in the Operator ID Message (Msg Type 0x5):

   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.  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/UAS.  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.

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10.3.  Key Derivation vulnerabilities

   [RFC7748] warns about using Curve25519 and Curve448 in Diffie-Hellman
   for key derivation:

   Designers using these curves should be aware that for each public
   key, there are several publicly computable public keys that are
   equivalent to it, i.e., they produce the same shared secrets.  Thus
   using a public key as an identifier and knowledge of a shared secret
   as proof of ownership (without including the public keys in the key
   derivation) might lead to subtle vulnerabilities.

   This applies here, but may have broader consequences.  Thus two
   endpoint IDs are included with the Diffie-Hellman secret.

10.4.  KMAC Security as a KDF

   Section 4.1 of NIST SP 800-185 [NIST.SP.800-185] states:

   "The KECCAK Message Authentication Code (KMAC) algorithm is a PRF and
   keyed hash function based on KECCAK . It provides variable-length
   output"

   That is, the output of KMAC is indistinguishable from a random
   string, regardless of the length of the output.  As such, the output
   of KMAC can be divided into multiple substrings, each with the
   strength of the function (KMAC128 or KMAC256) and provided that a
   long enough key is used, as discussed in Sec. 8.4.1 of SP 800-185.

   For example KMAC128(K, X, 512, S), where K is at least 128 bits, can
   produce 4 128 bit keys each with a strength of 128 bits.  That is a
   single sponge operation is replacing perhaps 5 HMAC-SHA256 operations
   (each 2 SHA256 operations) in HKDF.

11.  Normative References

   [NIST.SP.800-185]
              Kelsey, J., Change, S., and R. Perlner, "SHA-3 derived
              functions: cSHAKE, KMAC, TupleHash and ParallelHash",
              National Institute of Standards and Technology report,
              DOI 10.6028/nist.sp.800-185, December 2016,
              <https://doi.org/10.6028/nist.sp.800-185>.

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   [NIST.SP.800-38A]
              Dworkin, M., "Recommendation for block cipher modes of
              operation :: methods and techniques", National Institute
              of Standards and Technology report,
              DOI 10.6028/nist.sp.800-38a, 2001,
              <https://doi.org/10.6028/nist.sp.800-38a>.

   [NIST.SP.800-56Cr1]
              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>.

12.  Informative References

   [drip-requirements]
              Card, S. W., Wiethuechter, A., Moskowitz, R., and A.
              Gurtov, "Drone Remote Identification Protocol (DRIP)
              Requirements", Work in Progress, Internet-Draft, draft-
              ietf-drip-reqs-18, 8 September 2021,
              <https://datatracker.ietf.org/doc/html/draft-ietf-drip-
              reqs-18>.

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

   [RFC5869]  Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand
              Key Derivation Function (HKDF)", RFC 5869,
              DOI 10.17487/RFC5869, May 2010,
              <https://www.rfc-editor.org/info/rfc5869>.

   [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>.

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Appendix A.  Feistel Scheme

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

   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.

Acknowledgments

   The recommended ciphers come from discussions on the IRTF CFRG
   mailing list.

Authors' Addresses

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

   Email: rgm@labs.htt-consult.com

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   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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